electronics projects volume 15.bak

Electronics ProjectsVol. 15

ELECTRONICSPROJECTS

VOL. 15

EFY Enterprises Pvt LtdD-87/1 Okhla Industrial Area, Phase-1

New Delhi 110020

© EFY Enterprises Pvt Ltd.First Published in this Edition, January 2003

Published by Ramesh Chopra for EFY Enterprises Pvt Ltd,D-87/1, Okhla Industrial Area, Phase I, New Delhi 110020.

Typeset at EFY Enterprises Pvt Ltd andPrinted at J.K Offset, C-21, DDA Shed,

All rights reserved. No part of this book may be reproduced in anyform without the written permission of the publishers.

ISBN 81-88152-08 -0

Okhla Industrial Area, Phase-1, New Delhi 110020

Reprinted in November 2000Reprinted in April 2003, February 2007

FOREWORD

This volume of Electronics Projects is the thirteenth in the seriespublished by EFY Enterprises Pvt Ltd. It is a compilation of 23construction projects and 77 circuit ideas published in ‘ElectronicsFor You’ magazine during 1994.

In keeping with the past trend, all modifications, corrections andadditions sent by the readers, have been included with each project,along with the replies and amendments, if any, proposed/suggestedby the authors themseleves. It is a sincere endeavour on our part tomake each project as error-free and comprehensive as possible.However, responsibility cannot be taken if readers are unable tomake a circuit successfully, for whatever reason.

This collection of a large number of tested circuit ideas andconstruction projects in a handy volume would provide all classesof electronics enthusiasts—be they students, teachers, hobbyists orprofessionals—with a valuable source of electronic circuits, whichcan be fabricated using readily-available and reasonably-pricedcomponents. These circuits could either be used independently orin combination with other circuits, described in this and othervolumes. We are sure that this volume, like its predecessors, willgenerate tremendous interest among its readers.

EFY Books & Publications

FOR YOUEFY is a reputed information house, specialising in electronics and information technology

magazines. It also publishes directories and books on several topics. Its current publications are:(A) CONSTRUCTION PROJECTS1. Electronics Projects, Vol. 1: A compilation of selected construction projects and circuit ideas Rs 120

published in Electronics For You magazines during 1979 and 1980.2. Electronics Projects, Vol. 2 to 19 (English version): Yearly compilations (1981 to 1998) of Rs 120 (each)

interesting and useful construction projects and circuit ideas published in Electronics For You.3. Electronics Projects, Vol. 20, 21 and 22 (with CD): Yearly compilations (1999 to 2001). Rs 150 (each)4. Electronics Projects, Vol. 16 (fgUnh laLdj.k): Yearly compilations (1995) of interesting and Rs 95

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Section A: Construction Projects

1. A Versatile Receiver for DXers ................................................................................. 32. Gongs and Alarm for CMOS LED Dial Clock .......................................................... 83. Hi-Fi Stereo Cassette Deck ........................................................................................ 124. Digital Chess Clock ................................................................................................... 205. Cordless Security System .......................................................................................... 246. Microprocessor-Based Capacitance and Resistance Measurement Meter ................ 277. BFO for Receiving SSB Signals ................................................................................ 308. Safety Indicators and Aids ......................................................................................... 339. Portable SMD Soldering Station ............................................................................... 3710. Digital Controller for Outdoor Lights ....................................................................... 4011. VOX for Ham Rigs .................................................................................................... 4312. Steady Hand Game ..................................................................................................... 4613. Teleconversation Limiter ........................................................................................... 4914. Programmable Number Lock..................................................................................... 5215. Humidity Controller ................................................................................................... 5716. Doorbell-Cum-Security Alarm .................................................................................. 5917. Improved Three-Phase Power Supply Card .............................................................. 6518. Solidstate Advertisement Display .............................................................................. 6719. Auto-Changing In/Out Indicator With Door-Bell ..................................................... 7020. Telex-Computer Changeover Switch ......................................................................... 7321. 60W Active Amplifier System ................................................................................... 7622. Impedance Meter ........................................................................................................ 8023. Interruption Counter cum Burglar Alarm .................................................................. 86

Section B: Circuit Ideas:

1. Digital Even and Odd Parity Checker/Generator ...................................................... 942. SW Booster For DXers .............................................................................................. 953. Intercom for Your Home ............................................................................................ 954. Slave Switch ............................................................................................................... 965. Morse Interpulse Time Period Counter ..................................................................... 976. Amplifier Taciturner .................................................................................................. 987. Remote Audio Level Indicator ................................................................................... 99

CONTENTS

8. Simple Karaoke Circuit ............................................................................................. 999. Transistorised ‘Walkman’ Radio Player .................................................................... 10010. Safety Circuit for Instant Water Heater ..................................................................... 10111. Long Duration Sequential Timer With IC MM5387 ................................................. 10212. Telephone Silencer ..................................................................................................... 10413. Control Circuit For In/Out Counter ........................................................................... 10514. Low-Cost Pulse Generator ......................................................................................... 10615. Top-Secret Code Lock ............................................................................................... 10716. Accurate Frequency-to-Voltage Converter ................................................................ 10817. Rolling Electronic Dice ............................................................................................. 10918. Gradually Turning-Off Courtesy Light ...................................................................... 11019. Digital On-Off Timer ................................................................................................. 11120. Rain-Operated Wiper ................................................................................................. 11321. Programmable 100-Day Timer .................................................................................. 11322. Phase Sequence Detector cum Single-Phase Protector ............................................. 11423. Multichannel Touch Switch ....................................................................................... 11524. Two-In-One Switch Circuit ....................................................................................... 11625. Stereo Audio Level Indicator ..................................................................................... 11726. Keyboard With Strobe Signal .................................................................................... 11827. Hybrid Audio Output Amplifier ................................................................................ 11928. Sawtooth Wave Generator ......................................................................................... 11929. Multichannnel Water Level Monitor-cum-Controller ............................................... 12030. Programmable 100-hour Timer .................................................................................. 12131. -30 to +30V Regulated Power Supply ....................................................................... 12332. SCR/Triac Tester cum Distinguisher ......................................................................... 12433. Thermometer Add-on to Multimeter ......................................................................... 12534. Listening Bug ............................................................................................................. 12635. Telephone Tapping/Misuse Indicator ......................................................................... 12636. Electronic Calling System ......................................................................................... 12737. Hen Sound Emulator .................................................................................................. 12938. Phone Line Monitor ................................................................................................... 12939. Improved Pest Repeller .............................................................................................. 13040. Logic Level Detector cum Continuity Tester ............................................................ 13141. Flora Caretaker ........................................................................................................... 13142. Electronic Candle ....................................................................................................... 13243. Motor Protector And Water Level Indicator .............................................................. 13344. 360W Hi-Fi Stereo Power Amplifier ......................................................................... 13345. Regulated Power supply With Short-Circuit Protection ........................................... 13446. Musical Greeting Cards ............................................................................................. 13547. Temperature Controlled Fan ...................................................................................... 136

48. Line Synchronised Oscillator .................................................................................... 13849. Electronic Fuse ........................................................................................................... 13850. Polarity Tester ............................................................................................................ 13951. Low-Cost 4- to 16-line Decoder ................................................................................ 14052. Automatic Volume Control ........................................................................................ 14153. Musical Continuity Tester .......................................................................................... 14154. Hi-Q Stereo Mixer ..................................................................................................... 14255. 1Hz Master Oscillator ................................................................................................ 14356. Super Simple Timer ................................................................................................... 14357. Stereo Disconnector With Timer ............................................................................... 14458. Frequency Filter for Playing Old Records................................................................. 14559. Static Starter ............................................................................................................... 14660. Headphone Adaptor for Hi-Fi Stereo ......................................................................... 14661. Linear Bulb Light Controller ..................................................................................... 14762. Electronic Automobile Horn ...................................................................................... 14863. Call Bell For The Impatient ....................................................................................... 14964. 0-10V Amplifier ......................................................................................................... 14965. A Sensitive Touch Switch .......................................................................................... 15066. Remote TV Tester ...................................................................................................... 15167. pH Meter .................................................................................................................... 15268. Cordless Intercom ...................................................................................................... 15269. A Simple Battery Charger .......................................................................................... 15470. Temperature Sensor for Power Amplifiers ................................................................ 15571. Telephone Monitor ..................................................................................................... 15672. A Binary Frequency Meter ........................................................................................ 15773. Caretaker For Home Appliances ................................................................................ 15874. 40W +40W Auto-Reverse Stereo Cassette Player ..................................................... 16075. Remote TV Headphones ............................................................................................ 16176. Infrared Remote Control Tester ................................................................................. 16277. Soft Radio/Cassette Selector ...................................................................................... 163

SECTION A:CONSTRUCTION PROJECTS

ELECTRONICS PROJECTS-15 3

D espite the increasing Ham fra-ternity in India, commercialHam equipment is still a

dream for an average amateur radioenthusiast. And truly, it seems beyondthe reach of an average zealot to pur-chase such sophisticated commercialequipment, specially the high-grade re-ceivers. This is why several enthusi-asts give up the hobby soon after get-ting their licences.

Here is a simple and versatile, low-cost receiver design for such readers. Itcan be built in a few spare eveningswith the minimum of effort and expen-diture. The receiver can be modifiedeasily for other bands (right throughtop band to 2-metre). It can be usedwith your existing transmitter or as astand alone receiver for your shack.The receiver, despite being low-cost, isvery dynamic and certainly beats thecommercial black-boxes, costing morethan ten times.

A DC (direct-conversion) design hasbeen selected for its low cost and dy-namic operation, with easy assemblingand requirement of minimum align-ment. Low-cost and easily-availablecomponents have been selected. PhilipsNE602 has been selected for its bal-anced operation for good AM suppres-sion, excellent IMD and very low noise.Further, the chip in its 8-pin DIL pack-age has an on-chip oscillator/buffer—another plus point over the other chips,such as 3039, 3028 and 1496.

The operation of the receiver is verysimple, as shown in the blockdiagram.The input signal is first am-plified in RF preselector stage and is

source configuration, providing a gainof approximately 12 dB. 2N3823 hasbeen selected for its low noise and hightransductance. But BFW10 can be sub-stituted in case of its nonavailability,compromising a little on the overallperformance.

Common-source configuration hasbeen used for little loading of the inputtank circuit and excellent gain. Bipolarstages have been tested in the proto-type, but these tend to be rather un-stable and nonlinear. FET in this stagealso helps better IMD, an importantfactor for any serious DXer.

Both input and output are tuned forgood front-end selectivity and spuri-ous rejection. A 50-ohm antenna termi-nation has been provided, using a linktapping on coil L1 for proper imped-ance matching.

Good results can be obtained withproperly tuned preselector stage on160M, 80M, 40M and 20M bands. Butabove these frequencies, a preselectorstage often creates a problem. So,above these frequencies, it could beleft out—using a tuned circuit directlyat the input of IC1, with an impedance

fed to a mixer, where it is mixed withthe local oscillator frequency of almostthe same frequency. The resultant out-put, which is AF, is amplified for thedesired power to drive a loudspeakeror a headphone set. An AF narrow-band filter can be introduced betweenthe mixer and the AF amplifier, sincesuch designs lack selectivity. Other-wise, the output would span over al-most the entire AF spectrum, whichcan be detrimental, specially when onethreads a weak CW out from a crowdedband.

The design is also popularly knownas a ‘homodyne’, since it is a hetero-dyne, with the same injection frequencyand zero IF.

As the overall sensitivity dependsupon AF gain, it is quite easy to achievea good sensitivity and a low noise fig-ure with a dexterous design. But add-ing too much gain will be troublesomeand a microphonic effect will result.This is because tiny currents will beproduced during vibrations and thesecurrent changes will be amplified byhigh-gain AF amplifier, producing ahowling sound.

The aimed per-formance has beenderived from thecircuit, which com-prises three inte-grated circuits andtwo field effecttransistors (FETs).

The preselectoris designed aroundTl, an FET. It op-erates in common Fig.1: Block diagram of the receiver.

A VERSATILE RECEIVERFOR DXERS

KANG K.P.S.

4 ELECTRONICS PROJECTS-15

18 t

urn

s+

4 tu

rns

Fig.2: Circuit diagram of the receiver.


transformation ratio of about 1: 3.5 forconnecting a dipole antenna.

The 8-pin chip (IC1) has an on-chip oscillator, two cross-wired differ-entials as double balanced mixer, and abuffer for oscillator.

The chip works with a 6.2V supply,which is obtained using zener diode Dlat its pin 8. The input is given at pins 1and 2 with a suitable impedance trans-formation by coil L2. Capacitors C6 toC11, together with coil LX form thetank circuit for the oscillator, whichoperates in the Collpitt configuration.

Plug-in band-changing has beenemployed in the prototype for its easein assembly and operation. Coil LX isactually a plug-in inductor which canbe changed for 20- and 40-metre bands.It works very well with the given com-ponent values. The receiver can be usedfor other bands (almost any band from60- metre to 2-metre), simply by modi-fying the oscillator tank circuit, com-prising capacitors C6 to Cll and coilLX.

The on-chip oscillator is tolerablystable, provided a sturdy wiring is used

in the oscillator tank circuit. The out-put (AF) is taken from pins 4 and 5 ofthe IC through a transistor-type drivertransformer, commonly available in thelocal market.

Suitable value for capacitor Cl2 canbe found by experimenting. It shortsout any trace of RF components at theoutput and also peaking X1’s primaryfor certain frequency around 700 Hz,aiding in CW DXing. Any value be-tween 0.001 and 0.47μF will do.

A low-noise preamplifier stage fol-lows the mixer, designed around tran-

Fig. 3: Actual-size PCB layout of the receiver.

Fig.4: Component layout of the PCB shown in Fig. 3.


C9 –– 10-100pF air trimmerC11 –– 50pF air variableC12 –– (0.001μ to 0.47μF)

ceramic discC13,C20,C23 –– 0.47μF ceramic discC14,C16,C18 –– 0.001μF, polyesterC15 –– 47μF, 16V electrolyticC19 –– 100μF, 16V electrolyticC21 –– 470μF, 16V electrolyticC22 –– 220nF ceramic discC24 –– 1000pF ceramic discC25 –– 220nF, 16V electrolyticC26,C27 –– 10μF, 16V electrolytic

Miscellaneous:X1 –– Driver transformer with

1.5k: 1.5k impedanceratio

L1 –– 2J type SW antennacoil with 24 SWG, 4turns link forantenna winding

L2 –– 22 turns of 24 SWGtapped 4 turns fromlower end primary and8 turns secondary onPVC 1.27 cms air coretube

LX –– (see text)S1 –– SPST toggle switch

–– 4.8cms x 8-ohmloudspeaker

–– Ribbon cable–– Suitable cabinet

Semiconductors:IC1 –– NE602 buffer/oscillatorIC2 (a,b) –– TL072IC3 –– LM386NT1 –– 2N3823/BFW10T2 –– BFW10D1 –– 6.2V zener

Resistors (all 1/4watt, +5% carbon unlessstated otherwise):R1 –– 220-ohmR2 –– 470-ohmR3 –– 180-ohmR4 –– 1.2-kilohmR5 –– 22-ohmR6 –– 15-kilohmR7 –– 18-kilohmR8 –– 27-kilohmR9, R13 –– 680-kilohmR10, R14 –– 24-kilohmR11, R15 –– 33-kilohmR12, R16 –– 1.8-megohmR17 –– 100-ohmR18 –– 100-kilohmR19 –– 2.2-ohmR20 –– 4.7-ohmVR1 –– 100-kilohm (log)

Capacitors:C1 (A,B) –– 2J gang (variable)C2 –– 0.02μF ceramic discC3,C5 –– 0.047μF ceramic discC4 –– 22μF, 12V electrolyticC6,C7,C10 –– 100pF polyesterC8 –– 0.01μF polyester

Fig. 5: Power supply arrangement for the receiver.

PARTS LIST

made to rove through a 2-pole, nar-row-band AF filter, wired around low-noise op-amp TL072. Switch Sl pro-vides a choice of selecting a single poleor two, depending on the type ofwork—AM/ SSB or CW, respectively.The overall circuit provides good filtercharacteristics at reasonable cost andhelps to reduce the detrimental QRMduring CW DXing on crowded bands.

LM386N is the useful amplifier withjust about 30 dB of gain at good signal-to-noise ratio—an elegant addition tothe circuit, which provides sufficientgain to drive a speaker. Capacitor C22and resistor R19 form a high-pass filterfor supersonics to avoid instability.

Coil LX may be wound on a com-monly available 11mm Alladin former.The winding will have 13 turns of 28SWG for 40-metre band, while 20-metre band can be worked with 7 turnsof 28 SWG. The cores of coils can beadjusted for proper band limits andsealed with Bee-wax.

A plug-in arrangement of bandchanging has been used for ease andreduced complexity. The coil, with itsbase, is positioned near the right of theback. Both the coils provide sufficientoverlappings near both band-ends.

The receiver is designed for 12-voltbattery operation, but it can be usedwith almost any well-regulated powersupply. Two RFCs of about 40 MHz ormore need be placed in both supplylines, as shown in Fig.3. Otherwise, apoignant hum will result.

An indoor or long-wire outdoor an-tenna will give good results, but a di-pole will certainly enhance the perfor-mance of the receiver.

Although the unit is quite simple indesign, extremely good results can beobtained, if it is used skilfully. The pro-totype was able to receive 1-microvoltinput signal from Marconi PM 2952 ra-diotelephone test set (G-land made). �

Readers’ Comments:Thank you very much for publishingsuch a good article on ‘A Versatile Re-ceiver For DXers’. Kindly clarify thefollowing:

1. The author has used IC NE602,a double balanced mixer. In its placecan IC NE612 be used? The pinconfigurations for both the ICs aresame, but the latter contains an inter-

nal amplifier.2. The Value of RFC is not clear.

At 12 volts how much current shouldbe provided?

3. The data of L2 is not givenclearly. Secondly, 50pF tuning capaci-tor is not available.

4. Can varactor diodes not be usedfor tuning?

5. Can T2 be dispensed with, as

FETs are costly, costing at least Rs40to 50 per piece?

6. Output from IC1 can be takeneither from pin 4 or 5 and not both.

I.S. RathodJunagadh

� In the article the author says that thegiven preselector cannot be used abovetwenty metre range. I think the prob-lem is due to the use of FET BFW10.

the source for optimum gain.Since the bandwidth of the receiver

at this stage is quite wide and spansalmost the full AF range, the signal is

sistor T2, with about another 15 dBelevation to AF signals. Resistors R5to R8 set the biasing of FET for linear‘Q’ point and capacitor C27 bypasses


In international data books, the fre-quency range of BFW10 is given asbelow 20 MHz. Therefore, for VHFoperation, BFW10 in the preselectormust be replaced by MPF 102 or asimilar VHF FET. Use of BFW10 at 2metre range is wrong. Also, at 2 metres,frequency modulation (FM) is used andnot AM.� Is IC NE612 equivalent to ICNE602? IC NE602 contains voltageregulator stage internally. What is theneed, therefore, of connecting an ex-ternal 6V zener diode in the circuit? Ithink a current limiting resistor shouldbe sufficient. Wherefrom can the ICNE612 be obtained?

Pradeep G.Kerala

� With reference to the circuit ‘AVersatile Receiver For DXers’, Iwould like to know how it can beconverted into a transreceiver? Theauthor is also requested to clarifywhether the circuit idea ‘SW Boosterfor DXers’ by Mr Pradeep G., publishedon page 102 in this book, can be incor-

porated in the circuit.Ginish K. Varghese

Kerala

The author, Mr K.P.S. Kang, replies:I, first of all, thank all the readers for

their valuable feedback and am pleasedto note their interest in the article.

The given circuit is a simple directconversion receiver.

IC NE612 is a good alternative andcan be substituted for NE602. Whileusing IC NE612, connect a 27k resis-tor from pin 7 to ground.

The value of RFCs is in millihenryand the circuit draws about 350mA at12V.

All the formers’ diameter aregiven. The 50pF capacitors are easilyavailable in Delhi. However in caseof nonavailability, there are two al-ternatives: (1) Pick a standard vari-able capacitor and count its vanes(moving plates). Divide the total ca-pacitance value by the number ofplates, so as to get capacity per plate.By this method, you can remove any

number of plates to get the desiredcapacitance. (2) Use a 47pF capacitorin series with the standard variablecapacitor.

I never recommend the use ofvaractors in HF circuits. FETs are avail-able at fairly cheap prices. Also, FETsare specially selected for their low noiseand very high input impedances. ICNE602 in this arrangement is used inbalanced configuration, so both tails ofmixer are used for output. This pro-vides maximum AM rejection.

A preselector often causes troubleat 20 metres and neighbouring bandswith a lot of QRN. So, it is better notto use a preselector stage in thesebands.

SW booster circuit need not be usedwith the project, as it is provided onboard itself (constituted around T1).The sensitivity of the receiver is suffi-cient for serious DX experimentation.

The project can be converted into afull-fledged transreceiver by inclusionof a transmitter with the receiver.


All the circuits are built around eas-ily available CMOS chips. This helpsto keep the current consumption low,maintain a wide supply voltage limit,economise the design and simulta-neously obtain the maximum reliabil-ity. A complete power supply, alongwith a battery backup facility, has alsobeen included to adapt the clock formains operation.

The circuits can, in fact, be inter-faced with any of the dial clockprojects, published in EFY after slightalterations. The block diagram of thecomposite clock is shown in Fig. 1.

The hourly gongThe feature of the hourly gong,

which was until now found only in theold mechanical clocks, can be added

T his article describes the func-tional and constructional de-tails of several features, such

as an hourly pendulum gong, a regularalarm, a digital calendar, a day indica-tor, and an AM-PM indicator, whichcan be added to the basic CMOS dialclock, published in EFY, May 1990 is-sue. These will enhance the clock’s util-ity to the maximum.

GONGS AND ALARMFOR CMOS LED DIAL CLOCK

AMRIT BIR TIWANA

An hourly gong, a regular alarm, a calendar, a day indicator and an AM-PM indicator—these are just a few of the facilities, described here, that

can be added to EFY’s basic CMOS LED dial clock.

Fig. 1. Block schematic of the composite dial clock.


When the alarm sounds, it may ei-ther be disabled manually through S3or it will automatically reset itself afterfive minutes.

CalendarThe calendar comprises a day and a

date display. The display circuit isshown in Fig. 4 and the date displaycircuit in Fig. 5.

The day display uses the CMOSoctal counter CD4022BE, which iswired to count up to seven. The ICdirectly drives the LEDs that displaythe day. The 1-ppd input is fed to clockinput of the IC, which counts up toseven pulses and resets on the eighth.Switch Sl is used to set the days andthe NAND gate is used to preventdebouncing.

The date display uses two CMOSCD4033BE counters, which are usedto both count and drive the displays.The carry output of the first counteris fed to the clock input of the next.

The total number of pulses that canbe counted are 99. The circuit needsto be reset after the end of eachmonth, using switch S5. The initialsetting is done through switch S6. Thereset function can be made automaticby the implementation of multi-inputNAND gates, such as 4011,4023 or4068.

Fig.2: The hourly gong for the dial clock

Fig.3: The alarm circuit.

quite easily to the dial clock as well.The circuit given in Fig. 2 producesthe sound of a huge hammer hitting abrass gong, which is made highly real-istic by the effective utilisation of theADSR envelope effect.

The circuit is based on a CMOSLSI chip. The 1-pph (pulse per hour)

output is fed to the enable input of theIC through the level dropper, compris-ing the 10k resistor and the 4.7μF ca-pacitor. The output signal is amplifiedby Tl, which drives the loudspeaker.

The number of times the ‘gong’strikes corresponds to the time—forexample, ten gongs at 10 o’clock andeleven at 11 o’clock.

Alarm

The circuit for an add-on alarm isgiven in Fig. 3. The alarm time can beset, using the miniature 12-wayswitches—Sl for hours and S2 for min-utes (minimum 5). S3 is used to enableor disable the alarm.

The circuit uses a CMOS timer7555 in astable mode, which is en-abled when pin 4 is at logic 1. GateNl is used to multiplex the signalsfrom the ‘time set’ switches. The re-set terminal of IC 7555 goes highwhen the time displayed on the dialis the same as that preset. The alarmtime may also be hard-wired.


AM-PM indicatorThe AM-PM indicator,

which is given in Fig. 6, isbased on the CMOSCD4017BE counter that iswired as a simple flip-flop.The circuit provides AM-PMindication and simultaneouslyprovides the 1-ppd output.The indication is providedthrough two LEDs. ICCD4017 can easily be substi-tuted with dual flip-flop ICs,such as CD4013 or CD4027.

Seconds flasherThe seconds flasher, or the

blinking/activity indicator, asit is commonly known, canalso be added to the clock.As a IHz pulse source is notavailable in the clock, an ex-ternal oscillator is used. Thecircuit is built around the un-used gate of CD4093 IC. VR1should be adjusted to makethe LED flash at a frequencyof 1 Hz.

Power supply

Although CMOS chipscan work off supply voltagesbetween 3 and 15 volts, astable and regulated powersupply (9V) should be usedto maintain timing stabilityand display brightness. A suit-able circuit is given in Fig. 8.The circuit is provided withan in-built battery back-up in

Fig. 4: The day indicator circuit.

PARTS LISTSemiconductors:IC1 — CD45273IC2 (N1-N4),IC7 (N5,N6) — CD4093 NANDIC3 — 7555 timerIC4 — CD4022 counter/dividerIC5,IC6 — CD4033, 7-segment

display driverIC8 — CD4017 decade counterIC9 (N7, N8) — CD40106IC10 — μA7809 regulatorTl — BC157 pnp transistorDl — 3.1V, 250mW zenerD2-D5 — 1N4001 rectifier diodeD6 — 1N4002 rectifier diode

Resistors (all 1/4watt, +5% carbon, unlessstated otherwise) :R1,R5,R24 — 10-kilohmR2 — 220-ohmR3,R6,R-R23,R25 — 1-kilohmR4 — 100-kilohmR7 — 150-kilohmR8 — 12-kilohmR26 — 68-kilohmR27 — 680-ohmVR1 — 22-kilohm (preset)

Capacitors:Cl — 4.7μF, 16V electrolyticC2 — 47pF ceramic discC3,C5 — 220μF, 16V electrolyticC4 — 0.01μF ceramic discC6,C8 — l0μF, 16V electrolyticC7 — lμF, 16V electrolyticC9 — 1000μF,250V electrolyticC10 — 0.1μF ceramic disc

Miscellaneous:X1 — 230VAC primary to 0-12V,

250 mA secondarytransformer

S1,S2 — 1-pole, 10-wayrotary switch

S3 — On-off switchS4- S6 — Push-to-on reset switchDis.1, Dis.2 — FND543 common

cathode display— 9V battery— 8-ohm, 0.5W loudspeaker— Green and red LEDs— General-purpose PCB

Fig 6: The AM-PM indicator cum 1-ppd oscillator.

order to maintain the counter status,even in the event of power failure. Thecircuit, which basically comprises afull-wave rectifier, followed by single-chip regulator μA7809, is fully self-explanatory.

ConstructionThe circuits may be made on a

general-purpose PCB of suitable size.Extreme care should be taken whilesoldering the ICs, as most of theseare of the CMOS type. The use ofsockets is recommended. A 10-wattiron with 60/40 type solder may be

Fig.5: Circuit diagram for the date display.

Fig.7: The seconds flasher circuit.


Fig 8: Power supply for the circuits.

used for soldering.The assembled circuits can be fixed

behind the main board, using 6mmspacers. The LED indicators can be ac-commodated on the main panel boarditself.

The circuits given here are actuallyindependent, and only the ones requiredneed to be constructed, except for Fig.6, which is required wherever the 1-ppd pulses are required.

The NAND gates can easily beinterchanged to facilitate easy inter-wiring. �


A hi-fi system is a completeaudio system, with a combina-tion of hi-fi stereo cassette

deck, hi-fi stereo integrated amplifier,hi-fi speaker systems, stereo tuner, ste-reo-turntable and compact disc-player.My earlier article on stereo integratedamplifier forms the heart of a hi-fi sys-tem. To make a complete hi-fi system,stereo tuner and a cassette deck are alsorequired—the remaining two, namely,stereo turntable and compact disc playerare beyond the scope of this construc-tion project. In this article, I present acomplete stereo cassette deck, to beused in conjunction with the hi-fi ste-reo integrated amplifier, for making acomplete audio system.

FeaturesSpecial features of this project are:

hi-fi recording and playback on nor-

mal as well as chromium dioxide (CrO2)

and metal tapes; recording level con-trol and automatic level control; mic.and aux. inputs with mic. mixing con-trol; remote muting and standby con-trols.

The circuit uses AC bias at 100 kHzfor recording and erasing. It providesrequisite networks for properly biasingnormal chromium (dioxide) and metaltapes in recording as well as playbackmodes, with correct amplitude and fre-quency emphasis. A visual indicationof the record-level is provided with tensegment resolutions, per channel.

The circuitFig.l shows the block diagram of the

circuit. The heart of the circuit is theequalisation amplifier, which hasequalisation networks corresponding tothe type of tape used, in its feedback

loop. The equalisation amplifier gets itsinput from low-level preamplifier, in-tended to amplify low-level signalsfrom magnetic head, mic. or aux. inputs.

The output of the equalisation am-plifier is made to drive the peak levelindicator, ALC (automatic level con-trol) circuitry and recording circuitrysimultaneously. The same output istaken as line output through the mutingcircuit. For recording purposes, therecord bias is generated by bias oscil-lator circuit, which can bias normal,chromium or metal tapes. 12-volt regu-lated supply is provided to variousblocks of the circuit.

Characteristics of tapesThe frequency characteristics of the

three types of tapes are different and soare their bias levels. The normal (Fe

2O

3)

tapes are characterised as type I tapes

HI-FI STEREO CASSETTE DECK

T.S. SHANKAR

Fig. 1: Block diagram for the HI-FI stereo deck

13ELECTRONICS PROJECTS-15

Fig. 2: Circuit diagram for the HI-FI stereo deck


Fig. 3: PCB layout for the HI-FI stereo deck.


Fig. 4: Component layout for the HI-FI stereo deck.


and have an equalisation time of 120μs. These tapes can handle frequenciesup to 15 kHz.

The chrome (CrO2) tapes are called

type II tapes and have an equalisationtime of 70 μs. These tapes can handlefrequencies up to 16 kHz. The metaltapes or metal alloy tapes are calledtype III tapes and have an equalisationtime of 70 μs, these being capable ofhandling frequencies up to 17 kHz.

Circuit diagramThe circuit diagram is shown in

Fig. 2. Low-noise op-amps LA3161 areused for low-level preamplifier andequalisation amplifier. ICs LM3914 areused to display recording level in ten-step resolution in both playback andrecording modes. IC 7812 is meant forregulated supply of 12-volts at 300mAcurrent capability.

The bias oscillator is a push-pullHartley oscillator wired around low-noise transistors BC149C. The ALC cir-cuit uses low-noise BC149C transis-tors to attenuate the signal proportionalto the average input voltage.

Low-level preamplifierThe dual op-amp LA3161 ampli-

fies low-level signals of the order of1.2 to 1.5 mV from the tape duringplayback and from mic. or aux. or bothduring recording. The amplifier is anon-inverting type, with a voltage gainof 48. The output of this amplifier isnow sufficiently high (around 75 mV)and can easily be used with attenuatingcircuit like ALC loops. As the ampli-fier has no equalisation, it has high-frequency linearity. The output is fedto the equalisation amplifier.

Equalisation amplifierLA3161 pre-amps are used as

equalisation amplifiers. The pre-ampsare wired as large signal amplifiers,with the output swinging to almost halfthe supply voltage. As the IC handleslarge signals, its quiescent input, andhence its quiescent output, is fixed tohalf the supply voltage by a resistordivider, using 270-kilohm resistors atthe non-inverting inputs.

The gain of the amplifier is fixedby a feedback resistor of 8.2 kilohmsand an input resistor of 330-ohms.Feedback is provided from the output

to the inverting input through variousequalisation networks, meant for nor-mal, chrome and metal tapes in play-back or recording modes.

The network comprises 150-kilohm,390-ohm, 2.2-kilohm and 1.8-kilohmresistors and 33nF, 2.2nF and 22nF ca-pacitors for normal tape-playback and47-kilohm and 750-ohm resistors andl0μF capacitors for normal tape record-ing. The network has 150-kilohm, 680-ohm and 1-kilohm resistors, and 22nF,1.8nF and 68nF capacitors for chromeand metal playback equalisation. It has47-kilohm and 1.8-kilohm resistors and8.2nF capacitor for chrome-recordingequalisation and 47-kilohm and 2.2-kilohm resistors and 6.8nF capacitorfor metal-recording equalisation. Thesevalues are in accordance with standardNAB equalisation values, and henceshould not be altered.

The output of equalisation ampli-fier is used for three purposes. The out-put is taken as line output, and the sameoutput is used for peak level indicatorand recording circuit.

Peak level indicatorLM3914 is an LED bar-graph dis-

play IC, specifically designed for peaklevel indicator. It has eleven op-ampsin it, ten of which are wired as com-parators and the eleventh as voltageamplifier. The resistors at pins 6,7 andground fix the LED current to 10 mA.

There are two modes of display—

the dot mode and the bar mode. Theformer is used for recording and thelatter for playback. This is only forconvenience, and can be changed tosuit one’s choice. Pin 5 is the input,which requires 1.25 volts for full-scaledeflection.

The display module gets the inputfrom the equalisation amplifier. The sig-nal is clamped by a 1N4148 diode, rec-tified and filtered by a 10μF capacitor.The capacitor, along with 27-kilohmpreset resistor, forms an RC network tosmoothen the display level excursions.

The 27-kilohm preset is meant forplayback level presetting. It comes un-der system alignment, which is dis-cussed later.

RecordingRecording can be done in two

ways— the ALC mode and the manualmode. Both the modes have their ownadvantages and disadvantages. In ALCmode, the ALC circuit automaticallyadjusts the output level to a relativelevel of 0 dB during normal recordingand to + 3 dB level during chrome andmetal recording.

An AGC or ALC delay of 1000 sec-onds is provided by two l00μF ca-pacitors and two 10-megohm resistors.This is a large delay and can be re-duced to as much as 10 seconds, de-pending upon one’s requirement.

The recording can be done in themanual mode where the ALC function

Fig. 5: Wiring diagram for HI-Fi stereo deck.


is disabled and the signal is attenuatedmanually.

The advantage of the ALC functionis that no attention is required duringrecording, especially when the record-ing is done through microphone. In theALC mode, the circuit can handle fairlylarge signal ranges. But in no signalcondition, the gain of the equalisationamplifier increases very much and as aresult noise appears at the output.Hence, ALC recording is not suitablefor dubbing.

ALC circuitThe output of the equalisation

amplifier is clamped by a 1N4148 di-ode, rectified by another 1N4148 diodeand filtered by a pair of l00μF capaci-tors. The time delay is fixed with two10-megohm resistors in parallel. Thecapacitors hold voltage proportional tothe average of the output of the equali-sation amplifier. This voltage decidesthe signal level attenuation.

The voltage across the 100μF ca-pacitors is mixed for keeping the rela-tive record level unaltered, and fed tothe attenuating transistors BC149C,wired as voltage-to-current converters.The collector current is now propor-tional to the average signal voltage.

This current varies the static resis-tance, i.e. the slope of the diode charac-teristic curve of diode 1N4148. Thisstatic resistance, along with series re-sistance of 22 kilohms from the outputof low-level amplifier, forms a vari-able voltage divider network to attenu-ate the signal entering the equalisationamplifier.

Bias oscillatorBias oscillator circuit provides a si-

nusoidal voltage output at 100 kHz atdifferent levels, suitable for differenttypes of tapes. The circuit is push-pullHartley oscillator, with tuned second-ary. The push-pull configuration isutilised here for its superior character-istics over conventional type.

L6 is a bias oscillator coil of typePhilips 38161.The prototype used sucha coil, but it may be substituted by anyother suitable coil. The circuit oscil-lates at a frequency, decided by thesecondary inductance and shunt capaci-tance, which in this case is 100 kHz.

The power is supplied to this cir-cuit by voltage regulating transistor

dates all the components of the circuit.The rec./ play switch is 18-pole, 2-waytype. Two such switches are employedhere, one for each channel. However, itis represented as single switch Sl in thecircuit diagram. S2 is an 8-pole, 3-wayswitch, usually used in 3-band radiosets. Four-core flat ribbon cable may beused to wire this switch to the PCB. Thewire length should not be more than 15cms. S3 is a 6-pole, 2-way switch usedto select ALC or manual adjustment ofrec. level control. S4 is a rec.-muteSPDT type switch. Use ribbon cablesfor S3 and S4 too, with lengths notexceeding 15 cms. Use two core-shielded wires for stereo-head, mic, aux.inputs, fine output, rec. level pots, mic.mixing pots etc as LM3914 ICs areassembled on the motherboard itself, a24-way ribbon wire may be used toconnect the LEDs. The wire lengthshould be less than 40 cms. Althoughthe circuit recommends AC erase head,a provision for DC erase head has alsobeen made. Short these terminals, if theAC type erase head is used. Groundingof all unconnected metal work shouldbe done including the cabinet.

Alignment

The circuit requires a bit of align-ment before it is put to use. The differ-ent adjustments are: playback level, biaslevel and ALC.

For adjusting the playback level,VR5 and VR6 are used. For adjustingthese presets, record a tone of 1 kHz ona good quality blank cassette in a goodquality cassette deck at ‘0’ dB. Verifyin the same deck for record level of ‘0’dB in the playback mode. Now, playthe cassette in this deck and adjust thepresets VR5 and VR6 such that thefirst six LEDs are lit. Now the sixthLED corresponds to the ‘0’ dB level.

For adjusting VR7 and VR8, con-nect a 1kHz source to ‘aux.’ terminalsand turn VR1 and VR2 completely to‘aux.’ side. Now, record this tone bypressing the record key in a blank cas-sette like Sony HF60. Now play therecorded tone and see whether the dis-play shows ‘0’ dB or not. If not, repeatthe procedure once again after adjust-ing VR7 and VR8 a little. Repeat untilthe recorded level shows ‘0’ dB, whenrecorded at ‘0’ dB. During recording,put ALC switch to manual positionand adjust record level control to ‘0’

BEL 187. The supply voltage is madehigh for metal tapes, medium for CrO

2tapes and low for normal tapes, as thesetapes have different bias levels.

The output of the oscillator is fedto AC type Erase head and also to therec./play (R/P) heads. The 100kHz biascomponent and the signal fromequalisation amplifier are mixed via100-kilohm presets and 6.8-kilohmresistors, respectively before feedingto the R/P heads. A parallel resonantcircuit (for 100 kHz) of l00pF capaci-tor and 23mH inductor blocks the biasoscillator output from entering theequalisation amplifier circuitry.

Rec. mutingWhile recording from microphones,

a howling sound is produced from thespeakers due to acoustic feedback fromthe speakers to microphones. The vol-ume of the power amplifier should belowered every time the record buttonis depressed. Hence, a rec. mute circuitis introduced, which automaticallymutes the output while recording. Thecircuit uses two BC149C transistors indeep saturation for the purpose. Rec.mute can be disabled by switch S4 dur-ing dubbing or aux. recording.

Power supplyVoltage is step-down from 230V AC

to 18-volt and 12-volt AC by trans-former X1. It has two secondary wind-ings, one for 18-volt supply and theother for 12-volt supply. Instead of onetransformer, one may use two separatetransformers for 18 volts and 12 volts.

The 18-volt AC is rectified by abridge rectifier, filtered by a 2200μF,40-volt capacitor and the resultant volt-age of around 24 volts is applied to theinput pin of regulator IC 7812. The 12-volt output of IC5 is supplied to vari-ous modules of the circuit.

The 12-volt AC is rectified by ahalf-wave rectifier and filtered by al000μF capacitor. The resultant 14-voltDC is given to motor through a leaf-switch and a ‘remote’ socket. Thesocket should be of normally-closedtype. Remote facility comes handywhile recording through microphones.

ConstructionA suitable PCB for the circuit is

shown in Fig. 3. The PCB accommo-


Semiconductors:IC1, IC2 — LA3161 preamplifierIC3,IC4 — LM3914 dot/bar

displaygenerator

IC5 — LM7812 regulatorT1-T4,T6,T7 — BC149C npn transistorT5 — BEL187 pnp transistorD1-D5, D16-D20 — 1N4148 silicon switch-

ing diodeD6-D15,D21-D30,D33 — Green LEDD31 — Red LEDD32 — Orange LEDD34 — Yellow LEDD35-D40 — 1N4007 rectifier diode

Resistors (all 1/4watt, ±5% carbon unlessstated otherwise):R1,R8,R27,R29,R31,R39,R45,R62,R65,R66 — 47-kilohmR2, R3, R40,R41 — 68-kilohmR4,R42 — 10-kilohmR5,R43,R76 — 10-ohmR6,R38 — 6.8-kilohmR7,R25,R26,R44R60,R61,R75,R80-83 — 1-kilohmR9,R30,R64 — 750-ohmR10,R47 — 10-megohmR11,R17,R46,R52 — 330-ohmR12,R21,R28,R48,R56,R63 — 2.2-kilohmR13,R49 — 22-kilohmR14,R19,R54 — 390-ohmR15,R16,R50,R51 — 270-kilohmR18,R53 — 8.2-kilohmR20,R24,R55,R59 — 150-kilohmR22,R32,R57,R67 — 1.8-kilohmR23,R58 — 680-ohmR33,R34,R68,R69 — 560-ohmR35,R70 — 1.2-kilohmR36,R71 — 12-kilohmR37,R72,R77 — 4.7-kilohmR73 — 270-ohmR74 — 910-ohmVR1, VR2 — 22-kilohm linear dual

potsVR3, VR4 — 100-kilohm linear dual

potsVR5,VR6 — 22-kilohm trimpotsVR7, VR8 — 100-kilohm trimpots

Capacitors:C1,C4,C23,C24,C27,C29,C46,C47,C54 — 10μF, 25V electrolyticC2,C26,C28 — 1nF ceramic discC3 — l00pF polystereneC5,C11 — 470μF, 25V electrolyticC6,C30 — 47μlF, 25V electrolyticC7,C10,C25,C32,C34,C48 — lμf, 25V electrolyticC8,C12,C31,C35,C53, C59 — 100μF, 25V electrolyticC9,C33 — 22pF ceramic discC13,C36,C58 — 4.7μF, 25V electrolyticC14.C37 — 33nF polyesterC15,C38,C52,C71 — 2.2nF polyesterC16,C19,C39,C41 — 22nF polyesterC18,C42 — 68nF polyesterC20,C43 — 6.8nF polyesterC21,C44,C57,C60,C65-C68 — l00nF polyesterC22,C45 — 8.2nF polyesterC29,C46,C47,C54 — lOμf, 25V electrolyticC49,C50 — 470p styroflex/

polystereneC51 — 2.7nF polyesterC55,C56 — 3.3nF polyesterC61,C63 — l00nF ceramic disc

Miscellaneous:L1,L4 — Record/play headL2,L3 — 23mH coilsL5 — AC erase headL6 — Philips 38161 coilL7 — DC erase headXI — 230V AC, primary to

0-18V, 500mA second-ary transformer

S1 — 18-pin record/playswitch (2 nos)

S2 — 8-pole 3-way switchS3 — 6-pole 2-way switchS4 — SPDT switchS5 — 250V, 2A SPST switchS6 — Leaf switchSK1,SK2SK5.SK6 — RCA socketsSK3,SK4 — Microphone JacksSK7 — Front load cassette deck

mechanism— Suitable cabinet— Connecting wires,

shield wires, mainscord, screws, nuts andsuggested PCB.

Readers’ Comments:The circuit of Hi-Fi Stereo CassetteDeck suffers from mistakes in thecomponents layout as well as the PCBlayout.

After going through the circuit andcomparing it with the components lay-out, I found that the circuit diagramand components layout do not match atmany places. Mismatch occurs nearswitch S2g and at IC2 pins 1 to 4 andthe associated circuitry. Diode D39 ismissing in components layout near thesupply of motor.

What type of mechanism was usedby the author?

What is the output of the circuit?And what is the number of the 23mHcoil used in the recording circuitry?

VishweshKarwar

� Could the author please clarify mydoubts?

There are no places for feedbackresistor (8.2k) and input resistor of 330ohms in equaliser amplifier section.

In equaliser amplifier IC LA3161receives supply voltage at pin 2 in thePCB layout, which is not correct.

The 270k bias-divider resistor atnon-inverting input has no connectionon the other side, i.e. positive.

There is no place for 100μF, 25Velectrolytic capacitors in the PCB.

Rajesh Kumar VermaVaranasi (UP)

� The circuit contains no details about23mH coils (L2 and L3).

Can we use any other IC, instead ofLM3914?

Benoy GeorgeWadakanchery

� The circuit of Hi-Fi stereo cassettedeck is really superb. The only disad-vantage is its cost, as LM3914 ICs havebeen used in display section. If LB1405,etc are used the cost of the project getsreduced.

What is the cost of the PCB and ofthe circuit?

Can we use the amplifier, using

PARTS LIST

dB. After adjusting VR7 and VR8, thebias levels for chrome and metal tapesget adjusted automatically. ALCadjustment is not needed, as the circuitis so designed as to get self-adjusted.However, if at all the circuit does notrecord the signal at ‘0’ dB, try differ-ent values of R13 (and R49) from 10-kilohm to 47-kilohm. At some value inbetween these two, the circuit shouldrecord the 1kHz signal up to ‘0’ dB

mark.In this circuit, I have not compro-

mised upon quality and sophistication.As such, the circuit uses somewhat ex-pensive components like LM3914 forpeak-level display and dual-stage pre-amplifier for the preamplifier section.The circuit also uses complex equalisa-tion networks and push-pull bias oscil-lator, which can otherwise be substi-tuted by simpler versions.

Even though a trifle expensive, thiscircuit is cost-effective and would costonly about one third of its equivalantcommercially available decks, manu-factured by reputed companies.

�


STK459 with the positive rail con-nected to regulator (IC 7812) input,thereby eliminating the need for a sepa-rate transformer, with common ground-ing?

G.Chandra MouliAnantapur

� On the upper right hand side in Fig.4a 390-ohm resistor ends nowhere,which cannot be possible.

Binoy K. Shah

The author, T.S. Shankar, comments:I am very glad to see the interest

taken by the readers in my article onHi-Fi stereo cassette deck.

Instead of LM3914 circuit, any dis-play circuit can be used with sensitiv-ity of around 1V for full scale display.But expensive LM3914 is worth thecost.

For R/P switch, use a tension string,such as radio dial cord with a suitablearrangement for operation.

I am sorry to say that a number of

mistakes were noticed in PCB layout.Please, verify the circuit diagram andcorrect the PCB layout with black inkbefore making the PCB.

For 23 mH coil, wind approximately500 turns of 45SWG copper wire on6mm ferrite bead. Adjust bias oscilla-tor coil for resonance.

This cassette deck can be used withthe integrated amplifier of Feb. '93 is-sue. It is advisable to always use a sepa-rate power supply to reduce interfer-ence, humming and motorboating, etc.

Please, make some pads and tracksfor the components R17, R18, R52,R53, C12 and C35 in the vacant spaceremaining near IC2 with a black pen.there is no mistake with S2g switch.Tracks near IC2 pins 1 to 4, are wrongand must be corrected. Diode D39 maybe fixed on motor terminals.

I used imported soft-touch deckmechanism. Playback head of 250-ohmimpedance is suitable.

There is no type number for 23mH

coil. It must be available in the marketas 25mH coil. Otherwise, it can be con-structed by winding 350 turns of48SWG copper wire on core, taken outfrom the coil of a 36cm B&W TV. afterremoving the original coil and magnetplaced on the linearity coil.

Since input impedance of thepreamplifier is very high, any head issuitable for the circuit. LM3914 can-not be substituted by any other IC. In-deed, no tracks have been provided forfeedback circuit of LA3161. I deeplyregret the mistake. Fortunately, enoughplace exists in the area around LA3161.Similar correction can be made for sup-ply pin of LA3161 after following thecircuit diagram.

For the 270k resistors, please shortcircuit the joint of 270k resistors to theadjacent pin 4 track of LA3161 with asolder bridge.

The project cost me around Rs2000, including cabinet, deck mecha-nism etc.


R ecently, one of my colleagues,who is an ace player of chess,mentioned the fate of the chess

clock of their recreation club. The oldmechanical clock is taken out of thecupboard once in a year, at the time ofthe annual tournament. With the humidclimate of Bombay, the clock is rustedand is no more reliable. He urged for aDigital Chess Clock, a novel idea and anew design for India. Before I agreedto his proposal, he gave me the follow-ing specifications:

1. There should be two indepen-dent clocks for both the players.

2. The umpire should be capable ofresetting the clocks to a reference time.

3. After thinking and playing themove, the player should operate a le-ver to stop his own clock and at thesame time start the opponent’s clock.

4. There should be a facility towatch the movement of the clock also.

5. If either of the player takes morethan 150 minutes to complete all themoves, there should be a time-out indi-cation.

6. The clock should be an accurateand reliable one.

PrincipleWith the above specifications in

mind, a block diagram was drawn, asshown in Fig. 1. To have an accurateand reliable time, a quartz-controlledmaster oscillator is utilised to providethe drive for the clocks. It will supplythe drive to either a Black Clock or theWhite Clock. The reset switch willbring the clocks to 000. With the helpof switch S2, an alarm at 2:30 hourshas to be set before the start of thematch in each clock. A common piezobuzzer will sound after the lapse of

150 minutes by either of the player.The dot in the running clock will beblinking as the second’s indicator. Thisway, it may meet all the above-men-tioned specifications.

Circuit diagramThe complete circuit diagram of the

chess clock is shown in Fig. 2. Thecommonly available 40-pin ICs,LM8361 or its equivalents, are usedfor identical Black and White clocks.The IC MM5369 along with the quartzcrystal is used as the Master Oscillator,which supplies 60Hz drive to theclocks. The operation of switch S8 willcut off the drive from one clock andextend to the other clock. As the clockis generally required for two hours and30 minutes, three-digit display is ad-equate.

Alarm outputs are ORed throughtwo diodes and directly extended tothe trigger terminal of a piezo buzzer.

Reset switch Sl extends the Vcc toSLOW SET, FAST SET and SEC. DISPLAY

(i.e. pins 33,34 and 32). This will resetthe clock having drive. Hence, both theclocks can be reset to 000, one by oneby operating switches Sl and S8. Please,note that pins 38 of IC2 and IC3 areconnected to Vcc, which makes theclocks run in the 24-hour mode. If pin38 is kept open, the clock will run in a12-hour mode and, while resetting, theclock will reset to 12:00 hour.

Using diodes D5 and D 12, it utilisesthe same switches S3 and S2 for ALARM

DISPLAY/ALARM OFF functions of Blackand White clocks, respectively. Thepower is provided through a conven-tional 12-0-12 volts transformer andfull-wave rectifier. The 9-volt batteryprovides the backup during power in-terruptions, if any. Hence, there are twobuses of supplies. Vcc (A) has batterybackup whereas Vcc (B) does not havebattery backup. Total current drain isabout 300 mA at 12 volts.

AssemblyThe complete circuit shown in Fig.2

DIGITAL CHESS CLOCK

S.BATRA

Fig. 1: Block diagram for the digital chess clock.


Fig. 2: Circuit diagram for the digital chess clock.

Fig. 3: PCB layout for the chess clock.


Fig. 4: PCB layout for the display.

Fig. 6: Component layout for the PCB shown in Fig. 4.



can be easily wired on two PCBs meas-uring about 12 cms x 10 cms and 12cms x 3 cms. The actual-size layout ofboth the PCBs is shown in Figs. 3 and4, respectively.

The PCBs are designed in such away that the segment outputs fromclock PCB are in exact alignment withthe segment of display PCB, which canbe mounted above the clock PCB atright angle. Extra room in the clockPCB will make the assembly morecomprehensive and even the trans-former, piezo buzzer, etc can bemounted on the clock PCB itself.

First of all, check all the compo-nents with the help of a multimeter. Tostart with, assemble the display PCBwith the six display chips. With thehelp of 9-12 volts test bench powersupply through a 1 -kilohm series resis-tor, check whether all the segments areglowing properly.

Now assemble the clock PCB. Sol-der all the resistors and check themsubsequently. Then solder all the di-odes and finally all the three ICs. Con-nect display PCB and clock PCB with43 fine wires at right angle and solderthem. Connect the flexible wires ofabout 15cm length to the clock PCBfor various switches.

Testing

Extend the power and test thecomplete circuit. As soon as the poweris switched on, the clock receiving thedrive may start blinking. Press the re-set switch and the clock will reset to000. Turn switch S8 to another posi-tion and reset the second clock aswell. With the help of SLOW SET andFAST SET switches, check the workingof both the clocks. Check that the sec-onds ‘dot’ blinks for the clock whichis ‘on’ and does not blink for the clockwhich is ‘off’.

Now, set the alarm and see that itsounds at the desired time for both theclocks.

The complete assembly can behoused in an acrylic cabinet. All theswitches should be mounted on the topside of the cabinet for the sake of con-venience. Further, care may be takenthat SLOW SET, FAST SET and RESET

switches of both the clocks are not pro-jected outside the cabinet, and they areaccessible only through a pen/pencil.This will prevent their accidental press-ing by anyone, and hence the time willnot be disturbed.

Instructions for umpire

1. Energise the clock with mainssupply and insert a 9-volt battery in thebattery terminals.

2. One of the clocks, depending on

PARTS LISTSemiconductors:IC1 — MM5369 oscillator/dividerIC2,IC3 — LM8361 clock chipD1-D4 — 1N4001 rectifier diodeD5-D14 — 1N4148 switching diode

Resistors (all 1/4watt, +5% carbon unlessstaled otherwise):R1-R42 — 1-kilohmR43 — 10-megohmR44 — 100-kilohmR45 — 330-kilohm

Capacitors:Cl — 470μF, 25V electrolyticC2 — 15pF ceramic discC3 — 0-22pF variable trimmer

Miscellaneous:X1 — 12-0-12 volts

500mA, sec. transformerS1-S7 — Push-to-on switchesS8 — 1-pole, 2-way ‘Anchor’ make

switchXTL — 3.579MHz crystal

— 12-volt trigger type piezo buzzer— 9-volt battery.

Fig. 7: Suggested cabinet for the digital chess clock.

the position of switch S8, may startblinking. Press the corresponding SLOWSET or FAST SET switch to stop the blink-ing. Bring switch S8 to the other posi-tion and similarly stop the other blink-ing display.

3. By pressing switches S2, S3 andRESET together, obtain 000 display foralarm. Now, keeping switches S2, S3pressed, adjust the display to 2:30 withthe help of SLOW SET/FAST SET switches.One by one set both the alarms to 2:30.

4. Bring switch S8 towards theWhite player and press RESET to get thetime as 000. Keeping switch Sl pressed,bring switch S8 towards the Blackplayer. This clock will also reset to000. The moment the RESET switch isreleased, the time for White starts.

5. After playing his move, the Whiteplayer should shift switch S8 towardshimself. This will stop his clock andthe clock of Black player will start.

6. If anyone of the players takesmore than 150 minutes, the alarm willsound and indicate the end of the game.

The analogue chess clock availablein the market has the problem of inher-ent delay due to inertia after operatingthe lever. This delay, which can be upto 2.5 seconds in a typical case, is elimi-nated with this digital clock.�


S ecurity is one of the major probl-ems encountered nowadays inbig cities. Security systems

available in the market are costly andcord-based. When such gadgets work,they produce unwanted noise throughthe speakers or buzzers. These devicesare generally light-operated or infraredlight-controlled, and are more suitedfor use in homes.

But think of a situation when youare out shopping, while your car is ly-ing unguarded in the parking lot. Onceinside the shop, you have no controlover your vehicle and may fall an easyprey to car-lifting. In such situations,cordless security systems come to yourrescue. While away from your valu-able gadgets, of course within a lim-ited range of distance, the cordless se-curity systems send out signals to youif your goods are interfered with.

This article enables you to fabri-cate such a type of cordless securitysystem.

Working principleThe basic circuit is a combination

of a signal generator, an amplifier anda radio transmitter. The signal genera-tor produces an alarm sound if a switchis left open. The amplifier magnifiesthis signal and feeds it to the transmit-ter. The radio transmitter transmits thissignal in the MW band. A pocket radioreceiver can be used as a detector.

The range of the transmitter variesfrom 30 to 50 metres, making the sys-tem convenient for use for cars parkedwithin the transmitter’s range, and alsofor door/window security. The elimi-nation of use of wires/cords makes thesystem cost-effective too.

Circuit descriptionThe circuit comprises a transis-

torised astable multivibrator, an IC am-

plifier and an RF transmitter.(a) Multivibrator: Transistors Tl

and T3 work as an astable multivibra-tor, whose frequency is about 1kHz.This signal is then amplified by tran-sistor T4 to get a higher output.

Transistor T2 works as an electronicswitch for the multivibrator. Whenswitch Sl is kept closed, due to thereverse bias on its base, transistor T2will be off. When switch S1 is released,transistor T2 supplies negative voltageto the multivibrator.

(b) IC Amplifier: BEL1895 isan eight-pin audio amplifier IC, gener-ally used in radio circuits. In this cir-cuit, it is used for amplification of the

1kHz signal available from themultivibrator. The signal available fromthe collector of transistor T4 is fed tothe noninverting input of BEL1895.This IC is wired in its noninvertingamplifier configuration. The output isavailable from pin 2of BEL1895.

(c) RF Transmitter: The mixer-cum-local oscillator circuit comprisestransistor T5 and its associated feed-back network. The tank circuit consistsof coil L1 and capacitor C16. By suit-ably varying capacitor C16, the fre-quency can be adjusted on the MWband. The feedback network comprisesresistor R13 and capacitor C13.

Capacitors:C1,C2 — 0.4μF ceramic discC3 — 0.2μF ceramic discC4,C5,C8 — 0.1μF ceramic discC6,C13 — 0.01μF ceramic discC7 — 200μF, 12V electrolyticC9,C12 — 470μF, 12V electrolyticC10 — 0.001μF ceramic discC11 — 10μF, 12V electrolyticC13 — 0.01μF ceramic discC14 — 100μF,, 12V electrolyticC15 — 33PF ceramic discC16 — 100pF trimmerC17 — 47pF ceramic disc

Miscellaneous:S1 — Push-to-on switchL1 — MW antenna coil (2J)

Ferrite rodSuggested PCBClampsTelescopic antenna (90cm)

Semiconductors:T1-T4 — BC548 npn transistorT5 — BF194B npn transistorD1 — 1N4001 rectifier diodeIC1 — BEL 1895 audio

amplifier

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1,R4 — 10-kilohmR2,R3,R5 — 220-kilohmR6 — 5.6-kilohmR7 — 1-megohmR8,R14 — 1-kilohmR9 — 4.7-kilohmR10 — 10-ohmR11 — 100-kilohmR12 — 470-ohmR13 — 47-kilohmR15 — 2.2-kilohmVR1 — 10-kilohm preset

PARTS LIST

CORDLESS SECURITY SYSTEM

PRADIP KUMAR BOSE

Fig. 1: Block diagram of the cordless security system.


The audio signal, when applied tothe emitter of transistor T5, modulatesthe carrier wave, to generate the radiosignal. The amplitude modulatedoutput is available at the collector oftransistor T5. This signal is then trans-mitted through a 90cm telescopicantenna.

An ordinary AM receiver is suit-able for receiving the signal. It would,however, be better to use a Philips makeportable radio rather than any low-costreceiver.

Easy fabricationThe PCB and component layout are

shown in Figs 3 and 4, respectively.The PCB can be made either by the

ion-etching process or with the help ofPCB making kits available in the mar-ket. For perfect soldering, use of goodsolder and soldering iron are essential.Use coloured wires for switch Sl, bat-tery, antenna coil, etc to avoid confu-sion. The coil can be fixed on the PCB.

Actual operationAfter complete fabrication, release

switch Sl and power the circuit. AdjustVR1 to mid-position and connect thespeaker between the negative terminalof capacitor C14 and the positive railof supply. A continuous beeping soundmust be heard. Now, disconnect thespeaker and connect a telescopic an-tenna, as shown in Fig. 2. Place a MWradio about three metres away and tuneit to a null position. Now, adjust ca-pacitor C16 and the ferrite rod of theantenna coil to get a loud beeping soundfrom the radio.

In order to get perfect sound fromthe radio, adjust capacitor C16 and theferrite rod and MW receiver gang. Af-ter getting a clear sound, adjust VR1 tomake it louder, but remember that thequality of sound must be the same asbefore. On pressing switch Sl, the soundshould cease. Only a small hissingsound may be heard in this state. Therelease of switch Sl should produce thesound again.

The whole system can be housed ina small cabinet. Use rectangular nickelcadmium cells, if you wish to use it fora long time. The MW receiver shouldbe kept with the user and turned ‘on’.The use of switch S1 depends upon theuser, and it can be employed to protectone’s valuables in several ways, thusFig. 2: Circuit diagram of the cordless security system.


circuit?

The author, Pradip Kumar Bose, replies:Regarding the letter of Mr

Ramanan, I would like to make thefollowing comments:

1. The range of the transmitter canbe increased by using a high power

RF amplifier unit. But this is not de-sired as high power transmitters needlicence.

2. The transmitter itself can beused as voice transmitter by simplemodifications. The frequency of the cir-cuit can also be changed by the tuningcircuit.

providing flexibility of use. �

Readers’ Comments:I want the author to clear the followingdoubts:

1. What modifications are neededto extend the range of the system up to1 kilometre?

2. How to modify the transmitter

Fig. 4: Component layout of the cordless security system.

Fig. 3: PCB layout of the cordless security system.


There are various analogue cir-cuits available for the measurement of capacitance and resis-

tance. Here is a microprocessor-basedcircuit which, instead, displays the re-sults digitally.

This circuit has an added advan-tage over the conventional analogue cir-cuits in that both resistance and ca-pacitance can be measured withoutmaking any changes in the circuit con-figuration. It is this simplicity of dualoperation made possible by the use ofa DPDT switch that makes this projectvery appealing and qualifies it as a tech-nical improvement over the other ana-logue measurement methods.

Circuit detailsThe circuit makes use of a timer IC

555, wired as an astable multivibrator.The output of the timer is fed to themicroprocessor. A DPDT switch is pro-vided for connecting standard values ofresistors and capacitors to the circuit.

When the capacitance is to be meas-ured, the standard value of resistor RBgets connected to the circuit throughthe DPDT switch. Now, the capacitancevalue is directly proportional to the timeperiod. This time period is measuredand the capacitor value is calculatedby the microprocessor, which .displaysthe reading. Here, the design is suchthat the displayed capacitor value is innanofarads.

Similarly, when the resistance is tobe measured, the standard value of thecapacitor gets connected to the circuit

by the DPDT switch. The displayedvalue of resistance is in ohms. Insteadof wiring the fixed values of resistorsRA and RB and capacitor C, variableresistors and capacitors can be usedto obtain accurate values. In this cir-cuit, a standard-value capacitor Cof value 54.17μF and resistors of stan-dard values RA= 53 kilohms and

RB = 54.17 kilohms are used.

Working principle

DPDT switch Sl shown in the cir-cuit diagram, when moved to position-1, connects the standard resistor RB inthe circuit. In this condition, if we nowconnect a capacitor of any value, the

MICROPROCESSOR-BASEDCAPACITANCE AND RESISTANCE

MEASUREMENT METER

VIVEKANANDA K.S. AND ZAMEERUDDIN

Fig. 1: Flow chart.


time period is proportional to the valueof that capacitor (Note that the stan-dard value of RA has already been con-nected.) From the time period, the valueof the capacitor can be obtained.

If the switch is now moved to theother position, the standard capacitorgets connected to the circuit and thestandard resistor RB gets disconnected.By placing the resistor whose resis-tance is to be measured, we can obtainthe value of the resistor as, in this case,the time period is proportional to theresistance connected.

We measure resistance or capaci-tance during the negative half cycle ofthe timer output. If we are getting posi-tive half cycle by the time the micro-processor connects, we wait till we ob-tain the negative half cycle. On theother hand, if we get a negative halfcycle by the time the microprocessor isconnected, we wait for the positive halfcycle to pass off and continue waitingtill the beginning of the next negativehalf cycle, as shown in Fig. 2. At thebeginning of the negative half cycle,the register pair HL resets to 0000. Inthe software, the counter loop startsfrom the instruction MOV A,L and endsat the instruction JZ AAAA, whichamounts to a total of 67 T-states.

After completion of the loop onceagain, the microprocessor checks thestatus of the pulse, i.e. after (67 x 1/1.785 x 106 = 37.54μsecs), 1.785 x 106,being the microprocessor frequency.

At ‘An’ (Refer Fig. 3), the counteris incremented to n and the total timeperiod of the negative cycle is n x 37.54μsecs.

For the astable multivibrator, thetime period of the negative cycle is

Fig. 2: Circuit diagram.

Fig. 3: Detection of negative-going edge by the microprocessor.

0.693 RBC, which gives:n x 37.54 x 10-6

C= –––––––––––––0.693RB

Substituting RB = 54.17 kilohmsn x 37.54 x 10-6

C= ––––––––––––––––––––0.673 x 54.17 x 103

So, C = n x l0-9

The value of capacitor in nanofaradsis therefore given by the relation-ship C = n.

Thus, the contents of the HL pairdirectly give the capacitor value innanofarads.

Similarly, for measurement of re-sistance,

n x 37.54 x 10-6

R= ––––––––––––––––––0.693 x 54.17 μF

So, R = n

The counter-number, thus, directlygives the value of the unknown resis-tance in ohms.

This project, thus, eliminates theneed for multiplication or division byany constant terms in the software.

Software

The software described here isstored in the EPROM, existing inthe microprocessor kit. The count isstored in the HL pair and register C.The first part of the softwaresynchronises the beginning of the ‘off’cycle of timer output to the counterprogram. The counter increments fora period of the ‘off’ cycle. The de-sign is such that the count directlygives the value of resistance in ohmsand capacitance in nanofarads. The

PARTS LISTSemiconductors:IC1 — NE555 timerD1 — 1N4001 rectifier diode

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1 — 560-ohmR2 — 100-ohmRA*, RB* — (See text)

Capacitors:C1 — 0.01μ ceramic discC* — (See text)

Miscellaneous:S1 — DPDT switch

LEDsμP 8085 kit


values are directly displayed.The circuit is designed to measure

capacitance, ranging from one nF to999μF and resistance, ranging from tenohms to one megohm. The port ad-dresses in the software are accordingto the ‘Anshuman 8085’ microproces-sor kit.

EFY Notes:1. For accurate results, styroflex

standard capacitor and resistor withleast tolerance should be used.

2. The project was tested on an8085 microprocessor training kit, whichoperates at a frequency of 3.07 MHz.Hence, a standard resistor of value,RB=31.62 kilohms and capacitor ofvalue, C = 31.62 μF were used. Theport addresses were changed in the soft-ware accordingly.

�

ADDRESS INSTRUCTION OP-CODE COMMENTSSynchronising program

6000 MVI A,82 3E,82 Load accumulator with control word 826002 OUT CWR D3,OB Output it to Control Word Register

XXXX 6004 XRA A AF Set accumulator to zero6005 IN PORT B DB,09 Read data from port B6007 ANI 01 E6,01 Mask bits except D06009 CPI 00 FE,00 Compare with zero600B JZ XXXX CA,04,60 Jump if getting negative pulse to XXXX600E LXI H,0000 21,00,00 Initialise the counter to zero6011 MVI C,00 OE,00 Initialise the counter to zero

YYYY 6013 XRA A AF Clear accumulator6014 IN PORT B DB,09 Read data from port B6016 ANI 01 E6,01 Mask bits except D06018 CPI 00 FE,00 Compare with zero601A JNZ YYYY C2,13,60 Jump if not getting negative-going

edge to YYYYCounter ProgramAAAA 601D MOV A,L 7D Transfer contents of L to accumulator

601E ADI 01 C6,01 Increment the contents6020 DAA 27 Convert to decimal6021 MOV L,A 6F Store back in L6022 JNZ ZZZZ C2,32,60 Jump if less than 99 to ZZZZ6025 MOV A,H 7C Transfer contents of H to accumulator6026 ADI 01 C6,01 Increment the contents6028 DAA 27 Convert to decimal6029 MOV H,A 67 Store back in H602A JNZ ZZZZ C2,32,60 Jump if less than 9999 to ZZZZ.602D MOV A,C 79 Transfer the contehts of C to accumulator602E ADI 01 C6,01 Increment the content6030 DAA 27 Convert to decimal6031 MOV C,A 4F Store back in C

ZZZZ 6032 XRA A AF Clear accumulator6033 IN PORT B DB,09 Read data from port B6035 ANI 01 E6M01 Mask bits except D06037 CPI 00 FE,00 Compare with 006039 JZ AAAA CA,1D,60 If not over, continue the process

Display program603C Push H E5 Store the contents of H603D Push B C5 Store the contents of B603E MOV A,C 79 Transfer the contents of C in A603F CALL DISA DT CD,E8,02 Call display routine6042 LXI D FFFF 11,FF,FF Fill the DE pair with counter6045 CALL DELAY CD,C7,06 Call delay routine6048 CALL DELAY CD,C7,06 Call delay routine603B POP B C1 Restore BC pair604C POP H E1 Restore HL pair604D JMP 6000 C3,00,60 Jump to start for next measurements


M ost amateur radio stationsare now using single side-band (SSB) equipment for

the very simple reason that it is muchmore effective vis-a-vis AM. Almostall the commercially available equip-ment are also basically SSB type.

Different receivercapabilities

An ordinary radio receiver intendedfor broadcast reception is inadequateto receive SSB or CW, and this poses areally major problem to the SWLs(shortwave listeners) in copying ama-teur stations. If supplemented by a BFO(beat frequency oscillator), an ordinaryBC (broadcast) receiver can be effec-tively used to receive CW and SSBsignals within the bandwidth limita-tions of such receivers.

Let us have a brief review of a com-munication receiver, compared to anordinary BC receiver (see Table).

From the Comparison Table, we cannotice that, apart from adding a BFO/product detector, if we make an effortto have a stable VFO-type local oscil-lator and employ a slow tuning rate,the performance of a BC receiver canbe greatly enhanced. Limiting the IFbandwidths to around 6 kHz can alsobe achieved by using cascaded IFTs inboth the stages of IF amplification.

This article, however, describesonly the construction of a BFO, to beused with a BC receiver. The BFO PCBcan either be fitted inside the receiveror it can be made an outboard unit.

Equipment requiredThe circuit is self-explanatory. Tran-

BFO FOR RECEIVINGSSB SIGNALS

P. R. VAIDYANATHAN

PARTS LISTSemiconductors:T1 — BC147 npn transistorT2 — BC148 npn transistor

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1 — 47-kilohmR2 — 27-kilohmR3 — 100-ohmR4,R7 — 1-kilohmR5 — 22-kilohmR6 — 18-kilohmR8 — 560-ohm

Capacitors:C1 — 180pF styroflexC2 — 2000pF styroflexC3 — 820pF styroflexC4,C7 — 0.01μF ceramic discC5 — 150pF ceramic discC6 — 0.05μF ceramic disc

Miscellaneous:XTL — 455kHz crystalS1 — On/Off switch

sistor Tl works as a Colpitts crystaloscillator and T2 as a buffer amplifier(for those who wish to drive a productdetector). As such, there is no productdetector in the circuit. The 455kHz RFenergy radiated from the BFO getsmixed at the IF/detector stage and boththe SSB and CW signals get effectivelyresolved.

It is better to use a 455kHz quartzcrystal, if available. A cheaper, but ef-fective, substitute will be one of thoseceramic crystals (resonators) availablein the market for less than ten rupees.

Ceramic IF filters could also be sub-stituted in place of the crystal. If quartzcrystals or ceramic resonators are notavailable, one can safely substitute anordinary IFT (primary only) as well.The circuit will oscillate perfectly.

A problem with using an IFT is thatthe stability of frequency is much less,

Fig. 1: Circuit diagram for the BFO.


compared to the crystal/resonator os-cillator.

The example of 455kHz IF has beenchosen because most BC receivers workat that frequency. But variants are alsoavailable, e.g. 462 kHz, 467 kHz, etc.So, before attempting to build a BFO,make sure of the intermediate frequencyof the receiver and only then buy theappropriate crystal/resonator.

If it is intended as an outboard unit, asmall plastic eliminator box can accom-modate the PCB. Power supply to theBFO could be from a set of four pen-torchbatteries, or it can be availed from thereceiver’s main power supply.

The current consumption of theBFO is less than 5 mA. The outboardunit can be provided with a short pieceof stiff wire (or a short telescopic an-tenna) to radiate the IF energy, and thewhole unit can be located at a conve-nient distance from the receiver.

Process of fabrication

One important point to be consid-ered in assembly is that if a ceramicresonator is used in the circuit, the legsof the resonator may be protected by along-nosed plier, while soldering. Evena slight amount of heating of the legsmay damage the resonator. The use ofa suitable socket may be ideal.

After assembly, check for any omis-sions or mistakes and switch on theDC power. If you have a DC mA meter,check for a current consumption ofaround 4 mA. The circuit should beoscillating.

If your receiver IF frequency ismatched with the BFO frequency, youwill hear a hzzz... sound in the re-ceiver. When an AM broadcast stationis tuned in, there will be pronouncedhowling (beat note) heard in the re-ceiver. The howling will be heard allalong the dial on all bands at all loca-

tions where the different stations aretuned in.

When a station is correctly tuned, itshould be possible to take an exact zerobeat. (Zero beat is a term familiar tomost hams.) The unit is now ready foruse.

Testing the unit

Now, keep the BFO switch in Offposition and tune in an amateur SSBstation. When you hear the characteris-tic SSB ‘monkey talk’, switch on theBFO and adjust the main tuning of thereceiver carefully till you hear the natu-ral voice of the person operating thestation.

Try this with strong SSB stations

also. If you experience a little diffi-culty in resolving the signal, keep theoutboard unit slightly closer. A fewhours of practice will be enough foryou to use the gadget effectively. Theunit built by the author (installed in-side a home-constructed receiver) poses

no problem in receiving both weak andstrong SSB stations.

Insufficient injection of BFO en-ergy indicates itself as a weak howlingand excess of injection apparently pullsdown the received station due to strongAGC action.

If an outboard unit is constructed,keep the unit at a convenient positionaway from the receiver to get the rightamount of BFO injection.

For units installed inside the re-

Fig. 2: Actual-size PCB layout for theBFO

Fig. 3: Actual-size compomentlayout for the BFO

ceiver, the extent of injection can becontrolled by selecting a proper posi-tion for the PCB inside the receiver.If an enhancement of the BFO signalis required, a piece of wire could besoldered to the output terminal of theBFO board and kept near the receiverIFT area.

The whole project can be completedin a few hours, provided you have allthe components ready. Last, but not theleast, it must be mentioned that theprerequisite for using the BFO is thestability of the local oscillator in yourreceiver. Should the LO be not suffi-ciently stable, a slight retuning may beneeded from time to time when listen-ing to long QSOs.

Mechanism of SSBreception

Single-side band, as the name indi-cates, contains only one side band ofthe modulated carrier. When the car-rier is modulated with an audio sig-nal, the resultant output is a complexwaveform which can be resolved intothree components, viz, the upper sideband, lower side band and the carrieritself.

Of these, the frequencies of the up-per and lower side bands vary with thefrequency of the modulating audio sig-nal. Thus, if ‘F’ is the carrier frequencyand ‘f’ is the audio modulating fre-quency, the upper and lower side bandswill have frequencies ‘F+f’ and ‘F-f’respectively.

In a side-band transmitter, only oneselected side band is transmitted—ei-ther the upper or the lower side band.The opposite side band and carrier aresuppressed.

At the receiving end, the side-bandsignal by itself gives no informationon the audio modulating frequency,because of the absence of the refer-

If supplemented by a BFO(beat frequency oscillator), an

ordinary broadcast receiver can beeffectively used to receive CW andSSB signals within the bandwidth

limitations of such receivers.

If your receiver IF frequency ismatched with the BFO frequency,you will hear a hzzz... sound in thereceiver. When an AM broadcaststation is tuned in, there will bepronounced howling (beat note)

heard in the receiver. When astation is correctly tuned, it should

be possible to take an exact zerobeat. The unit is now ready for use


No. Communication Receiver BC Receiver1. NarrowIF bandwidth Relatively large IF bandwidth

(typically 2.3kHz) (typically 10kHz)2. Employs RF amplifier front No RF amplifier in majority of cases. Sensi-

end and hence, highly sensitive tivity is adequate for BC reception only3. Local oscillator(s) are carefully Separate local oscillators are absent in most

made VFOs and hence, cases (oscillating converters are used in thedrift-free front end) and hence, frequency stability is

poor4. Employs product detectors for Uses only AM envelope detector

reception of SSB and CW signals5. Effective AGC circuits are Simple AVC circuits are used

employed6. Frequency coverage per band is very Frequency coverage per band is relatively

much limited and tuning rate is slow very large and turning rate is high

pling to BFO signals at different IFstages and detector stage.

The reference carrier can also bereintroduced at the input stage at thereceived frequency, instead of at theIF 1evel in a superhet receiver. Rein-troducing the reference carrier at theoperating frequency itself will call fora change of frequency of the refer-ence carrier for each station received.IF being a fixed frequency, it is onlynecessary to generate the referencecarrier at one single frequency (equalto intermediate frequency), irrespec-tive of the frequency of station re-ceived.

ence carrier ‘F’. When detected in aconventional AM diode detector, anSSB signal results in characteristic‘monkey talk’.

To make this gibberish intelligible,the SSB signal has to be detected in aproduct detector where the missing ref-erence carrier (F) is mixed with theSSB signals to retrieve the audio modu-lating frequency ‘f’. The purpose ofthe BFO is to generate this referencecarrier locally.

It is found in practice that a mixer-type product detector is not really amust. Receivers with AM detectors alsoresolve SSB signals through stray cou-

Advantages of SSBtransmission

It is calculated that for 100 per centmodulation of the carrier, 50 per centpower goes on the upper and lowerside bands together, and the balance 50per cent on the carrier itself. Hence,while transmitting 100W of RF power,50W goes on carrier (F) and 25 W oneach side band.

As the audio (information) is carri-ed in the side bands, the useful part ofthe signal is the side bands only. Anysingle side band will suffice to retrievethe audio. The carrier frequency (F) inwhich 50 per cent power is dumped isused only for reference at the receivingend, which is considered to be waste-ful of energy.

Hence, if the carrier and one of thesidebands are suppressed and only oneside band is transmitted, the entiretransmitter power can go on a singleside band.

For a 100W transmitter, the full100W are available for one side band inthe SSB transmitter, whereas only 25Wwere available in an AM transmitter.Hence, the SSB transmitter is consid-ered to be as effective as an equivalentAM transmitter with 400W power. �


W hile dealing with electricalequipment, be it forindustrial or domestic pur-

poses, safety against fire and shock haz-ards is of utmost importance. For de-tection of and protection against bothshock and fire hazards, numerous add-on circuits, which can easily be incor-porated in the equipment, are describedhere. These are designed to protect boththe operators and the equipment.

Detection and protectionThe most common causes of shocks

are in the event of the cabinet becom-

ing ‘live’, interchanging of the live andneutral wires, earth disconnection,mains leakage, or at times even due tothe ‘blowing’ of an improperly con-nected fuse. Hence, protection can beensured by detection of the above faultsand then either manually or automati-cally disconnecting the supply line. In-most cases, audio or visual indicatorsserve the purpose and automaticdisconnectors need to be used only incertain cases.

Shock warning indicatorsShock warning indicators can be

SAFETY INDICATORS AND AIDS

A.B. TIWANA

used to indicate that the operator mayrecieve an electric shock if the cabinetis touched. Circuits for shock warningindicators, using neons, LEDs, flash-ing neons, are given in Figs 1(a), 1(b)and 1(c). The resistor values are for220V mains, and may be appropriatelyreduced for 117V operation (in somecountries).

Blown-fuse indicatorsBlown-fuse indicators are used to

indicate a blown fuse. The circuitsshown in Figs 2(a) and 2(b) give visualand audio indications of a blown fuse.

Fig 1(b): Safety indicator using LED.

Fig. 1(a): Safety indicator using neon. Fig. 1(c): Safety indicator using flashing neon.

Fig. 2(a): Visual indicator of a blown fuse.


PARTS LIST

Semiconductors:IC1,IC2 — NE555N timerIC3 — LM709/μA741IC4 — LM748 op-ampIC5 — 7912 regulator chipTl — BEL187 pnp transistorT2, T4, T6 — BC147B npn transistorT3,T7 — BC158 pnp transistorT5 — SL100B npn transistorD1-D3, D5-D11 — 1N4001 rectifier diodeD4 — 5.1 V zener diode

Resistors(all 1/4 watt, ±5% carbon unlessstated otherwise):Rl, R4, R10 — 820-kilohmR2 — 22-kilohm,2WR3 — 1.8 megohmR5, R13, R28 — 100-kilohmR6,R16,R21,R36 — 1-kilohmR7, R27 — 1-megohmR8,R12,R19,R20, R32, R33 — 10-kilohmR9 — 47-kilohm,5WRll — 22-kilohmR14 — 150-ohmVR15 — 680-ohmR17,R18,R22 — 10-kilohm (thermistor)R23 — 10-megohmR24 — 220-ohmR25 — 12-kilohmR26 — 180-kilohmR27 — 1-ohm, 10WR30 — 8.2-kilohmR31 — 820-ohmR34 — 1.2-kilohmR35 — 4.7-kilohmVR 1 ,VR3-VRS — 10-kilohm presetsVR2.VR6 — 100-kilohm presets

Capacitors:C1, C2 — 22μF, 25V electrolyticC3 — 1μF, 250V tantalumC4, C7 — 0.1μF ceramic discC5 — 47-μF, 25V electrolyticC6, C14 — 220μF, 16V electrolyticC8 — 100μF, 10V electrolyticC9 — 1000μF, 25V eictrolyticC10 — 220μF, 10V electrolyticC12 — 2.2μF, 10V electrolyticC13 — 470μF, 16V electrolyticC14 — 200μF, 10V electrolytic

Miscellaneous:RL1 — 230V, 10mA relayRL2 — 9V or 6V, 300mA relayRL3 — 9V, 300 mA relayRL4 — 12V, 300-ohm relay

— VK27CT piezo buzzer— Piezo buzzer (1PB27B)— 230V AC piezo buzzer— 230V AC bell— 8-ohm loudspeaker— Neon lamps— Fuse wire etc— LEDs

Fig. 2(b): Audio indicator of a blown fuse.

Fig. 3: Earth disconnection alarm with visual indicator.

Fig. 4(a): Polarity reversal cutout with alarm.

Fig. 4(b): Audio-visual polarity reversal alarm.


Earth disconnectionindicator

The safety system of a device be-comes ineffective if the earth line ofthe device is disconnected. The circuitshown in Fig. 3 indicates disconnec-tion of the earth wire, by lighting up aneon lamp. The neon lamp can easilybe replaced by a nixie tube.

Polarity reversal indicatorsThe interchanging of the neutral and

the live wires can pose a major threat,as it brings the cabinet close to the‘live line’ potential. The circuit givenin Fig. 4(a) is designed to give an alarmand disconnect the mains, should sucha situation occur. The circuit given inFig. 4(b) gives both an audible andvisual alarm without taking any action.

Overheating/fire-hazard alarms

The circuits shown in Fig. 5 aredesigned to forewarn of any chances offire in sensitive equipment. The cir-cuits, which can be clipped onto thepower component which tends to heatthe most, say a power transistor or anIC, gives an alarm when the componentheats up to a preset level. The circuitsgiven in Figs 5(a), 5(b) and 5(c) givevisual and audio indications respec-tively and disconnect the supply.

Mains LeakageMains leakage is the cause of

electrocution in a majority of cases.Two circuits are given to detect and

Fig. 5(a): Visual overheating alarm.

Fig. 6: Mains leakage alarm circuit.

Fig. 5(c): Overheating cutout circuit.

Fig. 5(b): Audible overheating alarm.


provide protection against the same.The circuit given in Fig. 6, providesan alarm upon sensing the leakage,whereas the circuit given in Fig. 7cuts off the supply in that case.

Electronic fuses

In most cases, conventional fusescan provide protection against varioushazards. But in certain instances, wherethe load consumes more current during‘switch-on’ but lesser afterwards, thefuse blows up when the device is turnedon. In such cases, the electronic fusewith an adjustable ‘blowing speed’ maybe used. The fuse, as shown in Fig. 8,

is based on an LM748 comparator. VR5is used to adjust the current level andVR6 to vary the speed of blowing inthe seconds range.

Essential precautions

1. Use only the right hand whilehandling high voltages, so that evenduring momentary accidental contact,the chances of cardiac muscularcontraction are reduced.

2. Keep the hands dry while han-dling voltages even above 50V.

3. It is advisable to wear rubbergloves while handling high voltages.

4. Try to avoid direct contact with

ground, if possible. It is recommendedthat one stands on a rubber or woodenblock while handling high voltages.At least, the left leg should be keptwell insulated.

5. Keep a neon pen tester handy.6. Never try repairing gadgets with

the mains plug connected. Persons withweak hearts or heart problems are ad-vised not to risk handling high volt-ages, as they are the most susceptibleto cardiac arrest.

Keeping these precautions in mind,even a beginner should be able to con-struct the useful safety gadgets, de-scribed above. �

Fig. 7: Mains leakage cutout circuit.

Fig. 8: Electronic variable 'blowing speed' fuse.


S urface mount technology(SMT) was introduced in thebeginning of the 1980s to make

circuits compact and small. It wasSMD (surface mounted device) whichcreated different layer PCBs to with-

stand the fast, advanced and smallcomponents in micro electronics. To-day, almost all components in the com-puter industry are surface mounted.Hence, modern electronic equipmentis very compact and small. Surface

mounted devices have revolutionisedthe electronics industry.

An SMD is a very small electroniccomponent. It may be an IC, a resis-tor a capacitor or a diode. Generally,ICs are labelled as SMDs when they

PORTABLE SMDSOLDERING STATION

PRADIP KUMAR BOSE

Fig. 1: Circuit diagram for the portable SMD soldering station.


are very small and have several ter-minals. Nowadays, single-chipm i c r o c o n t r o -llers in keyboards, FDC, XT/AT I/O,display cards and other add-on cardsare almost all SMDs. Modern elec-tronic wave soldering assembly is usedto solder these devices on PCBs. Butfor servicing purpose an SMD solder-ing station is necessary. A com-mercially available station costsaround Rs 700 while the station de-scribed here costs around Rs 400 andit can do all the work, the one avail-able in the market can do.

The soldering station is nothing buta temperature controlled soldering iron.A low voltage (12V) and low wattage

PARTS LISTSemiconductors:IC1 — LM3914N bargraph displayTl — BC148 npn transistorT2,T3 — SL100 npn transistorT4 — 2N3055 npn transistor

(on 3mm thick heatsink)D1-D4 — 1N5402 rectifier diodeD5 — 8.6V, l00mW zenerD6 — 1N4001 rectifier diodeD7 — 6.1V, 400mW zenerD8 — 1N4148 silicon switching

diode

Resistors (all 1/4 watt, ±5% carbon unlessstated otherwise):Rl — 560-ohmR2 — 100-ohmR3,R7,R8 — 8.2-kilohmR4,R6 — 2.2-kilohmR5 — 1.5-kilohmR9 — 3.9-kilohmR10,R11 — 1-kilohmVR1 — 22-kilohm potVR2 — 2.2-kilohm potVR3 — 4.7-kilohm linear pot

Capacitors:Cl — 1000μF, 25V electrolyticC2 — 10μF, 12V electrolyticC3 — 220μF, 25V electrolytic

Miscellaneous:RL1 — 12V, 300-ohm SPDT relaySl — ON/OFF switchX1 — 0-12V, 2A step-down

transformerFl — 1.5A fuse

— 12V, 10W/15W SMD iron— LEDs

(10/15W) soldering iron is necessaryfor SMDs. As it would be very compli-cated to use a temperature sensor, avariable voltage source is used to con-trol temperature of iron tip. Since ther-mal load is very small for SMDs, thismethod is suitable. Besides, an over-voltage sensor is there to monitor themains voltage. If voltage increaseslargely, this section disconnects the ironfrom the supply to avoid damage to theelement.

A virtual indication of temperatureby several LEDs is used here. The LEDsglow like a bargraph in audio amplifieraccording to the rise in voltage, i.e. therise in temperature. Though this is avirtual method, it is quite suitable for

such cost-effective equipment.

The circuitTransformer X1 steps down AC

supply voltage. After filtering and recti-fication, the voltage is applied acrossan over voltage cutout (OVC) circuit,built around transistors Tl and T2. Af-ter a predetermined voltage set by VR1,zener D5 fires and transistor Tl con-ducts. As a result, its emitter voltageincreases, T2 becomes ‘on’ andenergises relay RL1. At the same time,LED2 indicates over voltage. RelayRL1 breaks the supply to the regulator.

The voltage regulation is achievedby a series regulator. Transistors T3and T4 are connected in Darlington ar-rangement. The base voltage of tran-sistor T3 is controlled by VR3. ZenerD7 makes voltage across VR3 constant.The variable voltage source is capableof giving 9 to 12V DC at 1A.

IC LM3914N is used here for vir-tual temperature indication. The volt-

Fig. 2: Actual-size PCB layout for the portable SMD soldering station.



age divider, made by resistors R8, VR2and R9, gives sensor voltage to LM3914. This sensor voltage is applied atthe input terminal of ICl (pin 5). Tenoutputs are taken from IC1 and tenLEDs are connected to them, forminga bargraph array. The voltage variationacross the soldering iron increases ordecreases the number of correspondingLEDs that glow.

SMD iron used here is a 12V, 10Wor 15W soldering iron. These irons aresmall and have a pin-type tip. Besides,a thin solder wire should be used. Theseirons are now available in the marketfor around Rs 140.

Construction, testingand alignment

The PCB layout and component lay-out of this project are shown in Figs 2and 3, respectively.

Use IC socket for IC 3914. Aftercompleting construction, disconnect re-

lay switch from anyone end of coil L1.Now, switch on Sl,observe and ensurethat LED1 is glowing.Apply about 250V tothe primary of X1.Now, adjust VR1 insuch a way that relayRL1 just energises. Inthis state, LED1should ‘go off’ andLED2 must glow.Check voltage acrosscapacitor Cl. It mustbe higher than 16V in this state.

Decrease primary voltage to 220V.Relay RL1 should de-energise now.Connect coil L1 in its place and adjustVR3 to get minimum output voltage.Check this voltage with a meter. If it isbelow 9V, replace resistor R7 by trialand error method.

In this condition, adjust VR2 suchthat LED3 just glows. Now, rotate VR3

to maximum position. All LEDs shouldglow. The soldering station is ready foruse now. A suggested cabinet is shownin Fig. 4.

To begin with, place VR3 to 3/4thof its total rotation and then after heat-ing place it near 1/2 position. Thiswill increase the life of iron as wellas the bit. �

Fig. 4: Suggested cabinet for the SMD soldering station.

Readers’ Comments:Please, correct some mistakes in thearticle:

Resistors R1 (560-ohm) and R5(1.5k) have been connected in paral-lel to each other across rectified volt-age filtering capacitor C1. With 12VDC, the flow of around 30mA cur-rent disspates about 360mW acrossR1, which exceeds the 1/4W specifi-

cation of the resistor. Please, clarifythe wattage of resistor. Why are weusing such low-value resistor? Is it ableeder resistor?

Ramesh HirparaGandhinagar

The author, Pradip Kumar Bose,replies:

Please, note that the two resistors

are in parallel, but current through themis unequal. Resistor R1 dissipatesaround 0.25W (IR1) = 0.021A approx.)and R5 dissipates 0.096W. However,R1’s rating may be changed to 0.5-watt, if required. Yes, R1 and R2 arebleeder resistors.


There is a variety of outdoor lightcontrollers available. Someswitch lights on at the fall of

dusk and off at the break of dawn. Oth-ers switch lights on at the end of duskand off after a few hours. But for do-mestic use, where we require light inthe early mornings also, these do notserve the purpose. The circuit describedhere solves this problem.

It offers the following features:1. It switches on lights at about 6.00

pm and switches off at about10.30 pm. Again, it switches on lightsat 4.30 am and switches off at about6.00 am.

2. If power supply is interrupted, itautomatically switches off the wholesystem. In this way, erratic switchingof the bulbs is avoided.

3. There are separate on and offswitches for easy operation of the sys-tem.

4. It uses LED indicators to detectdifferent conditions of the circuit.

5. Preset control to set the triggertime.

The circuit comprises six sections.

1. Autopower offThe autopower off system is based

DIGITAL CONTROLLER FOROUTDOOR LIGHTS

UNNIKRISHNAN P.R.

Fig. 1: Block diagram for the digital controller.

on IC1, an NE555. A fixed voltage,lower than 2/3 Vcc, is fed to pin 6(threshold) of IC1. If triggered, its out-put stays in the on state as long as itsreset pin 4 does not get a negative pulseor the power supply is interrupted. Theoutput goes low when the supply isswitched on again. The output of IC1provides power supply for the secondsection.

By pressing Sl, a negative pulse isfed to pin 2, and thus it can be used toswitch on the other sections. S2 is usedto switch off other sections by reset-ting IC1.

When power supply is interrupted,output of IC1 goes low and switchesoff the other sections. When power sup-ply is resumed, S1 must be pressed toswitch on the circuit. By observingLED1, we can know whether the cir-cuit is on or off.

2. TriggeringThis section is used to trigger the

circuit at the fall of dusk. It is config-ured around IC2, which is working inthe monostable mode. A variable volt-age is fed to trigger pin 2 of IC2 viaLDR1 and VR1. In the monostablemode of operation, the output of IC2

stays high as long as its trigger inputstays below 1/3 Vcc.

At night, the resistance of LDR1 ishigh, so the voltage at pin 2 is at a lowlevel and hence the output of IC2 goeshigh at night. The output condition ofIC2 can be detected from LED2. Theoutput of IC2 gives power supply tothe next three stages.

3. ResettingThe next section is used to reset

the two 4017 decade counter ICs usedin the counter section. This is alsobased on a 555 timer (IC3). Its trig-ger pin 2 is connected to capacitorC4 and resistor R7. In the absence ofa supply, there is no voltage acrossthe capacitor. When supply is on, IC3is triggered because voltage across ca-pacitor is below 1/3 Vcc and its out-put goes to a high state. The voltageacross capacitor C4 increases, so thatthe voltage at pin 2 crosses 1/3 Vcc.The time period for which the outputstays in the high state depends uponresistor R8 and capacitor C5, and isgiven by the relationship 1.1 x R8 xC5.

The output of IC3 is given to thereset pin 15 of decade counters IC5


and IC6. These ICs are reset when IC3is triggered, and as long as the outputof IC3 is in the high state, the countersstay in the reset position.

4. OscillatorThis section is used to produce a

square wave output. It is based onIC4, an NE555. Its output frequencydepends upon resistors R9, R10 andcapacitor C7, and is calculated by theequation

1.443––––––––––––––––

(R9 + 2 R10) C7The output frequency is fed to the

counter section.

5. CountingThe counter section is used to count

the output frequency from the oscillat-ing section. It uses two CMOSCD4017BS ICs. The frequency fromoscillating section, IC4, is fed to theinput pin of IC5. IC5 works as a fre-quency divider and its output is fed tothe input pin of IC6.

The four outputs of IC6 are com-bined by using diodes D1 through D4.This combined output is given to thelast section. If any of the outputs ishigh, then the load is in ‘on’ state.The condition of this combined out-put is detected by LED3.

6. OutputThe output section is used to switch

the relay on and off. It is based onBEL187 transistor Tl. The relay is con-nected at the collector of transistor Tl.Diode D5 is connected to protect tran-sistor T1. The output from the count-ing section is fed to the base of T1through resistor Rl1. If the output fromcounting section is high, it biases thetransistor and thus the relay is acti-

Capacitors:Cl,C3,C6, C8 — 0.0lμF ceramic

discC2 — 100μ, 16V

electrolyticC4 — 47nF ceramicC5 — 470μF, 16V

electrolyticC7 — l000μF,16V

electrolyticC9 — l00μF,25V

electrolyticCl0 — 10μF,16V

electrolytic

Miscellaneous:RL1 — 9V, 200-ohm,

SPST relayS1,S2 — Pushbutton type

switch

PARTS LIST

vated.

Working

By pressing S1, the triggering sec-tion gets the power supply. At the fallof dusk, the resistance of LDR1 in-creases, so a large voltage is droppedacross it, resulting in a decrease in thevoltage at pin 2.

When the voltage at pin 2 drops

Fig

.2:

Cir

cuit

dia

gra

m f

or

the

dig

ital

co

ntr

olle

r.Semiconductors:IC1 - IC4 — NE555 timerIC5, IC6 — CD4017B decade

counterT1 — BEL187 npn

transistorD1 - D5 — 1N4001 rectifier

dicode

Resistors (all 1/4 watt, ±5% carbon unlessstated otherwise):Rl, R13 — 10-kilohmR2 — 56-kilohmR3 — 27-kilohmR4, R6, R12, R14 — 680-ohmR5 — 100-kilohmR7 — 2.2-megohmR8 — 470-kilohmR9 — 120-kilohmRI0 — 330-kilohmR11 — 15-kilohmVR1 — 470-kilohm,

variableLDR1 — Light dependent

resistor


below 1/3 Vcc, IC2 is triggered andstays in the high state till morning.Since the output of IC2 is high, thenext three stages get power supply.

When C4 is fully charged, IC3 istriggered and so the reset pins of IC5and IC6 both get a positive pulse.Therefore, the first outputs of both ICsare high. This condition changes whenthe voltage across capacitor C5 crosses2/3 Vcc, because then the output ofIC3 goes to a low state and the resetpins of IC5 and IC6 get a negativepulse each. In this condition, IC5 getsready to accept pulses from the oscil-lating section and its outputs arechanged.

For each set of ten pulses from IC4,the outputs of IC6 are changed one byone. The output time period of oscillat-ing section is about 540.5 seconds, i.e.

about nine minutes. Since IC5 dividesthe output frequency from IC4, IC6gets a pulse of time period 9 x 10 = 90minutes, i.e. one and a half hours. So,the outputs of IC6 are changed everyone and a half hours duration.

When the controller is switched on,the output Q0 at pin 3 of IC6 is highand it turns on the load for first oneand a half hour. Then, the next outputQ1 at pin 2 is high, which switches onthe load for the second one and a halfhour. Next, output Q2 at pin 4 goeshigh, which switches on the load forthe same period again.

When the output Q3 at pin 7 ishigh, it switches off the relay as pin 7has no connection. Then, the output ofIC6 changes to Q4, Q5, Q6 for eachone and a half hour, and when it reachesQ7 (i.e. pin 6) the load is again switched

on. After one and a half hour, theoutput is changed to Q8 (pin 9).Since pin 9 of IC6 has no connec-tion, it again switches off the relay.

At the break of dawn, the re-sistance of LDR1 decreases, andthe trigger pin of IC2 gets a posi-tive voltage greater than 1/3 Vcc,so that its output goes low which,in turn, cuts off power supply tothe next three stages.

AssemblyThe circuit may be assembled

on a general-purpose PCB or thePCB whose pattern is shown inFig. 3. Front panel can be madeaccording to one’s choice. A modelis shown in Fig. 5.

A 9V power supply can be as-sembled, using a step-down transformerand a rectifier, and connected to the cir-cuit as shown in Fig. 2. Relay connec-tions are also shown in Fig. 2.

The front panel bears various con-trols and indication LEDs. In the pres-ence of power, LED1 glows. On press-ing S1, LED4 will glow.

At day time, adjust VR1 to set thetrigger LED2 to off position by trialand error method.

Under normal conditions, onlyLED1 and LED4 will glow. After dusk,trigger LED2 and load LED3 will alsoglow. After four and a half hours, theload and load LED3 are off. The loadLED3 is again activated after six hours,to be turned off after another one and ahalf hours.

To turn the system off, press S2. �

Fig. 3: Actual-size PCB layout for the digital controller.


Fig. 5: Front panel layout.


V OX or voice operated trans-receive control is very conve-nient in operating a transmit-

ter, especially in SSB mode. Commer-cially available units are expensive.Most of the available circuits in Hamjournals use complex circuitry, whicheither is highly expensive or most ofthe components are not easily avail-able in the Indian market. The unit de-scribed here is based on compact andstraightforward design, using easilyavailable discrete devices, and is quiteinexpensive. The unit is ideal to usewith almost any commercial or home-brewed VHF or HF trans-receiver thatdoes not have the facility of VOX op-eration. Further, the performance of this

unit is an improvement over the previ-ously available designs.

Block diagram

The block diagram is given in Fig.1.A part of microphone input is capaci-tively coupled to a preamplifier. The

VOX FOR HAM RIGS

KANG K.P.S

Fig. 1: Block diagram for the VOX.

Fig. 2: Circuit diagram for the VOX.

preamplifier is provided with a sensi-tivity control, which is used to adjustthe sensitivity of VOX. The output ofthe preamplifier is rectified and a DCsignal proportional to the available mi-crophone input is applied to VOX/antivox sensitivity control.

Antivox signal is derived from re-


ceased.

Circuit

The input stage to the preamplifieris designed with an FET to achievehigh input impedance (around 1-me-gohm) and low current drain. Capaci-tor C2 shunts traces of any RF pickupat the input of this stage since the unitis designed to operate near transmitter/receiver unit. Resistors Rl, R2 and R3bias the FET. The frequency responseof the stage is determined with emitterdecoupling capacitor C6. In case falseoperation due to hum pickup from sur-rounding mains wiring is observed, itcan be eliminated by reducing the val-ues of capacitors C6 and C9.

Transistors T2 and T3 constitute thenext two stages of the preamplifier.BC149C transistors are used at low Qpoints to achieve the required perform-ance from the circuitry. The emitterresistance VR1 of transistor T3 deter-mines the overall gain of the preampli-fier and so the microphone sensitivitycan be controlled with it.

Diodes Dl through D4 constituterectifier/detector sections for both VOXand antivox signals. Diode 1N34 isused, but OA79 or similar germaniumunits will render better service in suchstages, and can be substituted for bet-

PARTS LIST

VR3 — 10-Kilohm pot.VR4 — 22-Kilohm pot.

Capacitors:Cl — 0.005μF ceramic discC2 — 220pF ceramic discC3 — 270pF ceramic discC4 — 0.lμF ceramic discC5 — 2200μF, 16V electrolyticC6,C8,C9 — 4.7μF, 16V electrolyticC7 — 3.3μF 16V electrolyticC10,C12,C14 — 10μF, 16V electrolyticC11,C13 — lμF nonpolar electrolytic

Miscellaneous:Sl — SPST toggle switchS2 — Push-to-on switchRL1 — 12V, 600-ohm SPST

relay— PCB, connecting wires,

suggested enclosure andjacks etc

Semiconductors:Tl — BFW10 (FET) transistorT2,T3 — BC149C npn transistorT4 — BC148B npn transistorT5 — BC157A pnp transistorT6 — BC147B/SL100 npn

transistorD1-D4 — 1N34/OA79 detector

diodeD5 — 1N4001 rectifier diode

Resistors (all 1/4 watt +5% carbon, unlessstated otherwise):Rl — 1-MegohmR2.R6,R10,R13 — 10-KilohmR3,R14 — 1-KilohmR4,R8,R12 — 100-KilohmR5 — 270-KilohmR7 — 820-ohmR9 — 150-KilohmRll — 4.7-KilohmVR1 — 1-KilohmVR2 — 220-Kilohm

Fig. 3: Actual-size PCB layout for the VOX.


transmitter. The delay circuit providesthe required delay to trans-receivechange-over.

In presence of antivox signal fromthe receiver’s speaker, a signal of equalbut opposite magnitude is developedacross the second arm of VOX/antivoxsensitivity control, which cancels outthe trace of VOX signal to DC ampli-fier and de-operates the transmitter.Thus, in presence of signals in receiver,false operations of transmitter are

ceiver speaker output and is applied tothe unit. This signal is also rectified toachieve a similar DC signal, but withopposite polarity to that of VOX signaland applied to the second arm of VOX/antivox sensitivity control.

In presence of VOX signal frommicrophone input, a proportional DCsignal is developed at the centre ofVOX/ antivox sensitivity control, whichis amplified, using a DC amplifier andit operates the relay driver to key the


ter performance, if available. The rec-tified outputs are filtered with capaci-tors C11 and C13 capacitors andapplied to the two arms of VR2, whichis used as VOX/antivox balance.The reverse polarities of diodesD3 and D4 in antivox section of recti-fier are meant to generate negativemagnitude with respect to VOX recti-fier. A positive potential is appliedto the VOX arm of VOX/antivox bal-ance through switch S2 and resistorR12 to effectuate manual operationwhen needed.

The generated emf is applied to aDC amplifier, built around transistorsT4 and T5. Capacitor C12, resistor Rlland potentiometer VR4 form the delaycircuit, which provides the requiredtime delay between trans-receive op-erations. Transistor T6 is a relay driver.BC147B is used, but BC187 can alsobe used. Another good substitute isSL100 or BD135. An SPST relay at itscollector is used to key the transmitter.Diode D5 protects transistor T6 from

transients during relay operations.

Construction

A PCB layout of the VOX unitis given in Fig.3. The PCB is suffi-ciently large and simple for easyassembling, even by beginners.The PCB accommodates almost allcomponents, except VR3, C13, C14,D3 and D4 which are mountednear the antivox input jack on the rearpanel. The complete unit may be housedin a metallic cabinet usedfor TV signal boosters, but can be in-cluded in the existing transmitter cir-cuit itself.

The PCB layout shows that aboutone third of the space is unused and soa smaller version is obviously possibleto fit into the commercial rigs withcrowded inners and a PCB to suit indi-vidual needs can be designed accord-ingly.

Though a metallic enclosure is usedto avoid stray RF pickup by preampli-

fier stages, additional safety measures,such as use of high quality shieldedwires (especially as small in length aspossible) should be taken too. It is ad-visable to use ferrite beads on all inputand supply leads.

No provision of mounting the relayon the PCB itself is made and thus therelay needs to be fitted wherever pos-sible. A fast relay is needed to avoidclipping of the first syllable spoken. Asmall unit is often better than a largeone.

The adjustment is simple. KeepVR2 approximately at its centre. Ad-just VR1, so that the relay is keyedwith each syllable spoken at a consid-erable and convenient distance fromthe microphone. VR4 should be ad-justed to an individual’s speech habits,but it is a must to give sufficient delay,so that the relay should drop out onlyat long pauses in speech. The antivoxcontrol VR3 should be adjusted to giverequired antivox action at listening levelof the receiver of one’s choice. �


T his is a game which demandsa lot of concentration andpatience. The mechanical ver-

sion of the game consists of a cylindri-cal container with a transparent flat

top and a smooth inner surface witha lightweight ball in it. The objectiveof the game is to manoeuvre the ballinto a shallow pit at the bottom of thecontainer.

As the container’s surface issmooth, it is very difficult to achievethis objective. And even if the ball landsup in the pit, it is very likely that theball would jump out soon, as the pit is

STEADY HAND GAME

HARINDER SINGH

Fig. 1: Circuit diagram for the steady hand game.


very shallow. Another version of thegame has more than one ball, all ofwhich have to be manoeuvered into thepit at the same time.

The electronic version of the game,taken up here, imitates the mechanicalversion having two balls. The rollingballs are represented by red LEDs andthe ball in the pit is represented by agreen LED. The balls have to be ma-noeuvred into the pit, using a joystick.To increase the excitement, timers havebeen included. The balls have to bemanouvred into the pit within a preset

time, and it is necessary that the ballsremain in the pit for quite some time(approx. 15 seconds) before the playeris declared a winner.

Circuit description

The circuit can be divided mainlyinto two sections, the display sectionand the timer section. The display sec-tion consists of ball LEDs, LED1through LED4, the ball in the pit, LED5(bicolour) and the joystick potentiom-eters VR1 and VR5. IC1 LM324 is anop-amp. Op-amps IC1 (a) and ICl (b)are wired as window comparators.

In a comparator if the non-invert-ing pin voltage is higher than the in-verting pin voltage, the output is posi-tive; if the voltages at both the invert-ing and non-inverting pins of the com-parator are same, the output is zero;else the output from the comparator isnegative. Window voltage for compara-tor IC1(a) and ICl(b) can be increasedor decreased with the help of potenti-ometer VR3(a).

When the voltage at the centre pinof potentiometer VR1 is within the win-dow voltage limits, both LED1 andLED2 are extinguished, else if voltageat centre pin of VR1 is higher thanvoltage at inverting pin of IC1(a), LED1lights up and LED2 is extinguished.On the other hand, if voltage at thecentre pin of VR1 is lower than thevoltage at non-inverting pin of IC1(b)then LED2 lights up and LED1 is ex-tinguished. Decreasing the windowvoltage by decreasing resistance of VR3makes it difficult to achieve the objec-tive of the game as the position of VR1at which both LED1 and LED2 areextinguished becomes more precise.

VR1 is used to manoeuvre the ball,i.e. LED1 and LED2 in two oppositedirections, east and west. A similar cir-cuit is built around comparators IC1(c)

and ICl(d), and VR5 manoeuvresthe ball, i.e. LED3 and LED4, innorth-south direction.

When any of the four ballLEDs is lit, transistor Tl con-ducts, so that transistor T2 is cutoff and the green colour part ofbicolour LED5 does not light,indicating that the ball is not inthe pit. As soon as all the ballLEDs are extinguished, Tl stopsconducting. Hence, T2 conductsvia resistor R13, resulting in

PARTS LISTSemiconductors:ICl (a-d) — LM324 op-ampIC2 — CD4040 counterIC3-IC4 — NE555 timerIC5(N1-N4) — CD4011 NANDT1,T2 — BC548 npn transistorD1.D2 — 1N4148 silicon

switching diode

Resistors (all 1/4 watt, +5% carbon, unlessstated otherwise):R1,R2,R11,R12 — 47-kilohmR3,R6,R7,R10R13,R21 — 10-kilohmR4,R5,R8,R9,R14,R15,R22 — 1-kilohmR16 — 680-kilohmR17 — 330-kilohmR18 — 180-kilohmR19 — 100-kilohmR20 — 4.7-kilohmVR1,VR5 — 100-kilohm pot.VR2,VR4 — 220-ohm pot.VR3 (a,b) — 1-kilohm dual pot.

Capacitors :C1,C2,C4 — 0.1μF ceramic discC3,C6 — 100μF, 16V

electrolyticC5 — 0.01μF ceramic disc

Miscellaneous :LED1-LED4,LED5 — Bicolour LED

(common-anode)LED6 — Red LEDSl — Bell pushbutton

switch

lighting up of green portion of LED5.The timer section consists of two

NE555 timer ICs. When the green por-tion of LED5 is extinguished, the resetpin 11 of binary counter IC2 CD4040is held high and hence IC2 is reset.When the green portion of LED5 lightsup, pin 11 of IC2 is grounded, so that itstarts counting the clock pulses pro-vided to it from IC3. IC3, is wired asastable multivibrator and providespulses of length 0.117 seconds. IC2counts these pulses upto 128, i.e. afterabout 15 seconds pin 13 of IC2 will gohigh for a period of 0.117 seconds, pro-vided IC2 is not reset, i.e. the ball doesnot jump out of the pit for full15 seconds. If the ball does jump out,IC2 is reset and the next count startsfrom zero till the time the ball remainsin the pit.

IC4 is wired as a timer whose timeis set by potentiometer VR6 and variesfrom 20 to 130 seconds. Switch Sl isused to start the timer. As soon as S1 ispressed, pin 3 of IC4 goes high andLED7 is extinguished. If during thistime, when pin 3 of IC3 is high, theball remains in the pit for 15 seconds,i.e. pin 13 of IC2 is high, the output ofNAND gate N3 goes low, thus turningon the red portion of LED5. A latch isformed around NAND gates Nl andN2. The latch is activated as soon asthe output of gate N3 goes low, so thatthe red portion of LED5 lights on. Now,if the ball jumps out of the pit, greenportion of LED5 will get extinguished,but red portion remains lit, indicatingvictory.Fig. 2: Arrangement of joystick elevation.

Fig. 3: Construction of joystick plate.

FOLD HERETO MAKE 90O

ANGLE BETWEENTHE TWO SIDES


If the player is unable to achievethe objective of the game within thegiven time, LED7 is lit, indicating time-out. The red portion of LED5 will notlight up even by placing the ball in thepit for 15 seconds if LED7 is lit, as thecondition for low output from gate N3will not be fulfilled in that case.

Construction

As the circuit is simple, it can easilybe constructed on a general-purposePCB. All the LEDs, potentiometers VR3and VR6 and the joystick are mountedon the front panel. To keep the cost ofthe game low, it is advisable to con-struct your own joystick.

Detailed diagrams, showing theconstruction of a simple joystick, aregiven in Figs. 2 and 3. This joystickwill have two degrees of motion alongtwo axes. VR1 is connected to the plateby using the spindle holder of a dam-aged potentiometer, having samespindle dia as VR1. This spindle holder

is screwed to the plate after removingits original spindle. The spindle of VR1is then placed in place of its originalspindle and glued to it, so that thespindle of VR1 moves, on moving theplate.

The second potentiometer VR5 isscrewed to the other hole in the plateand its spindle is glued to a handleformed by the back portion of an oldball pen. Now, the handle can be movedin two directions perpendicular to eachother, thus changing the resistance ofpotentiometers VR1 and VR5, depend-ing on the direction of motion.

Before placing the joystick in thecasing, make sure that wipers of boththe potentiometers VR1 and VR5 aremidway, when the handle is almostperpendicular to the panel.

This arrangement is screwed to thepanel, so that only the handle of thejoystick is outside the casing. The holemeant for the joystick handle shouldbe somewhat larger in dia than the diaof the handle, so that the handle can be

moved easily. Using a commercial balltype joystick will make the gameslightly more difficult, since the mo-tion of joystick handle in slightly slant-ing direction results in movement ofspindles of both VR1 and VR5.

Alignment and operation

The only adjustments that have tobe made are in the presets VR2 andVR4. Keep the resistance of potenti-ometer VR3 minimum and adjust thejoystick, so that all the red LEDs areextinguished. If two opposite directionLEDs (1 and 2 or 3 and 4) are litsimultaneously, increase the resistanceof VR2 (for LED1 and LED2) or VR4(for LED3 and LED4) until only oneof the two opposite-direction LEDs islit, as the ball can only be in one ofthe two opposite directions at an in-stant.

Potentiometer VR6 may be adjustedto give the required delay time toachieve the objective of the game. �


Gossiping over the telephone,especially among youngsters,has become a very common

trait nowadays. The verbal instructionto “be brief on phone”, hardly has anyeffect. The electronic gadget discussedhere may force the telephone user tobe brief, or else disconnect the lineafter 9 or 10 minutes.

Working principleThe principle of the gadget can be

very well understood from the blockdiagram shown in Fig. 1. The gadgetconsists of a watchdog circuit, sensorand switch circuit, one-minute pulsegenerator, decade counter/divider andrelay circuit.

Watchdog circuit/sensor circuit willdetect whether the telephone is in use.If so, it extends the information to theswitch circuit which in turn suppliesVcc to one-minute pulse generator/de-cade counter. Conversation period iscounted and displayed by LEDs(LED1—LED10). After nine minutesthe relay circuit is enabled to discon-nect the telephone instrument from theline.

TELECONVERSATION LIMITER

S.BATRA

Capacitors:C1,C4 — 2.2 μF, 16V electrolyticC2 — 47μF, 16V electrolyticC3 — 0.0 lμF ceramic discC5 — 220μF, 16V electrolyticC6 — l00μF, 16V electrolytic

Miscellaneous:X1 — 230V AC primary to 0-5V,

100 mA secondary trans-former

RL1 — 5 or 6V mini relayLED1-LED5 — 3 mm green LEDLED6-LED8 — 3 mm amber LEDLED9,LED10 — 3 mm red LED

Suitable acrylic cabinet

Semiconductors :IC1 — 4N35 or MCT2E optocouplerIC2 — 7555 CMOS timerIC3 — 4017 CMOS decade counterT1,T2 — BC109 silicon npn transistorD1-D4 — 1N4004 rectifier diodeD5 — 15V, 250mW zener

Resistors (all 1/4 watt, +5% carbon, unlessstated otherwise):R1-R10,R14,R15,R19 — 1-kilohmR11,R12 — 10-kilohmR13 — 1-megohmR16,R18 — 100-kilohmR17 — 33-kilohmVR1 — 47-kilohm presetVR2 — 470-ohm preset

PARTS LIST

Circuit description

The complete circuit diagram of thegadget is given in Fig. 2. Under theidle condition, i.e. when the telephoneis on-hook, the potential differenceacross the telephone line is about 48V.The diodes D1 through D4 in the bridgeconfiguration, 15V zener diode D5 and100k resistor R18 form the watchdogcircuit. The circuit will detect whetherthe telephone is on-hook or off-hook.

Under on-hook condition, the linevoltage, i.e. 48V, enables zener to breakdown. Hence, a voltage of 15V devel-ops across the zener and about 33Vdrops across R18, which is applied toopto-LED pins 1 and 2 of theoptocoupler IC1(4N35) through 33klimiting resistor R17 and the opto-LEDlights up.

Under off-hook condition, a loop isextended through the telephone instru-ment, reducing the line potential to

Fig. 1: Block diagram for the teleconversation limiter.


about 9V. This voltage fails to operatethe 15V zener, and hence no potentialis developed across resistor R18 andno light is emitted by the opto-LED.

Light energy, if emitted, is sensedby the photo-transistor pins 4 and 5 ofoptocoupler IC1 and in turn drives tran-sistor Tl to saturation. The potential atthe collector of transistor Tl will there-fore be almost zero. In case the light isnot sensed, the transistor will be cutoff and the supply potential, about 5V,will be available at the collector of Tl.

The collector of transistor Tl extendsthe reset voltage to pin 4 of one-minutegenerator IC2 (7555) and Vcc to de-cade counter/divider IC3 (4017). Obvi-ously, when the telephone is on-hook,transistor Tl supplies zero potential toboth the ICs and they are inactive.

The moment the telephone is off-hook, supply voltage of about 5V isextended to IC2 and IC3. Every minuteis counted by the counter and displayedby the respective LEDs.

The first five minutes are indicatedby five green LEDs, one by one. Subse-quent three minutes are indicated bythree amber LEDs. The ninth minute isindicated by a red LED.

In case, the call is not concludedtill the end of the ninth minute, start ofthe tenth minute will operate LED 10and the mini relay circuit, disconnect-ing the telephone instrument throughN/C contact of the relay.

The watchdog circuit continues get-ting potential lower than 15V. The ca-pacitor C4 and resistor R12 help inresetting IC3 as soon as the telephoneis off-hook. Capacitor C6 provides adelay of two to three seconds as aprotection against any irregularity dueto tapping of the hook-switch by some-one. As soon as the telephone is on-hook, transistor Tl saturates and IC2and IC3 become inactive.

In case, the time-limit has to be setto other than nine minutes, vary thetime-period of IC2 by varying the val-ues of preset VRl, resistor R13 andcapacitor C2.

Assembly and testing

This project can be assembled on aPCB of about 80 mm x 65mm size.The PCB and component layouts aregiven in Figs 3 and 4, respectively. Af-ter the assembly is completed, beforeconnecting it to the telephone line, the Fig. 2: Circuit diagram for the teleconversation limiter.


Fig. 3: Actual-size PCB layout for the circuit shown in Fig. 2.

rent of the relay. Now, the gadget isready for connection to the telephoneline.

6. When connected to the telephoneline, no LED should light up, exceptthe power indicator LED 11. In caseunder on-hook condition LED1 lights

up, check either the optocoupler ortransistor Tl.

7. As soon as the instrument is off-hook, the first green LED should lightup. Subsequently, all LEDs should lightup one by one.

8. After about nine or ten minutes,

circuit could be tested as follows :1. Before switching on the main

supply, check for any short circuit be-tween Vcc and ground. When switchedon, the supply voltage should be be-tween 6 and 7 volts.

2. With the telephone line not con-nected, there will be no potential at thebase of transistor Tl and about 5Vshould be available at the collector ofTl. LED1 will light up.

3. After about a minute, LED2should light, if the one-minute pulsegenerator is working properly. If not,check output pin 3 of IC2 for 50 per-cent duty cycle. Otherwise, check thecomponents connected to IC2.

4. If the counter is working prop-erly, it should light up all the LEDs oneby one.

5. When the last LED lights up, therelay will operate. Keep preset VR2 atthe minimum value. Increase VR2 tillthe relay remains in the operating condi-tion. Try to adjust before the tenth LEDgoes off. This will economise the cur-

when the relay operates, the line willbe disconnected from the instrumentfor about a minute. The telephone callwill also get disconnected during thisone-minute interval.

After the circuit has been satisfacto-rily tested, the complete gadget canbe housed in a suitable white acryliccabinet and installed near the telephoneinstrument. A suggested front panel lay-out of the cabinet is shown inFig. 5.

Incoming ringing current will haveno effect on the gadget, as IC2 and IC3are not active at that time.

Any other disturbance on the linewill not affect the working of the gad-get, as it is completely isolated fromthe telephone line.

In case of any fault (such as failureof optocoupler or power supply) in thegadget, the normal operation of the tele-phone will not be affected as the tele-phone instrument continues to be con-nected to the line through the normally-closed (N/C) contact of the relay. �

Fig. 5: Suggested acrylic cabinet.



PROGRAMMABLE NUMBERLOCK

S. HARI KUMAR

Fig. 1: Circuit diagram for the main programmable number

Aprogrammable versatile elec-tronic number lock circuit isdiscussed here. It offers facili-

ties to set a nine-digit code number forthe lock. The code can be changed veryeasily by the legitimate user. A numberis entered and the lock is set. The lockopens only if the same number is en-

tered again. The memory is protectedagainst power failures, and is not losteven during battery replacement.

The circuit is built entirely aroundCMOS ICs, and hence consumes verylittle power under steady state condi-tions. With slight modification, a 4-digit number can be used to set the

lock, if one finds the 9-digit numbera bit too long.

Operation of the lock

A keyboard is provided to enter thenumbers. To set a number in the lock,first put switch S3 on, to activate the


displays. Display 1 shows the numberentered, and display 2 the position ofthat number.

Press the reset or retry switch S2 toinitialise the counter and flip-flop. Now,display 2 reads zero. Display 1 alsoreads zero, as no number is entered.Close switch Sl to enable ‘Write’ op-eration. Now, enter a 9-digit numberthrough the keyboard, taking care to

note the value and the position of eachdigit from the displays. For example, ifthe number entered is 748326781, whenthe digit 2 is entered display 1 will show2 and display 2 will read 5. After thenumber is set, open switch S1. Thedisplay can be put off by opening switchS3. Press switch S2 to reset the lock.

To open the lock, push switch S2and enter the number. If the number

entered is correct, the relay operates toopen the lock. If it is not, the relay isnot activated.

Working

The number entered at the keyboardis converted to BCD form, using thestandard diode encoder matrix shownin Fig. 2. The output of keyboard unitis fed to the lock control unit. IC2MM74C14 is a hex inverter schmitttrigger. The resistors and capacitorsalong with IC2 serve a dual purpose.They provide the time delays requiredto adjust the pulse timings for IC4 andIC6, and also serve as key debouncer.

The output of IC2 goes low whenthe input voltage reaches the positivegoing threshold V

T+ of the IC, and re-

mains so until the input voltage fallsbelow its negative going threshold V

T-.

IC6 is a decade counter, whichcounts on the negative going edge ofclock pulse. When any key is pressed,IC6 is given a clock pulse and it incre-ments its count.

The BCD output from the keyboardis inverted by IC2, and is again in-verted by IC3, a 7404 hex inverter, togive the original BCD form to IC4(MM74C89).

IC4 is a 64-bit tristate random ac-cess memory. Data is stored in wordsof 4-bit length and so a maximum ofsixteen 4-bit data can be stored. To storea 4-bit data in the memory, the data isgiven to data inputs and the address isfed to address inputs. Then CS (chip

PARTS LISTR15,R17,R18, R20,R21,R26R38 — 10-kilohmR16 — 22-kilohmR19,R36 — 1-kilohmR22, R23,R25 — 100-kilohmR24,R31-R34 — 2.2-kilohmR35 — 4.7-kilohmR37 — 1.2-kilohm

Capacitors:C1 — 1000μF, 25V electrolyticC2-C8 — 0.1μF ceramic discC9 — 0.01μF ceramic disc

Miscellaneous:S1 — SPST switchS2 — Push-to-on switchS3 — SPST switchS4-S13 — Keyboard with 10 keysRL1 — 6V, 300-ohm relayX1 — 230V AC primary to 12V,

500mA secondary trans-former

DIS1, DIS2 — FND500 or LT543 com-mon cathode display

B1, B2 — 6V battery (two 3V lithiumcells)

— PCB, chassis etc.

Semiconductors:IC1 — 7806 6V, 500mA regulatorIC2 — MM74C14 hex inverter

schmitt triggerIC3 — MM74C04 hex inverterIC4 — MM74C89 64-bit tristate

random access read/writememory

IC5, IC8 — CD4511 BCD to 7 segmentlatch decoder/driver

IC6 — MM74C90 4-bit decadecounter

IC7 — MM74C86 quad 2-inputEX-OR gate

IC9 — MM74C32 quad 2-input ORgate

IC10 — MM74C08 quad 2-inputAND gate

IC11 — MM74C74 dual D-type flip-flop

T1 — BC147 npn transistorT2 — SL100 npn transistorD1, D2D32-D36 — 1N4001 silicon rectifier

diodeD3-D31 — 1N4148 silicon switching

diode

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1-R14 — 370-ohm

Fig. 2: Circuit diagram for the keyboard of programable number lock.


Fig. 3: Actual size PCB layout for the circuit shown in Fig. 1.



select) and WE (write enable) pins 2and 3 are brought low.

In the read operation, WE is kepthigh and address is given to addressinput. When CS is brought low, thedata stored at the address location isavailable at the output. The address toIC4 is provided by the counter (IC6).The address is in binary form.

By keeping WE low, a number isstored in the memory chip. Now WE iskept high. During ‘Read’ operation,when each number is fed through key-board the corresponding number is readout from the memory and comparedwith the given number using IC7, aquad EX-OR gate.

If the numbers do not match in any

one of the four bits, one of the outputsof EX-OR gates goes high. All the fouroutputs are ORed to get output at pin 8of IC9, which is a quad OR gate. Amismatch in numbers results in a logic1 at pin 8 of IC9, which is given to theD flip-flop, IC11. Resistor R19 and ca-pacitor C9 are used for pulse stretch-ing. The databit at the input is latched

Fig. 5: Upper track side layout for the PCB shown in Fig. 3.

Fig. 6: PCB layout for the circuit shown in Fig. 2.


to the output at the positive transitionof clock pulse. Clock pulse is givenfrom CS pin of IC4. If pin 8 of IC9 ishigh during clock pulse, the output Qof IC11 goes high and Q goes low.When Q goes high, clock input is kepthigh and no more input data is acceptedby the flip-flop.

IC10 (MM74C08) is a quad ANDgate. When counter output is 1001 (9),pin 3 of IC10 goes high. Q output of IC11 and output at pin 3 of IC10 areagain ANDed to give trigger to the re-lay. So, when the count is 9 and Q is

low the relay does not operate. Thenpress S2 and try again.

The 9-digit number can be reducedto a 4-digit number by making someminor changes in the connections ofIC6, by making it operate as a ‘divideby five’ counter. IC5 and IC8 are BCDto 7-segment latch decoder/drivers. Thedisplay is latched till the next clockpulse arrives. Switch S1 is placed suchthat it is accessible only after openingthe lock.

The lock can be used as a hardwarelock for electronic equipment.

Battery E2 shown in the circuit com-prises two lithium cells of 3V each,which safeguard the memory duringpower failure. A 6V regulated powersupply circuit is shown in Fig. 2. As allthe ICs used are of CMOS type, powersupply can range from 3 to 15V. A 6Vpower supply is chosen for convenienceand the 7-segment LED displays arebiased according to that. Both the PCBsare double sided with plated throughholes. The displays can be located at aconvenient place and are not attachedto the main PCBs. �


Fig. 8: Upper track side layout for the PCB shown in Fig.


S ummer in India is very hard tobear. Everyone wants a breezeof cool air, whether at office,

workshop or at home. It is also a wellknown fact that everybody cannot af-ford an airconditioner. In cities, peoplemostly use desert coolers that provide

HUMIDITY CONTROLLER

DINESH KUMAR RAHEJA

some relief from the scorching heat.In many houses, rooms are quite

small and ventilation is also inadequate,and if coolers are used continuouslyfor a long time, excessive humidity re-sults. This can cause serious diseasesrelated to lungs and skin.

PARTS LIST

VR1, VR2 — 100-kilohm linear, dualpot. (tandem type)

Capacitors:C1, C2 — 470μF, 16V electrolyticC3 — 0.01μF ceramic discC4 — 1000μF, 16V electrolytic

Miscellaneous:RL1 — 6V, 100-ohm SPST relayX1 — 230V AC primary to 6V

0-6V, 250mA step-downtransformer

Semiconductors :IC1 — NE555 timerT1 — AC187/SL100 npn tran-

sistor with heatsinkD1,D2 — 1N4148 silicon switching

diodeD3-D5 — 1N4007 silicon rectifier

diode

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1 — 1-kilohmR2 — 10-kilohmR3 — 8.2-kilohm

Fig. 1: Circuit diagram for the humidity controller.

In many coolers fan speed control-lers are provided, but there is no provi-sion for humidity control. Here is alow-cost and easy-to-build humiditycontroller, which can easily be installedin any desert cooler, and the user canget the desired level of humidity.

Circuit description

This controller is designed to switchon/off the motor of the water pumpautomatically. A commonly available555 timer IC is used as the basic build-ing block of the controller. The timer isconfigured in astable multivibratormode, and generates a train of squarewave pulses at its output.

A dual potentiometer (tandem type)is used to control the duty cycle (ontime/cycle time ratio) of the square


wave generated by the timer. Annpn transistor (AC187/SL100) is usedto interface the relay to the timer IC,to avoid overloading of the output stage.

Working

The on time and the off time de-pend on the values of resistor Rl andcapacitor C1, used in the charging anddischarging paths, respectively. Here,two silicon diodes Dl and D2 (1N4148)are used to split the charging and dis-charging currents through dual poten-tiometer VR1 and VR2, respectively,so that capacitors Cl and C2 chargethrough +Vcc, resistor Rl, potentio-meter VR1, diode Dl and resistorR2 up to two-third times the value of

decreased by the same amount, andvice-versa. This increases the on timeby an amount (say +t sec.), and simul-taneously decreases the off time by thesame amount (-t sec.), keeping the cycletime constant. In other words, the pulsewidth of the square wave is controlledby the position of the dual pot.

The relay switches the 230V ACsupply to the motor of the pump, whichsupplies water only during the on time,when the timer output (at pin 3) is high.For the rest of the cycle time, the pumpmotor remains off. This action is re-peated automatically every minute. Inthis way, the excessive humidity is con-trolled. The humidity control action canbe set anywhere between 10 to 90 percent of the maximum humidity obtainedwithout a controller.

Installation

Considering the lack of space inthe front panel and excess of watervapour inside the cooler, it is recom-mended to install this controller in asmall box outside the cooler. However,the potentiometer can be fixed withinthe front panel. �

Fig. 2: PCB layout for the humiditycontroller.

Fig. 3: Component layout for thehumidity controller.

+Vcc during the time Ton, andthen discharge through resistor R2, di-ode D2, potentiometer VR2 anddischarge pin 7 of timer IC1 up toone-third times the value of +Vcc,during the time Toff, given by the equa-tions:

Ton = 0.69 x (Rl + VR1 + R2) x Clseconds

and Toff = 0.69 x (R2 + VR2) x Clseconds

Also, the cycle time of the squarewave is given by:

Tc = Ton + Toff = 0.69 x (Rl + VRl+ VR2 + 2 x R2) x (C1 + C2) seconds.

In this circuit, the cycle time is setone minute approx. The dual potenti-ometer (tandem type) is wired in sucha way that if VR1 is increased, VR2 is


M ost security systems sufferfrom the drawback thateither they require a num-

ber of keyboard switches to be mountedoutside or they need a hidden switch(prone to be detected), which defeatsthe very purpose of authorised entry.The security system described here re-quires only one doorbell push-buttonswitch to be mounted outside. And italso serves as a musical doorbell.

The doorbell-cum-security alarmsystem so designed is specially meantfor use in domestic flats and apart-ments, etc.

The salient features of the systemare:

1. It can be used as a musical door-bell and security alarm system simulta-neously.

Fig. 1: Block diagram for the door-bell-cum-security alarm

2. Multizone operation. Open/closeloop conditions of a number of entry-exit points or zones can be sensed andmonitored. LEDs on the front panel ofthe main control unit indicate the condi-tions of corresponding zones.

3. Multimode Operation. It oper-ates in two modes: (a) maximum se-curity mode, and (b) minimum secu-rity mode. In maximum securitymode, fault conditions cause a wail-ing alarm which is loud enough to at-tract the neighbours and/or deter theburglars psychologically. In minimummode, the irritation or chaos oftencaused by carelessness to defeat thealarm by authorised persons can beavoided. However, entry and exit areannounced with soft musical tunes ofshort duration.

4. Use of strong metallic enclosureand key switches as mode changeoverswitches and reset enable switch makethe control unit virtually invulnerable.Once activated, the control unit cannotbe deactivated without reset enable key,as it is provided with a battery in itsmetallic enclosure.

5. Absence of vulnerable keyboardswitches mounted outside or detection-prone ‘hidden switch’ realise the pur-pose of alarm for authorised entry.Only a single doorbell switch servesthe purpose.

6. Changeable code. The single-digit code number to defeat the alarmcan be changed periodically.

7. Self-resetting audible alarmalong with manual resetting facilitywhich also resets the zone indication

DOORBELL-CUM-SECURITYALARM

A.U. AHMED


Fig. 2: Circuit diagram for the doorbell-cum-security alarm.

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Circuit

The block diagram of the circuit isshown in Fig. 1. The circuit has fivesensors. Each sensor may be a pushswitch, limit switch, micro switch or aclosed-loop wire connection whichopens with the opening of a door or awindow.

Open condition of the sensor re-mains stored in the bistable memorytill it is reset. The positive going edgeof the bistable output causes a sharppulse after the blocking capacitor andtriggers the alarm monostable.

In maximum security mode themonostable produces a long durationoutput which enables the wailer cir-cuit. In minimum security mode it givesa short-duration high output which en-ables the musical alarm.

Each time the musical alarm is acti-vated by doorbell push switch, a pulsereaches the counter IC and its decimalcount output is incremented. At a de-sired count, selected by 10-way switchS7, reset monostable is triggered. Itsoutput provides the necessary resetpulse to alarm monostable and bistablememories.

Reset monostable can be triggeredby reset pushbutton also, only after be-ing enabled by switch S2.

Sensors

Sensors Al to A5 may be any con-tacts that open up with the opening ortampering of the corresponding doorsor windows they guard. These may bepush switches, limit switches or microswitches mounted on the door and win-dow frames in such an ingenious waythat a little opening of the door/win-dow causes the switch contacts to open.

With the prototype, spring-loadedbayonet contacts salvaged from elec-tric bulbholder, partly embedded withinthe hole drilled in the woodendoorframe and a small brass strip fixedon the door were used as a substitutefor limit switch.

The sensor may even be a closedloop of wire, criss-crossed across weakportions of wooden door or glass panes,fixed with the help of small nails orpasted with suitable glue or narrow-width cellotape. Thus, it can guard weak

PARTS LIST

and vulnerable sections of a door orwindow.

For better utilisation of the circuit,a sensor which may be a combinationof closed-loop wire, or a number ofpush/limit switches connected in seriesto give adequate protection to all thedoors and windows in a ‘zone’, may beused. In the prototype, though only fourzones had been used, each ‘zone’ cov-ered several doors and windows.

More ingeniously, the circuit canbe interfaced with some other type ofsensors, such as modulated infraredlight beam, proximity sensors, andmovement detectors. In these cases theN/C contacts of the output relays oropen-collector output transistors (nor-mally conducting) of the said devicesshould replace the sensors.

The circuits of such devices are be-yond the scope of this article. How-

ever, old issues of EFY can provideone with numerous such circuits.

A sensor (say Al) can be replacedeasily with an LDR or a photo-diode toconstruct an unmodulated light beam(ordinary or infrared) arrangement. Inthat case, resistors R29 to R33 shouldbe replaced with presets in the range of100-kilohm. However, the actual valueof these resistors depends on factors,such as the intensity of light beam used,ambient light and focusing arrangementof the beam.

Memory

If a door/window is closed after abrief opening, it becomes necessary tohave the fault condition stored in amemory, till reset, for identification ofthe faulty zone. IC4 through IC9 arewired as bistable multivibrators. An

VR2 — 47-kilohm miniaturepreset



Capacitors:C1,C6-C10 — 0.1μF ceramic discC2 — 0.0047μF ceramic discC3-C5,C24.C26 — 100μF, 25V electrolyticC11-C15,C18,C22 — 1μF, 25V electrolyticC16 — 10μF, 25V electrolyticC17 — 33μF, 25V electrolyticC19 — 47μF, 25V electrolyticC20 — 3.3μF, 25V electrolyticC21 — 0.001μF, ceramic discC23 — 33pF, ceramic discC25 — 2200 μF, 30V electrolytic

Miscellaneous:S1,S2 — Key switches: 2-pole, 2-

way (See text)S3 — Push-to-off switchS4, S5 — Push-to-on switchS6 — On-off switchS7 — 1-pole, 10-way switchLS — Speaker 8-ohm

— 5-way sockets orterminal strip

— micro/limit/pushswitches for sensor(see text)

— Suitable metal enclosure— Heatsinks— Knob for switch S7— Connection wires,

solder, PCB, nuts,bolts etc

LED1-LED2 — Green LEDLED3-LED6,LED7,LED11 — Red LED

Semiconductors:IC1-IC9 — 555 timer chipIC10 — CD4017 decade counterIC11 — UM3482A musical chipIC12 — 7812 three-terminal

voltage regulatorT1,T12 — SL100 npn transistorT2-T11 — BC148 npn transistorT13 — SK100 pnp transistorD1,D2,D7-D14 — 1N4007 silicon rectifier

diodeD3,D6 — 1N4148 silicon

switching diodeD4, D5 — 4.7V, 300mW zener

diode

Resistors: (all 1/4 watt, +5% carbon unlessstated otherwise):R1,R29-R33R48, R50 — 4.7-kilohmR2 — 5.6-kilohmR3, R42 — 47-kilohmR4, R8 — 1.5-kilohmR5 — 33-kilohmR9 — 6.8-kilohmR7, R34, R38, R43,R44.R49, — 100-kilohmR10-R12,R24-R28,R36,R40 — 1-kilohmR13,R45,R54 — 470-kilohmR14 — 2.2-megohmR15, R37 — 10-kilohmR16 — 3.9-kilohmR17,R41,R47 — 220-kilohmR18 — 120-ohmR19-R23 — 1.2-kilohmR35, R6 — 220-ohmR39 — 680-kilohmR46, R52 — 8.2-kilohmR51 — 82-kilohmR53 — 2.2-kilohmR55 — 150-kilohmVR1 — 220-kilohm miniature

preset



Fig. 3: Actual-size PCB layout for the doorbell-cum-security alarm


open sensor triggers the correspondingbistable to make its output high andcause the corresponding LED to gloweven after the fault is rectified.

The high outputs of the bistablesare blocked by capacitors C6 to C10.Only a sharp positive pulse producedat the positive-going edge of anybiastable output reaches transistor T3,which triggers the alarm monostableIC3.

Use of pin 6 as ‘reset pin’, instead ofthe conventional pin 4, prevents IC4from resetting as a long fault conditionpersists. This facilitates visual monitor-ing of all the zones, while keeping reset‘on’ to disable audible alarm. This alsoprevents IC3 from retriggering after areset with fault condition persisting.

Capacitors C11 through C15 areused to suppress electrical interferencespicked up by long sensor wires, whichmay otherwise cause a false alarm.

Alarm monostable

IC3 is wired as a monostablemultivibrator. The destination and dura-tion of its output are controlled by modeselection switch Sl.

In maximum security mode, thisoutput enables the wailer for approxi-mately five minutes. In minimum se-

only, speaker is connected to both thesections simultaneously through diodesDl and D2.

Doorbell switch S5 can also triggerIC11.While IC11 is activated from IC3output through diode D3, transistor T10pulls down pin 7 of IC11. It causes alittle change in the pitch of musicaltune. Though barely distinguishable,this change of pitch can differentiatethe doorbell tune from the alarm tune.

A small regulator constructed aroundtransistor T12 provides the necessary 3.5volts for IC11. Forward voltage dropacross LED3 and LED4 determines theregulator voltage and also serves as‘power on’ indicator. Diode D4 is usedas an additional protection for IC11.

Counter

Each time IC11 emits a musicaltune, its pin 10 goes low. This activelow pulse, after being suitably changedin phase and voltage by transistors T9and T8, is fed to clock input of IC10.

IC10 counts the number of timesthe doorbell rings; its decimal countoutputs are incremented accordingly.At the desired count selected by S7,transistor T4 conducts and triggers re-set monostable IC9.

IC10 normally kept reset by con-ducting transistor T6, is enabled onlywhen clock signal goes high and tran-sistor T7 discharges through capacitorC19 and diode D6. If another clocksignal does not arrive within the next20 seconds, C19 gets charged up toreset voltage level through resistor R45,and IC10 gets reset.

IC10 is also kept reset by transistorT5 while the alarm activating signal ishigh.

Reset monostable

IC9, also wired as a monostablemultivibrator, provides the necessaryreset pulse. It serves the dual purposeof resetting and defeating the systemfor 30 seconds for authorised entry-exit. However, this duration can bechanged by changing the values of re-sistor R39 and capacitor C17 as perrequirement.

IC9 can be triggered by switch S6or S4 only when enabled by switch S2.With S2 in RST disable position IC9can only be triggered by transistor T4at the desired count (selected by switch

Fig 5: Power supply for thedoorbell-cum-security alarm.

Fig 6: Suggested front panellayout for the security alarm.

curity mode, theoutput is fed to en-able the musical ICUM3482A for 45seconds. However,a reset during theseperiods can stop thealarm by resettingIC3.

These durationsmay be changed as

per individual choice bychanging the values of resis-

tors R13 and R14. To make thealarm of non-self-resetting type, oneshould simply remove resistor R14.‘Max. security mode’and ‘alarm active’ conditionsare indicated by LED6 and LED5,respectively.

Wailer

High output from IC3 enables IC1and IC2, both wired as squarewaveoscillators in maximum security mode.Output of IC2 is used to modulate thefrequency of IC1.

The modulating frequency, depth ofmodulation and modulated frequencycan be adjusted as per individual choicewith the help of presets VR3, VR2 andVR1 respectively. The output power isincreased by transistor Tl.

Musical alarm and doorbell

High output from IC3 enables musi-cal IC11 (UM3482A) through diode D3and resistor R52 only in the minimummode. Resistor R53, diode D5 and ca-pacitor C20 keep the voltage at pin 4 ofIC11 within safe permissible limit.

The pitch of the music is determinedby resistor R34 and capacitor C23. Out-put from ICl1 is fed to transistor T11-T13 combination through volume con-trol preset VR4. Since both the wailerand musical ICs provide square waves


S7) of doorbell ring. ‘Reset active’ and‘reset enable’ conditions are displayedby LED1 and LED2 respectively.

Power supplyThe circuit in Fig. 5 provides the

necessary 12 volt regulated power sup-ply to the circuit. Current rating of thetransformer is kept on the higher sideto prevent heating due to continuousoperation.

A 12-volt battery is essential as abackup in case of power failure or atampered power cord.

ConstructionThe prototype was assembled on a

veroboard. However, a suggested PCBlayout and a component layout aregiven in Figs 3 and 4, respectively.

Use of IC sockets is recommendedonly for IC4 through IC8, so that onlythe required ICs can be put into thecircuit. IC12, Tl, T12 and T13 must bemounted on heatsinks. Five-way sock-ets or terminal strips may be mountedat the back of enclosure for connec-tions of sensors and switch S5.

It is suggested that only a section ofthe circuit should be assembled andtested before proceeding to next section.

Housing of the full circuit within astrong metallic enclosure is of vital im-portance to make the main unit tamper-proof. Suggested front panel layout isshown in Fig. 6. The loudspeaker, withsmall holes, should also be mountedinside the enclosure. The enclosureshould have good ventilation, but at thesame time it should be sufficiently pro-tected against tampering from outside.

In prototype, the use of keysswitches was avoided due to their highcost. Instead, ordinary, low-cost drawer

locks—mechanically linked with pushtype 2-pole, 2-way switches—were in-stalled. Though little mechanical ad-justment was needed to position theswitch and lock, it made an effectivelow-cost substitute for key switch.Mounting of drawerlocks, push-switches, PCB, transformer, speaker,etc are illustrated in Fig. 7.

The main unit should be mountedrigidly in a suitable indoor location, sothat neighbours’ and guards’ attentioncan be attracted by the wailing sound.The location should be close to themain entrance, so that doorbell actu-ated music can be heard outside as well.The preset volume control VR4 shouldbe adjusted likewise.

Since the circuit is designed to beused indoor only, for an intruder it maynot be possible to get access to themain unit without activating the alarm.Even then, suitable iron angles, firmlyfixed in the wall, may be used to mountthe main unit rigidly.

A burglar needs a lot of courage totry to dismantle a wailing box from thewall or to open the enclosure screws totamper with the circuit. Even if he suc-ceeds in detaching the unit from thewall, it seems highly unlikely that hewould prefer to carry a wailing boxunder his arm!

Each soldering and fitting shouldbe strong enough to withstand impacts,caused by hard blows on the enclosure.Though not necessary, one zone maybe engaged to booby-trap the main unit,if used in other than the ‘domestic’ envi-ronment.

Operation

The alarm system can be kept de-feated by keeping switch S6 closed with

S2 locked in ‘enable’ position. It makesnormal entry-exit possible, but it shouldbe used only when security problemscan be overlooked.

Prior to leaving home unattended,the mode must be selected as ‘Max’.Just before the last person leaves, oneshould open switch S7, activate resetpulse for 30 seconds by pressing switchS4 (reset active indication comes on),disable further reset by switch S2, exitand shut the door.

Alarm can be defeated for entry fromoutside only if the number ‘N’ selectedby switch S7 is known. To defeat thealarm, press doorbell switch S5 to pro-duce a musical tune. Only after the tunecomes to its end (but within next 20 sec-onds) again press switch S5.

The procedure should be repeatedfor ‘N’th time. At the beginning of the‘N’th tune, the alarm deactivates itselffor 30 seconds. This duration is suffi-cient for one to enter and again shutthe door. Then the alarm system can bedefeated for any length of time by en-abling ‘reset’ by switch Sl and keepingswitch S7 closed.

The defeating process itself acts asan attraction to neighbours’ attention ifa high value of ‘N’ is selected. Selec-tion of number ‘0’ and ‘1’ is naturallyprohibited.

Switch S3 tests all the zonessimultaneously. It should cause all thefaulty zone indicating LEDs and alarmactive LED to glow along with the au-dible alarm. A reset should cause theabove LEDs to extinguish and ‘resetactive’ LED should come on.

Maximum security mode operationwith reset disabled is essential to leavethe house unattended. This mode mayalso prove useful at night, with occu-pants present in the house, dependingon security risks.

In day time, the irritating proce-dure during frequent entry-exit may beavoided with the minimum mode opera-tion. With occupants (usually house-wives) busy in their household chores,frequent exit-entry of children and ser-vants makes it impossible to lock theentrance. Min. mode gives necessary,but soft musical warning to check theidentity of the intruder.

A switch mounted indoor and con-nected in parallel with a sensor can beused to bypass that particular sensor.However, the switch should be openedfor normal use of the sensor. �

Fig. 7: Internal view of the doorbell-cum-security alarm.


addition, the circuit provides a soft on/off switch (manual) for up to 5kW load,avoiding any mechanical contacts.A common requirement in the

industry is that of a single/dual-phase preventer. The cur-

rent practice is to use a separate circuitfor each instrument. Such circuits do

not have any provision to take care ofrapid on/off switching transients in themains power supply. The circuit, de-scribed here, overcomes such drawbacksin a simple and economical manner. In

IMPROVED THREE-PHASEPOWER SUPPLY CARD

CHHAJED MAHAVIR D.

Fig. 1: Block diagram for the three-phase power supply card.

Fig. 2: Circuit diagram for the card.


PARTS LISTSemiconductors:IC1 — 555 timer1C2 — CD4027dualJKflip-flopIC3 — MCT2Eopto-couplerTl — BC157BpnptransistorT2 — SL100npntransistorTriac — BT136triac(4A,400V)D1-D4,D6,D7 — 1N4001rectifierdiodeD5 — 1N4148siliconswitching diode

Resistors (all 1/4 watt, +5% carbon, unlessstated otherwise):R1,R2,R4-R7 — 10-kilohmR3 — 100-kilohmR8 — 4.7-kilohmR9,R11 — 1-kilohmR10 — 1-megohm

Capacitors:C1,C8 — 470µF,25VelectrolyticC2,C5,C6C7 — 0.1µFceramicdiscC3 — l00µF,25VelectrolyticC4 — 0.01µFceramicdisc

Miscellaneous:S1,S2 — Push-to-onswitchX1 — 230VACprimary,0-12V, 350mAsec.transformerX2 — 230VACprimary,12V-0- 12V,250mAsec. transformerRL1 — 230V25A,3-pole,1-way relay — SuggestedPCB — ICbases

Principle of working

Block diagram of the circuit isshown inFig.l.Thecard tobeplacedbetweenthemainspowersupplyandthe

bealteredbychangingthe value of resistorR3 or capacitor C3.The time duration isgivenbytherelation-ship:

T=1.1(R3xC3)Theoutputpinof

IC 555 is connectedto clock pin of JKflip-flop as clock in-put. The T input offlip-flopistiedtoVcc.Also, the power-on-reset is provided tothe JK flip-flop sothatwheneverthe‘B’phase resumes, theoutput Q of the flip-flopshouldbehigh.

Also,amanualon/offswitchisprovidedinthissystem.Assum-

ingallthephasesarepresentinthesys-tem,when‘on’switchismomentarilypressedtherelaygetsenergisedandtheloadsgetconnectedtothesupplylines.When‘off’switchispressed,therelaygetsde-energisedandtheloadsgetdis-connectedfromthesupplylines.

Themainadvantageofthisarrange-mentisavoidanceofbulkymechanicalswitches, and the problems of arcingassociatedwiththem.

The+12Vsupplyderivedfromthe‘Y’phaseisconnectedtotransistorofopto-isolator.So,ifthe‘Y’phaseisnotpresent,thetransistorwillnotturnon,inspiteofgettingbasedrive.The‘R’phaseisconnectedtotherelayanditsassociatedcircuitry.

ConstructionThePCBandcomponentlayoutof

thecircuitareshowninFigs.3and4,respectively.

Forperfectsoldering,useofagoodsolderinggunisessential.Also,handleCMOS ICCD4027with utmost care.Use IC bases for 555, MCT2E andCD4027toavoidpossibilityofdamageduringsoldering.

The circuit achieves its desiredobjectives with minimal components,and is very inexpensive.As such, thecircuitshouldgainreadyacceptanceintheindustry.

Fig 3: Actual-size suggested PCB layout for the card.


loadhasatimercircuitryandswitch,anopto-isolatorandrelaycircuitry.

Assuming that only ‘B’ phase ispresent(outofR,YandB), thetimercircuitoutputwillbecomehigh(whenpower on the system output becomeshigh after 10-sec. delay), and thetransition from low tohigh is applied to clock input of JK flip-flop, which is edge triggered. But since the ‘Y’phase isabsent, theopto-isolatordoesnotturn‘on’inspiteofgettingthebasedrive.Thustherewillbenooutputfromtheopto-isolator(andthereforefromre-lay)aslongasonly‘B’phaseispresent.

Ifonlytwophases‘Y’and‘B’arepresent,theoutputoftheopto-isolatorbecomes high and gets applied to therelay circuitry. But since ‘R’ phase isabsent,therelaydoesnotturnon,andhencetheloaddoesnotgetthesupply.

Fromtheabovediscussion,it’sclearthat,fortheloadtobeconnectedtothemains,allthethreephasesmustbepres-ent.Interchangingthephaseconnectionsto the three blocks does not alter thecircuitaction.

Circuit descriptionIn order to prevent the load from

respondingtorapidon/offswitchingofmains supply, aprovision ismade foradjustable deadband setting. (Duringthisperiod the loaddoesnotget con-nectedtothemainswhenthepowersup-plyresumes.)ThisisachievedbyusingIC555inmonostableconfiguration,asshowninFig.2.Thetimedurationcan


A dvertisements have become apopular means of communica-tion through newspapers and

television for increasing sales of anyproduct. Nowadays, people prefershops that put up bright and beautifullights to attract people.

The advertisement display de-scribed here is inexpensive and easyto construct. The circuit uses readilyavailable low-cost components. To dis-play a seven-letter word ‘WELCOME’,

SOLIDSTATEADVERTISEMENT DISPLAY

PRADEEP G.

Clock pulse Letters Illuminated Remarks0 X X X X X X X (All off)1st W X X X X X X (I letter on)2nd W E X X X X X (2 letters on)3rd W E L X X X X (3 letters on)4th W E L C X X X (4 letters on)5th W E L C O X X (5 letters on)6th W E L C O M X (6 letters on)7th W E L C O M E (7 letters on)8th X X X X X X X (All off)9th W X X X X X X (1 letter on)10th W E X X X X X (2 letters on)Note: X indicates off state of a bulb

Fig. 1: Circuit diagram for the solidstate advertisement display.


PARTS LISTSemiconductors:IC1 — NE555 timerIC2 — 74HCT164 8-bit shift

registerIC3 — 7805 5V regulatorT1-T8 — BC147B npn transisorT9 — 2N3439 npn transistorDl — lN4001silicon rectifier

diodeD2-D5 — 1N4007 silicon

rectifier diodeTriac1-Triac7 — BT136

400V, 4A triac

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):Rl — 10-kilohmR2-R9 — 2.2-kilohmRI0 — 1-kilohmR11-R17 — 330-ohmR18 — 4.7-kilohmR19 — 15-kilohm, 10-wattVR1 — 100-kilohm

Capacitors:C1 — 4.7μF,10V electrolyticC2-C5 — 0.1μF ceramic disc

Miscellaneous:— IC sockets— PCB— LEDs

Fig. 4: Circuit diagram for arrangement of bulbs.


Fig. 2: Actual-size PCB layout for the advertisement display.

letters are made with multicolour semi-transparent plastic sheets and each ofthe letters is separated by black opaquesheets (cardboard, mica, hylam etc).

Each letter is displayed one by oneand finally all letters of WELCOME orany other word are displayed. The cir-cuit automatically resets after the lastletter. Then again, the circuit begins tofunction by displaying the letters. Thisprocess is repeated endlessly.

CircuitThe heart of the circuit is an 8-bit

TTL shift-register 74HCT164. It isavailable in the market for about Rs18.

IC1 555 functions as a clock-pulsegenerator in free-running mode. Thepulse rate of IC1 can be varied by poten-tiometer VR1, and hence the speed ofdisplay of the letters can be varied. Foreach clock pulse from IC1, IC2 shiftsits output one by one till the seventhoutput.

After the seventh pulse from IC1,the word WELCOME is completelydisplayed. Thereafter, the eighth out-put goes high. Then transistor T8quickly conducts and holds pin 9 (re-


set) of IC2 at ground level. All outputsgo low and all bulbs are switched off.

As IC1 is continuously operating,the circuit begins to function again by

displaying the letters, one by one. Thedisplay may be stopped by switchingoff the power supply.

All triacs are triggered by using dis-

crete transistor stages. The use of tran-sistor T8 eliminates the need for anextra IC inverter like 7404.

The complete sequence of operationis given in the truth table.

AssemblyThe complete circuit for the

solidstate advertisement display can beassembled on a 12cm x 7.5cm PCB.All ICs and triacs are soldered on thePCB shown in Fig. 2. The ground lineof PCB is connected to neutral wire ofAC mains. One terminal connecting230V AC bulbs is connected to the livewire of AC mains. Remaining termi-nals of all bulbs are connected to theseven triacs. In the prototype, IC sock-ets were used for IC1 and IC2.

IC3 (7805), a 5V regulator, is in-cluded in the circuit to power the TTLIC. The entire circuit can be poweredfrom an ordinary 9V or 12V batteryeliminator. Don’t forget to use adequateheatsink for all triacs. If one wishes touse series of LEDs, the triacs can bereplaced with high voltage 2N3439transistors as shown in Fig. 5. All seventriacs can be replaced with 2N3439transistors. �

Fig. 5: Circuit for optional LED display.

Fig. 6: Display board setup.

Readers’ Comments:I am very thankful to Pradeep G. for hiscircuit. Due to non-availability of IC74HCT164, I used IC 555 as clock pulseand a CMOS IC CD4017 as counter inplace of IC 74HCT164. But only oneLED is glowing continuously. Could theauthor suggest any modifications?

Himangshu MajamdarMidnapore

� The solidstate advertisement displayis no doubt an inexpensive circuit. I had

to use as many as seven of 555 ICs formaking the same display work (EFYJan’91). Each letter is lit up one-by-onefrom left to right, giving a visual effectof words. Why can’t we give a wipingeffect also? Instead of connecting lastoutput(pin 13 inverted by transistor T8)to reset pin 9, it can be connected to oneof the serial input pins (1 or 2).

Reset pin can be clamped topositive rail. With this small changewiping effect is given to the total

display and the circuit works endlessly.K.P. Viswanathan

Calicut

The author Pradeep G. Replies:The direct replacement of IC

74HCT164 with counter IC 4017 is notproper. While 4017 is a decade counter,the 74HCT164 is an 8-bit shift register.For wiping effect the circuit may bemodified, as described by MrViswanathan.


AUTO-CHANGING IN/OUTINDICATOR WITH DOOR-BELL

UNNIKRISHNAN P.R.

Fig. 2: Circuit diagram for the auto-changing in/out indicator with door-bell.

G enerally, whether a person is‘in’ or ‘out’ is indicatedthrough a cardboard indica-

tor which has to be turned for eacharrival or departure. Instead of this,electronic indicators can also be used.However, these also need to be set forchanging the display. So, if you forgetto change the display the indicator be-comes useless.

The circuit described here solvesthis problem. It does not need any set-ting or adjustment for each change. It Fig. 1: Block diagram for the auto-changing in/out indicator with door-bell.


Semiconductors:ICI,IC3 — 555 timerIC2 — 4017 decade counterT1 — BC149C npn transistorT2-T5 — BEL188 pnp transistorD1,D2 — 1N4001 rectifier diode

Resistors (all 1/4 watt, ±5% carbon unlessstated otherwise):R1,R3 — 10-kilohmR2 — 470-kilohmR4 — 1-kilohmR5 — 6.8-kilohmR6 — 47-ohm, 1 wattR7.R8 — 5.6-kilohm

R9, RIO — 8.2-kilohmR11-R26 — 680-ohm

Capacitors:C1,C6,C7 — 100μF,25V electrolytic

C2,C5 — 0.0lμF ceramic discC3 — 3.3μF,16V electrolyticC4 — 0.1μF ceramic disc

Miscellaneous:DIS1-DIS3 — LTS 543 common

cathode displayLS1,LS2 — 4-ohm speakerSl — Push-to-on switch

PARTS LIST

automatically changes the display whenyou enter or leave your office or home.It has the following features:

1. The ‘IN’ and ‘OUT’ signs areindicated by using LED displays.

2. It displays ‘IN/OUT’ only whena guest presses the calling bell switch.This will avoid unwanted wastage of

energy.3. If you are inside then a press on

the switch displays ‘IN’ and sounds abell inside the home for each press ofbell switch.

4. If you are out then a press of theswitch will display ‘OUT’ and the bellwill ring inside.

Principle of operation

The circuit works on the basic prin-ciple of a touch switch. The input ofthe circuit carries a touch plate (keyplate). You must place the key of themain door on the surface of this plateonce, before you take it away, so thatthe display keeps showing ‘OUT’.

When power supply is switched on, thedisplay shows ‘IN’. When you leave homeyou must take the key. Then, the touchswitch is enabled because the key is placedon the plate. So the display is changed to‘OUT’. When you reach home, you placethe key on the plate. And again, the touchswitch is activated and the display is flippedto ‘IN’.

CircuitThe circuit uses two 555 ICs and

one decade counter IC 4017. IC1 andIC2 make the clap switch. The base of

Fig. 3: PCB layout for the main circuit. Fig. 4: Component layout for the PCB shown in Fig. 3.

Fig. 5: PCB layout for the display circuit. Fig. 6: Component layout for the PCB shown in Fig. 5.


Fig. 7: Suggested front panel layout.

is enabled. If you are out then the sec-ond speaker (LS2) is enabled.

Working

When you touch the key plate,transistor Tl gets biased. So, IC1 istriggered. This changes the output ofIC1 from low to a high state. This highstate remains for about 52 seconds.Since output of IC1 is given to IC2, theoutput of IC2 also changes from thelow to high state, with change in theoutput of IC1. This changes the dis-play and rings the bell through thespeaker.

Installation

The circuits of the internal speaker(LS2) and switch Sl may be fitted in a

suitable cabinet. This box may be fixedin place of the calling bell switch oranywhere in the sit-out, so that a guestmay see it at the first look. The exter-nal speaker can be fixed anywhere in-side the house. The key plate shouldalso be fixed in the house. It is betterif the key plate is fitted near the maindoor. The key plate must be kept at asafe place to avoid unwanted touch-ing. It should be fixed at a height tokeep it away from children.

Figs. 3 and 4 show the main PCBand the component layout, respectively.Figs. 5 and 6 show the display PCBand the component layout, respectively.

Make a suitable cover and fit themain PCB, display PCB, LS2 and Sl init. A model of the front panel is shownin Fig. 7.

�

transistor Tl carries the touch plate (keyplate). IC3 functions as an oscillator. Itproduces a frequency of about 1kHzwhich is applied to two speakers. Ifyou are in, then the first speaker (LS1)


T his circuit is very useful fordata communications in a datacommunication centre which

does not have a telex dialing system. Ifleased communication channels becomeunserviceable, but telex is in workingcondition, then data from computer canbe sent through the telex line, using thiscomputer - telex changeover switch.This switch is useful also in situationswhere data or messages stored in com-puters need to be transmitted on telexline through the interconnection be-tween a computer and a telex.

TELEX - COMPUTERCHANGEOVER SWITCH

SANJOY N. ROY

Fig. 1: Block diagram for the telex-computer changeover switch.

Fig. 2: Circuit diagram for the telex-computer changeover switch.


Fig. 3: PCB layout for the computer-telex changeover switch.

Fig. 5: Internal structure of the polar relay.


General description

The telex-computer switch has twoinputs and one output. There is an in-put selecting switch. With the opera-tion of this switch, it appears that anyone of the inputs is electronically

shorted to the output. One of the inputsis connected to the transmission sideof the Line Terminating Unit [LTU],working at 50 Bauds. The other inputis connected to the transmission leg ofthe telex machine. Output of the telex-computer switch is connected to the

PARTS LISTSemiconductors:IC1 (a-e) — 7404 hex inverterIC2 (N1-N3) — 7400 quad 2-input

NAND gateIC3 — 7805 +5V regulatorT1 — BC107 npn transistorT2 — SKI 00 pnp transistorT3 — SL100 npn transistor

Resistors (all 1/4 watt, +5% carbon unlessstated otherwise):R1,R2 — 10-kilohm, l-wattR3,R4 — 1-kilohmR5,R8 — 47-kilohmR6 — 1.2-kilohmR7 — 4.7-kilohmR9,R10 — 680-ohm, 0.5-wattR11,R12 — 470-ohm, 1-watt

Capacitors:C1,C2 — 0.1μF ceramic disc

Miscellaneous:RL1 — A15,T Bv 3000/36

polar relay— PCB etc.

sending line of the telex, as shown inthe block diagram (Fig. l).

OperationIf it is desired to send data through

telex, then telex-computer selectingswitch should be in telex position. Now,operation of telex is restored. If it isdesired to send data from computerthrough telex line, then, first keep theselecting switch in telex position anddial the number to which data is to betransmitted. After the line connectionis established with the number dialed,i.e. ending of ring back tone, etc, se-lecting switch should be flipped to com-puter position. At this stage, transmis-sion of command data from computerwill take place through this switch totelex line. The dialed station will nowreceive data from computer in theirtelex machine.

Technical descriptionThe +60V or -60V telegraph signal

either from LTU or from telex machineis converted to +5V or 0V by a clippercircuit. The output of the clipper is fedto one of the inputs of a two-input ANDgate, as shown in Fig.2. The other in-put of AND gate is fed from a (High-Low) bounce-free switch.

The bounce-free switch is made upof inverters and a toggle switch. Theoperation of bounce-free switch does


not change its states when the tongue(pole) of the toggle switch jumps fromone contact to the other, and also whenit bounces back on its arrival from theother. The output changes only whenthe tongue shifts and makes firm con-tacts with terminals.

Whenever one input of NAND gatesN1 and N2 is kept high from high-lowbounce-free switch, the 5V or 0V tele-graph signal at the other input will beavailable at the output of the NANDgate, in inverted form.

The outputs of two NAND gatesare ORed (NAND gate N3 acts as anegative input logic NOR gate) andfed to an electro-mechanical polar tele-graph relay through transistors T2 andT3. When the output of gate N3 is high,the collector of transistor Tl is low andthat of transistors T2 and T3 are logic

high. When collector of T2 is high, it isconducting, and there is a current flowfrom pin 8 to pin 5 of polar relay coil.If collector of T3 is high, it means thatit is not conducting. Hence, there is nocurrent flow through pin 1 to pin 4 ofthe polar relay. This causes relay tongueto move in some particular direction.

Similarly, when collector of transis-tor Tl is high, collectors of T2 and T3are low, hence T3 conducts and T2 doesnot conduct. Now, there is a currentflow in the relay coil from pin 1 to pin4 and no current flow in coil from pin 8to pin 5. This causes relay tongue tomove in the reverse direction than thatof current flow from pin 8 to pin 5.

With this arrangement, polar relayis being actuated in both directions froma single supply. The polar relay hasbeen used to convert the 5V or 0V

telegraph signals into +60V or -60Vtelegraph signals. The tongue (pole) ofpolar relay generally known as ‘A’toggles between terminals known as‘Z’ and ‘T’. Terminals (contacts) ‘Z’and ‘T’ are connected to +60V and -60V terminals of telegraph power sup-ply. At tongue A, where +60V and -60V telegraph signals, either from com-puter or from telex machine, depend-ing on selection, are available and fedthrough the send side of telex line.

The actual PCB for the purpose isshown in Fig. 3 and component layoutis shown in Fig. 4. Spark quenchingcircuit is provided at the output of re-lay tongue to suppress any spark, dueto relay tongue’s movement. TTL chipsare powered through a 5V regulator ICchip (LM7805), derived from a 12Vsupply. �


I n a conventional hi-fi system, theoutput from the power amplifierdrives two or three units. The

drive unit, which is the speaker, is thedevice that actually transforms the elec-trical signal from the amplifier intosound pressure waves. Ideally, wewould use a single unit to cover thewhole frequency range, but in practicewe have to use two or three units to

60W ACTIVEAMPLIFIER SYSTEM

DORAISWAMY VISWESWARAN

Fig. 1.: Block diagram for the active amplifier system.

Fig. 2: Circuit diagram for the active amplifier system.


Fig. 3: PCB layout for the active amplifier system.

Fig. 4: Component layout for the active amplifier system.


cover the desired range of 50 Hz to 20kHz, because of the following facts:

The useful frequency range of aspeaker drive unit over which a linearoutput can be expected is primarily gov-erned by two things. The low frequencylimit is defined by the resonance fre-quency of the cone in the suspensionsystem. The cone has a mass which iscoupled with a spring, combining thecompliance of the suspension systemwith the compliance or ‘springiness’ ofthe air in the enclosure.

Below the resonant frequency, theunit is very inefficient and thereforerequires a lot of power from the ampli-fier to gain any appreciable output.Above the resonant frequency, the coneoperates in the ‘compliance’ region,giving a reasonably linear and efficienttransfer of energy. It is in this rangethat the drive unit can be used.

As the frequency increases, a pointis reached when the wavelength of thesound reaches half the circumferenceof the speaker cone. Above this fre-quency, different areas of the cone startmoving in different ways, resulting inphase anomalies and unwanted reso-nances. This then forms the upper fre-quency limit of the useful range of thedrive units.

Considering the above limitations,the reproduction of the lower frequen-cies, especially the base, requires a mas-sive cone. For example, a 20cm bassunit may have a resonant frequency ofaround 40 Hz, but the upper frequencylimit would be 3 kHz. The reproduc-tion of the middle order and higherfrequencies requires a lighter cone.

So, in the simplest system, we wouldhave two drive units: one for reproduc-tion of the bass frequencies, which wecall the ‘woofer’ and the other one suit-able for high frequencies, which we callthe ‘tweeter’. In the most popularlyused approach, we use a ‘crossover net-work’, which splits the audio spectrumand feeds the relevant drive units.

The crossover network consists ofa passive filter network of inductors,capacitors and resistors, dividing thefrequency range into a low frequencyband which is fed to the woofer, a midfrequency band which is fed to thesquawker, and a high frequency bandwhich is fed to the tweeter.

While this system works perfectlyand adequately, and witnesses a num-ber of excellent passive speakers in the

Fig. 5: Power supply for the circuit shown in Fig. 2.

market, it has several inherent disad-vantages. The problem arises in the de-sign of such a filter.

To design such a filter, two factorsneed to be known—the impedance ofthe source and the impedance of theload. The source impedance is the out-put impedance of the amplifier and theconnecting loads. With good qualityleads, this should be less than a tenthof an ohm, and no problems present.

However, the load impedance is theimpedance of the drive unit itself. Fig.6shows the impedance of a typical loud-speaker. As you can see, the imped-ance varies considerably with fre-quency, and is, in fact, a filter designer’snightmare. As you can imagine, cross-over design is an art in itself.

The second problem arises from thehigh currents involved in driving a loud-speaker, which can reach tens of amps.At low currents, capacitors, inductorsand resistors are linear components.However, at high currents these can befar from linear, introducing their owndistortions.

This is particularly true when highvalues of inductance are required. Toconstruct a coil with an inductance ofover, say, l0 mH, a ferrite core is re-quired, if the size of the inductor is notprohibitive. A ferrite core will imposeits nonlinear hysteresis curve onthe circuits, which could introduceserious distortion at high currents.

As we have seen, the imped-ance of a drive unit by itself is farfrom linear, having an impedancevalue and phase angle that is vari-able with frequency. However, ca-pacitors and inductors also intro-duce considerable phase (voltage)lags and leads. A crossover net-work, while having a linear fre-quency response, could present avery complex load to the amplifier.

Another major problem con-

cerns damping factor. A drive unitworks just as effectively as agenerator’s motor. When a transientpeak occurs, the amplifier drives thecone outwards, and then applies a brak-ing force. However, the cone will in-evitably overshoot, and as it settlesback, it generates a current that is fedback to the amplifier. If the impedancepresented by the amplifier is very low,or damping factor high, then the cur-rent will disappear quickly. If, how-ever, the impedance of the amplifieroutput is fairly high, the current willaffect the performance of the ampli-fier. Effectively, then, a high dampingfactor increases the control of the am-plifier over the movement of thespeaker cone.

In a passive speaker the crossovernetwork forms a part of the impedancethat the drive unit ‘sees’ as it looksback at the amplifier output. In the passband of the crossover filter, the imped-ance of the crossover is fairly small,and the damping factor high. However,in the cut-off regions, the impedanceof the crossover rises, as it cuts out thefrequencies outside the pass band. Thismeans that the damping factor de-creases, and the amplifier progressivelyloses control over the speaker cone.

On the other hand, in an active sys-

Fig. 6: Characteristics of impedanceof a typical speaker.


PARTS LISTSemiconductors:IC1,IC2 — TDA2002TI,T3,T5 — BC547 npn transistorT2,T4,T6 — BC557 pnp transistorT7 — 2N3055 npn transistorD1,D2 — 1N4001 rectifier diode

Resistors (all 1/4 watt, ±5% carbon unlessstated otherwise):R1,R2 — 220-kilohmR3,R9,R14 — 5.6-kilohmR4,R10,R15 — 2.2-kilohmR5,R11-R13 — 10-kilohmR6 — 3.9-kilohmR7,R8 — 150-kilohmR16,R19 — 220-ohmR17,R20 — 2.2-ohmR18,R21 — 1-ohmR22 — 680-ohm, 0.5 wattVR1,VR2 — 10-kilohm pot.

Capacitors:C1,C22,C27 — 100μF,25V electrolyticC2-C4,C20,C21C26,C28 — l00nF ceramic discC5 — 470nF ceramic discC6,C10,C15 — 4.7nF ceramic discC7-C9 — 5600 pF ceramic discC11,C16,C17,C23 — 10μF, 25V electrolyticC12-C14 — 1.5nF ceramic discC18,C24 — 470μF, 16V electrolyticC19,C25 — 1000μF, 16V electrolyticC29,C31 — l000μF, 25V electrolyticC30 — 47μF, 25V electrolytic

Miscellaneous:X1 — 230V AC primary to

15V-0-15V, lAsec.transformer

— 8-ohm tweeter— 4-ohm woofer— PCB, heatsinks etc.

pre-amp. The job of the crossovernetwork is simply to ensure thatthe amplifier receives only thosefrequencies that its drive unit cancope with in a linear fashion. Theblock diagram of an active sys-tem is given in Fig. l.

A study of Fig. 1 will indi-cate that the frequency divisionis done before the power ampli-fier stage by means of two elec-tronic filters. The divided signal is fedto two individual power amps, whichthen drive the woofer and tweeter. Thisapproach, enunciated in the early 80’s,did not find wide acceptance, mainlybecause of reasons of economy, thoughthis system offers several exciting ad-vantages over the passive system, suchas:

1. The individual amplifiers forwoofer and tweeter can be designed,keeping the power requirements and thecharacteristics of the drive units in mind.

2. The power supply design is sim-pler and smaller.

3. Short circuit protection circuitrycan be done away with.

4. A 220W passive amplifier wouldbe required to give the same effect as a3x25W active unit.

5. The loss in the inductors and thephase shift in the capacitors of thecrossover network is avoided, leadingto better damping factor at all frequen-cies (even in the proximity of the reso-nances of the drive units).

6. It is possible to compensate fordifferences in sensitivity between thedrive units, simply by increasing or de-creasing the gain of a particular ampli-fier.

The interesting aspects of this cir-cuit (Fig.2) are:

1. The electronic crossover consistsof the low pass section and the highpass section. The crossover frequencyis 2 kHz. The filter slopes are set at 18dB/octave. This steepness is requiredfor hi-fi reproduction.

2. The power amplifiers used arebuilt around the rugged IC amps TDA2002, which can withstand wide volt-age fluctuations. The use of these ICsmakes the circuit suitable for fitting intoautomobiles, having a 12V battery.

3. A smooth supply, using 2N3055power transistor as series regulator, isused for providing DC to the filter andthe amplifier.

4. Because the power requirementsof the tweeter are less than those ofwoofer, we have used drive unit with 8ohms impedance for the tweeter and40 ohms for the woofer.

5. The dimensions of heatsinks forthe ICs and the series regulated tran-sistors are given in Fig.7.

The recommended PCB, sized12.5cm x 10cm, is shown in Fig. 3.The PCB is designed to accommodatethe power supply as well.

It is advisable to build the circuiton an aluminium chassis and in-corporate it into the speaker cabinetitself. In such, a case we only need apreamplified signal. �

Fig. 7: Construction of heatsinks.

tem described here, the amplifier outputis connected directly to the drive unit,and one amplifier is used for each driveunit. The crossover network becomeselectronic or ‘active’ and is connectedbetween the power amplifiers and the


A nybody working with linearcircuits in the high frequencyrange often comes across the

problem of matching a tunable part ofhis circuit to the operating frequency.Some examples are: matching the frontend of a receiver to the desired fre-quency signal or, conversely, match-ing the output unit of a transmittingequipment to the transmitting fre-quency, or matching the feedline forhigh-frequency signal transmission.Numerous other problems, such asthose of knowing the antenna-systemimpedance at a particular frequency(resonant or otherwise), establishingthe resonant frequency of a tuned cir-cuit, measuring the characteristic im-pedance of a coaxial or flat cable oreven of the lamp cord at a particularfrequency, might come up.

In a nutshell, whenever one needsto know the resonant frequency, thecharacteristic impedance of any cir-cuit at resonance or any other particu-

lar frequency, the multimeter devicefails, since it is basically a DC instru-ment. And if at all it works on AC, itsoperating frequency is nominally 50Hz.

The meter described here is per-haps the simplest device as far as con-struction, cost and operation are con-cerned. Yet it covers a wide range ofimpedances from below 10 ohms tobeyond 1 kilohm quite accurately, evenat sufficiently high frequencies.

The principle of operation

The device is based on the well-known ‘bridge’ which is used to mea-sure accurately all ranges of resistances,from the fraction of an ohm to thegegaohms range. Even the reactances,such as inductance and capacitance, canbe measured by this arrangement.

The bridge is said to be balancedwhen Ra:Rb = Rc:Rx and no currentflows through the closing diagonal at

IMPEDANCE METER

KULWANT SINGH

PARTS LIST (for Fig. 1)

Semiconductors:IC1 — 741 opampDl — OA79 or equivalent

diodeD2 — 9.1V zener diode

(see text also)

Resistors(all 1/4 watt, ±5% carbon unlessstated otherwise):Rl — 10-kilohmR2,R3, R4 — 4.7-kilohmR5 — 100-ohm, 0.5 wattVR1 (Ra, Rb) — 470-ohmVR2 — 100-kilohmVR3 — 10-kilohm (linear)

Capacitors:C1,C2 — 0.01μF ceramic discC3 — 0.047μF ceramic discC4 — 0.02μF ceramic discC5 — 0.1μF ceramic discC6 — 100μF, 25V electrolytic

Miscellaneous:— VU meter— Sine wave signal

generator.

Fig. 1: Circuit diagram for the impedance meter.


Author’s prototype, including RF generator on the left

this stage. This is termed as ‘null’ andis the precious state. Knowing Ra, Rband Rc, the value of Rx can be deter-mined easily through the above-men-tioned relationship.

To determine the impedance of anydiscrete inductive or capacitive compo-nent, or an LCR circuit, an RF signal isapplied to the bridge in place of a DCpotential required for resistive network.

Since all other components, viz, poten-tiometer and resistor in the bridge arenon-reactive to AC, only Rx introducesany impedance to the applied RF sig-nal. So, the impedance of Rx at theapplied frequency is determined by thebridge in terms of the other three armsof the bridge, directly in ohms, sinceimpedances of all other three arms arein ohms. Besides, in this case, the cal-culations are cut short by pre-calibra-tion of the meter by using a single po-tentiometer for Ra and Rb.

The heart of the device is a directcoupled amplifier, built around the fa-miliar operational amplifier IC 741.This DC amplifier serves three func-tions:

1. It increases the sensitivity of thebridge, making it more accurate andsuitable for a wider range of imped-ances.

2. As very small RF signal of theorder of 0.25 volt rms is required, lowwattage resistances and potentiometer,etc. can be used.

3. A small VU meter may be usedas an ‘eye’ of the instrument, making asmall and compact independent unit all-inclusive. However, an ordinarymultimeter, put in the range of 2.5 voltsor 0.25 volt, serves the purpose equallywell.

Circuit description

The bridge in its basic form isshown in part one of the circuit dia-gram. With a moderate RF signal ofthe order of 0.5 to 1.0 volt, it workswell as such and no personal powersupply is needed.

Ra and Rb in the circuit are the twoparts of a single 470-ohm potentiom-eter which must be linear and carbon-film type. Of course, 220-ohm or 100-ohm pots are still better if one’s inter-est is in measuring low impedances of50-ohm range.

Rc, as used in the prototype, is a100-ohm, 1/4-watt carbon resistor. Thismakes the bridge most sensitive in 50-to 200-ohm range and sufficiently accu-rate in 5-ohm to 2-kilohm range. For25 to 100-ohm most accurate range,take the value of Rc as 50 ohms. (Itcould be taken as 200 ohms for high-impedance type bridge.)

Diode Dl is BEL OA79 or any point-contact type germanium diode, such as1N34A. Silicon junction diode is not

Fig. 2: PCB layout for the impedance meter.



advisable since it has a forward volt-age drop of about 0.5 volt, thus affect-ing the accuracy of the bridge and re-quiring a higher RF signal injection.

Diode Dl rectifies the signal of theunbalanced bridge and the envelopepotential charges up capacitor C3. Thecharged level shows up as a deflectionin the meter. A high-resistance-sensi-tive galvanometer movement is re-quired. The circuit has been tested andfound to work satisfactorily with ordi-nary 175μA multimeter put in the 0.25-volt range, with above 1-volt rms sig-nal infeed.

Part two of the circuit is an improve-ment over the basic bridge circuit ofpart one. It incorporates a DC ampli-fier stage, built around operational am-plifier IC 741, with the galvanometerbeing shifted to the output of the IC.

The IC has been wired to operatefrom a single supply, stabilised to 9volts in the prototype. Zener D2’s volt-age could be raised to 12V or evenhigher, keeping in mind that the powersupply must always be sufficientlyhigher than the stabiliser voltage.

There is a potential divider arrange-ment, comprising resistors R3 and R4.A common supply of 12 volts for thesignal generator and the impedancemeter has been used. A small VU meterhas been used to read the null positionon the bridge. Two types of VU meteravailable in the market are 400μA 325-ohm and 250μA 1-kilohm.

The 10k potentiometer VR3 meantfor offset-null can be substituted with aresistor of around 1k between pin 5 andthe negative line, by trial and error.

The gain of the IC could be in-creased, if desired, by increasing feed-back variable resistor VR2 to 470k or,still better, a 470k pot. be used for avariable gain. However, the prototypehas a fixed 100K feedback resistor.

All components can be mounted onthe PCB itself to reduce stray capaci-tance and inductance effects in the RFstage. The 470-ohm pot. should be sol-dered on to the PCB. A long spindlecan be used to have the dial outside thecabinet. The meter can be fitted any-where, but the prototype shows themeter too on the PCB itself.

Sensitivity control can be achievedby the 470k feedback variable resistor.It can also be achieved through a vari-able RF signal, injected from the sig-nal generator by means of another small

470-ohm pot, which also serves as aload to the. signal generator, besidesgiving a wider range of sensitivity. Thismode is used in the prototype.

CalibrationTo make a direct-reading instru-

ment, avoiding calculation of imped-ance each time, one should use a singlelinear potentiometer of 470 ohms forRa and Rb.

For calibration of the instrument, athick white sheet of paper may bepasted under the 470-ohm pot.’s knob.Switch on the power, providing a volt-age well above the zener level. Adjustthe 10k pot. to produce null in the meter.Now, move the 470-ohm pot. to itsmaximum side and apply a low-ampli-tude RF signal of a suitable frequency.Since the bridge itself does not containany reactive component, the supply fre-quency is immaterial at this stage. (Ofcourse, up to a limit, lest the circuitleads, etc. start producing their ownappreciable inductance and capaci-tance.) Connect 1k resistor (preferably1 per cent tolerance type, at most 5 percent) for Rx and adjust 470-ohm pot.to get the null. Increase the sensitivityby increasing RFsignal/feedback resistance, and adjustthe pot. for null. This is called ‘finetuning’.

Now, mark 1k on the paper scale,below the knob position indicator.Switch off RF and change the resistorto another suitable value, say 750 ohms.Repeat the process and mark the knob’sposition as 750 ohms. Carry on withlower-value resistors, like 500, 390,250, right up to 5 ohms. Avoid longseries of resistor combinations, since itwould amount to an inductance in ad-dition to the resistance even at ordi-nary frequencies, because carbon-filmresistors are also toroidal conductorsin real sense.

AF-RF signal generatorAn integral part of the impedance

meter, the RF signal generator, is oth-erwise too a necessity for everyone,related to the consumer as well as theexperimental field of electronics.

The general problems which arisein the construction of a signal genera-tor are: a complicated circuitry, tediouscoil windings with strict turns ratio thatlead to non-signal generation, a lim-

ited frequency range, and above all thenuisance of harmonics beyond rea-sonable limits. All such problems havebeen taken care of while presentingthis miniature hut problem-free AF cumRF signal generator. The usage of coils,the most dreaded part of circuits, hasbeen kept to a minimum.

The circuit is a linear one, usingthree transistors—one each for masteroscillator, driver and output stage—giv-ing about 1-volt rms on all bands, testedup to 40 MHz (on a 20 MHzBW oscil-loscope).

The RF signal is sinusoidal, excepta bit flattening of crests in the lowestfrequency band (below 1 MHz), whichis, of course, rarely used, except for IFstages of AM radio where the wave-form is immaterial. Harmonics areamazingly low, even though the circuitis linear and no filters are used.

The circuit fires on the very clickof the switch, even if your coil turnsratio varies within limits.

This has been achieved, first, byusing a field effect transistor in placeof a bipolar transistor in the oscillatorsection, second, by putting another FETas buffer driver through a 5pF load onthe oscillator, and third, by applyingaudio modulation at the collector ofthe output stage transistor.

Circuit

C1 is an AM radio insulation gangcondenser, which may even be PVC type.Both the stator plate sections have beenconnected in parallel for broader bands.For higher frequency bands, however,a single section may be used for betterspread.

L1 is one of the five fixed coilswound on readily-available 6mm plas-tic formers, each with four pins and atorroidal ferrite core. The coils could beselected by means of a double-pole, six-way switch. But to save space and avoidmechanical problems, an 8-pin IC socketcould be accomodated in the PCB. Thefour pins of the formers of the coils fitinto the IC socket after slight bending ofthe pins. Just plug in the desired coil andyou get the desired band.

Coil 1 among the set of five coilsmay be an ordinary MW osc. coil. Itsoutput coil has seven turns, while theinput coil has about 80 turns (with themiddle pin cut-off) linked in series, thebase fits into the IC socket. Coils 2, 3


and 4 may be wound, using 36SWGwire, while coil 5 may be wound, us-ing 26SWG wire.

Coils made with any availableformer and almost any wire with nearabout the same turns ratio as given herewill oscillate well; the frequency rangeof course will change. The length ofwinding in each case should be 8 mm.Excessive turns, where required, maybe wound in layers.

For those who desire to use 8mmformers, two coils with ranges given inthe Table (Coil Data) have been testedwith the prototype. Other coils may bedesigned for 8mm formers, keeping inmind the specifications for 6mm coils.The frequency ranges of the coils wereselected, so that they had frequencyoverlap and gave frequency ratios of1:3 or. 2:5 for easy dial marking, ex-cept, of course, the first coil, whichwas not self-wound.

Diode Dl, used in the prototype, isOA79 germanium type. It limits thegate potential of transistor T1, the pur-pose being amplitude stability. Omis-sion of Dl or use of silicon diode makesno other difference.

Similarly, diode D2 used in the pro-totype is a 9.1 V zener, but it could beof any other voltage rating from 6V to12V, the purpose being frequency sta-bility. The FETs are used in common-

drain mode, whereas BF959 is used incommon-emitter mode.

Audio section

The audio section may be used formodulating the RF carrier signal or itmay be used for audio gadget checking.

IC 555 gives out square waves ofabout 1.35 kHz, with the suggestedR10-Cl1 combination. The filter cir-cuit, comprising resistor R11 and ca-pacitor C13, makes the wave almostsawtoothed. When it is fed to the col-lector of BF959 through C14-L2 com-bination, it nearly resembles a sinewave.

The feeding of audio at the collec-tor of BF959 helps in prevention offrequency modulation and hence a pure,amplitude-modulated, harmonics-freeRF signal is obtained. Moreover, sucha modulation is found to have no ad-verse effect (as compared tounmodulated sine wave) while usingwith the impedance meter, which is ourmain concern here.

The RFC(L2) can be made by wind-ing about 300 turns of fine wire on a6mm or 8mm former, covering a lengthof 3 mm to 5 mm. With this AF-RFcombination, nearly 50 per cent modu-lation of the carrier is obtained, whichis just right for most purposes.

Some applications

The impedance meter is a versatiletest instrument, whose capability canbe utilised in various ways. Here aresome of its applications:

To determine the characteristic im-pedance or the terminating load imped-ance of a transmission cable: Differ-ent cables pose different impedancesto RF passing through it. This imped-ance is independent of the signal fre-quency, if the load matching is proper.

This fact is very important for theselection of cable for proper signaltransmission. It helps to avoid reflec-tion of signal from the antenna or theinput circuit back into the cable or viceversa. If the impedance of source,transmission line and load is proper, nosignal reflection or absorption takesplace in the transmission line and,above all, a transmission line of anysuitable length may be used.

To use, connect one end of the cableto the bridge at Rx position by the short-est length of leads. To the other end ofthe cable, connect a known resistor ofthe order of 200 ohms.

Feed low-amplitude RF signal ofany frequency from the signal genera-tor. Adjust the bridge pot. to get thenull point, i.e. zero on the VU meter.As the reading may not be exactly zero

Fig.4: Circuit diagram for the AF-RF signal generator.


due to stray effects, take the minimumof meter as null point.

Let’s say the pot. is at 150 ohms atnull. When you vary the signal genera-tor frequency, the VU meter will showvariance in null position. Change theterminating load resistance to 150ohms. Vary the frequency from signalgenerator again. The meter now willnot move from its minimum position.(You might have to change a bit ofresistance further but generally you getthe matching in the very first instance.)

Now at this position your imped-ance meter reads 150 ohms with anylength of the cable and at almost anyfrequency, with a load of 150 ohmsitself, and your cable is posing practi-cally zero resistance to the signal. This150 ohms is the characteristic imped-ance of the cable, i.e. this cable willmatch exactly into 150-ohm load.

It will be found that a flat TV cablegives the free null point with 300-ohmimpedance while the round coaxialcable has 75-ohm impedance. Imped-ance matching of cable is importantbecause then any suitable length of the.

cable (not necessarily an integer mul-tiple of half wavelength) may be usedwith the matched load.

Antenna and feeder matching:Theoretically, a half-wave dipole an-tenna, fed at the centre, by a half-wave-length sized feedline cable (or an inte-ger multiple of the half wavelength)gives a proper antenna system with theminimum impedance. But, practically,a bit less is required for proper match-ing.

To ascertain this fact, connect theinput of the antenna system (free endof feedline cable) to the impedancemeter at Rx position. Feed low-ampli-tude RF and rotate the bridge pot. toget the null. The bridge reading givesthe antenna systemimpedance directly.

As an example,27MHz frequency an-tenna system shouldrequire 5.555 metresof feeder cable (of anycharacteristic imped-ance) feeding into adipole with each arm

of 2.777 metres. But with the above-mentioned theoretical lengths, nearly150-ohm antenna system was obtainedat 27 MHz. With 5.10 metres (100-ohm type) flexible cable feeder and2.70 metres each of balanced 16 SWG

wire dipoles, an antenna system of only25-ohm impedance was obtained.

A final test was done by feeding inexactly 27.045 MHz (fixed) signal froma small transmitter of a toy car, and theabove said impedances were confirmed.

Parallel and series tuned circuits:Just connect the tunable circuit to themeter (in Rx position) by short leads.Let it be a series LC circuit. As weknow, series circuit is an acceptor typeof circuit and will provide the mini-mum (but not zero!) impedance at reso-nance to the signal frequency, which isgiven by the relationship

1f =

2�LCOur aim is to tune the circuit so that

it provides a minimum impedance at aparticular frequency. Feed in the de-sired frequency low-amplitude signal.Say we get a null point at 150-ohmposition. Move the pot. knob to a lesserimpedance position, say 100 ohms. Re-tune the circuit, by changing the in-ductance or capacitance to come to thenull position again. Come down fur-ther on the impedance scale and re-tune, and so on, till you get the mini-mum possible impedance. Increase thesignal from the signal generator for amore pronounced null and more cleartuning.

If the natural frequency of the al-ready tuned circuit is to be established,connect as usual and vary the frequencyfrom the generator till you have theminimum possible reading on the im-pedance scale. A final reading shouldalways be taken with elevated signalfeed.

Further, we know a parallel tunedcircuit is a rejector circuit, as it pro-vides the maximum impedance to the

COIL DATA

Ciol Turns Former Descrp. Band LimitsNo. (1-2):(2-3)1 AM osc. coil 10 mm IFT 0.4 to 1.2 MHz2 15:150 6mm, Full core 0.55 to 1.8 MHz3 10:60 6mm, Full core 1.6 to 5.5 MHz4 06:20 6mm, Halfcore 4 to 15 MHz5 05:8.5 6mm, Halfcore 10 to 38 MHz1(i) 10:40 6mm, Full core 1.8 to 6.0 MHz2(i) 06:15 6mm, Half core 5.4 to 18 MHz

Fig. 5: PCB layout for the AF-RF signal generator.

Fig. 6: Component layout for the AF-RF signal generator.


gave an impedance of 2000 ohms whenthe same very components were put inparallel, at proper resonance.

In rejection type circuits, the VUmeter does not read zero exactly, dueto reflected feed signals.

Tuning the output filter circuit: Tothe output end of the filter circuit, con-nect a properly selected load resistorof the same value, as that of the an-tenna system or the load to be con-nected. It should be of suitable wattage(generally 50 ohms or else as requiredby the circuit design). Connect the in-put end of the filter circuit to the im-pedance meter (at Rx position). Applylow-amplitude RF signal of the desiredfrequency and obtain null. Move theknob to the desired impedance posi-tion, the same as that of the terminat-ing load (50-ohm position or else), andtune the filter circuit (through core orcapacitor, as the case may be) till youget your meter to the null position again.Increase the signal level for an exactand final tuning. Now, your filter cir-cuit poses the minimum possible resis-tance at the desired frequency and thestipulated operating load, and hence hasthe best filtering efficiency.

Some more interesting applicationsof the impedance meter are suggestedbelow, but you may try others yourself.

Measuring L and C: A bridge canmeasure (in fact compare) inductancesand capacitances. But our present meterhas diminished accuracy due to thepresence of filtering capacitors C3 andC4, following the basic bridge circuit.The way out is: replace 100-ohm resis-tor Rc with a fixed inductor. (For ex-ample, an ordinary radio IFT, withoutcapacitor, has an inductance of around

PARTS LIST (for Fig. 4)

Semiconductors:IC1 — 555 of any seriesD1 — OA79 detector diodeD2 — 9.1V zener diodeTI,T2 — BFW10 (BEL)T3 — BF959 (BEL)

Resistors (all 1/4watt, ±5% carbon unlessstated otherwise):R1,R2 — 100-kilohmR3,R4,R5 — 470-ohmR6 — 27-kilohmR7 — 330-ohmR8 — 10-ohmR9 — 1-kilohmR10 — 2.2-kilohmR11 — 220-ohmR12 — 47-ohmVR1 — 1-kilohm pot.

Capacitors:C1 — 2J air gangC2 — 100pF ceramic discC3, C4, C11 — 0.1μF ceramic discC5 — 5pF ceramic discC6,C7,C8,C12 — 0.01μF ceramic discC9,C10 — 47μF, 25V electrolyticC13 — 0.47μF ceramic discC14 — 4.7μF, 25V electrolytic

Miscellaneous:Sl — 1-pole, 2-way switchCoil 1 (L1) — One of the 5 coils (please

refer text)L2 — 300 turns of fine copper

wire on 6 or 8mm diaformer.

— Output jacks etc.

600 μH.)Now, you can calibrate your meter

for various inductances and capaci-tances at Rx position, as you did forresistances. Of course, the scale willbe direct for inductances and inversefor capacitors.

As mentioned earlier, the VU metermay not read exactly zero in this casetoo.

Check yourself the functioning ofbalun: The input of a TV set has 75-ohm input impedance. But we gener-ally use 300-ohm antenna, with a 300-ohm flat cable. Sometimes, 75-ohm co-axial cable is used. Let us see how abalun converts impedances in suchcases.

Connect 75-ohm end of the balunto the impedance meter. To the other(300-ohm) end of the balun, connect a300-ohm resistor directly or with apiece of (300-ohm) flat cable. You willsee that your bridge nulls at 75-ohmposition, although the terminating re-sistor is of 300 ohms. It means thebalun has converted the impedancefrom 300 ohms to 75 ohms.

Conversely, connect the 300-ohmend of the balun to the meter. Now, youwill see that you have to connect 75-ohm resistor to the other end of thebalun, directly or through a piece of75-ohm coaxial cable to get the nullposition on the impedance meter at 300-ohm position, thus showing that thebalun has converted the impedancefrom 75 to 300 ohms. �

tuned frequency. So, you have to go infor the maximum possible impedancein this case. Rest of the procedure re-mains the same.

A 27MHz tuned circuit, with in-ductor and capacitor in series, and thesecondary open, was tuned by core,and the impedance was found to be 5ohms at resonance. The same circuit


INTERRUPTIONCOUNTER CUM BURGLAR

ALARM

S.S. CHENDAKE

Now-a-days the electronic digi-tal interruption counter is verypopular. It counts and records

automatically the number of objectspassing on a conveyor belt or the peopleentering or leaving any place through aparticular gate (permitting only one-way entry). The interruption of lightbeam in this circuit is counted in twodigits—up to 99.

The burglar alarm provides aninexpensive yet effective form ofprotection against intruders. Whenthe invisible ray of light is interruptedmomentarily by any person, the alarmrings up to indicate entry of someunidentified person. The alarm stopsautomatically after a preset timeperiod.

Such burglar alarms are widely usedin homes, banks and offices as a secu-rity measure. This compact and por-table gadget is easy to operate and canbe set up in minutes.

The circuit can be easily understoodby dividing it into sevensub-units, as shown in Fig.1. Two very importantsub-units are source anddetector, which are com-mon to both—interruptioncounter and burglar alarm.

The source has anLED, which emits the in-visible infrared light,when a small DC voltageis applied across it. Thislight beam is made to fallon the phototransistor in-side the detector stage.The detector also has anop-amp (used as com-

parator) and an npn transistor. The com-parator switches from low to high statewhenever the incident light on thephototransistor is interrupted, produc-ing a clock pulse at the collector of npntransistor. These pulses are the output

of detector stage.The two-way selector switch en-

ables one to use the gadget as either aninterruption counter or a burglar alarm.When the selector switch is at position‘A’, the counter converts the clock

Fig. 1: Block diagram for 2-digit interruption counter cum burglar alarm.

Author’s prototype


pulses into BCD and its equivalent deci-mal number is displayed on the 7-seg-ment display.

When the selector switch is at posi-tion ‘B’, the clock pulses at the detec-tor output are given to the timer, usedin a monostable mode to switch thetimer output high. This high outputstarts the alarm and the ion-buzzer os-cillates, producing a pleasant sound. Aprecise 5-volt power supply is recom-mended for the circuit.

Circuit operation

The heart of the source stage is aninfrared (IR)-LED. The source is usedto emit infrared light, using a small DCvoltage since the IR-LED requires avery small current (about few mA).Thus, series resistor Rl is used as acurrent limiting resistor.

The main part of the detector stage

is the phototransistor. Although the IRlight beam is invisible, it behaves likeordinary light and is focused on thephototransistor by using a convex lens.When IR light beam is focused onphototransistor Tl, the resistance of T1becomes low. Hence, the voltage acrossresistor R2 increases. An op-amp (IC1)is used in the circuit as a comparator.

When the IR light stops falling onphototransistor Tl, the resistance ofphototransistor becomes very high. Inthis state, Tl and R2 act as a resistivevoltage divider at the inverting termi-nal of the comparator (IC1). With re-sistance of Tl being very high, the volt-age at inverting terminal is very low.Using variable resistance VR1 (preset),the voltage at non-inverting terminalof comparator is so adjusted that it isslightly greater than the voltage at in-verting terminal. As a result, the outputof IC1 becomes high.

Similarly, the output of ICl becomeslow when voltage at its non-invertingterminal is slightly lesser than voltageat the inverting terminal. Thecomparator’s output changes from highto low or from low to high, with smallchanges of a few millivolts at its inputterminals.

When output of IC1 becomes high,it gives sufficient base bias to drivenpn transistor T2 to conduction throughdiode Dl and resistor R3. Since Dl isforward biased, it conducts only whenoutput of IC1 is high. Resistor R4 keepstransistor T2 ‘off’ when output of IC1goes low.

When the IR radiation is incidentalon transistor Tl, its resistance decreases.This increases the voltage drop acrossresistor R2 sufficiently to change theoutput of IC1 from high to low state,and so transistor T2 turns ‘off’. Thus,the collector of T2 becomes more posi-

Fig.2: Circuit diagram for the interruption counter cum burglar alarm.


Inside view of author’s prototype

tive through resistor R5. No currentflows through LED2 and it remains‘off’. When LED2 turns ‘on’, it indi-cates that the IR beam has been inter-rupted.

When IR beam is interrupted mo-mentarily, as explained earlier, the out-put of IC1 becomes high and transistorT2 starts conducting. Thus, collectorof T2 is grounded for a moment, LED2lights up momentarily and a clock pulseis produced at point ‘C’. Each time theIR light is interrupted, another clockpulse (square shaped) is produced atthe collector of transistor T2.

When selector switch S1 is at posi-tion ‘A’, the pulses produced at collec-tor of T2 indicate the number of inter-ruptions through the counter chain forcounting and displaying the actualcount digitally.

The first decade counter IC4 pro-cesses the pulses and gives a binaryoutput to IC6 which drives a 7-seg-

Fig. 3: PCB layout for the circuit shown in Fig. 2.


ment, common-anode display andshows the corresponding decimal num-ber. After counting up to 9, the tenthpulse overflows to IC3, whose outputgoes to IC5 that drives another 7-seg-ment display. The two displays togetherenable a count of up to 99. On 100th

pulse, the combined display shows ‘00’.The pushbutton switch S2 resets the

circuit and returns the display to ‘00’.Hence, the maximum numberof interruptions that can be countedis 10n-1, where ‘n’ is number of digits.So, in this case the number is 102-1, i.e.

99.The number of digits can be in-

creased by cascading more counterswith decoder drivers and displays, toshow a larger number of interruptions.

When selector switch Sl is movedto ‘B’, capacitor Cl being initially dis-


Fig. 5: PCB layout for the display. Fig. 6: Component layout for the display.


charged, trigger pin 2 of IC2 gets Vccthrough resistor R6. When the lightbeam is interrupted by someone, theoutput of IC1 becomes high and pro-duces a pulse at the collector of T2.This pulse triggers IC2 through capaci-tor C1 and makes its output high. Thisoutput is given directly to the ion-buzzer which produces a pleasant alarmsound. After a preset interval, the out-

put of IC2 goes low and the buzzerstops producing sound. If another inter-ruption takes place, the alarm againrings for a few seconds and stopsautomatically, ready for the next inter-ruption.

The current consumption of thealarm circuit being only a few milli-amperes, IC2 can easily withstand thisload. As use of relay at the output of

IC2 has been eliminated, the circuit’soverall cost gets reduced.

Power supply

Power supply for the ciruit is verysimple as it requires only +5 volts. Ituses step-down transformer X1 to re-

Fig. 7: Wiring diagram.

Fig. 8: IR-LED and phototransistor arrangement.

PARTS LISTSemiconductors:IC1 — CA741 op-ampIC2 — NE555 timerIC3,IC4 — 74LS90 decade counterIC5,IC6 — 74LS247 BCD to 7-

segment decoder displaydriver

IC7 — LM7805, +5V voltageregulator

T1 — TIL81 phototransistorT2 — BC148 npn transistorLED1 — Infrared light emitting

diodeLED3,LED4 — 5mm general-purpose LEDD1 — IN4148 switching diodeD2-D5 — 1N4007 rectifier diodeB1 — PB27 ion buzzerDIS1,DIS2 — LTS542 common-anode

displayLED2 — D.P. of DIS-1 display

Resistors (all 1/4 watt, ±5% carbon unlessstated otherwise):Rl — 220-ohmR2 — 6.8-kilohmR3 — 2.7-kilohmR4, R5,R8, R10 — 1-kilohmR6, R7 — 10-kilohmR9 — 270-ohmR11 to R24 — 330-ohmR25, R26 — 680-ohmVR1 — 4.7-kilohm presetVR2 — 220-kilohm preset

Capacitors:C1,C5,C6,C8 — 0.1μF ceramic discC2 — 0.01μF ceramic discC3 — l0μF, 16V electrolyticC4 — l000μF, 25V electrolyticC7 — l00μF, 16V electrolytic

Miscellaneous:X1 — 230V AC primary to 12V,

500mA sec. transformerF1 — Fuse with holderPCB-1 — PCB for componentsPCB-2 — PCB for displayS1 — SPDT switchS2 — Push-to-on switchS3 — SPST switchJ1,J2 — Jumper wires

— IC sockets, DIL (two 8-pin,two 14-pin and two 16-pin)

— LED holder— Heatsink for IC7 (TO-220

package)— Convex lens and plane

glass— Nut bolts and screws— Wooden cabinets (three)


Fig. 10: Back view of the cabinet.Fig. 9: Front panel layout.

duce mains voltage to 12 volts, diodesD2 through D5 for converting AC volt-age to DC, capacitors C4 through C7for filtering the DC and 3-terminal posi-tive voltage regulator (IC7) for a con-stant voltage. C5, C6 and C8 also actas surge capacitors. LED4 monitors theworking of the circuit.

Since TTL ICs require a regulatedpower supply, it should be ensured that

the regulated voltage remains within4.5 to 5.5 volts.

Installation

To simplify installation, the unitmay be split up into three parts: (a)source of light, (b) phototransistor,and (c) the rest, comprising detector,counter, timer with alarm, display and

power supply.The IR-LED and phototransistor

can be fitted on either side of the con-veyor belt or the entrance gate (about90 cms above ground level). Convexlens may be used to focus the IR lightbeam on phototransistor. The box, con-taining the rest of the circuit, may bekept elsewhere. �

Readers’ Comments:The author may please clarify some ofmy doubts:

Can a 6V stepdown transformer beused, instead of a 12V transformer?

How could this project be used asa burglar alarm and interruption countertogether? Can it be used by connectingthe points B and C together with pointA?

What should be the maximum dis-tance between the IR transmitter andthe receiver?

Could the buzzer be replaced witha speaker?

Sandeep MathurDelhi

� In the counter there are two ICs usedto drive one display and a total of 4 ICsfor the two displays. This will makethe PCB more complicated which willthus cost more. Instead of using ICs74LS247 (BCD to decimal decoder)and a 7-segment display for counter 1,only a CD4033 can be used for onedisplay. It has both decade counter aswell as BCD to decimal decoder andin-built circuit, limiting resistors anddrives a common-cathode 7-segmentdisplay, thus reducing the size and costof the PCB.

R. Sasidhara ReddyBangalore

The author, Shamsundar Chendake,replies:

As explained in my article, TTLICs require regulated +5V supply. Ifvoltage across TTL ICs increases be-yond 5.5 volts the ICs will get dam-aged, and if voltage falls below4 volts the ICs will not give properperformance. Thus, this project needsregulated voltage and the input volt-age (through the regulator) mustbe 3 volts higher than the output volt-age.

This circuit can be used bothas burglar alarm as well as interruption

counter.The maximum distance between the

IR transmitter and the receiver shouldbe 2.4 metres, but if you install anotherconvex lens at IR diode (LED1) thedistance can be increased up to 7.6metres.

The buzzer may be replaced afterthis modification as shown.

Here IC 555 is used as a stablemultivibrator which produces 1kHz au-dio tone at its output.

IC CD4033 (or CD4026) can beused instead of two ICs (7490 and74247).

ELECTRONICS PROJECTS92

SECTION B:CIRCUIT IDEAS


output is ‘low’. The sum odd outputis the complement of the sum even out-put.

The input signals are applied byclosing switches S1 to S9, which isindicated and confirmed by red lightof the bicolour LEDs. Absence oflogic high input signals is ensured byopening of switches S1 to S9, indi-cated and confirmed by green light ofthe bicolour LEDs.

N1 to N9 are the inverting gates of

7404 ICs to provide alternate light ofbicolour LED. IC1 accepts nine inputs.Its outputs are obtained at the fifth pin aseven and at the sixth pin as odd. DiodesD1 and D2 and display LTS-543 areused to show E (even) and O (odd),according to the sum of input signals.Red light of bicolour LED is consideredfor total logic high inputs to decideparity signal at the output. This circuitworks well off 4.5V DC supply for anycombination of input signals.

DIGITAL EVEN AND ODD PARITYCHECKER/GENERATOR

J.P. SHARMA

This circuit checks and generateseven and odd parity of nine in-

put signals. It is wired around IC74HC280.

The 74HC280 is a high-speed par-ity generator/checker that accepts ninebits of input data and detects whetheran even or an odd parity exists, andwhether these inputs are ‘high’. If aneven number of inputs is high, the sumeven output is high. If an odd numberof inputs is high, the sum even


Resistors R10 to R16 limit the cur-rent to display. Luminosity of display

I ntercom systems are becomingincreasingly popular these days.

Persons having big bungalows with ga-rages, or a flat with several rooms, of-ten prefer an intercom set. So, here isone to fulfil such requirements.

This intercom is designed to ensureprivacy in conversation. It basicallyconsists of a 4-stage transistor ampli-fier.

When a set’s DPDT switch is inTalk-Listen mode, the audio signal from

the condenser microphone is first fedto the base of transistor amplifier Tl,via volume control VR1. The audio sig-nals from transistor Tl to T2, T2 to T3and T3 to T4 are coupled by capacitorsCl, C2 and C3, each of 10 microfarads.

Resistors R1-R2, R3-R4, R8-R9 andR12-R13 determine the base bias totransistors T1,T2,T3 and T4. ResistorsR5, R7, Rll and R14 act as emitterstabilisers. Capacitors C4, C5, C6 andC7 bypass spurious distortions to the

ground level. X1 is an output-match-ing transformer of the type commonlyused in radio sets.

The circuit is powered by 9V bat-tery.

After assembly, power the unit andslide the switch to Talk-Listen posi-tion. Now, talk through microphoneMIC.1 and listen through loudspeakerLS1. If you want to listen throughloudspeaker LS2 and talk through mi-crophone MIC.2, slide the switch to

Amateur Radio enthusiasts andshortwave listeners will find this

circuit very useful. Using the circuit,even weak shortwave stations can beheard clearly through an ordinary SWradio set. The circuit should be con-nected between the antenna and re-ceiver. The RF signals picked up bythe antenna are first amplified by thisbooster and then fed to the receiver.

The circuit is a single-stage, com-mon-source amplifier built around BELdual-gate MOSFET 3N200. Any othersimilar MOSFET can also be used.

L1 is a shortwave antenna coil usedfor radio receivers. Commercially avail-able coils with movable ferrite beads(slug-tuned types) can be used.

The RFC coil may be wound onresistor R4 with thin enamelled (40SWG) copper wire of 180 turns. Usea 1-watt resistor as R4 for conve-nience.

Receiver’s 9-volt supply can be

SW BOOSTER FOR DXERSPRADEEP G.

used for this booster but it should bewell filtered. Otherwise, use a 9-volt,PP3 battery. As the current consump-tion of this circuit is very low, the bat-tery life will be long.

Necessary care should be taken

while soldering the MOSFET. Beforesoldering, all the leads of MOSFETshould be short-circuited with a cop-per wire. Shorting should be removedonly after completing the assembly ofthe unit.

INTERCOM FOR YOUR HOMEIYER MAHESH NAGARAJAN

may be decreased by increasing the se-ries resistance or vice versa. If output

is tapped off pins 5 and 6, it is used asparity generator.


Listen-Talk position. The wiring be-tween two rooms should be carried

out with a shielded cable.This intercom unit can also be used

for other applications, e.g., as a babyalarm to monitor the baby’s move-ments.

SLAVE SWITCHP.K. MISHRA

W hile using a computer, both themonitor and the CPU have to be

switched on or off. The same is thecase while using a VCR with a TV.This low-cost circuit switches off theslave (monitor or TV) when the master(computer CPU or VCR) is switchedoff, provided the two are being pow-ered through the slave and master sock-ets respectively in this circuit.

Whenever a load (equipment)is connected to the master socketin circuit, the voltage developed acrossthe load resistor R8 saturates transis-tor T1. At the same time, theAC mains voltage––rectified by diodeD1 and reduced by resistors R1and R2––gets applied to transistorT1. This voltage saturates transistor T2also and thus helps to switch the SCRon.

Coil L1 is made by winding 9 turnsof 36 SWG enamelled copper wire on


a 10mm dia. ferrite rod. It helps toprotect the circuit against radio inter-ference. Capacitor C4 and resistor R6

filter the spurious waveforms to pro-tect the SCR from high-voltage tran-sients. The SCR should be mounted on

Aperson learning Morse codeknows the importance of time gap

between words. This time should benearly the same throughout the trans-mission of the Morse pulses.

The circuit given here enables oneto check this timing. It can count timefrom 0.01 to 0.8 seconds. Generally,the time between two words never ex-ceeds 0.8 seconds.

The circuit is intended for use witha tape recorder. The Morse signals arerecorded on an audio cassette and thenreplayed. This circuit is connected tothe tape recorder, with a wire taken

a proper heatsink.The circuit costs around Rs 25

only.

MORSE INTERPULSE TIMEPERIOD COUNTER

KULJEET SINGH


from the extra speaker connection.The circuit detects the audio sig-

nals and produces a square waveformaccordingly. A time-base producesl00Hz pulses, which are fed to acounter, after resetting it. As soon as apulse disappears, the counter keepscounting until the next pulse is received.Then it displays the count for a fixedshort period. In normal condition, onlya dot in the deci-second display getsilluminated, and the whole display re-mains off. The display starts glowingonly when the gap is more than a cer-

tain specific time.The circuit skips off the gaps between

two characters as these are too small.For this purpose, a timer is incorporated,which is triggered by each positive go-ing pulse. If the next pulse arrives be-fore its time period, that pulse will notbe acknowledged. This time period canbe set by preset VR1 and depends on thecode-sending speed. The time for whichthe display holds the counted time canbe adjusted by VR2. Generally, 0.2 sec-onds are sufficient. The time is shown indeci-seconds.

Note: On connecting pin 9 of IC4538 to pins 1 and 7 of both 4543 ICs,the display works in latched mode. Thedisplay in normal condition will beblank, except for a dot (dp) on the leftside digit. This circuit displays the peakvalue of the time lapsed between twosuccessive words (not letters) of theMorse signals. It displays that peakvalue for a while, but internally, it startscounting the next time period, sincethe two 4543 ICs are used in the latchedmode. That is why the display does notglow all the time.

M ore often than not, one hears thedoor-bell ring when the audio

rack is turned up to its maximum vol-ume range, causing inconvenience tothe visitors. In case you are concernedabout this, this simple circuit will aptlysolve the problem by turning the vol-ume to nearly zero level the moment it

hears the door-bell ring, and then backagain to its normal level after an ad-justable time gap.

The circuit is based on a fairlyoverused quad CMOS inverterCD4093, whose inputs are wired to

make two of its four used gates tofunction as inverters with a hysterislevel acting at the inputs. When thebell (mains-operated) rings, a parallelconnection input sends a voltagethrough the voltage dropper compo-nents Dl, D2, Rl, R2 and Cl. Theseapply a 9-volt DC pulse at the input

of gate N2, which triggers gate Nl,whose output goes high for a time pe-riod, depending upon the setting ofVR1. As long as the output remainshigh, the two transistors remain con-ducting. As these are connected to the

AMPLIFIER TACITURNERAMRIT BIR TIWANA

audio amplifier inputs, the amplifier’svolume is dropped to zero for that pe-riod. This is indicated by the LED.

The circuit can be used with am-plifiers running at up to 60 volts; forhigher voltage types, transistors Tl andT2 should be replaced by higher volt-age devices.

The current intake of the circuit be-ing very low, the need for a separatepower supply will never arise. The cir-cuit can also be tailored to suit theamplifier’s power supply by varyingthe ratings of resistor R3 and zener D3.


T he normal level-indicator circuitswhich are available in the market

require connections to be made to theoutput of the player, which may not beeasily accessible. The circuit describedhere removes this restriction as it maybe placed close to the player’s speak-ers and yet the desired effect can berealised.

As shown in the circuit, signals arepicked up by the condenser micro-phone, which get further amplified bythe non-inverting amplifier built aroundone of the four op-amps of LM324.The remaining three, along with fourop-amps of the second LM324, are usedas seven comparators to work as thelevel detector, giving seven output lev-els through seven coloured LEDs.

The sensitivity of the circuit maybe improved by varying the 220k po-tentiometer. If a fine adjustment is de-sired, a 4.7-kilohm potentiometer maybe connected in series with the 220kpotentiometer.

REMOTE AUDIO LEVEL INDICATORRAJESH L. CHHABRIA

quired, it should be attenuated. For thispurpose, a potmeter (VR1) is provided.

Transistor T2 is meant for micro-phone. The value of resistor R18 liesbetween 4.7-kilohm and 22-kilohm, de-pending upon the sensitivity of the mi-crophone. The passive network is foran auxiliary input, such as a keyboard.

The op-amp is built in the ‘buffer’mode. Output from the op-amp is to bemaintained at the ‘recording’ level. Pre-set VR4 is set by trial and error to getundistorted recording. Output from

C ostlier audio systems generallyhave ‘sing along’ facility now-

adays. Generally, all these circuits em-ploy a simple mixer, which acceptshead signal from a recorder as well asa microphone signal simultaneously.Familiar songs enable us to sing in sucha way that the singer’s voice in therecording is eliminated.

This circuit utilises easily availablecomponents and its performance is re-ally satisfactory. Only mono versionhas been given here. A replica can be

used for the stereo version. The ‘fan-in’ facility of op-amp 741 is employed.This circuit is to be connected in be-tween a cassette player and a cassetterecorder.

Recording output or ‘external’ ter-minal of cassette player A is to be used.As the output from the terminal is suf-ficient for recording, it need not befurther amplified. Hence, an emitter fol-lower is constructed, using transistorT1.

If the output level is more than re-

SIMPLE KARAOKE CIRCUITL. SRIDHAR


VR4 is fed into the input of tape B,kept in the recording mode.

A very good power supply withleast ripple level should be used toenergise the circuit. Tmhe signal-carry-ing wires should be good-qualityshielded cables to avoid noise pick-up.

In the stereo version, by maintaining aneffective control over the inputs, actualstereophonic effects can be produced.

Author’s Comments:In my circuit for ‘Karaoke System’ re-

sistor R18 had been used for bias-ing of the condenser microphone.However, if one inserts a dynamicmic., it may get damaged due tothe current flowing through R18.So, before using a dynamic mic-rophone, resistor R18 should be re-moved.

TRANSISTORISED ‘WALKMAN’RADIO PLAYER

PRADEEP G.

tion. The radio stage is common forboth channels. Use high-qualityshielded wire for the tape’s play headand the volume controls. The length ofthese wires should be as short as pos-sible.

Connect the motor to supply railsthrough the leaf switch of the mecha-nism. The motor will be operated onlywhen the Play, R/W or F/W switch isoperated.

Use a separate on/off switch for bat-tery. Otherwise, the switch of the vol-ume control can be used.

As already stated, this circuit can

stage is made with BEL 187 and 188matched pair. 250mW power can beobtained at 3-volt supply with thisamplifier.

The outputs of the head preampli-fier and the RF stage are selected by atwo-pole, two-way slide switch andgiven to the main amplifier throughthe volume control.

This circuit has comparatively goodsensitivity for the local MW stations.In the circuit, only one channel isshown.

All stages, except the RF stage,should be made twice for stereo opera-

Afully transistorised circuit isgiven here for a ‘walkman’ radio

player. The circuit comprises a singletransistor RF stage, a two-stage headpreamplifier and a four-stage mainamplifier. The entire circuit is devel-oped for 3-volt operation, meant tobe used with compact ‘walkman’mechanisation.

The circuit employs seven cheapsilicon transistors. The head preampli-fier is the popular two-stage npn-pnpDC amplifier.

The main amplifier is a four-stage,direct-coupled amplifier. The output


deliver 250 mW power into each chan-nel. So, instead of a headphone, mini-speakers can also be used.

For on/off indication, a red LEDwith a 470-ohm resistor can be con-nected across the power supply lines.

over the ferrite bar.L3 — Philips interstage (IFT) with-

out parallel capacitor. (Primary wind-ing only.)

The switch connections are alsoshown in the circuit.

The specifications of the coils (Ll-L3) used are :

LI — 100 turns of 42SWG enam-elled copper wire on flat ferrite bar.

L2— 10 turns of 42SWG enam-elled copper wire near top end of L1

Instant water heater is a very usefulhousehold device. Inside its attrac-

tive body, there is a metal containerwith an insulated heater coil. Inlet andoutlet water pipes are fixed to the con-tainer. A thermostat is placed outsidetowards the top of the container to pro-tect the gadget against excessive tem-perature.

The circuit needs a continuous flowof water while working. The inlet and

SAFETY CIRCUIT FORINSTANT WATER HEATER

H.M MARUTHI RAO RAYKAR

outlet pipes should be connected prop-erly. There is no protection to the heatercoil, if there is an inlet water flow fail-ure, owing to the pipes being reverseconnected or otherwise. This can dam-age the heater coil.

Here is a unique circuit to protectthe heating coil. Two probes arefixed inside the container, just 2.5 cmsabove the coil through an insulatedwatertight tube with a gap of 7.5 cms

for each probe.The circuit has a single-stage ampli-

fier built around transistor Tl, whichoperates relay RL1. When switch Sl isturned on, step-down transformer X1provides 12-volt AC. This is rectifiedby diodes Dl and D2, filtered by ca-pacitor Cl and applied to the base oftransistor Tl through water probes. Posi-tive voltage to its base passes throughresistor Rl and the water probe, tran-


sistor Tl conducts and the relayenergises. Diode D3 and capacitor C2are connected in parallel to relay RL1to avoid chattering of the relay. Resis-tor R2 is used to limit the current flowto LEDs.

The heater coil gets supply throughthe relay’s N/O contacts. The green

LED indicates safe working. As thewater falls down below the probe level,due to absence of base voltage to Tl,the transistor stops conducting, and therelay de-energises. The heater is turned‘off’.

In this circuit, 12-volt, 250-ohm re-lay with 10-amp DPDT contacts is used.

LONG DURATION SEQUENTIALTIMER WITH IC MM5387

A.U. AHMED

The circuit becomes operationalonly when switch Sl is in position 3.Precisely at the alarm set time, a mo-mentary high state of alarm output trig-gers IC1, which is wired as amonostable multivibrator. IC1’s outputpulse is used to set the internal ‘sleepcounter’ of the clock IC by providingnecessary high state to the ‘sleep dis-play input’ and ‘fast set input’ simulta-neously. The pulse length, and hencethe sleep counter setting, is adjustableby potentiometer VR2.

The resulting high sleep output isused to drive the relay through tran-sistors T2 and T3. Sleep display input

is also provided with the necessaryhigh state to display the countdownprocess. At the end of the duration,IC2, triggered by the negative goingpulse edge, triggers the tone genera-tor IC 555 for approximately oneminute. However, an alarm reset push-button switch is also provided. Manualon and off switches are provided tofacilitate manual operation. Withswitch Sl in position 1, the clock func-tions as usual.

To calibrate VR1 and VR2, keepswitch Sl in position 2. In this mode,the circuit can be test-operated with-out disturbing the relay output. Now,

W ith the addition of the circuitdescribed here, your digital

clock, built around IC MM5387, canbe used to automatically switch on anelectrical appliance at the alarm settime. It can also be used to switch itoff after a certain time duration, ad-justable between 1 and 59 minutes.The circuit also features display of thecountdown (in minutes) to the comple-tion of the process, which is an-nounced by a self-resetting audiblealarm. So, it can prove very usefulfor the automatic operation of a vari-ety of electrical appliances, such asheater, geyser, TV, VCR, etc.

One set of contacts is used for indica-tion and another for heater supply.When relay is de-energised, the redLED indicates that there is no waterinside the container.

The circuit can be built on a smallgeneral-purpose PCB, housed in a smallmetal box, fixed very close to the equip-ment.


keep VR2 at its zero-resistanceposition. Adjust VR1 and press themanual on switch to get the sleepcounter setting displayed. Then pressthe manual off switch briefly to tryagain. Achieve the required maximumsetting of 59 minutes by trial and er-

ror. Similarly, keeping VR2 at its maxi-mum-resistance position, adjust VR4for a setting of one minute. VR2’s dialmay be marked accordingly. However,while setting the timer forreal operation, it is advisable to cross-check VR2 dial-setting by operating

manual on/off switches in the testmode.

The relay output can drive low-power loads like TV and VCR directly.However, the optional circuit to driveloads with higher power rating isalso given.


S uppose you are in the bathroom orenjoying a TV program while your

telephone is ringing. Someone is call-ing you on your phone. This is a veryirritating situation. Perhaps you lift thetelephone handset off the cradle andput it on the table to avoid this kind ofsituation. But there is a chance youmay forget to place the handset backon the cradle. This simple circuit solvesthis problem.

Connect this gadget to your tele-phone line. Press a switch wheneveryou want to disconnect your line. Thetelephone will get reconnected aftera preset time, set by switch S1. Any-body trying to call you during thisperiod will get a busy tone in his tele-phone.

The basic idea of this circuit is thatit puts a resistive load equal to that of

your telephone set in the telephone linewhen you press switch Sl. The loadgets disconnected after a preset time.The load forces the telephone exchangeto think that the line is busy. So, any-body trying to get your number willreceive a busy tone. The time periodcan be adjusted according to one’schoice.

The heart of the circuit is a 555 IC,which is wired in one-shot monostableon-timer configuration. On pressingswitch S3, the IC gets power throughthe switch. At the same time, a nega-tive-going pulse is applied to the trig-ger pin of 555 through another part ofswitch S3. So, the timer starts work-ing, making the output pin high. As aresult, relay RL1 energises.

On energising relay RL1, the cir-cuit gets its power through one of its

contact points. So, releasing switch S3will not disconnect circuit from the sup-ply. Another contact point of the relayis used to apply a resistive load on thetelephone line. A bridge rectifier is usedto avoid the problem of polarity. TheLED should glow, indicating that thesilencer is working. The value of Rlmay be changed to suit different tele-phone lines.

The time interval is set by switchS1 to 5, 10, 20 or 40 minutes. After thistime, the line will be normal. SwitchS2 is used to reset the circuit. After thetime interval set, the circuit will getdisconnected from the supply, causingno power consumption. A 9V batterymay be used to power the circuit.

The circuit can be housed in a smallbox. Use miniature relay to save space.Transistor Tl needs a good heatsink.

TELEPHONE SILENCERPRADIP KUMAR BOSE

1-POLE4-WAY

SWITCH

S1TIME

SELECTOR

R21K

S2RESET

SWITCH

C31000μ25V

BRIDGERECTIFIER

TOTELEPHONE

LINES


T he following is a reliable andeasy-to-build transistorised control

circuit for In / Out counter. There arevarious object counters available, butthey cannot be used for simultaneousin and out counting. In most cases, thereis only one door for entrance and exit,so false counting is likely. The circuitpresented here solves the problem.

The circuit, built around transistorsTl through T4 and the LDRs, controlsthe relay circuit. When LDR1 and

LDR2 are simultaneously obstructed,relays RL1 and RL2 get energised andfinally activate relay RL3, through tran-sistors T5-T9.

The LDR-controlled relay circuit isvery simple. Here, only potentiome-ters VR1 and VR2 are adjusted for abetter triggering of the LDRs. WhenLDR1 and LDR2 are obstructed, therespective relays RL1 and RL2 get en-ergised at once. When RL1 gets ener-gised, T5 is biased. T5 biases T6 and

capacitor C3 gets charged. Similarly,when RL2 gets energised, it connectsthe base of transistor T7 to the emit-ter of transistor T6, and thus T7 is bi-ased. This activates transistor T8, andT8 biases transistor T9 through relayR5. Finally, relay RL3 is energised.The on-time of RL3 is about seven toeight seconds and depends on the val-ues of resistor R3 and R4. An LDR-controlled object counter may be con-nected to RL3, which acts as the On/

CONTROL CIRCUITFOR IN / OUT COUNTER

BHASKAR BANNERJEE


Off switch for the counter.If the persons entering are to be

counted, then only one circuit is neces-sary. But when the persons enteringand leaving are both to be counted,then two such circuits are needed. Youmust be careful about the settings ofthe lights and the LDRs. LE1 and RE1of the entry counter should come firstwhen a person enters, then come LE2,RE2 and LE3, RE3. On the other hand,LX1 and RX1 are chosen first, when aperson exits. This is shown in the dia-gram below the circuit. LE and RE

represent the lights and LDRs for theentry counter, while LX and RX rep-resent the lights and LDRs for the exitcounter.

If you want to keep LE3, RE3 andLX3, RX3 in separate places, then in-crease the on-time of the relay RL3 byincreasing the values of resistors R3and R4. Be sure that a person takeslesser time than the on-time of relayRL3 to reach that point. For conve-nience, keep the distance shorter andon-time of RL3 higher. Keep LEI, LE2,RE1, RE2 and LX1, LX2, RX1, RX2

close, so that a single-light source maybe used.

The advantage of this circuit is that,if RE2 or RX2 or RE3 (LDRs of thecounter-circuit) or RX3 is obstructedfirst, relay RL3 will remain off. Thus,false counting is prevented.

The circuit can be constructed on averoboard or a general-purpose PCB.If possible, use lenses for perfect andaccurate operation. The light must befocussed just on the LDRs. Low-costlenses available in the market can beused without hesitation.

LOW-COST PULSE GENERATORMUKESH AMBWANI

H ere is a low-cost pulse generatorwith four independent controls for

pulse, i.e. On and Off periods and Onand Off levels.

IC LM393 has two comparators. Wecan use any comparator IC. The firstcomparator is used as an astablemultivibrator and the other as a zerocrossing detector. We can calculate

TON, TOFF, VON and VOFF from thefollowing equations:TON = VR1 x CTOFF = VR2 x CVON = Vcc x R5/(R5 + VR3)VOFF = VDD x R5/(R5 + VR4)

The circuit was tested with VON =+15V and VOFF =-15V, up to 300kHz.The circuit can be energised by a single

supply of up to +30V or dual supply of+17V > Vcc and -17V > VDD.

The output pulse is obtained frompin 5 or pin 7 of IC1. The characteris-tics of the waveform obtained areshown in the figure besides the circuitdiagram. The values of the variableresistors and the capacitor can be cal-culated from the equations, given inthe text.


M ost electronic code locks areprovided with a number of push

switches. These are pushed one by onein a particular sequence to activate the

lock. However, if somebody else ispresent while we are operating such alock, that person can easily manage tograsp the code by noting the fingermovements.

The code lock described here elimi-nates this drawback. It has only twopush switches for entering the codenumber. The mode of operation dependsupon the number of times the switch ispushed. Since the finger movementsare minimised, it is impossible for an-other person to copy the mode of op-eration. Besides, the electronic part ofthis lock switches off automatically af-ter each operation.

When reset switch S1 is pushed,capacitor C1 gets charged and givesenough bias to operate transistor Tl.The positive supply is connected to the

ICs through transistor T2. IC1 is resetat this time.

The second push switch S2 ispressed to give positive pulses to the

input of IC1 , which is a decadecounter. Each time switch S2 is pushed,the output of IC1 increments one step.On the sixth step, pin 5 (Q6) goes highand charges C2, providing bias fortransistor T3. Transistor T3 bringsdown the trigger pin 2 of IC2 to nega-tive level. But its output does not gohigh because reset pin 4 remains at alow level. If switch S2 is pushed again,transistor T4 shorts the base of tran-sistor T3 to ground and pin 2 of IC2will be brought to positive again. Thisshould not happen for a successful op-eration.

After the sixth push of switch S2,IC1 is reset by pushing S1 again. Foursuccessive pulses are given to the in-put of IC1 to hold the Q4 output (pin10) high. In this position, reset pin 4 of

IC2 gets positive voltage and its outputpin 3 goes high. This condition is indi-cated by the glow of LED2. The powertransistor 2N3055 conducts and the

motor starts rotating. This is a DC mo-tor, rotating both in clockwise and anti-clockwise directions, depending uponthe position of switch S3.

In the beginning, set the slide switchS3 in open or close mode. Then pushS1 once. LED1 glows . Push switch S2six times. Then push switch Sl onceand then switch S2 again four times.LED2 glows. Now, the lock is closedor opened.

The code numbers 6 and 4 are withrespect to the diagram given. Thesemay be altered by reconnecting the out-puts of IC1 with diodes D2, D3 andpin 4 of IC2. The door lock can beopened or closed with the help of themotor, which moves the latch insideforward or backward with the help ofa toothed wheel.

TOP-SECRET CODE LOCKK.A. SAKTHIDHARAN


The author, Mr K.A. Sakthidharan, re-plies:

1. To clarify the doubt, I am illus-trating it with the help of a simple dia-gram, as shown in the figure. Leaf

switches L1 and L2 are intended forstopping the circuit. These are to beconnected on both sides of the lock

lever (see fig) in such a way that eachtime the lever is moved to and fro,either L1 or L2 gets closed.

2. The DC motor can be replacedby a relay switch to drive any type of

locking system. In thiscase, 2N3055 may be re-placed by SL100.

The positive pulseavailable at the output ofIC2 may be used for trig-gering other electronicgadgets. In this case, amotor is not necessary.

If the mechanicalparts of the lock aresmooth running and lightweight, an ordinary mo-tor serves the purpose.Otherwise, a powerful

motor should be connected through arelay switch.

Readers’ Comments:Please convey my thanks to the authorof the above circuit, published in EFYMarch’94.

1. I could not understand the use ofthe two leaf switches L1 and L2, con-nected in parallel.

2. May I connect a relay at the out-put in place of the motor? Can I use tran-sistor BC 148 in lieu of transistor2N3055, when I am using the relay only?

Simon PhilipThiruvalla

� I want to clarify the following pointsfrom the author regarding the abovecircuit.

1. Can the circuit be used without amotor?

2. Does the circuit need an ordi-nary motor as used in tape-recorders ora special one?

Ritesh KumarRatlam

The reference clock, phase detectorand frequency comparator can be im-plemented by using a single CMOSPLL IC CD4046. This IC consists of

H ere is a circuit of an accuratefrequency-to-voltage converter

using analogue phase detectors andmultiplexers.

The analogue-type phase detectorcompares the two signal input frequen-cies fin and f0, and provides an outputsignal, which comprises the sum anddifference of the two signal frequen-cies fin ± f0.

The frequency comparator giveslow output (0V) when fin < f0 andhigh output (+Vcc) when fin > f0, andthis controls the 2-to-l analogue multi-plexer. The multiplexer selects the com-ponent fin ± f0 when its control is lowand selects Vcc/2 when it is high.

Initially, let fin < f0. The compo-nent fin ± f0 is given to the low-passfilter, to filter out the component fin +f0, and its output DC voltage is propor-tional to the input frequency fin forconstant f0. When fin becomes >f0,the output goes to saturation, i.e. toVcc/2.

two phase detectors and one linear volt-age-controlled oscillator. The phasecomparator I is a digital type, employ-ing an X-OR gate and the phase com-

ACCURATE FREQUENCY-TO-VOLTAGECONVERTER

C. SELVAM


parator II is of the analogue type.The analogue phase comparator II

compares the frequencies of the twoinput signals and it has two outputs.One output (at pin 1) provides an aver-age voltage, proportional to the com-ponent fin ± f0 and another (at pin 13)provides low when fin < f0 and highwhen fin > f0. The linear voltage-con-trolled oscillator in IC 4046 can beused as reference clock for constant f0by giving constant input voltage to it.The 2-to-l multiplexer can be imple-mented by another CMOS IC CD4053.

This IC has three, 2-to-l multiplexers.Out of three, one can be used here. Forthe low-pass filter, a simple RC filtercan be used. The complete implemen-tation is shown in the circuit diagram.

The graphical characteristics ofphase comparator at pin 1 of IC1 areshown alongside. From this, it is obvi-ous that when fin=f0, the average out-put voltage is maximum, i.e. Vcc/2.This is given by:

fin x VccVo = –––––––– where fin < f0

2f0

While testing, f0 was first set to1kHz by adjusting the value of re-sistor VR1. Then the input frequencywas varied from 100 Hz to 1 kHz,and the corresponding DC outputvoltage measured. The output DCvoltage can be attenuated to a re-quired level and can be given to aPMMC meter for analogue indicationor to a DVM for digital indication,corresponding to the unknown inputfrequency fin. Hence, this circuit canbe used for frequency measurementalso.

wired as a six-state counter, the countersequence being 0-1-2-3-4-9-0.

Each state of the counter is used toobtain one of the possible states, i.e.outcomes of the dice. It can be seenfrom the table that the QA bits invertedcorrespond to the states of LED7, QCbits inverted correspond to the states of

ROLLING ELECTRONIC DICESANJAY MALJURE

M ost popular indoor gamesrequire a dice. The circuit for

electronic rolling display given heresimulates an actual dice very closely.

Seven LEDs are used in the display.These LEDs are driven by a decoder cir-cuit of 3 NOR gates, which in turn aredriven by the decade counter (IC7490)

the LEDs 2 and 5, QD bits directly cor-respond to the states of the LEDs 3 and4. The QB and QC bits are NORed toobtain the states of the LEDs 1 and 6.IC555 is used in the astable mode toprovide the clock to the counter. Thefrequency in this case is 22 Hz. The dis-charge pin 7 of the IC 555 is connected


cuit becomes foolproof even at low fre-quencies and the use of this model atplay becomes exciting.

The plots of capacitor voltage and

State of IC7890 Condition of LEDs Count of

QD QC QB QA LED7 LEDs3,4 LEDs1,6 LEDs2,5 Dice

1 0 0 1 ON OFF OFF OFF 10 0 0 0 OFF ON OFF OFF 20 0 0 1 ON ON OFF OFF 30 0 1 0 OFF ON ON OFF 40 0 1 1 ON ON ON OFF 50 1 0 0 OFF ON ON ON 6

to an RC combination through a switchas shown. The 330μF capacitor chargesup to the supply voltage through the100-ohm resistor, and when the switchis released this capacitor dischargesthrough the 10-megohm resistor.

This mode of connection gives asignal of decaying frequency. This ef-fect is seen on the display as a continu-ously slow change of count of the dicetill it settles down to some steady state.Since the count of the dice continues tochange even after the switch is releasedby the person who tries a roll, the cir-

output of IC 555 are given below forbetter comprehension. Switch S2 ispushed on at t =Tl and released at, sayt =T2.

T his simple circuit enables theconstruction of an inexpensive and

compact courtesy light controller. Un-like the other controllers, this one doesnot switch off the light abruptly, put-ting the user in instant darkness. In-stead, it provides the user full intensitylight for a fixed time, which then gradu-ally dims and finally goes off. The cir-cuit dispenses with the bulky electro-mechanical relay and the stepdowntransformer by using power MOSFETas the control element. The circuit isconfigured such that the normally dis-charged timing capacitor ensures thatthe circuit does not start timing se-quence when the mains supply failsand then resumes.

The line voltage is rectified by di-odes Dl to D4. Op-amp LF351 is ener-gised by the supply Vcc, developed byresistor R1, diode D5, capacitor C1 andzener D6. Voltage VNI, at thenoninverting input of the op-amp, has aDC component derived from voltageVcc, scaled down by resistors R4, VR1and R3, and a ripple component derivedfrom bridge rectifier output, scaleddown by resistors R2, VR1 and R3.

When switch Sl is closed, the volt-age VI, at the inverting input-pin 2 ofIC1, is made zero and the capacitor C2is charged to Vcc volts. Op-amp outputswitches to Vcc volts, switching the

MOSFET and the lamp ‘on’ simultane-ously.

When switch Sl is opened, C2 isallowed to discharge through resistorR5. Full-intensity light is produced tillVI is less than VDC, the DC componentof VNI. During the period, VI is com-pared with the ripple component of VNI.The duty cycle of the pulses at line

frequency at the op-amp output de-creases from 100 per cent to zero per-cent, causing gradual dimming of theintensity of the light to zero. When VIexceeds the ripple peak value, the lampbecomes fully off.

Potentiometer VR1 provides an ad-justment for off delay from about 40seconds to 20 minutes.

GRADUALLY TURNING-OFFCOURTESY LIGHT

M. S. NAGARAJ


in this circuit gives on-off timing prop-erty, which avoids the need ofmechanical switches. Programmablelamp flasher is another feature ofthis circuit. In PLF working, the relay

T he circuit described here is of adigital on-off timer. This timer is

capable of dual-mode operation. It alsoprovides a wide range of timings (upto 9999 states) in seconds or minutes,

DIGITAL ON-OFF TIMERBIJUKUMAR J.

ranging from both the On and Off pe-riod control of the relay.

The circuit consists of eight ICs, aset of four thumbwheel switches andsix logic indicators. Automatic loading


turns ‘on’ and ‘off’ alternately, withequal periods as in the case of an astablemultivibrator. A four-stage display isreplaced by five logic indicators, whichis another advantage of this circuit.

The four 74190 presettable decadeup-down counter ICs are the heart ofthe circuit. IC 74190 is more suitablefor developing timer circuits. It has fourdata input pins labelled A, B, C and Dand four output data pins labelled QA,QB, QC and QD. Its pin 11 is used to setthe input data into the output. When alow pulse is applied to this pin, thedata at the input pins is transferred tothe output pins, i.e. QD, QC, QB, andQA become D, C, B, and A. This opera-tion has a priority over others and isdone independently in all the cases.This has an ENABLE pin 4. To operatethe counter, this pin is connected atlow level.

When the pin goes to a high level,the IC stops counting and disables allthe operations except load operation.

To set the counters in the count-down mode, keep the up/down inputpin high or open. This pin is labelledDown/Up and is pin 5. The clock isapplied to pin 14. QD is taken as theclock output.

This also has two outputs, whichindicate the conditions of outputs QD,QC, QB, and QA. When the counteroutput reaches 0000 in the count-down mode and also 1001 in thecountup mode, the maximum/minimumpin 12 goes to the high level. Hence, inthis system, the data output is used toidentify the maximum and minimumcount states of the counter outputs.When the output of any counter reaches0000, its pin 12 goes to a high level.

IC 7408 contains four two-inputAND gates. Three gates are connectedto get a four-input AND gate. The fourthgate is used for ANDing the load pulse,which is given from power on the resetswitch and output of four-input NAND

gate.IC 74LS00 contains four two-input

NANDgates. Two gates of this IC areused as inverters. One of these invertersis connected at the output of the four-input AND gate and results in a four-input NAND gate configuration. Thesecond inverter is used to invert a lowpulse given by power-on reset switchwhen power is on. This single highpulse is used to clear the subcounterIC 7493.

IC 7493 is also a subcounter whichis used in the timer unit to drivethe relay transistor. The QA output isused for this purpose. The QB output isused to operate the presettable counters.This output is connected to ‘enable’input of all presettable counters. Hence,whenever a high appears in the QB out-put, the counters stop counting opera-tions and remain in disable state. Thissub-counter is cleared by applying asingle high pulse while power is on.

A low pulse of time duration above1 ms is generated when the poweris on. The electrolytic capacitor C1charges through the resistor R10.After maximum charging, the output isheld in high state, but initiallyno charge condition produces a lowpulse. To get the desired result, a resis-tor of one-kilohm and 10μF capacitoris used.

Thumbwheel switches are used togive the desired time to the counters.The thumbwheel switches have one in-put terminal and four output BCD lines.The actual method to select the desiredBCD code using thumbwheel switchesis to connect the input terminal to theoutput when high output is required.To get this result, connect VCC supplyto input terminal and connect 16 resis-tors from each output of four thumb-wheel switches to the ground. The out-put data is taken from the BCD out-puts.

In the connection arrangement de-scribed above, whenever we need a highdata, the connection occurs, and hencethe voltage across the resistor is +5V.But for a low pulse, no connection oc-curs, and hence there is no source cur-rent. So, the above arrangement givesground through the resistor.

After arranging the desired On timein thumbwheel switches, press thepower-on switch. The power is givento the timer unit and the common cir-cuits. The load pulse applied to thecounters is done in two ways. Due tolow detecting operation, an AND gateis used. Pin 12 of all counters is con-nected to the four inputs of the four-input NAND gate. This gate is realisedby using three two-input AND gatesand an inverter.

Whenever the output of all thecounters becomes 0000, pin 12 of allcounters goes to a high level. Hence,the output of NAND gate becomes low.This output is always connected to the

clock input of the subcounter IC 7493to count a single pulse after the firstprogramed time in the counter. In thisway, the first programed time in allthe counter outputs becomes 0000 anda low pulse acts at output of NANDgate. The subcounter counts up a singlepulse, and hence its output becomes0001. The relay is turned on at thistime.

The output of NAND gate and inte-grator output are ANDed using a two-input AND gate and the resulting out-put connected to load inputs of allcounters. Due to this arrangement, af-ter the operation for first programedtime completes, an automatic loadingis done, and hence the second time orOff time is automatically loaded intothe counters.

After automatic loading of Off time,the clock runs and the output becomescountdown with the clock. In thisway, after the second programed time,another low pulse appears at the NANDgate output. This results in nextloading and single pulse counting bythe sub-counter. Hence, its outputbecomes 0010. At this time, the relayturns off due to low level in QA anda high pulse in the QB results in a holdstate of the system operation. Thenall presettable counters stop counting,and hence all counter and subcounterICs’ output states remain in the abovestate. Due to this reason, after theOn time, the relay works and then afterthe Off time, the relay turns off. Hence,the system automatically remains in thedisabled state.

The clock generator unit is usedto give one-second and one-minutepulses for the operation of the timer. Itis wired around a CMOS ICCD 4060, due to its advantages overall other clock generators. It containsan oscillator section and a divideby 214 counter. The oscillator sectionproduces high frequency clock andthen the counter section divides it tosuitable one-second and one-minuteperiods. The basic clock has atime period of 2.2 RC seconds, andhence the final output has a muchlarger time period of 2.2 RC x 214 sec-onds. Due to the smaller values ofthe capacitor and resistor used, thestability of the clock period becomesconstant.


T he circuit shown in the figure isdesigned to operate wipers fitted

in vehicles. These are special types ofwipers that turn on automatically whenit begins to rain and turn off when therain stops.

The circuit is based on the versa-tile, multipurpose timer chip 555. Thetimer 555 is used in the monostablemode in the circuit. The timer is trig-gered when a voltage below 1/3 Vcc isapplied at its trigger pin. The sensor isconnected between the trigger pin andground.

The circuit can also be turned on oroff, using switch S1. As the sensor de-tects rain, the wiper starts operating.

RAIN-OPERATED WIPERUNNIKRISHNAN

One can make a suitable sensor accord-ing to one’s choice. A model of thesensor is shown in the figure. The dis-tance between the rails is made as small

as possible, so as to have a more com-pact and reliable sensor. The sensorshown here is made from resistorleads or small metal foil pieces.

T his is a timer circuit designed togive five different outputs. The

output can be delayed up to 100 daysby varying the clock frequency. Thecircuit has an advantage that it pro-vides flexibility in the output range. Ifthe five outputs are taken at a timewhen the clock is adjustedfor 0.5 Hz, the outputs aregiven by the time periodsof 10,20,30,40 and 50 days.By adjusting VR1 now, ifthe clock frequency is made0.1 Hz, the output time pe-riods change to 20, 40, 60,80 and 100 days. If onlythe fifth output is taken, thetimer will give an outputafter 100 days.

The circuit makesuse of a 555 clock IC, two12-bit binary counters

(CD4040), a four-input AND gate(CD4082) and a decoder (CD4515).The clock frequency of 555 is adjust-able and can be varied by adjustingVR1 suitably. When the first 12-bit bi-nary counter (IC2) gets a count of 900,the second binary counter (IC3) is

driven by IC2 via IC5, which is a four-input AND circuitry. The count of IC3is 960 when all the outputs from IC3,i.e. Qo to Q3 become high and theoverall count is 864000.

When a clock frequency of 1Hz isgiven, the output time period of the

PROGRAMMABLE 100-DAY TIMERBABU M.G.


Frequency (Hz) Outputs (After days)Output1 Output2 Output3 Output4 Output5

0.5 10 20 30 40 500.1 20 40 60 80 1001 2 4 6 8 102 1 2 3 4 510 1/5 2/5 3/5 4/5 1

fifth output will be ten days. With aclock frequency of 10 Hz, the outputtime period at output terminal 5 be-comes one day. Thus, by changing thevalue of VR1, the clock frequencychanges accordingly, which in turn in-troduces a change in the output, as de-picted in the table.

PHASE SEQUENCE DETECTORCUM SINGLE-PHASE PROTECTOR

HARINDER SINGH

F or three-phase motors, it isessential that all the three phases

are present in a particular sequence.This circuit cuts off the supply to theequipment if any of the phases is ab-

sent, or if the phases are not in a cor-rect sequence.

The problem with most electronicequipment is that the supply frequencyshould be precisely 50 Hz for correct

operation, but in actual practice the sup-ply frequency is anywhere between 47Hz and 53Hz. This circuit is able toaccommodate such changes in the sup-ply frequency.


Rectified voltage from each phaseis stepped down, using voltage divid-ers formed around resistors Rl to R6.This stepped-down voltage is used todrive transistors T1-T3 from phasesB,Y and R, respectively. Schmitt-trig-gered NAND gates N1-N3 shape upthe signals from transistors T1-T3, re-spectively. IC2 is a positive edge-trig-gered, clocked, dual-JK flip-flop. Thesequence is detected, using IC2 andN4.

When the red-phase(R) voltage VRrises just above 146V, the output ofNAND gate N3 goes from low to high,resulting in clearing of both FF1 andFF2 through capacitor Cl.

While VR is still above 146V, theyellow-phase(Y) voltage VY risesabove 146V, resulting in the output ofgate N2 going high, and hence provid-ing a clock pulse to FF1.

Thus, the output of FF1 goes high(as the J input of FF1 is already high atthe time of the clock pulse leading edgearriving at the triggering terminal).Now, when the blue-phase(B) voltageVB goes above 146V, the output ofgate Nl goes high, while the output ofgate N2 is already high, resulting inthe output of FF2 going high.

The above process repeats itselfonce in each cycle. When the Q out-puts of both flip-flops FF1 and FF2 arehigh, i.e. the phase sequence is correct,the output of gate N4 goes low, result-ing in the triggering of IC3, and henceenergising the relay which controls thesupply to the machine.

IC3 is wired as a retriggerablemonoshot timer, whose time period isset by resistor Rt and capacitor Ct. Ifthe monoshot is not retriggered withinthis period, the relay is put off, i.e. the

equipment is disconnected. The timeperiod is given by:

T=1.1 Ct+RtT is set to approximately 25ms, i.e.

the circuit will operate flawlessly atfrequencies as low as 42 Hz. Diode D4protects T5 from the back emf, gener-ated due to the switching of relay RL1.

If any of the phase supply fails, thephase sequence is disturbed, resultingin de-energising of the relay, and thusswitching off of the machine.

The circuit is so small that it canbe fitted even within an existingswitching unit. The circuit costsaround Rs150.

Note: As some of the parts in the cir-cuit contain fatal voltages, special careshould be taken during testing and in-stallation. The circuit should be properlyisolated from all metallic parts of themachine and its switching units/casing.

T his is a circuit of atouch switch which op-

erates by simply touchingthe touch-plates. The relaysare operated sequentially ontouching TP1 to TP7.

The heart of this circuitis a seven-channel Darlin-gton array IC ULN2004.Seven touchplates are pro-vided, one each at the inputof each Darlington stage. Andseven relays are connected atthe outputs.

When we bridge anytouchplate with the mainplate be a finger (connectedto the positive supply), thecorresponding Darlingtontransistor saturates and thecorresponding relay getsenergised. That is, when TP1is bridged to the main plateby a finger, relay RL1 isenergised. On bridging TP2with the main plate, relayRL2 is energised. This pro-cess continues all through the

seven channels.One of the interesting

applications of this circuitis a touch-sensitive tape-re-corder. The tape recorder’sPlay, Fast, Rewind, Stop andEject switches may be me-chanically or electronicallyconnected to the relay con-tacts. So, different relays canbe operated by touching thecorresponding touchplates.

Reverse-biased diodesconnected across the relaysinside IC1 protect the ICfrom back emf, generatedduring the relay coil breaks.

Electrolytic capacitorsconnected across the relayshelp to prevent relaychatterings.

The IC ULN2004 costsaround Rs15.

Note: The spacing be-tween the touchplates to themain plate should be asshort as possible (2mm inprototype).

MULTICHANNEL TOUCH SWITCHPRADEEP G.


H ere is a circuit which providestwo types of switches, i.e. touch-

switch and staircase switch. Mosttouch-switch circuits published in ear-lier issues of Electronics For You havethe disadvantage of having two sepa-rate touchplates for the On/Off opera-tion. The circuit presented here uses asingle touchplate. On touching theplate, the gadget connected throughthe relay will switch on and on nexttouch, the relay will switch it off.Therefore, a single touchplate per-forms both the functions. An electri-cal device can be controlled from anumber of switch locations with thehelp of switches S2, S3 ... Sn.

TWO-IN-ONE SWITCH CIRCUITTARUN IQBAL SINGH

the output on pin 15 is high, it be-comes low on touching the touchplateor by pressing S2,S3 ... Sn and viceversa. This makes the relay activateand deactivate, thus switching on andoff the device connected to the relaycontacts. S1 is an SPDT switch, whichis used here as a selector switch.

Please, note that CD4027 has twoindependent JK flip-flops, and only oneof these two has been used here. If bychance one of these gets damaged, theother one can be used by simply study-ing the pin configuration of CD4027and then suitably changing the pin con-nections.

IC1 has been wired up in mono-stable mode, with the time period givenby 1.1RC. Each touch causes it to per-form one monostable operation. A simi-lar action takes place when any of thepush-to-on type switches, i.e. S2, S3...Sn is pressed.

The output of IC1 from pin 3is connected to the clock input (pin13) of JK master slave flip-flopCD4027. Its truth table is also given.Out of the four conditions, the lastone is used in this circuit. J and K,i.e. pins 10 and 11 of IC2 are madehigh. Now, for every positive-goingclock edge obtained from IC1 (pin 3),the output of CD4027 toggles. So, if

Readers’ Comments:The circuit of ‘Two-in-one Switch’is very interesting. I have assembledit on the PCB designed by my-self. But when I switched on thesupply the relay got energisedwithout my touching the touch-plate or pressing the switches. ThePCB designed is matching the circuit.

Please suggest remedies for pro-per operation of the circuit and the PCB.Also, I used transistor CL100 insteadof SL100. Are these transistors simi-

lar?Sandeep Mathur

Delhi

The author, Tarun Iqbal Singh, replies:I am thankful to Mr Mathur for tak-

ing interest in my circuit.Transistor CL100 is a suitable re-

placement of SL100. The PCB designsent to me is correct in all respects.Please, try to increase the value of R4to 2.2M or even higher, if necessary.

Connect a voltmeter (0-10V) be-

tween pin 3 of IC1 and ground. Bytouching the touch plate or pushingthe push-button switch, the metershould read about 8V or so for threeto ten seconds. If this happens, the ICis good.

Now, connect again the voltmeterbetween pin 15 and ground. On touch-ing the plate or push-button switch, themeter should now read about 9 voltsuntil we touch the plate or press thepush-button switch again.


STEREO AUDIO LEVEL INDICATORUNNIKRISHNAN P.R.

Audio level indicators are usedwith most audio systems and can

be easily incorporated into any deck.There are many audio level circuits pub-lished in earlier issues of ElectronicsFor You. The circuit presented here de-scribes a stereo version of audio indi-cators. The first ten LEDs are used toindicate the right channel and the sec-ond row of LEDs indicates the leftchannel.

The circuit uses a bar graph displaydriver ICs LM3914N, 555, 4017 and4066. The input signal is given to twopresets to set the voltage at a correctlevel. This is then fed to the CMOSswitches inside IC 4066. Only two ofthe four switches are used in the circuit.

The two switches go On/Off accordingto the output levels of the 4017 IC.

IC 555 works as an astable multivi-brator to generate a rectangular waveat a frequency of about 55 Hz. It is fedto the input of IC 4017, CMOS decadecounter. This IC is working in the togglemode, because the Q2 output (pin 4) isconnected to the reset pin.

The output from the CMOS switchis amplified by a single-stage amplifier,

built around transistor Tl. It is then fedto the input of IC4 (3914). The outputsof IC 3914 are connected to 20 LEDs.The anodes of the first ten LEDs areconnected to the collector of transistorT2. The anodes of the other ten LEDsare connected to the collector of transis-

tor T3. Transistors T2 and T3 are alsocontrolled by the outputs of IC 4017.

When the first output of IC 4017 (pin3) is high, the first switch in IC 4066 isenabled. Since transistors T2 and T3 areboth pnp type, therefore transistors T3is cut off when transistor T2 goes intosaturation. The anodes of the first set ofLEDs get the positive supply, so LEDs1 to 10 glow according to the right in-put, since the first switch in IC 4066 is

connected to the right channel.When the output of IC2 (pin 2) is

high, the second switch in 4066 is en-abled. So, the left input is given to theinput of IC 3914. Transistor T2 is cutoff and transistor T3 goes into satura-tion. Thus, the second set of LEDs


glows according to the voltage level.The only adjustment required is

T he circuit shows a keyboard whichcan generate a key-code along with

handshaking signals. The circuit useshardware detection of depressed keyand generation of its code. By generat-

of IC 7430 goes high, whenever anykey is pressed. This triggers one ofthe monostable sections(A) of IC74123, which activates IC 74374latch by enabling it (LE). This latches

ing the strobe signal STB, it can loadthe key-code into the input port of themicrocomputer system.

In this circuit, eight keys of thepushbutton type are used. The output

Readers’ Comments:Regarding the circuit ‘Stereo AudioLevel Indicator’ by Mr UnnikrishnanP.R., the author has multiplexedboth the channels on the same bar-graphdisplay IC LM3914. The approach isexcellent, but the circuit is not eco-nomical. The cost of other ICs (555,4017, 4066) and transistors (BC188 etc)exceeds the cost of IC LM3914.

It would be better to use anotherLM3914 IC for the second channel toavoid wiring over the other ICs andtransistors. Using a separate IC

LM3914 for each channel will also savespace on the PCB.

Pawan Kumar KhatriJodhpur

The author, Mr Uhnikrishnan P.R., re-plies:

ICs 4017, 4066 and 555 cost aroundrupees twelve each and transistorBC188 costs rupees three. So, the totalcost for stereo indication does not ex-ceed rupees forty two. On the otherhand, a single IC 3914 costs about ru-pees fifty five. So, the reader’s fears

are ill-founded.Moreover, the technique of multi-

plexing enables the circuit to handleup to ten channels, which means it cangive ten different outputs. The circuitcan thus be used to indicate:

1. 4- or 8-track stereo signal levels.2. Play back and recording levels

in double cassette deck at the sametime, which is very useful for synchro-nous dubbing.

3. Different channel inputs in a pub-lic address system simultaneously.

KEYBOARD WITH STROBE SIGNALPRATAP HALKARNIKAR

that of the two presets VR1 andVR2.Both the presets are adjusted to a suit-

able level to display the LEDs in eachchannel.


key-code into IC 74374. At the sametime, it triggers the next monostablesection(B) of IC 74123 which givesthe STB signal. This signal loads thevalid data latched into IC 74374 into

input port.This circuit is useful where hand-

shaking signals are necessary for key-board interface. Handshaking signalscan be made compatible by adjusting

the R-C value of the second mono-stable. Active high (STB) or activelow (STB) signal is generated by se-lecting the Q or Q output of themonostable.

P resented here is a hybrid typehigh-fidelity audio output am-

plifier. The design allows it to deliver8W into an 8-ohm load, and 16W intoa 4-ohm load. IC1(a) is a preamplifierwith input impedance of about 100kilohms. This is followed by the vol-ume control VR1. The signal is thenfed to the voltage amplification stage,based on IC1(b). The high slew rate ofthe opamp (9V/μs) is ideally suited tothis application, since the voltage ex-

cursion at the output of the opamp isabout ± l2V at full-drive condition. Thisvoltage drives the current amplifica-tion section, based on a complemen-tary Darlington emitter-follower con-figuration. The coupling between thevoltage and current amplification sec-tions is formed by just four resistors.The final output is taken out through azoebel network for stability. The out-put voltage at full drive is around 8V.The output current must not exceed 2A.

Transistors T2 and T4 are to be mountedon heat sinks. The circuit needs a sym-metrical power supply capable of pro-viding 1.5A, with an 8-ohm output loadand 3A with 4-ohm output load. Thecircuit has been tested at an 8-ohm out-put load. The cost of the circuit is aboutRs 60.

Note: The circuit was tested at EFYLab with two 741 opamps, instead ofNE5532, and was found to be workingwell.

HYBRID AUDIO OUTPUT AMPLIFIERA.S.V. RAVI SHANKAR

utilises this principle.The switch is realised, using the

4053 analogue multiplexer. IC1 is a V-I converter (integrator) that forces ca-

pacitor C to charge at a rate set by I =Vi/R. For proper operation, this cur-rent must always flow out of IC1’ssumming junction; hence, Vi must

Asawtooth wave is generated bycharging a capacitor at a specified

rate and then rapidly discharging itwith a switch. Here is a circuit which

SAWTOOTH WAVE GENERATORC. SELVAM


TABLEC Practical Calculatedin μF frequencyin Hz frequency in Hz

0.1 1710 17060.2 1047 08530.3 0824 05690.4 0708 04270.5 0635 03410.6 0584 02840.7 0546 02430.8 0515 02130.9 0491 01901.0 0471 0170

Note: -Vi = 476.5mV; R = 558.4 (1%); andVT= 5V; C is Laboratory StandardCapacitor

always be negative. Resistor R2 andR3 establish a threshold voltage

VT = Vcc x R3(R2+R3)

During capacitor charge, the com-parator output is in Low state and theswitch is in Off state. The output of theintegrator will be

1 t ViVs = –––– Vidt = –––– t ...1.

RC o RC

When the output of the integratorexceeds the threshold voltage VT, thecomparator output becomes high andthe switch shorts the capacitor C, andhence Vs falls down to 0V.

The comparator is prevented fromresponding immediately to this changebecause of the charge accumulated incapacitor Cl during the comparator’soutput transition from 0V to +7.5V. Thisone-shot action, whose duration TD is

MULTICHANNEL WATERLEVEL MONITOR-CUM-CONTROLLER

AJITESH ARUN AND ANIMESH ARUN

proportional to product R1 x C1, is de-signed to ensure complete discharge ofcapacitor C by the switch. With the com-ponents shown, TD is less than 20μs.After timing out, the comparator outputreturns to low state and the switch be-comes Off again, allowing capacitor Cto resume charging. The cycle, there-fore, repeats itself. The waveforms gen-erated are shown below the circuit.

From the waveforms shown hereand from the equation (1), at time t =T, Vs = VT

Vif = –––––––– ...2.

VT RCThe above circuit was tested in our

laboratory (Measurements & Instru-ments Lab, IIT, Madras) and the obser-vations during the testing of this cir-cuit are as under:Voltage : HIL 2665 DMMmeasured (0.5% accuracy)Waveforms : KIKUSI COM 7101Aand frequency 100 MHz digitalmeasured oscilloscope.

T he circuit differs from standardwater level monitors in that it is

more user-friendly, economical andeasier to construct. Its main featuresare—mains operation, a 64-note ROM-based music generator to provide au-

dio indication of a full tank, auto-offfor the pump and an inverter logic gateto prevent dry running of the motor.

The circuit is configured aroundthree parallel BEL188 pnp transistoramplifiers. The note generator com-

prises the UM66 musical IC and thelogic gate turns off the pump whenwater is not available for pumping tothe upper tank.

Assembling can be easily done ona 5 cm x 5 cm veroboard. The circuit,

~


speaker and transformer can be housedin a plastic remote-bell cabinet.

A normally-closed solidstate relaycan be used in place of the musicalcircuit to turn off the pump automati-cally.

Probe 1 goes to the top of the uppertank, to indicate full water level.

Probe 2 goes to the middle of theupper tank to indicate normal waterlevel.

Probe 3 is kept at the bottom of thelower tank. A chime and turning off ofpump automatically, accompanied bybicolour LED showing yellow, meansupper tank is full.

TABLELED Display Inference DirectorsYellow Water levels normal in Pump may be turned on, if

both tanks. more water is to be pumped up.Green Upper tank short of Turn-on pump.

water; lower tank normal.Red Lower tank has no water; Turn-off pump.

upper tank normal.

Readers’ Comments:I am a new reader of EFY and havefound the circuit ‘Multichannel WaterLevel Monitor-cum-Controller’ byMessrs Ajitesh and Animesh Arun veryuseful. I used two types of UM 66 ICs,with annotation UM66 (TOIL) 8719and UM 66 (TI9S) 911. The first ICworks very well, but the second onegenerates music only once. It does notrepeat the music, unless the power sup-ply is disconnected first and then re-connected across pins 2 and 3. Is itpossible to generate a continuous toneform UM66 (TI9S) 911?

Bimlendu ShekharBihar

The authors, Messrs Ajitesh andAnimesh Arun, reply:

We thank Mr Shekhar for the inter-est shown in our project. In the proto-type we have made, we used UM66 ICavailable in TO-92 package. Appar-ently, the first type of UM66 IC usedby you gives a continuous output,whereas the second one needs to betriggered each time to give a series ofnotes. The relay works in both the cases.

PROGRAMMABLE 100-HOUR TIMERUNNIKRISHNAN P.R

T his is a simple timer, designedwith readily available ICs. It can

be programmed up to 100 hours. Whenit reaches the preset time, the timer

cuts off the load automatically, resetsthe counters and gets ready for the nextoperation. The circuit uses a single 4060oscillator cum binary divider, two 4017

decade counters, one 4011 quad NANDgate and one 7555 CMOS timer IC.

IC 4060 produces a pulse everyhour. This is fed to the input of first


4017 (IC2). The carry output from IC2is given to the input of the next 4017(IC3).

Both the ICs are connected to ro-tary type 10-way switch. The time pe-riod is preset, using these switches S2and S3. S2, is meant for hour positionand S3 for ten-hour position. That is, ifswitch S2 is in the third position (cl), itstands for third hour, and if switch S3is in fourth position (d2), it stands for 4x 10 hours. You can get the time byadding the two, i.e. 40 + 3=43 hours. Itmeans, the load is disconnected when43rd hour is reached.

The common points of switches S2and S3 are connected to the two inputsof NAND gate IC5(a), whose output isgiven to the reset pin 4 of IC4. Pin 7

(discharge) of IC4 is open. So, if IC4 istriggered, its output stays in the on stateas long as it is not reset.

Switch S5 is used to trigger IC4 andswitch S4 is used to reset IC4. The condi-tion of the load is detected by watchingLED21. The output of IC4 is fed to the baseof transistor Tl. The relay is connected atthe collector of transistor Tl.

For each pulse from IC 4060, theoutput of IC2 is changed in a sequen-tial mode. After 10 pulses, IC2 givesa pulse to IC3. Thus, for each set of10 pulses from IC1, output of IC3changes sequentially.

When the (high) outputs of IC2and IC3 reach the preset value set byswitches S2 and S3, both the inputsof NAND gate IC5(a) go high, so that

its output goes to a low state. This re-sets IC4 and the output of IC4 goeslow. This de-energises the relay. At thesame time, the input of NAND gateIC5(b) goes low, so that its output ishigh. This high output resets ICs 1, 2and 3 and helps them to stay in thereset state.

When switch S5 is pressed to startthe timer, the timer’s output goes high,so that the output of second NANDgate IC5(b) goes low. This enables thethree ICs to count. The twenty LEDshelp to detect the counting of the two4017 ICs. The only adjustment neededis to set the value of preset VR1. It isadjusted to get exact one-hour pulsefrom the third pin of IC1.

Readers’ Comments:In ‘Programmable 100-hour Timer’ it isnot possible to set switches S2 and S3

for 1 to 10 hours. By inserting an SPDTswitch, as shown, one may fulfil thisrequirement.

The position of the switch can bechanged as per one’s requirement.

V. Shailaja KumariBangalore

The author, Mr Unnikrishnan P.R.,replies:

The modification suggested in

the letter is not necessary becausethe timer itself can work from 00to 99.

The confusion has arisen per-haps from the example cited in the de-scription, wherein to get 43 hours thecommon pins of switches S2and S3 are to be in contact with thethird and fourth outputs of respectiveICs.


D ual power supply circuit pre-sented here provides 1A regulated

supply from -30V to +30V.The variable resistors VR1 to

VR6, used in the circuit, are dualtype pots. The power supply’s out-put voltage can be increased or de-creased by VR3 and VR4, and moresmoothly and accurately by VR1 andVR2. The variable resistors VR5 and

-30V TO +30V REGULATEDPOWER SUPPLY

SUNIL KUMAR

VR6 are used to limit the value of cur-rent.

This circuit has a short-circuitprotection. If we connect together theoutput terminals of the power supply,its output voltage will be zero, thoughsome voltage drop occurs across resis-tors R5 and R6. Transistors T5 andT6 conduct and a negative currentwill flow towards the cathode of zeners

and a positive current will neutralise it,and no current will reach thebase of transistors Tl and T2. Hence,transistors Tl and T2 remain cut-off.Since emitter terminals of T1 and T2are connected to the bases of transis-tors T3 and T4, these will be cut-offtoo and the supply would be cut-off.Thus, short-circuit protection is en-sured.


T he size and shape of an SCRand a triac being similar, it is

sometimes difficult to distinguish be-tween the two, particularly when thedevice number is not visible. When thedevice is marked properly, it is of courseeasy to consult a data book to distin-guish it. To distinguish an unmarkeddevice, a multimeter or some other sys-

tem can be used, but this is not a straightand easy method. To solve this prob-lem, a simple and reliable system with-out any complicated operation is re-quired. Such a circuit is given here.

X1 is a 6V, 250mA sec. transformer.Resistor Rl is a current limiter for theseven-segment displays. Diodes Dl andD2 are half-wave rectifiers. B, t, G-1and G-2 are the seven-segment displaysthat display the letters B, t and G. G-1

and G-2 will display ‘G’. Displays B,G-1 and G-2 are of common-anodetype, whereas display ‘t’ is of com-mon-cathode type. Switch Sl is used todistinguish the SCR/triac under test.Resistor R2 limits gate triggering cur-rent to SCR/triac. IC socket is providedfor fixing the device under test. SwitchS2 is press type (Bell switch) and tests

the device for its working order.As 230V AC is applied to trans-

former X1, 6V AC gets applied to di-odes Dl and D2 for half-wave rectifi-cation. When switch Sl is at position 2then negative DC voltage is applied toseven segment t, G-1, G-2 and anode/T2 terminal of SCR/triac, respectively.One end of transformer X1 goes to cath-ode/ Tl of IC socket directly.

If an SCR is fixed in the IC socket

and switch S2 is pressed, then none ofthe seven segments operate. Whenswitch S1 is at position 1 and S2 ispressed then B and G-1 displays oper-ate; B display indicates that the deviceunder test may be an SCR/triac. BothG-1 and G-2 displays indicate the de-vice is good and ‘t’ display indicatesthat the device is a triac.

When a triac has been insertedin the socket and switch Sl is atposition 2 and S2 is pressed, then Band G-2 displays operate. When Slis at position 1 and S2 is pressed, t andG-1 displays will operate. In thisway the device can be tested anddistinguished and an indication ofwhether the triac/ SCR is good or badis given by the displays.

For ‘B’ display (common anode

SCR/TRIAC TESTER CUMDISTINGUISHER

B. S. MALIK


type) short pins a,b,c,d,e,f and g. For‘t’ display (common cathode type) shortpins d,e,f and g. For G-1 and G-2 dis-plays (common anode type) short pinsa,c,d,e and f.

Insert a good SCR in the IC socketand switch on the mains supply. Putswitch Sl to position 1. Press switchS2. Displays B and G-1 should oper-ate, showing that the device may be anSCR or a triac, but it is good. Now putswitch Sl to position 2 and press Sl.None of the displays should operate

now. It confirms the device is an SCRand not a triac.

Now, insert a triac in the socket andput switch Sl to position 1. On pressingS2, displays B and G-l light up to indi-cate that the device (SCR/triac) is good.Turn switch Sl to position 2 and pressSl. Display shows t and G-2, i.e. thedevice is a triac and is good.

Without pressing S2, none of thedisplays should operate.

Following points may be noted:(a) No display should operate with-

out pressing switch S2.(b) If displays operate without press-

ing S2, then the SCR/triac is internallyshort.

(c) If the device is open internally,then no display will operate in any con-dition of switches Sl and S2.

(d) All the connections and fittingsshould be firm and proper.

The approximate cost of the circuitis Rs 125.

Readers’ Comments:The circuit for ‘SCR/Triac Tester CumDistinguisher’ is very interesting, buthas become expensivedue to use of the four 7-segment dis-plays.

I propose use of four LEDs, insteadof the displays to make it economical.Green LED could be used to indicategood SCR/Triac in position ‘1’ ofswitch S1. No indication of LEDs inposition ‘2’ of S1 would indicate a goodSCR on test.

Similarly, a red LED could be usedto indicate a good triac in position ‘2’of switch S1. If the LEDs light up with-out pressing S2, then the triac/SCR isinternally short. And if none of theLEDs light up in any position of S1and S2, then the device under test isopen internally.

Jayanta Kumar ChakrabortyJamatra (Distt Dumka)

The author, Mr B.S. Malik, replies:The reader’s suggestion is correct

as far as economy is concerned. Butwhereas the LEDs will indicate the sta-tus through a set of combinations, the7-segment display will show letters B,t and G to present a clear, confusion-free display.

Though my circuit is costlier, itsperformance would prove much betterin the long run. Initially, I had alsodeveloped the circuit with LEDs, butlater I decided to use the 7-segmentdisplay instead. I thank the reader forpointing out this possibility

T he compact add-on describedhere, when used in conjunction

with a multimeter (analogue), will en-able it to measure temperatures in the-50° to +150° centigrade range.

The whole thermometer is basedon the National Semiconductors’ op-amp type LM10, which suitably ampli-fies and regulates the low current pro-duced across the sensor. The sensor, anIC type LM134, produces a small cur-rent of the order of a few microamps at150° centigrade, and this varies lin-early with the change in temperature.Preset VR1 is used to correct linearityof the incremented output.

The final output is fed via points Aand B to a multimeter or milliammeter,switched to the 100μA range.

The circuit needs no on/off switch,

THERMOMETER ADD-ONTO MULTIMETER

AMRIT BIR TIWANA

since its current demand is so low thateven the mercury cell used in elec-

tronic watches (1.2V, button type) willlast fairly long.


LISTENING BUGPRADEEP G.

Using this circuit, one can listen tothe conversation going on in a

room from a remote place. The circuitis highly sensitive. Sound signals arepicked up by a condensermicrophone and con-verted into electrical sig-nals which are then am-plified to a suitable level.The sound is reproducedthrough a loudspeaker.

The circuit mainlyconsists of two amplifiers.One of these is a micro-phone preamplifier andthe other is a mini poweramplifier. The preampli-fier stage is of high-gaintype. It is a two-stage di-rect-coupled complemen-tary-stage, which has again of about 100.

The R8/R5 ratio gives the gain ofthe preamplifier stage.

At the power amplifier stage, ICLM386 has been used. Different typesof LM386 ICs are available. These canbe operated at a supply voltage of 6Vto 12V. The difference between differ-ent versions of LM386 lies in the maxi-mum output power delivered by them,at different maximum supply voltages.

The circuit can be easily assembledon a general-purpose PCB of dimen-

sions 6x4 cms. Volume control and thespeaker are fitted externally.

A shielded two-wire ribbon cable isrecommended to connect the micro-

phone. The wire should be as short aspossible, otherwise AC hum will begenerated. Shielded wire should be usedfor volume control also. A simple 9Vunregulated but well filtered power sup-ply is recommended.

Check all the connections thorou-ghly. If everything is correct, keep theloudspeaker a few metres away andswitch on the power supply. Turn onthe volume control slowly and adjust itto mid position. Sound can be heard

through the loudspeaker, if somebodytalks into the microphone. Log presetVR1 is used to control the input, givento the power amplifier.

Next, place the circuit in an adja-cent room where a conversation is go-ing on. Keep the loudspeaker in yourown room. Two-core ordinary plasticinsulation cable can be used betweenloudspeaker and the amplifier. One canhear the conversation very clearly. Thecircuit can pick up sounds from a dis-tance of several metres. Do not placethe amplifier and the loudspeaker in asingle room or nearby to avoid howl-ing feedback.

TELEPHONE TAPPING/MISUSEINDICATOR

H. RAJEN SINGH

W hen the phone is in idlecondition (on hook), the full

voltage of 48V DC from the exchangeappears across the line. When the phone

is engaged by lifting the hand-set, thevoltage across the line falls down toaround 12V. Thus, under idle condition(48V), VR1 is adjusted to give a posi-

tive biasing voltage to the base of tran-sistor Tl. As transistor Tl conducts, tran-sistor T2 gets into reverse bias and stopsconduction. If the phone is off-hook,


Tl stops conducting due to voltage dropacross the lines and subsequently tran-sistors T2 and T3 start oscillating andbeeping is heard.

The on/off switch S1 may be a pushto on switch. It can be fitted inside thetelephone instrument in conjunctionwith the hook switch, so that unwantedbeeping can be avoided during use ofthe phone and beeping can be heardonly when the phone is tapped, discon-nected or faulty. Thus, this circuit canbe used as an indicator for telephonetapping/misuse by an unauthorised per-son.

Readers’ Comments:I have made a little modification in itand found that it can work as a tele-phone misuse indicator as well as atelephone amplifier. Just a simple 2-way, 1-pole switch is to be connected,as shown, for the purpose.

Sasank BoruaGolaghat (Assam)

� I would like to request the author ofTelephone Indicator circuit to clarify:

1. Whether the +Vcc supply canbe taken from phone line itself?

2. Can an LED be placed with aseries resistor in place of R3, eliminat-ing the other components?

B. ChatterjeeCalcutta

The author, Mr H. Rajen Singh, re-plies:

Mr Sasank’s idea is nice. The mostimportant point that we have to keep inmind is the basic working principle ofthe circuit. Once we know the working

principle, the circuit can be modifiedin many ways for various purposes/indicators.

Regarding Mr Chatterjee’s ques-tions:

1. The +Vccsupply can alsobe taken fromthe phone line it-self through acircuit to controlthe requiredlower voltagelevel. However,this gadget notonly indicates thetapping/misuseof the phone, butalso monitors thehealth of thephone line. So,use of separate power supply is rec-ommended. Moreover, as far as pos-sible, we should not disturb the tele-

phone line voltage for satisfactory op-eration of our phone.

2. Yes, an LED can be placedas suggested, but an audio indi-

cator is more efficient than a visualindicator.

ELECTRONIC CALLING SYSTEMPRAVIN KRISHNARAO RAUT

N/O point to the alarm unit. Similarly,connect the N/C point of relay RL-A tothe triac and N/O point to the displayboard.

Flashing unit is configured aroundthe timer IC 555. VR1 is connectedbetween pins 6 and 7. The output pin 3of this IC is connected to triac BT162

play boards are placed on the door ofthe cabin and the alarm is placed onthe display board.

Relay driver unit comprises a step-down transformer, rectifier diodes, afilter capacitor, a 6V DPDT relay and aswitch. Connect the N/C point of relayRL-B to the flashing unit and

T his calling system is useful todoctors, officers etc for use in

their cabins. The system comprises fourunits—the relay driver unit, the flash-ing unit, the alarm unit and the displayunit.

Switch Sl in this circuit is to beplaced on the user’s table. The dis-


through a diode D7 and a 220-ohmresistor R2. This diode is used to pro-tect the IC from back emf.

When the supply is connected, ca-pacitor C2 begins to charge. As capaci-tor C2 gets charged to a voltage lowerthan 1/3 Vcc, pin 2 of IC1 is triggeredand the output goes high, turning thetriac on. When capacitor C2 getscharged to 2/3 Vcc, pin 6 gets trig-gered and the output goes low, the triacturns off. On-off period of the triac canbe adjusted by VR1.

The alarm unit is built around tran-sistors BEL187 and BC147. An 8-ohmspeaker is used. Inplace of this alarmcircuit, you may use any other musicalsound circuit or COB with the ampli-fier.

There are two display boards in the

last unit. These boards are made up ofLEDs. Each board consists of 130 to175 LEDs connected in series.

Board I indicates the message ‘WAITPLEASE’. Power supply is connected tothis board from the triac through R9, a1.8k, 2W resistor. Board II flashes themessage ‘COME IN PLEASE’. It gets sup-ply from the N/O terminal of relay RL-A through R10, a 1.8k, 2W resistor.The brightness of these LEDs can bechanged by using suitable values forresistors for R10 and R9.

Make two plywood boxes of thesize of the display boards and fixthe boards inside the plywood boxes.Close the front side of these boxeswith transparent glass. Fix the alarmcircuit in another small box. Fix thetwo boards and the alarm unit on the

door of the cabin. Put switch Sl onthe table of the user.

When the supply is connected andS1 is off, the relay is in off state. There-fore, 9V DC appears across the flash-ing unit. Now, the first board startsflashing ‘WAIT PLEASE’. The flashingperiod can be varied through potenti-ometer VR1.

When Switch Sl is pressed, the re-lay is energised and the second boardindicates ‘COME IN PLEASE’, as thisboard gets supply from N/O terminalof relay RL-A. The alarm unit also turnson as it gets supply from the N/O con-tact of relay RL-B.

As the visitor enters the cabin, theuser releases switch Sl, so that the sec-ond board and alarm are cut off andboard I again indicates ‘WAIT PLEASE’.


HEN SOUND EMULATORK.A. SAKTHIDHARAN

T his circuit is capable of prod-ucing a sound which is similar to

that which a hen usually produces whenit is alarmed or after laying an egg.

The first two NOR gates (a and b)of IC1 are wired as a low frequencyoscillator. The frequency, whichshould be below 1 Hz, can bevaried by adjusting preset VR1.The output of this oscillator isapplied to the clock-input pin14 of IC2, which is a decadecounter.

The first three alternate out-puts of IC2 are connected to pin7 of IC3 through preset VR2 andthree diodes D1-D3. The seventhand ninth outputs are given tothe set and reset inputs of a flip-flop, which is formed by the com-bination of two other NOR gates(c and d). When pin 8 of gate ‘c’receives a high pulse, it will re-tain a high voltage at pin 11 ofgate ‘d’, until its pin 13 receivesa high pulse. The audio frequencyproduced by the timer IC 555depends upon the voltage re-

ceived at its pin 7.The sound produced will be like

that of a Morse code, containing threedots followed by a dash (…-). The fre-quency and pitch of the dash should behigher than that of dots. This can beachieved by adjusting presets VR2 and

VR3. To get an exact replica of hen orcock sounds, try different values forcapacitor C3.

The circuit can produce a numberof sound effects, like those of robots,computers, birds, space talk etc afterminor changes.

S everal telephones, when connectedto a single line, are generally con-

nected in parallel. Sometimes, it is nec-essary to know the number of phones,connected to the line at a particularmoment. The circuit described here canperform this function. It does notinterrupt normal telephone operationand draws power from the telephoneline itself.

Telephone lines operate at 40

to 48V DC. A load of about 200-ohmappears across the line when asingle handset is lifted. This voltagesuddenly drops to 9V with one handsetlifted and to about 5V with two hand-sets lifted. The monitor indicates thisstate by glowing two different LEDs.

The basic circuit is a constant cur-rent source (CCS) that remains off atvoltages greater than 35V. At 9V theCCS switches on and at 5V, a single-

transistor switch is activated that lightsup another LED.

When the line voltage is more than35V, transistor Tl switches onthrough base bias from diodes D5 andD6, and switches off the CCS transis-tor T2, so that no LED can glow. Thisis the idle state of monitor when itdraws about 1 mA.

When the supply voltage drops toabout 9V, transistor Tl becomes off and

PHONE LINE MONITORPRADIP KUMAR BOSE


IMPROVED PEST REPELLERPRADEEP G.

T he circuit employs four CMOSICs. IC1 works as a clock pulse

generator for IC2, which is a CMOSshift register. IC3 is 7555, the populartimer wired as astable flip-flop whichoscillates at about 80 kHz frequency.

frequency of 40 kHz. This frequency issufficient to repel pests and at the sametime is inaudible to human beings.

IC4 also drives an ultrasonic trans-mitter-transducer. All presets VR1 toVR4 are of 10k type, but have been setto different values.

The frequency of oscillator is variedby about 10 kHz in four steps. Thecircuit works effectively up to fivemetres.

IC4 is a D-type flip-flop for obtain-ing symmetrical output at an average

lifted, the voltage becomes 5V. Thus,transistor T3 switches off and causesLED1 to glow. This indicates anotherreceiver has been lifted. The currentthrough LED now is 5 to 7 mA.

A bridge rectifier is employed as apolarity guard. The values of resistorsRl, R4 and R5 may need to be ad-justed. As the current through LEDs isvery small, high efficiency LEDsshould be used.

The gadget can be housed in asmall plastic box and may be placednear a telephone terminal. This equip-ment is very useful for PCO andEPABX users.

The circuit can be modified, us-ing high-power load, instead of LEDs.It can also be used as a voltage moni-tor to measure low voltage on line, ifany.

switches on the CCS transistor T2. Buttransistor T3 being on, only LED2glows to indicate that a handset is lifted.

In this state the current supply is about2 to 3 mA.

As soon as the second receiver is


Readers’ Comments:With reference to the circuit idea, someclarifications are needed:

1. Most of the pest repellers avail-able in the market are not very effec-tive against mosquitoes and cock-roaches. I want to know whetherthis repeller is an improved one to repellmosquitoes and cockroacheseffectively?

2. Four diodes are used across thepresets VR2 to VR5, but no specifica-tions are there in the circuit about thetype of the diodes.

used to repel pests like mosquitoes andcockroaches. All diodes in the circuitare 1N4148, presets VR2 to VR5 are10k type.

An ultrasonic transducer is used atthe output. Ordinary paper cone loud-speakers are not recommended.

The frequency range of this trans-ducer should be 30kHz to 50kHz formaximum effectiveness.

A frequency counter may be usedto adjust presets VR2-VR5, otherwiseit can be done by trial and errormethod.

3. The last line about the circuitmentioned “All presets VR1 to VR4are of 10k type, but have been set todifferent values,” whereas in the cir-cuit diagram VR1 is shown to be of1M and VR2 to VR5 are 10k. Again,nothing is mentioned about how to setthe presets.

4. In the circuit, no informationis given about the type of the speakerused.

The author, Pradeep G., replies:My pest repeller can be effectively

LOGIC LEVEL DETECTOR CUMCONTINUITY TESTER

ASHISH AND PRAKASH MEHROTRA

T his is a logic level detectioncircuit, with the facility of conti-

nuity testing.The circuit is built around two

BC148C transistors, used as NOT gates(RTL). When the input at probe ‘A’ islow or probe ‘A’ is disconnected, thedisplay (LT543) shows ‘L’, indicating‘low or zero logic’. But when probe‘A’ is at high logic, the display shows‘H’, indicating ‘high or logic 1’.

The circuit can also be used as acontinuity tester by using probes B andC. When there is a continuity betweenB and C, the display will show ‘C’.

The given circuit uses FND LT543display, but any common-cathode dis-play can be used instead.

FLORA CARETAKERAMRIT BIR TIWANA

Next time you will not have to thinktwice before going out on a holi-

day, just because of your plants, whichmight dry up without your care. Here

is a flora care taker that will take ad-equate care of your plants.


Two sensor probes, about 40 cmeach, are inserted into the soil at asuitable location. The preset VR1 inseries, with the soil resistance betweenthe probes, forms one arm of a poten-tiometer, while resistor R1 in serieswith preset VR2 forms the other arm.The resistance between the probes in-creases as the moisture in the soil de-creases.

When the moisture in the soil islow (below some preset limit set byVR1), the input voltage to gate N1 goeslow and its output goes high.As a result transistor T1 begins to con-duct and activates the spray pump,through the relay. At the same timecapacitor C1 begins to charge.After lapse of a certain amount of timecalled ‘spray time’, determined bypreset VR3, the voltage across capaci-tor C1 reaches the logic high level.This causes the output of gate N2 to golow, cutting off transistor T1 andthe relay. As a consequence, the spray

ner next time. Presets VR1 and VR2are used to set the sensitivity. VR3 isused to set the time for which the cen-trally placed pump sprays water.

pump turns off.A 20 mega-ohm resistor, which is

in parallel to C1, ensures that the cir-cuit is ready to work in the same man-

T he circuit uses a bulb in place ofthe candle-flame, which can be

‘lighted’ with a matchstick and ‘blownout’ like a conventional candle.

An LDR is used for sensing lightfrom a matchstick. When illuminated,the LDR causes the voltage at pin 2 ofIC2(NE555) to go lower than one-thirdVcc. The output voltage from IC2 goeshigh and switches on the bulb.

On blowing at the thermistor, itstemperature falls, though slightly, andthe voltage across it increases by about0.4V.

IC1 (μA741) is wired as a compara-tor. The reference input is obtained froma potential divider circuit, built aroundresistor R2 and preset VR1. Under nor-mal conditions, the output of the com-parator is the lower saturation voltage.Due to the voltage rise across the ther-mistor, the output of the comparatorswitches to the upper saturation volt-age (+Vsat >2Vcc/ 3.) This voltage ap-pears at pin 6 of IC2, causing its output

to go low, and thus switching off thebulb.

It takes one to two seconds for thethermistor to heat up again and the out-put of the comparator to settle to thelower saturation voltage. During thisperiod, the candle cannot be lightedagain.

Mount the bulb, the thermistor andthe LDR on the top of a candle, withthe light-sensitive face of the LDR away

ELECTRONIC CANDLEMANGESH M. KASBEKAR

from the bulb. As the three compo-nents have a common ground terminal,only four wires are to be taken upthrough the hole in the candle.

After switching on the supply, al-low a preheating period of at least 21/

2minutes. Then, adjust the preset suchthat the output of the comparator justreaches the lower saturation voltage.

The circuit does not cost more thanRs35.


T he circuit protects motor pumpsrunning under no-load condition,

i.e. when there is no water in the pump.The working of the circuit is facilitatedby the indication of bulb 1, which lightsup only during presence of water in thepump.

When there is no water in the pump,probes 1 and 2 are open and transistorT1 remains non-conducting. Generally,when the motor is switched on, it takes5 to 6 seconds for water to reach thedelivery pipe end A. During this periodtransistor Tl does not conduct. As wa-ter reaches point A, probes 1 and 2 getclosed and Tl conducts, causing bulb 1to glow.

Glowing of bulb 1 indicates that

MOTOR PROTECTOR AND WATERLEVEL INDICATOR

R. BALAJI

the pump is running on load. But if thebulb does not glow even after 5 or 6seconds, this is an indication that thepump or borewell is dry and the motorshould be switched off under such con-ditions.

In a similar way, bulb 2 glows when

the water in the tank is full. Probes 1and 3 form a closed circuit path as thetank becomes full. Transistor T2 con-ducts and results in the bulb 2 glowing.When bulb 2 glows, the motor is to beswitched off, so as to prevent the over-flow of water.

This Hi-Fi amplifierproduces 360W (180+

180) PMPO into a 4-ohm load.The amplifier is built aroundthe STK 461 chip, which needsa dual supply of +21V at 5Acontinuous rating.

The 42V DC symmetricalsupply is used, so that no elec-trolytic capacitors are neededin the output stage. Use oflarge, thick and fanned heatsinkis recommended for cooling theSTK chip within its permissiblelimit(105OC max.).

The 15-0-15V, 5A supply isrectified by a high-currentbridge rectifier and filtered by

360W HI-FI STEREO POWER AMPLIFIERN.S. HARISHANKAR


suitable capacitors.STK 461 is protected against short

circuit by a 2.5A fuse in series with theloudspeaker. The speakers should be

Readers’ Comments:Please clarify the following doubts re-garding the circuit idea for ‘360W Hi-Fi Stereo Power Amplifier’:

1. The author has recommended50W RMS speakers, whereas the out-put is of the order of 125W RMS (180WPMPO) in each channel!

2. In the beginning of the text, it ismentioned that the amplifier needs a+21V @5A supply, but later the authorstates that a 15V-0V-15V supply @5Acan be used.

Chetan KhannaNew Delhi

of 50W RMS rating for good qualityand long life. The circuit was testedwith the universal 240W (120+ 120)premplifier.

STK459 chip can be used in thesame way to produce 300W PMPO.STK459 needs a DC supply of + 16Vat 5A.

� The author has used very highly rated1μF, 63V capacitors for C2 and C9,while a lower rate 1μF, 25V capacitorswould have sufficed. Similarly, a 100μF,25V rated capacitor would suffice forC11 (in place of 100μF, 63V).

What’s the cost of IC STK461 andwhere from can it be obtained?


The author, Mr N.S. Harishankar,replies:

The output at 1 kHz is 45W RMS.So, 50W RMS speakers are all-right.

The maximum supply recom-mended for STK461 is +21V for 8-ohm load. A 15V-0-15V supply wouldproduce the required +21V DC afterrectification by diode and filterationby capacitor.

Regarding Mr Khatri’s letter, lowervoltage rated capacitors may be usedfor C2, C9 and C11, provided these areof good quality.

IC STK461 may cost anywhere be-tween Rs 300 and 450. For its pin con-figuration, one may refer Sanyo 87/88Semiconductor STK Data and Appli-cation book.

REGULATED POWER SUPPLY WITHSHORT-CIRCUIT PROTECTION

PRADIP KUMAR BOSE

connected to a voltage divider at theoutput.

Thus, in the beginning, the differ-ential voltage between the inverting andnon-inverting pins switches onTl. With a variation in the input

voltage the output of IC1 changes tosuit the base current of transistorT1, which in turn controls the outputvoltage.

The short-circuit protection systemis built around transistor T2. If a large

Aregulated power supply is veryessential for the proper functioning

of a circuit. The regulated power sup-ply described here is an IC based,ripple-free power supply which keepsits output voltage constant even withvariations in the input voltage. It alsoprotects the supply by disconnectingitself when any short circuit occurs inthe output line.

The power supply is configuredaround the versatile op-amp IC LM741.The voltage at its non-inverting inputpin 3 is maintained constant withthe help of zener D2, which determinesthe output voltage. Resistor Rl is usedto limit the current through zener D2.

The output of IC1 at pin 6 is fed tothe base of series regulator transistorTl through resistor R2. The base volt-age of Tl, determines the voltage dropacross the emitter and collector (Vce)of T1, thus resulting in the selection ofthe output voltage.

The inverting pin 2 of IC1 is


current flows through the output, i.e. incase of a short circuit, the voltage dropacross resistor R3 increases sufficientlyto switch on transistor T2. T2 appliesreverse bias to Tl, which in turnswitches off the output supply, and in

this way safe-guards against any shortcircuit.

All the components are readilyavailable in the market. The circuit canbe constructed on an IC veroboard.

The circuit provides flexibility in

the output voltage and maximum shortcircuit current limit by proper selec-tion of zener D2 and resistor R3. Thesecomponents can be changed accordingto the user’s requirement.

Readers’ Comments:Could the author please suggest the useof this circuit for conversion from 24VDC to 12V DC? However, if this is notpossible then please suggest any othercircuit.

If we use 24-48V DC as input inthe given circuit, can we get 12V DCregulated at the output after varyingVR1?

G.D. RajpaliGhaziabad

� I want the author to comment on thefollowing queries:

1. Can I feed maximum of 15volts unregulated DC to obtain vari-able DC voltage ranging 0V through12.5 volts.

2. Are R1 and D1 proper to get theabove required voltage range?

3. I think the output voltage can bevaried from 1.5 volts to 12.6 volts withthe help of VR1.

4. What is the maximum value ofcurrent to be drawn from the output at12.6 volts?

5. Please mention the changes inthe modified circuit diagram.

6. Can we replace SL100 transistorwith 2N3055, if more current (say 500mA) is to be drawn?

P. GhoshHowrah

� In the above circuit, the author usedonly one SL100 transistor to power thecircuit. I think it is better to use twotransistors in Darlington mode to boostthe current.

I want the author to comment.Pawan Kumar Khatri

Jodhpur.

The author, Pradip Kumar Bose, re-plies:

Regarding the queries of Mr Ghosh,the circuit can be used with up to 15-volt input. You can make a variablepower supply by varying voltage at pin3 of IC 741. This can be done in sev-eral ways; however, a zener with a pot.(variable) can be used.

The circuit was basically designed

for short circuit protection. The currentrating depends upon T1 and hence tran-sistor 2N3055 may be used. But it isalso necessary to adjust R3 for shortcircuit protection.

Regarding Mr Rajpali’s queries,the circuit may not work in 24-28VDCrange, but he can try the circuit witha series resistor in supply of IC 741only to control the output. Otherwise,he may try to use a linear regulatorfor 12V regulated output. But a linearregulator will waste much power ifsufficient current is drawn. Here,we can use a 7812(12V) regulatortoo. VR1 is used as dummy load andhence it will affect the output by itsmovement, but to the minimum extent.Keep VR1 at half of its value in thiscase.

I agree with the reader fromJodhpur with regard to connectionof a Darlington pair, instead ofsingle SL100 for higher current con-sumption.

H ere is an electronic way ofgreeting people through an elec-

tronic ‘musical greeting card’, basedon the UM series ICs.

The UM musical series ICs offersa wide range of musical applicationIC chips for use in greeting cards, in-terlude players, intercoms, toys etc. Itrequires a very low power supply,ranging from 1.5Vto 3V, so that asmall button cell can be placed insidethe greeting card as a power supplyto the circuit.

Some of thecommonly usedUM series chipsare UM3166,U M 5 1 6 2 ,UM3561, UM66,etc. The circuitsusing UM3562and UM66 havealready been pub-lished in earlier is-sues of EFY.UM3166 series is

MUSICAL GREETING CARDSIYER MAHESH NAGARAJAN


T he system described here is quiteuseful during summers, since it

automatically controls the speed ofa fan. Speed control is determinedby the outside temperature. Normally,a fan operates at three to fivespeeds. This circuit offers upto fivespeeds for a fan. Besides, it providesa visual indication of the speed ofthe fan.

The electronic temperature con-trolled fan given here is cheap, quiteaccurate and effective. The tempera-ture is easily set to cover a widerange, viz, 10OC to 30°C or 3OC to22°C. This increases the reliability andversatility of the device.

Temperature is converted intovoltage by LM335 which holds itsvoltage at 2.73V at 0°C. As the tem-perature increases, the voltage increases

linearly @ 10mV/°C.This voltage is passed through

a buffer (IC2) and then appliedto the non-inverting inputs ofIC3 through IC7 (CA3140). A refer-ence voltage obtained from resistor R7,the 3.3V zener D3 and the laddercircuit configured around resistorsR8 through R14, is applied tothe inverting inputs of IC3 throughIC7. The ladder resistances shouldhave one per cent tolerance foraccuracy.

In order to reduce the size ofthe system and to have greater reliabil-ity, triacs have been used in place of re-lays.

Triacs 2, 3 and 4 should be con-nected to points 2, 3 and 4 of the regu-lator respectively, which is not shownin the circuit. A 12V supply for the

circuit is obtained throughRl, Dl, D2, Cl and C2.

The voltages at the respective pointsare as shown in Table I. These voltagesshould be measured accurately with adigital multimeter. Voltages at no twopoints should be equal, which meansthat no two LEDs should glow at thesame time.

The 100-ohm resistors (R9 to R13)can be replaced by 100-ohm presets toget more accurate voltages at points Ato E.

TABLE I

Point Voltage

A 2.94VB 2.97VC 3VD 3.03VE 3.06V

TEMPERATURE CONTROLLED FANCHHAJED MAHAVIR D.

Readers’ Comments:I request the author of the very inter-esting ‘Musical Greeting Cards’ circuitidea to answer:

1. What’s the approx. cost of theUM3166 IC and the piezo buzzer?

2. How can I replace the piezobuzzer with and 8-ohm speaker?

3. What use are the pins 5, 6 and 8of the UM3166?

Bimlendu ShekharBihar

The author, Mr Iyer Mahesh Nagar-ajan, replies:

The piezo buffer and the UM3166ICs are available in a small plastic pack-age which is as thick as a card. So, it iseasily hidden in a greeting card. Thepackage costs around Rs 25 in Bombay.

In my original manuscript, I hadalso included a diagram for using theIC as a melody generator/doorbell,with an external amplifier, which was

not published. It is now being repro-duced above.

Pins 5, 6 and 8 of the IC 3166 haveto be grounded.

a complete melody generator chip, ca-pable of directly driving a small piezobuzzer.

The UM series ICs function with in-

built oscillators and memory programs.Since the circuit incorporates a

single IC with a resistor, battery and aleaf switch, no PCB is required. The

leaf switch is fixed inside the greetingcard in such a way that on opening thecard the two terminals of the leaf switchmake contact with each other.


Readers’ Comments:The circuit for ‘TemperatureControlled Fan’ is excellent, butthe author has not taken care ofcost. Instead of the six CA3140op-amp ICs, it is better to use oneLM324 (quad op-amp) and oneLM1458 (dual op-amp). This will re-duce the cost as well as the space re-quired on the PCB, besides reducingthe wiring.

What is the cost of LM335 andwherefrom can it be obtained?


� I want the author to kindly clarifythe following problems concerning theabove article.

1. How can we adjust the tempera-ture range and its setting?

2. Is there any problem if the phaseand neutral wires are interchanged?

3. Specifications for the triac arenot mentioned in the circuit; please

name the triacs which can be used inthe circuit.

Saji K.M.Cochin

� The circuit idea of Temperature Con-trolled Fan is very interesting. The au-thor is requested to clarify the follow-ing:

How can we replace the tempera-ture sensors (LM335) in the circuit? Ifthe sensor is kept near the fan, it willcool down by itself. Thus, the circuitwill not work properly.

The author, Chhajed Mahavir D.,comments:

In temperature controlled devices,accuracy and stability of the circuit ismore important than the size and costof the circuit.

As in my circuit the output ofIC2 increases linearly @ 10mV/oC,the input offset voltage is of moreimportance. The input offset volt-

age for CA3140B (Bi-MOS op-amp) is2mV (max.) while it is 9mVfor LM324 and 7mV for CA 1458,which may bring instability to thecircuit.

LM335 may be obtained from Kits‘n’ Spares, 303 Dohil Chambers, 46Nehru Place, New Delhi 110 019.

IC LM335 is a temperature sensingdevice, which should be kept far awayfrom the fan. It should be placed at aproper place to sense the temperatureof the room.

Regarding Saji’s letter, the temp-erature range of the circuit may bechanged by altering the voltage levelsat points A and E. The given voltagelevels in Table I are the tempera-ture range of 21oC to 33oC. Thephase and neutral wires can be inter-changed.

You can use any 4A, 400V triac asmentioned in the circuit, viz, ST044,BT136, etc.


LINE SYNCHRONISEDOSCILLATOR

V. VIDYALAL, K. RAJASREE AND V. SIVANAND

T his circuit is useful for triggeringcircuits which need a 50Hz square

waveform derived from AC mains.Such sources are being increasinglyused and incorporated inmodern oscilloscopes andwaveform generators.

ICl is a 741op-amp,configured as a comparator.It squares the 50Hz signalreduced by the transformerand resistor network. Off-set voltages of the op-ampcan be trimmed by pot.VR1. The output from thecomparator is fed to a resis-tor - diode network, com-prising resistor R6 and di-ode Dl, which cuts off thenegative part of the signaland ensures only positivetriggering of IC2.

This waveform is fed tothe clock input of decadecounter CD4017B(IC2).The output Q5 at pin 1 ofIC2 is connected to the re-set pin 15, and clock enable pin 13 isgrounded. Q0 output is taken from pin3. This configuration finally results inan undistorted 50Hz square wave.

A monitor output is provided at pin3 of IC2. The signal is buffered and isavailable at the collector of transistorT2 with slight amplification. LED1 in-

dicates presence of the signal and isalso useful for optical fibre coupledapplications and/or for opto–couplercoupled isolated mains circuits.

This circuit, due to its compactness,lower component count, and rugged-ness, can be easily incorporated intoolder model oscilloscopes which do not

have this facility. Moreover, the signalsource is fully isolated from the mainsand can be optically routed.

T his gadget trips the supply in caseof a short circuit or an overload.

The circuit is based on the factthat when a current flows throughresistors Rl and R2, there is a voltagedrop, proportional to the amount

ELECTRONIC FUSEB. JAYAPRAKASH

of current passing.The voltage drop across Rl or R2 is

compared against the reference volt-age set at the non-inverting input of the741 IC, used as comparator. The cur-rent limit can be set by adjusting VR1,

according to one’s requirement.If the current to load exceeds the

predetermined level, the output ofcomparartor (IC1) goes low. At thesame time, transistor T2 is cut off,which in turn cuts off transistor Tl.


Relay RL1 is thus de-energised. Sinceno current flows into the load, the volt-age drop is zero. In this condition, theoutput of IC1 goes high once again,which makes the Ql output of IC2(CD4017) to go high.

As Q1 goes high, LED1 glows.Thus, whenever a current greater than

the predetermined level flows into theload, the relay is de-energised andLED1 glows to indicate this state.

To activate the relay again, pushswitch Sl which would reset IC2,thereby energising the relay if, and onlyif, the current is less than the pre-determined level.

Calibration can be done accordingto one’s current requirement. If a cur-rent limit of 5A, say, is chosen, thevoltage drop across resistors Rl and R2will be 2.5V, which requires the refer-ence voltage to be set at 2.5V, so as tocut off the supply to load, whenevercurrent exceeds 5A.

POLARITY TESTERPRADIP KUMAR BOSE

T his polarity tester can detect thepolarity of a supply voltage of 3

to 30V and indicate the same with anLED. The tester can be used for digi-tal circuits to check the polarity ofthe supply unit.

The circuit is basically a constantcurrent source, which drives an LED.The CCS (constant current source) isconnected to a transistor switch whichturns on when the correct polarity ofsupply is connected.

Diode D2 makes the base voltageof transistor T2 constant, as a result ofwhich the collector current of T2 be-comes a constant current source. Onconnecting correct polarity of input test

voltage, transistor Tl turns on andswitches the CCS to drive LED 1.

If reversepolarity is con-nected, diode Dlconducts. As a re-sult, transistors Tland T2 both switchoff and LED1 doesnot glow. Theswitch Sl is meantfor connectingLED1 to internalor external circuit,as required.

The tester canbe assembled on a

small veroboard, in a small cabinet.


T his is a 4- to 16-line decodercircuit that uses a minimum num-

ber of, cheap and easily available com-ponents. The circuit provides a suit-able replacement for the 4- to 16-linedecoder IC 74154, which is quite ex-pensive.

The circuit comprises four ICs. IC1NE555 is wired in the astable mode.Its frequency can be varied by the 1Mpotentiometer VR1. The clock pulsesfrom IC1 are fed to IC2 SN7493, whichis a 4-bit binary counter.

Two SN7442 ICs (IC3 and IC4) pro-vide the 4 to 16-line decoding func-tion. IC3 decodes the 4-bit inputs from0 (0000) to 7 (0111) and IC4 decodesthe inputs beyond 7, i.e. from 8 to 15.

LOW-COST 4- TO 16-LINE DECODERPRADEEP P. KAVI


Transistor Tl BC548B functions as aninverter (NOT gate) for the ‘D’ inputto IC4.

For inputs beyond 7, i.e. from 8(1000) to 15 (1111), IC4 counts from 0to 7, since the input to IC4 is DCBA.

T he circuit described here is veryuseful in checking the continuity

of low resistance coils, speakers, IFTs,small audio transformers, etc. A spe-cial feature of this continuity tester is alow supply voltage (2-3V) and a lowquiescent current (1.5mA at 3V).

IC1 TLC251 (or TLC271) is an op-amp, wired as a comparator. When anexternal low resistance is connectedacross the probes, output of IC1 goeshigh and IC2 UM66, the popular melodygenerator, gets supply voltage. Musicsignals, generated at the output of IC2,are amplified by transistor Tl, whichdirectly drives a low wattage speaker.

This continuity tester can be used tocheck low resistances only, and not highresistances or semiconductor devices.

T his automatic volume control canbe incorporated into any audio

equipment, using a few discrete com-ponents, as described below.

The circuit is connected with theoutput and volume control of the AFamplifier section of the equipment.When the input signal strength rises,diode Dl will conduct and tend to shortthe upper terminal and ground of thevolume control, hence reducing the out-put.

The reverse happens in case the in-put signal drops. Hence, the output vol-

Hence, for an output of 8 (1000) fromIC2, the input to IC4 will be 0 (0000),and so on.

The truth table for the circuit is alsogiven. At any instant, only one LEDwill glow.

The speed of the running LEDs canbe adjusted by potentiometer VR1. Thewhole circuit can be assembled on ageneral-purpose PCB, at a cost of Rs60 to 70 approx.

AUTOMATIC VOLUME CONTROLAMRIT BIR TIWANA

ume is maintainednearly constant. Acapacitor, rangingfrom 220nF to 4.7μF(16V), may be con-nected across thebase of transistor Tlto attenuate thechanges.

Preset VR1 setsthe sensitivity andlevel of automaticvolume control.

MUSICAL CONTINUITY TESTERPRADEEP G.


T his two-channel stereo mixer usestwo LA3161 ICs and some related

components. These ICs are commonly

used in the preamplifier stage of carstereo amplifiers.

The circuit was originally designed

to satisfy the needs of a classicalmusician for recording on his stereodeck, with at least four microphones,

HI-Q STEREO MIXERK.K. MURTY


ern low-impedance microphones well.Its output is connected to the mic. in-put of the main amplifier or stereo deck.

The power supply required for it isstraightforward, regulated and hum-free

9V DC. Diode-capacitor combinationsof D6-C20 and D7-C21 decouple ICland IC2.

and it performed well.It can be converted into a four-chan-

nel mono mixer by closing switch S1.We can stack more LA3161 ICs to getmore inputs. The circuit suits the mod-

T he circuit presented here is of aninexpensive 1Hz master oscilla-

tor. It uses only a few components, butgives satisfactory results.

The multivibrator, wired aroundgate Nl, produces 10Hz pulses. Theseare divided by 10 to produce 1Hzpulses.

Capacitor Cl is connected betweenthe input of the gate and ground. Theoutput of N1 is fed back to the inputvia potentiometer VR1 and resistor Rl.The capacitor is either charged or dis-charged, depending on the logic levelat the output of Nl.

Whenever voltage across the capaci-tor reaches the threshold trigger

level, the output of schmitt gate Nltoggles. The values of Rl and Cl are sochosen that they produce pulses @l0Hz, which are then divided by IC2 to

get 1Hz output.In order to get accurate output

pulses, the l0Hz generator must be ad-justed to the correct frequency by meansof potentiometer VRl.

1Hz MASTER OSCILLATORVIBIN ANDREWS K.

H ere is a very simple, low-cost anduseful timer circuit. It will auto-

matically turn off power to your TV,VCR, radio, tape recorder or any otherelectronic equipment after a predeter-mined period.

The circuit basically uses only twotransistors as a Darlington pair and a6V relay.

When you push switches Sl andS2 at the same time, capacitor C1charges to about 9V and this voltagegoes through preset VR1 and R2 tothe base of transistor Tl. The relayenergises and its N/C contacts openand the common point gets connected

SUPER SIMPLE TIMERSUPRAVAT DAS


to N/O contacts. Due to base currentdrawn by Tl, capacitor Cl dischargesafter some time and the relay cuts off,connecting the relay’s common point

to the N/C points.This circuit can be housed in a plas-

tic or a metal cabinet. A 3-pin socketmay be fitted on this cabinet for the

‘E’, ‘N’ and ‘L’ points, and the deviceto be controlled may be plugged intothis socket. You can vary the time pe-riod by adjusting the 1M preset VR1.

W hen you are enjoying loudwestern music or other songs on

your stereo, and if a visitor comes andpresses the calling bell switch, you maynot hear the bell due to loud sound from

the stereo deck. The circuit shown hereprovides a remedy for this problem.

The input of this circuit is connectedin parallel to the electric bell of your

STEREO DISCONNECTORWITH TIMER

PRADEEP G.

house. So, when the electric bell is rung,a low DC voltage gets applied to theLED of opto-coupler IC1. As the opto-coupler’s LED glows, the photo-tran-sistor inside IC1 conducts. As a result,

IC2 (555), which is in monostable mode,gets triggered. So, output of 555 goeshigh and the relay gets activated. Thisswitches off the stereo set conn-

ected through N/C contacts of the relayfor a few seconds. The switch-off pe-riod is determined by potentiometerVR1, connected in the 555 timer circuit.

Thus, you can hear the bell clearly

even when your stereo is on. Besides,the red LED connected across the relaylights up to indicate that the timer is‘on’.

Readers’ Comments:The circuit of ‘Stereo Disconnectorwith Timer’ is very interesting and use-ful, but has some drawbacks:

1. The circuit requires contin-uous power supply, while the stereo

system is on.2. The opto-coupler further in-

creases the cost of the circuit.The circuit shown here excludes

opto-coupler, but bears all the featuresat much lower cost. Also, the trans-

former is controlled via relay RL1in such a manner that it connects to themains only when the push-button ispressed, otherwise it remainsdisconnected from the mains. Thus, thetransformer works only for a short time,


which is equal to the on-time period ofthe IC 555. Thus, the misuse of trans-former and power both are avoided.

Generally, double-pole push-to-onswitches are not available; use of twosingle-pole pushbutton switches, con-nected with a common yoke, is recom-mended.

Vidyut Sarkar

The author, Pradeep G., replies:Use of opto-coupler can indeed be

avoided by using a transformer. Con-sumption of electricity is minimisedwith this modified circuit, but the cir-cuit can be modified in many ways.However, I agree with the modifica-tions, done by Mr Sarkar.

W hen old records become dusty,the non-linear movement of sty-

lus over the record produces a variable

and irregular voltage, which producesnoise when amplified. With this filter,you can now play worn-out records,

FREQUENCY FILTER FORPLAYING OLD RECORDS

PRADIP KUMAR BOSE

with much better results. This device isbasically an RC-coupled two-stage am-plifier with several capacitive and re-


sistive controls for frequency selection.A ganged band switch with several fil-ter capacitors is used for selection. Also,a ganged potentiometer is used for thedesired level. Several other networksare used for perfect filtration.

Connect this device between the

preamplifier and the power amplifierof the record player’s circuitry. For astereo player, two units are required.The supply may be drawn from the DCsupply of the record player.

Play an old record and place switchSl in such a position that the noise be-

comes minimum. Now, adjust dual po-tentiometer VR1 and VR2 to get properand desired level of sound. Every timeyou play old records, you will have toadjust VR1/ VR2 and switch Sl.

Keep the connecting wires as shortas possible to reduce humming noise.

Conventional starters used withfluorescent tubelights are often the

cause of much interference and hum.These are also prone to become defec-tive soon, just for the reason that thegas may leak out or the contacts mayburn out.

The circuit presented here can beused as a direct replacement for thesame. It can fit within the same enclo-sure.

The circuit uses two series-con-nected diacs and a diode, all in parallelacross a filter capacitor . These pro-

vide the same action of switching cur-rent as the starter. The diac may beselected for general use, depending on

HEADPHONE ADAPTOR FORHI-FI STEREO

PRADIP KUMAR BOSE

If you wish to listen to your hi-fistereo through the headphone of your

personal stereo (such as a Walkman),here is an adaptor you can use for thepurpose.

The adaptor basically comprises adivider network with a simple delayline. It simulates cross field for stereoeffect and introduces delay for identi-cal stereo sound from both channels.The circuit has been designed for anaverage 8cm difference headphone.

Coils LI and L2 should be closelywound on 5mm dia ferrite cores, with100 turns of 33SWG enamelled copper

STATIC STARTERAMRIT BIR TIWANA

the mains voltage ratings (28V diac for220V AC).


wire in each case.After assembly, connect the adaptor

between your hi-fi stereo amplifer andthe headphones of your personal stereo.Keeping the volume of stereo amplifierlow, adjust VR1 and VR2 slowly till

you hear equal volume in your bothears. The balance control (VR3) shouldbe kept at its mid position.

Sometimes, a better result can beobtained by changing the values of net-work resistor and capacitor. If the sound

in headphones is too high or too low,try changing the values of resistors Rlthrough R4, one by one, till you get thesound of your choice. You may alsoadjust potentiometers VR1 and VR2for the purpose.

LINEAR BULB LIGHT CONTROLLERB.S. MALIK

firing angle of SCR and makes thebulb glow brighter. The oppositehappens if T1 conducts less. In thisway, the light intensity of a 100Wbulb gets varied from minimum to

maximum. So, it can be used as a15W or lower wattage bulb, while go-ing to bed, and a 60W bulb or sowhile reading.

The circuit can be wired on a smallgeneral-purpose PCB or veroboardof about 5.5cms x 4cms and fittedwithin the base of a table lamp or in-side a switchboard in a room. Pot. VR1may be fitted firmly on the PCBand controlled through a small knob. Asmall heatsink should be used for theSCR.

To test the circuit after wiring,switch-on Sl and vary pot. VR1 slowly.The intensity of bulb light shouldvary linearly (and not abruptly) withthe rotation of the pot’s knob. If it doesnot vary gradually, replace capacitorC2 with a lμF, 63V capacitor. Switch

the 100W bulb on for about 15minutes at its maximum intensity, andthen switch it off and disconnect themains supply. Check the temperaturesof T1 and SCR. They should be

at their normal temperatures, and notin an overheated state. Resistors Rl andR2 should not be too hot either. Other-wise, change their values to 10k, 2Weach.

Some of the precautions that needto be taken are:

1. Do not use an airtight box for thecircuit; leave a few holes in the box forair circulation.

2. Do not forget to disconnect thecircuit from mains, while checking thecomponents.

3. The circuit should be fitted awayfrom a radio receiver to avoid interfer-ence in the latter.

4. Use a suitable heatsink with theSCR.

The circuit will cost approx. Rs 90to build.

As light dimmers generally avail-able in the market are non-linear,

users find it inconvenient to use them,as they cause abrupt change in inten-sity. The circuit for linear control oflight is given here, with which a100W bulb may be used to workas 15W, 40W or 60W bulb, asper requirement.

On/off switch Sl for the con-troller is fitted on potentiometerVR1. Resistors Rl and R2, ze-ner D5 and pot. VR1 providethe emitter-bias circuit for UJTTl. Capacitor Cl is a waveshaper. Zener D5 operatesabove 12 volts and regulates thebiasing voltage for transistor Tl.Capacitor C2 smoothes DCvoltage at the emitter of Tl. Re-sistor R2 provides bias to B2terminal and R4 to Bl terminalof Tl. Capacitor C3 smoothesgate triggering voltage of SCR andavoids false triggering.

When S1 is switched on, 230VAC is rectified by the bridge, formedby diodes Dl through D4. Positivevoltage gets applied to one end ofthe load (a 100W bulb), while thenegative goes to cathode of SCRthrough a 1A fuse. DC current, flowingthrough resistors Rl and R2, breaksdown zener D5 to regulate the 12VDC.

Pot. VR1 varies the emitter volt-age of unijunction transistor Tl, whichin turn varies the conduction ofTl. So, voltage drop across resistorR4 varies accordingly. This is appliedto the gate of SCR to control itsangle of firing. Increase in voltageacross resistor R4 increases the


T he commonly used automobilehorns are either electrical or air-

pressure type. While these horns aregenerally unpleasant to hear, the air-pressure type horns in particular, con-

tribute a lot to noise pollution.The pleasant and distinct tone,

generated by the electronic horn de-scribed below, cannot be ignored evenby an absent-minded pedestrian. It alsoabides by the regulations, according tothe author, but it would be safer torecheck with the local authorities be-fore putting it to actual use. The horncan be used with any vehicle, fittedwith a 12-volt or 6-volt battery.

The circuit is built around a CMOSIC CD4001 or CD4011, which con-tains four NOR/NAND gates. Thesegates are wired as two sets of astablemultivibrators. One set works in au-

dio range, while the other oscillatesat about 2 Hz.

Both multivibrators are intercon-nected through resistor Rl, so that thelow-frequency oscillator forces the au-dio oscillator to change its output fre-quency continuously. Resistor R3 andcapacitor C2 determine the low fre-quency, while resistor R2 and capaci-tor Cl decide the audio frequency. Sincethese frequencies are dependent on the

Readers’ Comments:Is it possible to use triac, instead ofSCR by connecting the bulb directly tothe AC mains?


� I have assembled the circuit, but get-ting a fluctuated output. The wattage isbelow 30 watts. Is it possible to usethis circuit as regular for an iron or atubelight?

One more problem occurs when wereplace 2.2μF, 63V capacitor with a1μF, 63V capacitor; the resistor (R1,R2), SCR and UJT get heated up. So,

the idea is not quite reliable!The cost of the circuit is more than

Rs 100 against Rs 90, mentioned in thecircuit.

Rajesh KothariChitardurga, Karnataka

The author, B.S. Malik, replies:I am very thankful to the readers

for their keen interest in my circuit.1. Triac can be used in place of

SCR. But SCR has higher current/volt-age rating, as compared to triac. Also,it is possible that the intensity of thebulb may increase. Triac should be con-

nected in series with the bulb to ACmains, otherwise SCR/triac will showthe same result.

2. Regarding the letter of MrKothari, the circuit can be used as aregulator for iron and tubelight etc, af-ter replacing rectifier diodes and SCRsuitably.

3. Cost of components varies fromtime to time and place to place; so itmay be costlier at your location.

4. Mr Kothari is advised to use ex-act valued components to operate thecircuit successfully.

ELECTRONIC AUTOMOBILE HORNK.A. SAKTHIDHARAN

supply voltage, different values of R2and R3 are required for 6 and 12-voltoperations.

The output of ICl is drawn from pin11 and applied to the input of IC2

through coupling capacitor C3. The au-dio amplifier is a monolithic TDA2002IC which can produce 6-watt output,when working on a car battery. Thegain of this amplifier is fixed at 100(R4/R5). The output is connected to a4-ohm, 10-watt tweeter.

The volume of sound can be ad-justed by preset VR1. Diode Dl, onthe positive line, prevents any dam-age due to reverse polarity connec-


tion. A separate rectifier circuit isgiven with whose help the horn canbe connected to a two-wheeler with-out a battery (with AC dynamo).

All components can be assembledon a general-purpose PCB. Use socketfor IC1. The amplifier IC does not needany heatsink. Use a thin plastic film to

insulate the PCB from the metallic partsof the loudspeaker. A round shapedplastic container can be used as thecabinet for the horn.

CALL BELL FOR THE IMPATIENTK.N. SUNIL KUMAR

S ome people keep ringing thecalling bell impatiently, without

giving adequate time for the concernedperson to respond. Thiscalling bell will deacti-vate for a minute afterit has already been rung.

Two 555 timers,which are built into the556 IC, get activated onpushing switch Sl.While one timer pro-vides the delay of about55 seconds—with resis-tor R3 and capacitor C3deciding the delay pe-riod—the other remainson for about five sec-onds. The 5-second de-lay of the second timerpermits the bell to ringfor upto five seconds,with resistor R2 and ca-pacitor C2 deciding thedelay period.

Power supply forthe IC is provided bythe half-wave rectifiercum stabiliser circuit,comprising rectifier diode Dl, zener di-ode D2, resistor Rl and filter capacitorCl.

The outputs of both the timers arecombined in an AND gate. (Onlyone AND gate of the triple 3-input

74LS11 is put to use.) The 5V supplyfor the TTL gate is provided from thepotential divider, comprising resistors

R6 and R7.Both the timers are operated in

monostable mode. A momentary push

on switch S1 makes the output to gohigh for the first five seconds. Thistriggers the triac and makes poweravailable for the calling bell circuit.

0-10V AMPLIFIERDILIP KUMAR DE

Azero to 10V amplifier is useful inline synchronisation where the

tacho or line speed referene voltagesource is too weak to drive a sync-

hronising machine. It may be used intaping head to synchronise and adjustany lay length of polyester tape wrap-ping on cable. Whenever line speed

increases or decreases, the ang-ular speed of taping head willchange accordingly on account of thisamplifier.


a veroboard. Keeping the centre-tapof 100k preset in the middle, openresistor R8 at point ‘2’ and connectpoint ‘1’ to ground. Set your multim-eter to the 10V DC range and con-nect across point ‘2’ and ground. De-pending on the direction of deflection

of the multimeter’s needle rotate the100k preset (with the help of a screw-driver) to the right or left till the metershows no deflection of its needle. Re-peat the process, this time keeping themultimeter at 2.5V range.

After the above zero-setting hasbeen done, disconnect point ‘1’ from

ground and reconnect resistor R8 topoint ‘2’. The output is available atpoint ‘3’.

For testing in the laboratory, thepotential divider circuit comprising22k preset VR3 and 12k resistor R12,at the extreme right of the circuit, may

be used. Connect VR3’s centre-tap(point ‘4’) to the input (Vin). Varythe VR3 preset from its minimum tomaximum position, which will alsovary the Vin voltage from its mini-mum to maximum. The actual inputto the amplifier at point ‘1’ is in therange of 0 to 100 mV.

Two op-amps are used in the cir-cuit—one as an amplifier and the otheras a scale changer. The offset null con-dition, besides being universal in na-ture, minimises the common modesignal.

The amplifier draws very feeble cur-

rent from the source for a full-scaleoutput voltage. A voltage divider, com-prising a 1k resistor R10 and a 100kresistor R11, is used at the output forshort-circuit protection. The circuit runson a simple capacitor filtered powersupply.

The circuit may be assembled on

A SENSITIVE TOUCH SWITCHSHASHANK WADIBHASME

T his switch will turn on an appli-ance on first touch and switch it

off at the second. The circuit has anoscillator built around IC NE555, whichis followed by a JK master-slave flip-flop (FF) IC CD4027.

On touching the metallic plate, thetrigger terminal of IC NE555 mono-

stable multivibrator performs onemonostable operation. Its pulse width,given by 1.1 x RC, is three to ten sec-onds. The timer’s output is connectedto the clock terminal of JK master slaveFF.

A master-slave JK FF is a combina-tion of two clocked RS latches. The

first latch is called the master, while thesecond is called the slave. The master isclocked by positive edge, but the slaveis negatively clocked. Hence, whenclock is high, the master is active andthe slave is inactive, and vice versawhen the clock is low.

When inputs J and K in the circuit


are high, the JK FF toggles.If Q output was high onthe previous pulse, it be-comes low. Hence,Q output changes tothe complement of laststate, when both J and Kare connected to Vcc leveland positive going clockpulse is fed to the clockterminal. With each fingertouch, a positive pulse of3 to 10 seconds is pro-duced across pin 3 ofNE555 IC, and this is ap-plied to clock pin 3 of ICCD4027.

The output voltage ofIC CD4027 is connected to base termi-nal of transistor BC148 (T2) through10k resistor R4 to energise the relay.The diode is connected in reverse biasmode to protect the relay from backemf.

The circuit also gives visual indica-tion of the touch switch, using a bi-colour LED. Before touching the touch-plate, the LED shows red colour. Ontouching the plate, the LED changes itscolour to green. An npn transistor (Tl) is

used for the purpose. As the voltage isapplied on the base terminal of transis-tor T1 through 10k resistor R6,it goes into saturation, and current flowsthrough the transistor (collector toemitter), instead of the LED. Thischanges the colour of the LED to green,indicating the operation of the touch-switch.

Some of the precautions that needto be taken are:

1. The metal used as a touch plate

should be a good conductor, so thatthe plate’s size can be limited to 15mmin dia.

2. The wire used to connect thetouch-plate and pin 2 of IC NE555should be as thin as possible.

3. A suitable capacitor may be usedto avoid sparking in relay. Alternatively,a triac may be used, instead of the re-lay.

4. The earthing of the ciruit shouldbe proper.

REMOTE TV TESTERSUPRAVAT DAS

the circuit and keep it at a distanceof three metres. Set the TV (B&W)on channels 5, 11 or 12 for betterreception of horizontal line patternand audio tone. If the reception

is poor, change the direction ofantenna towards the TV set, and yourremote TV tester starts workingperfectly.

I t’s a very useful circuit for all TVtechnicians and service engineers.

The circuit can be assembled on apiece of veroboard. Basically, it is ahi-fi VHF oscillator, which oscillateswith the help of transistor BF194Band some other parts. A horizontal linepattern and audio tone is produced onthe TV screen, which makes the re-pairing of VIF stage faults quite easier.The circuit works on a 3V battery.

By changing the value of 4.7μF ca-pacitor C2 and the number of turns ofL2, the number of horizontal lines canbe changed.

The tester can work up to fivemetres distance. To get a wider range,the length of the telescopic antenna hasto be increased.

For testing purposes, assemble


T he intercom circuit described hereis simple, low-cost and cordless.

It operates on AC mains. The co-mmunication between two units is car-ried out through the supply lines.

As we know, in single phase ACsupply, the potential between neutraland earth is zero under ideal condi-tions. But if the insulation resistance

between phase, neutral and earth is in-sufficient, due to poor and old wiring,a potential difference may be foundbetween the neutral and earth lines. Thesafe difference is 1 volt to 2 volts. Ifthe voltage difference is more, the in-tercom may get damaged; use of a sepa-rate wire is recommended in that situa-tion.

The signal is carried through theneutral and earth wires. In case, a wrongconnection is made while servicing, thehigh line voltage can prove very harm-ful for the intercom. It would burn allcomponents of the set. So, an auto-shut-off system is provided in the setwhich disconnects the communicationline when such a mistake occurs.

pH METERNAVEEN SHARMA

K nowledge of pH value of a givensolution is often needed in dye-

ing and pharmaceutical industries, etc.The term ‘pH’ is a measure of the de-

gree of acidity or alkalinity in a solu-tion.

The circuit has two op-amp stagesin which IC1 is used as a voltage fol-

lower, while IC2 is configured as aninverting amplifier, with a gain of 10.

Pot. VR2 is an offset cancellationcontrol. The offset control is adjusted

to set output to 7volts when the inputvoltage is zero.

VR1 is a multiturn pot. It isused for calibration of the pH meter.

For calibration, the pH electrodesare dipped in a fluid of known pH value,and VR2 is adjusted till a correct read-ing is obtained on a DVM/DMM. The

power supply for the circuit is straight-forward. It uses regulators of type 7812and 7912 in standard configuration.

CORDLESS INTERCOMPRADIP KUMAR BOSE


According to the normal domesticwiring regulations, a 3-pin top’s righthand pin facing the switchboard is con-nected to the live wire and the left handpin is connected to neutral wire. Thebig remaining pin gets connected toearth wire. Under normal conditions,when all connections are according tothe rules, relays RL1 and RL2 areenergised, while RL3 is de-energised.This is indicated by glowing of LEDl

and LED2.The wiring is such that only in this

condition the circuit is complete forconversation. Any irregularity in con-nections switches off the line. So, incase of a wrong connection, thecommunication line disconnects itselffrom the fatal voltage. Two capacitorsare provided for preventing slow dis-charge in this condition.

But a momentary high voltage may

also damage the equipment, if it isswitched on with a wrong connection.This is prevented by switch Sl whichshould be kept in ‘on’ position afterconnection of the relays. This ar-rangement prevents the swing of volt-age that may damage the circuit. Be-sides, an alarm rings to indicate pres-ence of high voltage in the communica-tion line. The switches Sl and S2 areganged together, enabling the alarm,


type of interchange may be possible.2. The circuit will not respond if N

and E terminals (from supply line) areinterchanged.

3. Possibilities are there that the di-odes may get burnt off.

4. The circuit may work with poorwiring also, but it is recommended toconnect the intercom with good wir-ing, having very low line leakage.

5. The circuit does not completelyisolate the intercom from supply, whichis necessary for safety. My circuit costsa little more, but is sufficiently safe touse with the intercom.

when high voltageis present. Using discrete components,the mains 230V AC is dropped to6V to power the circuit.

It is, however, dangerous to touchany part of the intercom in working con-dition. IC CA810 and two transistorsform an audio amplifier. However, onemay use any audio amplifier circuit forbetter performance. The supply for am-plifier may be taken from a good ripple-free and filtered power supply unit.

The microphone should be kept

away from the speaker to avoid howl-ing sound due to feedback during use.A separate switch for feedback path isprovided to call on the other set. Thespeaker should be placed in front ofthe cabinet.

The optional alarm circuit is builtaround IC 741, which is wired as asquare wave generator. The feedbackresistor determines the frequency ofsound. Output available at pin 6 of IC741 is amplified by push-pull transis-tors T1 and T2 and fed to the speaker

via capacitor C20. As this circuit alsouses the 230V AC to 6V DC convert-ers, it is dangerous to touch any of itsparts while working.

There should be no sound from thealarm on switching on the set. If thereis a sound, check the line polarity. Plugin the two units in the same neutral andearth wires.

It has been found that the range ofthis intercom is 50 to 70 metres, with amedium volume. Each unit may costaround Rs 300 to make.

Readers’ Comments:The transmission through mains wir-ing is not new for EFY readers. Anarticle by Amrit Bir Tiwana on thissubject has already been published inJan.’92 issue. The only new thing inthis circuit is protection against anywrong connection.

Here is a circuit which protects theintercom against any wrong connec-tion and also maintains potential dif-ference between neutral and earthwithin a required level. Here the voltage drop for one diode = 0.6V, twodiodes = 1.5V, three diodes = 3V, whenconnected in series.

If in the circuit, the phase andearth wires are interchanged (bymistake) a short circuit occurs andthe fuse blows off. During blow-offthe circuit will not be damagedbecause the maximum potentialdifference between neutral and earthis 3V (can be decreased to 0.6V byusing one diode and 1.5V by using twodiodes in series between N and E).The diodes between N and E will notaffect the audio frequency passing

through them. These diodes also pro-tect the circuit if the potential differ-ence between N and E increases andmaintains it at 3V.

C.H. Swa Raghu RamGuntur

The author, Pradip Kumar Bose,replies:

Regarding the letter of Mr RaghuRam from Guntur, I would like to pointout:

1. In his circuit it is assumed thatonly phase and earth may be inter-changed by mistake. In practice, any

A SIMPLE BATTERY CHARGERSUPRAVAT DAS

T he circuit presented here cancharge two pencil type nickel-cad-

mium or carbon-zinc dry battery cellswith a single charger. In this charger a

6V-0-6V, 300mA step-down trans-former provides 12V unregulated powersupply. Regulator transistor Tl is usedto produce 4V regulated supply to

charge the cells. The charging currentof this circuit is not more than 50 mA,while the charging time is approxi-mately 20 hours.


The circuit may be assembled eas-ily on a piece of veroboard and en-closed in a small battery eliminator box.A small heatsink is needed for transis-tor Tl.

Before charging a cell it is neces-sary to measure the charging current.If it is more than 50mA then the valueof resistor R2 should be increased ac-cordingly. A dry battery below 1V maynot be charged with the help of thischarger. Overcharging of cells shouldbe avoided.

TEMPERATURE SENSORFOR POWER AMPLIFIERS

V. VIDYALAL, K. RAJASREE AND V. SIVANAND

A part from MOSFET-driven outputstages, the danger of thermal run-

way also exists in semiconductor basedpower amplifiers. This danger mani-fests itself when the current through asemiconductor creates so much heat asto raise its temperature above a criticallevel. Since the semi-conductor has anegative temperature coefficient of re-sistance, the increase in current leads tothe increase of temperature and finallyto the destruction of the device. Thiseffect can be avoided by incorporatingdevices with positive temperature coef-ficients, such as thermistors, or by suit-able feedback circuits.

However, a need often arises for anearly warning (audio/visual) to preventsuch a disaster.

The circuit described here providesan audio as well as a visual indicationwhen the temperature increases beyonda certain limit.

IC LM311 is configured as a com-parator. Its reference level is set bypreset VR1. The other two arms of thebridge are resistors Rl and R2 whereR2 is the ‘sense’ resistor. A 0.5-wattcarbon resistor is used, instead ofsemiconductors, for the sake of higherreliability and ruggedness. This alsomakes the unit inexpensive. A 5.1V ze-

ner diode (Dl) provides the constantvoltage for the bridge.

An inexpensive and easily avail-able comparator (LM311) is chosen forrapid switching. When the sensor re-sistor R2 gets hot (due to its proximityto the semiconductor) its resistance de-creases. This trips the comparator dueto the difference in voltage settings atthe inputs and its output goes high.This is visually displayed by LED1 andan audible note from the buzzer.

IC LM311 can drive loads requir-ing currents up to 50 mA (max.),whereas in case of LM741 only 25 mA(max.) is possible. Therefore, relays

Readers’ Comments:With this circuit, even below 1V re-chargeable batteries can be chargednicely. Could the author please suggestthe use of this circuit as an AC adaptor,so that it can perform as a dual-pur-pose circuit?

T. RameshBangalore

The author, Supravat Das, replies:I agree with Mr Ramesh that this

circuit can also be used as an AC adap-

To perform such a function, somemodifications are necessary. The cir-cuit given alongside can be used as anAC adaptor.

With the help of this battery charger,Ni-Cd batteries below 1V can becharged very well. But in case of car-bon-zinc dry battery, it may not chargethe batteries well.

tor for radio receivers or other elec-tronic gadgets.


TELEPHONE MONITORRAJAN GUPTA

W ith this circuit connected to thetelephone line, it will be pos-

sible to detect whether any unauth-orised telephone set is connected toyour telephone line.

The heart of this circuit is a quadcomparator IC (LM339). The DC volt-age level on a telephone line drops from48V to 12V when a telephone is in use,and it further drops to 8 volts whentwo telephone sets are connected inparallel. IC1 detects this voltage level.LED1 lights up when one telephonereceiver is picked up. LED2 also lightsup when another telephone is pickedup.

To adjust VR1 and VR2, pick upone telephone set and adjust VR1 untilLED 1 glows. Now, hold the firstset and pick up the second set and ad-just preset VR2 until LED2 glows. Usea 9V transistor battery to power thecircuit.

adjustment can also be done by fixingit to the actual heatsink whose tem-perature is to be monitored and preset-ting pot. VR1, so that the buzzer soundsand the LED lights up when tempera-ture reaches the danger point.

mometer. Adjust pot. VR1 for thebuzzer to sound and the LED to lightup. (This temperature should be equiva-lent to that from the heatsink on whichthe semiconductor is fixed.) Air coolthe sensor and lap it to the heatsinkwhose temperature is to be monitored.This is only a crude method and an

drawing less than 50 mA can be di-rectly connected to the output. This re-quires costly relays, and an alternativeis to use a transistor to drive the relay.

Setting this up is fairly simple. Keepthe sensor resistor (R2) in close proxim-ity to some hot material whose tem-perature can be monitored by a ther-


A BINARY FREQUENCY METERNARESH TYAGI

connected in cascade form. Their out-puts are connected to 24 red LEDs,which display the unknown input fre-quency in binary form.

The binary counter typically countsin 1,2,4,8,16... sequence, i.e. 2n. So,in this case, the 24-stage binary

counter can count as much as 224, i.e.up to 16.7 MHz.

IC 555 generates 1 Hz pulse,which is fed to the decade counter IC4017. The output Q0 from IC 4017 isfed to a NAND gate of IC 4011 and

Q9 is used to reset the counter ICs(IC1 and IC2).

Transistor Tl prevents the clock in-put of the counter from accepting toohigh values.

Now, for instance, if the readingof your binary frequency meter is

10001001, the frequency in decimal isgiven by 27+23+l=l28+8+1=137 Hz.

A regulated power supply shouldbe used with this circuit. The level ofthe input signal at buffer transistor Tlshould not be less than 2V rms.

For an average electronics hobb-yist, it is not always possible to

buy a frequency meter because it ispretty expensive. Here is an alterna-tive with one difference that this meterdisplays frequency in binary form,rather than decimal. But that is pre-

cisely what makes it so cost-effective.However, with a little practice, it isnot so difficult to understand the bi-nary readings.

At the heart of this circuit are two12-stage binary counter ICs (CD4040),

Readers’ Comments:‘The circuit of Binary FrequencyMeter’ is very useful and interesting,but it has some limitations. Forinstance, it cannot be used for mea-surement of analogue frequenciesof such waveforms as sinusoidal, trian-

gular or sawtooth.The impedance at the input of tran-

sistor T1 is very low, producing load-ing effect on the circuit under mea-surement, so further amplification ofthe signal is needed.

Use of one NAND gate (CD4011)

with IC 555 is uneconomical. It is bet-ter to use a NAND gate multivibrator.

These disadvantages can be over-come completely by using the follow-ing modified circuit.

For measuring analogue frequency,the circuit using a schmitt trigger (IC


CD4093) is described here.Also, we can use IC CD4011, in-

stead of IC 555 for generating 1Hzpulse, since the oscillator wired withNAND gates is very stable.

D.S. VidyasagarAkola

� The circuit idea ‘A Binary FrequencyMeter’ is very good, but instead of thesimple LEDs can we use 7-segmentdisplays? If yes, what will be the re-quired modifications?

Atal V. KulkarniBeed

� Can I use this circuit for countingsine wave frequencies?

Can I increase the power supply to9 or 12 volts, as CMOS ICs are used inthe circuit?

Can I replace VR1 with a fixed re-sistor, using the given power supply?What should be the exact value of the

fixed resistor?Sudarshan Hegde

Bangalore

The author, NareshTyagi, replies:

We can use this cir-cuit for counting sinu-soidal frequencies, pro-vided their amplitude >2V rms.

We can use 9 or 12volts power supply, butwe will have to changethe current limiting resistor R5 to 120-ohm, 1 watt.

Theoretically, we can calculate thevalue of resistance with the help of theformula:

f = 1.44(R1 + 2R2) C1

But in practice, it did not work when I

tried because of the electrolytic capaci-tor.

Tantalum capacitors are easily avail-able. I designed the circuit, keepingthe cost in mind. But a 7-segment dis-play may be used after modifying thecircuit. For this, please refer to suchcircuits, published earlier in EFY.

T his circuit is an improved versionof the ‘Auto-Cut’ circuit used nor-

mally for protecting the home appli-ances against very high and very lowvoltages. Its advantages are that its in-ternal components are also safeguardedand it uses the more reliable andcheaper silicon transistors.

In the circuit, RL1 and RL2 aredouble-contact relays. S1 is a switchfor the panel meter. Switch S2 is meantfor resetting the system.

X1 is a step-down, centre-tappedmains transformer. Diodes Dl and D2form a full-wave rectifier. CapacitorCl is for smoothing the rectified volt-age. Transistor Tl is for high voltagetripping. Diodes D3 and D4 avoid backemf effect in relay coils. Capacitors C2and C3 avoid chattering effect in re-lays and provide a delay time for theirre-energising states.

Resistors Rl and R4 provide emit-ter bias to transistors Tl and T2 respec-tively and limit current flow for safety.

CARETAKER FORHOME APPLIANCES

B.S. MALIK


Presets VR1 and VR2, which supplybase currents to the transistors, canbe adjusted as per requirements.Resistor R2, zener D5 and resistorR3 provide base bias for Tl. Similarly,resistor R5, zener D6 and resistorR6 provide base bias for transistor T2.Red and green neon are used as indica-tors.

The functioning of this circuit cen-tres around the two relays. If line volt-age goes below 170V, relay RL2 de-energises. Due to this, its RL2(a) con-tacts open and disconnect the load,whereas the RL2(b) contacts open anddisconnect the line voltage from theprimary of transformer X1. So, the con-trol circuitry as well as the load are

safe. The red neon glows to indicatethe out-of-range voltage.

If line voltage rises above 250V,relay RL1 energises and RLl(a)’s cen-tre contact shift to disconnect the load.The centre contact of RL1(b) opensline voltage path to X1’s primary. So,the circuit and the load are once againsafe. The red neon glows again. IfRL2’s de-energising time (due to ca-pacitor C3) is greater than that of RL1(due to capacitor C2), then the relaysmay chatter. To avoid this, the value ofC2 is kept larger than that of C3.

To test the circuit, take a variableAC voltage source and connect it tothe input. Connect an AC voltmeter tothe output of the circuit. Apply 230V

AC to the input and press switch S2momentarily. Relay RL2 should oper-ate. If it does not, adjust preset VR2.Also, ensure through the use of presetVR1 that the relay RL1 remains unaf-fected. The voltmeter connected at theoutput should indicate 230V and thegreen neon should glow.

Now, test the circuit for out-of-rangeoperation by varying the voltage toslightly less 170V and then slightlymore than 250V, respectively. Do pressS2 each time. In both cases, the redneon should glow and the output of thecircuit should be zero.

If relays chatter, try using a capaci-tor of 22μF, 25V for C2, instead of10μF, 25V.

Readers’ Comments:I was really glad to see the articleon ‘Caretaker for Home Appliances’.But it really misled me. It was onlya primary cut-off arrangement with tworelays. The very same purpose can beachieved, using only one relay.

With one SPDT relay (RL2), onemay just change the connections as fol-lows:

AC input L(phase)goes to RL2(b)common contact. Contact N/O (nor-mally open) goes to appliance. In be-tween these contacts, connect S2 andthe green neon. Between common con-tacts of RL2(b) and N/C (normallyclosed) contact, connect the red neon.Avoid RL1 and allied components. Letthe neutral stay as it is.

Connect collector of T1 to base

of T2. Give supply, press S2 and ob-serve. You can see that RL2 keepsenergised in between the predeterminedvoltages of 170V and 250V AC.Beyond these limits, the relaystays de-energised and the primary ofthe transformer stays cut off till S2 is

pressed again.Another circuit captioned ‘Auto

Protection for Refrigerator’, which waspublished in July ’93 issue, also usestwo relays. I suggest that an auto-gad-get, which is designed to control theline voltage to a motor, should have asuitable delay.

Narain P. MenokeeCalicut

� The circuit for ‘Caretaker forHome appliances’ is very good, butit is expensive due to use of two

relays, whereas one relay is sufficientfor the same work. A simple andeconomical circuit is given in the fig-ure below.

I have used only one DPDT relaysuccessfully. High voltage settingis done by VR1 and low voltage

setting by VR2.When a low voltage occurs, zener

D3 will not break down and transistorT2 will be off, relay is de-energisedand the gadget is ‘off’. For a high volt-age condition, zener D1 will breakdown, transistor T1 will be ‘on’ andrelay again gets de-energised. So, thegadget is isolated from the line volt-age. The circuit controls gadgets from175V to 230V AC.

Jayanta Kumar ChakrabortyDumka


The author, B.S. Malik, replies:Mr Menokee has suggested the use

of a single relay after some modifica-tions in the article. And the same ideawas also suggested by Mr Chakraborty.I have not tried their ideas, as I have

been using my circuit since last eightyears without any complaint.

The ideas of these readers seemsomewhat convincing theoretically, butpractice may reveal the truth. How-ever, Mr Menokee should have sent his

circuit with the letter for a clear under-standing. Also, he has suggested a suit-able delay for controlling theline voltage to a motor of the fridge.This is done already in the publishedarticle.

40W+40W AUTO-REVERSESTEREO CASSETTE PLAYER

MOHIT MISHRA

T his circuit will reverse the direc-tion of movement of the audio cas-

sette tape automatically after the sidehas been fully played. This permits acontinuous play of music without hav-ing to reverse the tape manually. Ofcourse, an auto-reverse mechanismwould be required.

The circuit comprises three stages—preamplifier, auto reverse and poweramplifier. The preamplifier is builtaround IC LA3161. It can also be builtaround ICs UPC1032, UPC1186H orM5152L.

The frequency response of thisstage is quite good. The auto-reverse

stage is built around IC UPC6304,which is a single-in-line IC. An npntransistor C2270 is also used in thisstage. The power amplifier is builtaround two UPC 1230H ICs, whichat 12V give an output of 40W at 4-ohm load. Pin 9 of both the ICs andpin 4 of the preamp IC should be con-


nected to the supply, i.e. point A. Acurrent of 2 to 4 amperes is giventhrough the supply, which is regulated

by the choke coil. Care should betaken of the pin configuration of ICs,since these are single-in-line ICs. For

proper reception, a good auto reversestereo head (glass ferrite) should beused.

T he circuit described here enablesone to listen to sound from a TV

set without disturbing others. It uses atransmitter and a receiver. The trans-mitter is of AM type, working in MWrange, which canbe kept near theTV set. Output ofthe audio ampli-fier (headphonesocket) can beconnected to thetransmitter.

Power supplyto the transmittercan be derivedfrom the TV set it-self. A separateDC supply canalso be used, if de-sired. The use ofa battery givesperfectly distor-tion-free output.

The receiver is based on 2Z414,a popular RF amplifier-cum-detector

REMOTE TV HEADPHONESPRADEEP G.

IC, meant for pocket radios. It isa compact receiver with 1.5-voltcell, housed inside a match-boxsized plastic cabinet. It has a holefor the headphone socket.

As the transmitter uses an iron coreoscillator coil, the harmonics will never

go to VHF range and so no disturbancewill be introduced in theTV picture or sound.

In case, you start hearing aprogramme from a radio station instead,

the frequency of operation of the cir-cuit can be changed.

Readers’ Comments:The circuit of ‘Remote TV Headphones’was very interesting. But I have somedoubts about assembling this circuit.

1. Please give the correct configu-ration of IC ZN414.

2. How can I get the complete kitor PCB? Please, mention its cost, etc?

3. What is the maximum rangeof operation between the transmitterand the receiver?

A.G. Poiya MozhiMadras

� The circuit is very interesting. Wecan listen to the TV, using this head-phone.

IC ZN414 is not available in the

market. However, YS414 is availablewhich costs about Rs 35.

I request the author to give internalstructure of ZN414 or YS414.

While using YS414, pins 1 and3 should be interchanged. In the re-ceiver circuit, one end of the MW an-tenna coil was connected to negativeterminal of the battery. This connec-tion should be disconnected for properfunctioning of the receiver.

What is the function of audio out-put transformer in the transmitter? Ifwe connect the secondary of the trans-former in parallel to the TV speaker,then the sound from the speaker de-creases. How can we avoid the use of

this transformer?MVS Rao

Hyderabad� This circuit is very useful forme but how much does the ZN414 ICcost?

I used BF194, instead of SL100.What is the rating of output trans-former—3 or 6 volt? What is the rangeof transmitter?

A.N. BabbuRajahmundry

� The circuit diagrams of the transmit-ter and receiver have some mistakes. Inthe circuit for L1, you have marked ‘SeeText’. But in the text, there is no men-tion of L1 and its description.


AllanMysore

� Please clarify the following queries:1. What is the value of L1 and

from which point the coil should betapped? Please, specify its completeconstruction method.

2. Specify the type of audio outputtransformer, used in the circuit.

3. What is the maximum outputpower of the receiver and the imped-ance of the headphones?

4. Specify the range of the system.M. Kumar

Hardwar� I have made the circuit, but it failed.

I used the transmitter circuitas usual, but replaced the receiverwith a MW pocket radio set. Ialso powered the transmitter separatelythrough a battery.

While operating, I found noisein the TV. However, on adjustment, thenoise disappeared.

On switching the receiver on, Igot a shrill sound. I selected a silentzone in MW band and the soundreduced, but I found that on placingthe receiver near (within 3 metres)TV set the sound increased. How-ever, I was able to receive the audiosignal, but it was backed by some noiseproduced by TV set. The backgroundnoise was very irritating. Also, thesound quality of transmitter was verypoor.

A reader from Calcutta

The author, Pradeep G., replies:I am thankful to all who have taken

keen interest in my circuit idea.L1 is made of 90 turns with 36SWG

copper wire over ferrite rod. The coilis tapped at 75th turn.

Capacitor C4 (0.01μ) should be con-nected between MW antenna coil andground. It should not be connected di-rectly to the ground.

IC YS414 can be used, insteadof ZN414. Pin configuration andinternal structure of IC ZN414 havebeen published on page 104 of Feb ’94issue.

Modulator transformer is a general-purpose audio output transformer. Cen-tre-tap of primary windings is not used.Transformer is 6-volt type.

The circuit can be operated up to 10metres, without any wire connection to10 metres and without any wire connec-tion to the TV set. Instead of TV set, atape recorder can also be used.

IC ZN414 can be obtained fromVisha Electronics, Bombay for Rs 30.

Audio transformer is pushpull type,which is serially used in audio receiverand ‘bird’ bells.

Output power of the receiver isa few milli watts. Any type ofheadphones or small speaker can beused.

INFRARED REMOTE CONTROL TESTERPRADIP KUMAR BOSE

T he circuit described here isbasically of an infrared light con-

trolled electronic switch, which can alsobe used to control a remote gadget.This switch will help detect faults andcheck the working of a remote controltransmitter.

Transistor T1 works as an electronicswitch, which makes the LED glowwhen any infrared signal is detectedthrough the sensor IR-LED. Diodes Dland D2 work as rectifiers to switch onthe LED-driver circuit. Capacitor Cl isused to produce a constant base biaswhen an infrared signal is detected evenfor a very short period.

Transistors T2 and T3 constitute aninfrared signal amplifier. The gain ofthis pair is suitable for detection andamplification of medium and strong in-frared signals. The interconnection be-tween the two stages is made by RCcoupling. Capacitor C4 is used tocouple the amplifier with the detectingdevice.

The main infrared detector in the

circuit is a silicon photodiode D3. It isused in reverse bias with a series resis-tance. The reverse current of this diodevaries with the infrared wave, and thisis the basic idea of this circuit. Thevariable signal is then amplified anddetected to light up the LED.

The circuit works on 9V supply forwhich a battery is recommended. Itdraws very little current in idle condi-tion.

After assembling the circuit prop-erly, take any remote control transmit-ter of any TV or VCR and focus itssignal on D3. The LED should glowand remain in the same state evenwhen the signal is removed. In case itdoesn’t glow, check the voltages atdifferent locations. If necessary, use aconvex lens and place D3 on its fo-cus. Now, it should work. The gadgetwill work from a distance of 15 cms


SOFT RADIO/ CASSETTE SELECTORNAVIN GAUTAM

Most radio-cassette players have amechanical radio/cassette se-

lector switch. Here is a simple elec-

tronic circuit, which selectsradio/cassette electronically with a softfinger touch. The circuit is simple andself- explanatory.

Transistors Tl and T2 are arra-nged as a bistable multivibrator. Bytouching the base of transistor T3 or

T4, one can change the state ofmultivibrator.

For switching of audio signal to

radio or cassette, CMOS bila-teral switch CD4066 is used.The four independent switchescontained within the IC are so ar-ranged that the IC works as a double-pole double-throw (DPDT) switch.This switch is controlled by the

multivibrator via transistors T5 andT6.

Transistors T7 and T8 provide volt-

ages to radio and its amplifier circuit,when the switch is in radio position.LED 1 and LED2 are used to indicatethe position of switch. Breakdownvoltage for zener diode D3 can beselected, as per requirement of theradio.

Readers’ Comments:I have made the project successfully. Itis working well. I found a range ofabout sixty centimeter without connect-ing any lens or filter.

H.B. TiwanaBangalore

� I successfully assembled the circuit

and it is working well. However, therange is very low!

Sudhir sinhaBidhan Nagar

The author, P.K. Bose, replies:The following was observed while

making the prototype:

1. The range of the tester is com-paratively low for testing purpose only.The range can be increased by addingIR amplifier.

2. The circuit can also be used as aremote control.

to 30 cms.If the LED glows with normal light,

an infrared transmitter filter should beused to cover D3. A red glass may be

used, instead. To drive high voltageload, a relay driver can be used in placeof the LED.

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