350 electronics for you projects

ELECTRONICS FOR YOU MAY 2001

C I R C U I T I D E A S

RUPANJANA

Here is a simple circuit to obtainvariable DC voltage from 1.25Vto 15.19V in reasonably smallsteps as shown in the table. The inputvoltage may lie anywhere between 20V

and 35V.The first section of the circuit com-

prises a digital up-down counter builtaround IC1 a quad 2-input NANDschmitt trigger (4093), followed byIC2 a binary up-down counter (4029).Two gates of IC 4093 are used to gen-erate up-down logic using push but-tons S1 and S2, respectively, while theother two gates form an oscillator toprovide clock pulses to IC2 (4029). Thefrequency of oscillations can be variedby changing the value of capacitor C1or preset VR1.

IC2 receives clock pulses from the os-cillator and produces a sequential binaryoutput. As long as its pin 5 is low, thecounter continues to count at the risingedge of each clock pulse, but stops count-ing as soon as its pin 5 is brought to logic1.

Logic 1 at pin 10 makes the counterto count upwards, while logic 0 makes itcount downwards. Therefore the countercounts up by closing switch S1 and counts

ing resistor across the relay contacts getsconnected to the circuit.

The table shows the theoretical out-put for various digital input combinations.The measured output is nearly equal tothe theoretically calculated output acrossregulator IC3 (LM317). The output volt-age is governed by the following relation-ship as long as the input-to-output differ-ential is greater than or equal to 2.5V:

Vout = 1.25(1+R2'/R1')Where, R1' = R15 = 270 ohms (fixed)

and R2' = R11 + R12 + R13 + R14= 220 + 470 + 820 +1500 ohms= 3,010 ohms (with all relays

energised)One can use either the binary

weighted LED display as indicated byLED1 through LED4 in the circuit or a74LS154 IC in conjunction with LED5through LED20 to indicate one of the 16selected voltage steps of Table I. The in-put for IC4 is to be tapped from points

down by closing switch S2.The output of counter IC2 is used to

realise a digitally variable resistor. Thissection consists of four N/O reed relaysthat need just about 5mA current for their

operation. (EFY lab note. The originalcircuit containing quad bilateral switchIC 4066 has been replaced by reed relaysoperated by transistorised switches be-cause of unreliable operation of theformer.) The switching action is performedusing BC548 transistors. External resis-tors are connected in parallel with thereed relay contacts. If particular relay con-tacts are opened by the control input atthe base of a transistor, the correspond-

NAVEEN THARIYAN

ELECTRONICS FOR YOU MAY 2001


TABLEBinary Equivalent LED4 LED3 LED2 LED1output dec no. R14 (W) R13 (W) R12 (W) R11 (W) R2' (W) Vout (V)0000 0 Shorted Shorted Shorted Shorted 0 1.250001 1 Shorted Shorted Shorted 220 220 2.270010 2 Shorted Shorted 470 Shorted 470 3.430011 3 Shorted Shorted 470 220 690 4.440100 4 Shorted 820 Shorted Shorted 820 5.050101 5 Shorted 820 Shorted 220 1040 6.060110 6 Shorted 820 470 Shorted 1290 7.220111 7 Shorted 820 470 220 1510 8.241000 8 1500 Shorted Shorted Shorted 1500 8.191001 9 1500 Shorted Shorted 220 1720 9.211010 10 1500 Shorted 470 Shorted 1970 10.371011 11 1500 Shorted 470 220 2190 11.391100 12 1500 820 Shorted Shorted 2390 11.991101 13 1500 820 Shorted 220 2540 13.011110 14 1500 820 470 Shorted 2790 14.171111 15 1500 820 470 220 3010 15.19

marked A through D in the figure. Thisarrangement can be used to replace theLED arrangement at points A, B, C, andD. This 74LS154 IC is a decoder/demultiplexer that senses the output ofIC2 and accordingly activates only one ofits 16 outputs in accordance with the

sets itself, and hence the output at pins6, 11, 14, and 12 is equivalent to binaryzero, i.e. 0000. The corresponding DCoutput of the circuit is minimum (1.25V).As count-up switch S1 is pressed, thebinary count of IC2 increases and theoutput starts increasing too. At the high-est count output of 1111, the output volt-age is 15.19V (assuming the in-circuit re-sistance of preset VR2 as zero). PresetVR2 can be used for trimming the outputvoltage as desired. To decrease the out-put voltage within the range of 1.25V to15.2V, count-down switch S2 is to be de-pressed.

Notes. 1. When relay contacts acrossa particular resistor are opened, the cor-responding LED glows.

2. The output voltages are shown as-suming the in-circuit resistance of presetVR2 as zero. Thus when the in-circuit re-sistance of preset VR2 is not zero, theoutput voltage will be higher than thatindicated here.

count value. LEDs at the output of thisIC can be arranged in a circular way alongside the corresponding voltages.

When the power is switched on, IC2 re-

ELECTRONICS PROJECTS Vol. 21 189

2-LINE INTERCOM-

CUM-TELEPHONE LINE

CHANGEOVER CIRCUIT

The circuit presented here can beused for connecting two telephones in parallel and also as a2-line intercom.

Usually a single telephone is con-nected to a telephone line. If another tele-phone is required at some distance, a par-allel line is taken for connecting the othertelephone. In this simple parallel line op-eration, the main problem is loss of pri-vacy besides interference from the otherphone. This problem is obviated in thecircuit presented here.

Under normal condition, two tele-phones (telephone 1 and 2) can be usedas intercom while telephone 3 is con-nected to the lines from exchange. In

changeover mode, exchange line is dis-connected from telephone 3 and gets con-nected to telephone 2.

For operation in intercom mode, onehas to just lift the handset of phone 1and then press switch S1. As a result,buzzer PZ2 sounds. Simultaneously, theside tone is heard in the speaker of hand-set of phone 1. The person at phone 2could then lift the handset and start con-versation. Similar procedure is to be fol-lowed for initiation of the conversationfrom phone 2 using switch S2. In thismode of operation, a 3-pole, 2-way slide-switch S3 is to be used as shown in thefigure.

In the changeover mode of operation,

switch S3 is used to changeover the tele-phone line for use by telephone 2. Theswitch is normally in the intercom modeand telephone 3 is connected to the ex-change line. Before changing over the ex-change line to telephone 2, the person attelephone 1 may inform the person at tele-phone 2 (in the intercom mode) that he isgoing to changeover the line for use byhim (the person at telephone 2). As soonas changeover switch S3 is flipped to theother position, 12V supply is cut off andtelephones 1 and 3 do not get any voltageor ring via the ring-tone-sensing unit.

Once switch S3 is flipped over for useof exchange line by the person at tele-phone 2, and the same (switch S3) is not

flipped back to normal po-sition after a telephonecall is over, the next tele-phone call via exchangelines will go to telephone2 only and the ring-tone-sensing circuit will stillwork. This enables theperson at phone 3 to knowthat a call has gonethrough. If the handset oftelephone 3 is lifted, it isfound to be dead. To maketelephone 3 again active,switch S3 should bechanged over to its normalposition.

ELECTRONICS PROJECTS Vol. 20

40-Metre Direct conversion receiver

Using the circuit of direct-conversion receiver described here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band.

The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifier-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (vari-able frequency oscillator) whose output

is injected into the gate of first FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency note in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios.

The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume

control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker.

The receiver can be powered by a 12-volt power-supply, capable of sourc-ing around 250mA current. Audio-output stage can be substituted with a readymade L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used in the circuit is given in the circuit diagram itself.


ELECTRONICS FOR YOUMAY 2002

Here is a low-cost circuit of Christ-mas star that can be easily con-structed even by a novice. The main

MAINS-OPERATEDCHRISTMAS STAR

SANI THEO

PRINCE PHILLIPS

advantage of this circuit is that it doesntrequire any step-down transformer or ICs.

Components like resistors R1 and R2,

capacitors C1, C2, and C3, diodes D1 andD2, and zener ZD1 are used to develop afairly steady 5V DC supply voltage thatprovides the required current to operatethe multivibrator circuit and trigger triacBT136 via LED1.

The multivibrator circuit is constructedusing two BC548 transistors (T1 and T2)and some passive components. The fre-quency of the multivibrator circuit is con-trolled by capacitors C4 and C5 and resis-tors R3 through R7. The output of themultivibrator circuit is connected to tran-sistor T3, which, in turn, drives the triacvia LED1. During positive half cycles ofthe multivibrators output, transistor T3energises triac BT136 and the lamp glows.

This circuit is estimated to cost Rs 75.


ELECTRONICS FOR YOUJULY 2002

This add-on circuit enables remoteswitching on/off of battery-operatedtoy cars with the help of a TV/video remote control handset operating at3040 kHz.

When the circuit is energisedfrom a 6V battery, the decadecounter CD4017 (IC2), which isconfigured as a toggle flip-flop, isimmediately reset by the power-on-reset combination of capacitor C3and resistor R6. LED1 connectedto pin 3 (Q0) of IC2 via resistor R5glows to indicate the standby con-dition. In standby condition, dataoutput pin of the integrated infra-red receiver/demodulator(SFH505A or TSOP1738) is at ahigh level (about 5 volts) and tran-sistor T1 is off (reverse biased).The monostable wired around IC1is inactive in this condition.

When any key on the remotecontrol handset is depressed, theoutput of the IR receiver momen-tarily transits through low state andtransistor T1 conducts. As a result,the monostable is triggered and ashort pulse is applied to the clockinput (pin 14) of IC2, which takes Q1 out-put (pin 2) of IC2 high to switch on motordriver transistor T2 via base bias resistorR7 and the motor starts rotatingcontinously (car starts running). ResistorR8 limits the starting current.

When any key on the handset is

INFRARED TOY CARMOTOR CONTROLLERT.K. HAREENDRAN

depressed again, the monostable isretriggered to reset decade counter IC2 andthe motor is switched off. Standby LED1glows again.

This circuit can be easily fabricated ona general-purpose printed board. After con-struction, enclose it inside the toy car andconnect the supply wires to the battery ofthe toy car with right polarity. Rewire theDC motor connections and fix the IR re-ceiver module in a suitable location, for

example, behind the front glass, and con-nect its wires to the circuit board using ashort 3-core ribbon cable/shielded wire.

Note. Since the circuit uses modu-

lated infrared beam for control function,ambient light reflections will not affect thecircuit operation. However, fluorescenttubelights with electronic ballasts and CFLlamps may cause malfunctioning of thecircuit.

SANI THEO


ELECTRONICS FOR YOU JULY 2002


MAINS MANAGERSHIBASHISH PATEL

Very often we forget to switch offthe peripherals like monitor, scan-ner, and printer while switching offour PC. The problem is that there are sepa-rate power switches to turn the peripher-als off. Normally, the peripherals are con-nected to a single of those four-way trail-ing sockets that are plugged into a singlewall socket. If that socket is accessible, allthe devices could be switched off fromthere and none of the equipment used willrequire any modification.

Here is a mains managercircuit that allows you to turnall the equipment on or off byjust operating the switch onany one of the devices; forexample, when you switch offyour PC, the monitor as wellas other equipment will getpowered down automatically.You may choose the mainequipment to control othergadgets. The main equipmentis to be directly plugged intothe master socket, while allother equipment are to be con-nected via the slave socket.The mains supply from thewall socket is to be connectedto the input of the mains man-ager circuit.

The unit operates bysensing the current drawn bythe control equipment/loadfrom the master socket. Onsensing that the controlequipment is on, it powers up the other(slave) sockets. The load on the mastersocket can be anywhere between 20 VAand 500 VA, while the load on the slavesockets can be 60 VA to 1200 VA.

During the positive half cycle of themains AC supply, diodes D4, D5, and D6have a voltage drop of about 1.8 voltswhen current is drawn from the mastersocket. Diode D7 carries the current dur-ing negative half cycles. Capacitor C3, inseries with diode D3, is connected acrossthe diode combination of D4 through D6,in addition to diode D7 as well as resistorR10. Thus current pulses during positivehalf-cycles, charge up the capacitor to 1.8

volts via diode D3. This voltage is suffi-cient to hold transistor T2 in forward bi-ased condition for about 200 ms even af-ter the controlling load on the mastersocket is switched off.

When transistor T2 is on, transistorT1 gets forward biased and is switchedon. This, in turn, triggers Triac 1, whichthen powers the slave loads. Capacitor C4and resistor R9 form a snubber network toensure that the triac turns off cleanly withan inductive load.

LED1 indicates that the unit is operat-ing. Capacitor C1 and zener ZD1 are effec-tively in series across the mains. The result-ing 15V pulses across ZD1 are rectified bydiode D2 and smoothened by capacitor C2to provide the necessary DC supply for thecircuit around transistors T1 and T2. Resis-tor R3 is used to limit the switching-onsurge current, while resistor R1 serves as ableeder for rapidly discharging capacitor C1when the unit is unplugged. LED1 glowswhenever the unit is plugged into the mains.Diode D1, in anti-parallel to LED1, carriesthe current during the opposite half cycles.

Dont plug anything into the master orslave sockets without testing the unit. If

possible, plug the unit into the mains viaan earth leakage circuit breaker. The mainsLED1 should glow and the slave LED2should remain off. Now connect a tablelamp to the master socket and switch iton. The lamp should operate as usual.The slave LED should turn on wheneverthe lamp plugged into slave socket isswitched on. Both lamps should be at fullbrightness without any flicker. If so, theunit is working correctly and can be putinto use.

Note. 1. The device connected to themaster socket must have its power switchon the primary side of the internal trans-former. Some electronic equipment havethe power switch on the secondary sideand hence these devices continue to drawa small current from the mains even whenswitched off. Thus such devices, if con-nected as the master, will not control theslave units correctly.

2. Though this unit removes the powerfrom the equipment being controlled, itdoesnt provide isolation from the mains.So, before working inside any equipmentconnected to this unit, it must be un-plugged from the socket.

SUNIL KUMA

R


The c i rcu i t of a 7MHz C W / A M Q R P t r a n s m i t -ter described here can be used to transmit e ither CW or audio fre-quency modulat-ed signal over a 7MHz carrier.

The carrier fre-quency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscilla-tor coil which can be tuned to 7MHz frequency with the help of J gang capacitor VC1.

Transistor T2 (with identical tank circuit connected at its collector as in case of transis-tor T1) serves as a power amplifier. The RF output from oscillator stage is inductively coupled to the power amplifier stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmis-

Reader Comments: I request the author for the following clarifications:

1. Please indicate the construction details of coils L1 and L3 as well as the inductor which is connected in parallel to VC2.

2. Can we use any other crystal in place of 7MHz crystal?

M.A. KamalGuwahati

What is the range of this transmit-ter and what is the output power of this circuit?

Vaibhav KumarSaharanpur

The author D. Prabaharan, com-ments:

In reply to the above queries, I would like to say that the transistor T1 is BF495. Power output of this circuit is about 150mW. It can be further increased by using separate power supply for the power-amplifier stage (24V, 1A).

The coil details are as follows L1 is short-wave oscillator coil; L2:14 turns on 1cm-diameter air-core tube using 26 SWG wire; L3 has 12 turns on 1.5cm-diameter

air-core tube using 26 SWG wire.The frequency allotted for amateur

radio operators is 7.0 MHz to 7.1 MHz. Hence, any crystal available within this frequency can be used. Range of this QRP transmitter depends on propagation condi-tions. If conditions are good, the range is about 500 kms in the CW mode and 100 kms in the AM mode.

It is possible to convert this transmit-ter to 20-meter HAM band. Any crystal available from 14 MHz to 14.350 MHz range can be used for the purpose. How-ever, this conversion needs following

7MHz CW/AM QRP TRANSMITTERD. PRABAHARAN

sion line for maximum power transfer. Suitable heatsink should be used for transistor T2.

Tuning adjustments may be accom-plished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 first through a coupling capacitor and tune J gang VC1 for maximum bril-liance. (Note: the bulb would light ac-cording to intensity of RF energy.) Same procedure may be repeated for power am-plifier stage and antenna tuning network for ensuring maximum power transfer.

For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be flipped to off position.

Any suitable mic. amplifier may be used to feed audio input to the audio driver transformer X1. (For transformer X1 you may use the transistor-radio type AF driver transformer.)


modifications on coils L1, L2 and L3L1: shortwave oscillator coil; L2: 11 turns on 1cm-diameter air-core tube using 26 SWG wire; L3: 9 turns on 1cm-diameter air-core tube using 28 SWG wire.

An ammeter with a range 0-250mA or a multimeter with 0-250mA can be

connected in-between the positive of the supply and the modulation transformer. Adjust VC1, VC2 and VC3 for maximum current through ammeter (CW-200mA, AM-125mA). The power input in CW and AM mode is calculated as shown below:

DC power input (CW mode) = 24V

x 250mA= 6watt (the power amplifier draws

250mA current).DC power input (AM mode) = 24V x

120mA= 2.8watt (the power amplifier draws

120mA current).

ELECTRONICS FOR YOU FEBRUARY 2001


S.C. DWIVED

I

This circuit is able to handle nineindependent telephones (using asingle telephone line pair) lo-cated at nine different locations, say,up to a distance of 100m from eachother, for receiving and making outgo-ing calls, while maintaining conversa-tion secrecy. This circuit is useful whena single telephone line is to be sharedby more members residing in differentrooms/apartments.

Normally, if one connects ninephones in parallel, ring signals are

heard in all the nine telephones (it isalso possible that the phones will notwork due to higher load), and out ofnine persons eight will find that the callis not for them. Further, one can over-hear others conversation, which is notdesirable. To overcome these problems,the circuit given here proves beneficial,as the ring is heard only in the desiredextension, say, extension number 1.

For making use of this facility, thecalling subscriber is required to initiallydial the normal phone number of the

called subscriber. When the call is estab-lished, no ring-back tone is heard by thecalling party. The calling subscriber hasthen to press the asterik (*) button onthe telephone to activate the tone mode(if the phone normally works in dial mode)and dial extension number, say, 1, within10 seconds. (In case the calling subscriberfails to dial the required extension num-ber within 10 seconds, the line will bedisconnected automatically.) Also, if thedialed extension phone is not lifted within10 seconds, the ring-back tone will cease.

The ring signal on the main phoneline is detected by opto-coupler MCT-2E (IC1), which in turn activates the10-second on timer, formed by IC2(555), and energises relay RL10 (6V, 100-

ohm, 2 C/O). One of the N/Ocontacts of the relay has beenused to connect +6V rail to theprocessing circuitry and theother has been used to provide220-ohm loop resistance to de-energise the ringer relay intelephone exchange, to cut offthe ring.

When the caller dials theextension number (say, 1) intone mode, tone receiverCM8870 (IC3) outputs code0001, which is fed to the 4-bit BCD-to-10 line decimal de-coder IC4 (CD4028). The out-put of IC4 at its output pin14 (Q1) goes high andswitches on the SCR (TH-1)and associated relay RL1. Re-lay RL1, in turn, connects, viaits N/O contacts, the 50Hz ex-tension ring signal, derivedfrom the 230V AC mains, tothe line of telephone 1. Thisring signal is available to tele-phone 1 only, because halfof the signal is blocked by di-ode D1 and DIAC1 (which donot conduct below 35 volts).

As soon as phone 1 islifted, the ring current in-creases and voltage dropacross R28 (220-ohm, 1/2W re-sistor) increases and operatesopto-coupler IC5 (MCT-2E).This in turn resets timer IC2causing:

(a) interruption of thepower supply for processingcircuitry as well as the ring

DHURJATI SINHA


A HIERARCHICAL PRIORITY ENCODER

Anormal priority encoder encodesonly the highest-order data line.But in many situations, not onlythe highest but the second-highestpriority information is also needed. Thecircuit presented here encodes boththe highest-priority information as wellas the second-highest priority informationof an 8-line incoming data. The circuituses the standard octal priority encoder74148 that is an 8-line-to-3-line (4-2-1)binary encoder with active-low data in-puts and outputs.

The first encoder (IC1) generates thehighest-priority value, say, F. The active-low output (A0, A1, A2) of IC1 is in-verted by gates N9 through N11 and fedto a 3-line-to-8-line decoder (74138) thatrequires active-high inputs. The decodedoutputs are active-low. The decoder iden-tifies the highest-priority data line and

that data value is cancelled using XNORgates (N1 through N8) to retain the sec-ond-highest priority value that is gener-ated by the second encoder.

To understand the logic, let the in-coming data lines be denoted as L0 to L7.Lp is the highest-priority line (active-low)and Lq the second-highest priority line

(active-low). Thus Lp=0 and Lq=0. Alllines above Lp and also between Lp andLq (denoted as Lj) are at logic 1. All linesbelow Lq logic state are irrelevant, i.e.dont care. Here p is the highest-priorityvalue and q the second-highest-priorityvalue. (Obviously, q has to be lower thanp, and the minimum possible value for pis taken as 1.)

Priority encoder IC1 generates binaryoutput F2, F1, F0, which represents thevalue of p in active-low format. Thecomplemented F2, F1, and F0 are ap-plied to 3-line-to-8-line (one out of eightoutputs is active-low) decoder 74138. Letthe output lines of 74138 be denoted asM0 through M7. Now only one line isactive-low among M0 through M7, andthat is Mp (where the value of p is ex-plained as above). Therefore the logic levelof line Mp is 0 and that of all other M

lines 1.The highest-priority line is cancelled

using eight XNOR gates as shown in thefigure. Let the output lines from XNORgates be N0 through N7. Consider inputsLp and Mp of the corresponding XNORgate. Since Mp = 0 and also Lp = 0, theoutput of this XNOR gate is Np = comple-

ment of Lp = 1. All other Ls are notchanged because the corresponding Msare all 1s. Thus data lines N0 through N7are same as L0 through L7, except thatthe highest-priority level in L0 throughL7 is cancelled in N0 through N7.

The highest-priority level in N0through N7 is the second-highest priorityleftover from L0 through L7, i.e. Nq=0 andNj=1 for q


ACCURATE ELECTRONIC STOP-WATCH

Here is a simple circuit which can be used as an accurate stop-watch to count up to 100 seconds with a resolution of 0.01 second or up to 1000 seconds with a resolution of 0.1 second. This stop-watch can be used for sports and similar other activities.

A 1MHz crystal generates stable frequency which is divided by two stages of 74390 ICs (dual decade counter) and another stage employing 7490 (decade

counter) IC to obtain a final frequency of 100 Hz or 10 Hz. Due to the use of crystal, the final frequency is very accurate.

The output of IC4 (7490) is counted and displayed using IC5 74C926 (4-digit counter with multiplexed 7-segment LED driver). Due to multiplexed display the power consumption is very low. Switch S2 (2-pole, 2-way) is used to select appropri-ate input frequency and corresponding decimal point position to display up to

either 99.99 seconds or 999.9 seconds maximum count.

For proper operation, first press switch S3 (reset) and then operate switch S2, according to the resolution/range desired (0.1 sec. or 0.01 sec.)/(100 seconds or 1000 seconds). Now to start counting, press switch S1. To stop counting, press switch S1 again. The counting will stop and display will show the correct time elapsed since the start of counting.



PRABHASH K.P.

RUPANJANA

The add-on circuit presented hereis useful for stereo systems. Thiscircuit has provision for connect-ing stereo outputs from four differentsources/channels as inputs and only oneof them is selected/ connected to theoutput at any one time.

When power supply is turned on,channel A (A2 and A1) is selected. If noaudio is present in channel A, the cir-cuit waits for some time and then se-lects the next channel (channel B), Thissearch operation continues until it de-tects audio signal in one of the chan-nels. The inter-channel wait or delaytime can be adjusted with the help ofpreset VR1. If still longer time isneeded, one may replace capacitor C1with a capacitor of higher value.

Suppose channel A is connected toa tape recorder and channel B is con-nected to a radio receiver. If initially

channel A is selected, the audio fromthe tape recorder will be present at theoutput. After the tape is played com-pletely, or if there is sufficient pausebetween consecutive recordings, the cir-cuit automatically switches over to theoutput from the radio receiver. Tomanually skip over from one (selected)active channel, simply push the skipswitch (S1) momentarily once or more,until the desired channel inputs getsselected. The selected channel (A, B, C,or D) is indicated by the glowing of cor-responding LED (LED11, LED12,LED13, or LED14 respectively).

IC CD4066 contains four analogueswitches. These switches are connectedto four separate channels. For stereooperation, two similar CD4066 ICs areused as shown in the circuit. These ana-logue switches are controlled by ICCD4017 outputs. CD4017 is a 10-bit ring

counter IC. Since only one of its out-puts is high at any instant, only oneswitch will be closed at a time. ICCD4017 is configured as a 4-bit ringcounter by connecting the fifth outputQ4 (pin 10) to the reset pin. CapacitorC5 in conjunction with resistor R6 formsa power-on-reset circuit for IC2, so thaton initial switching on of the powersupply, output Q0 (pin 3) is alwayshigh. The clock signal to CD4017 is pro-vided by IC1 (NE555) which acts as anastable multivibrator when transistorT1 is in cut-off state.

IC5 (KA2281) is used here for notonly indicating the audio levels of theselected stereo channel, but also for for-ward biasing transistor T1. As soon asa specific threshold audio level is de-tected in a selected channel, pin 7 and/or pin 10 of IC5 goes low. This lowlevel is coupled to the base of transistorT1, through diode-resistor combinationof D2-R1/D3-R22. As a result, transis-tor T1 conducts and causes output ofIC1 to remain low (disabled) as long asthe selected channel output exceeds thepreset audio threshold level.

Presets VR2 and VR3 have been in-cluded for adjustment of individual au-dio threshold levels of left stereo chan-nels, as desired. Once the multivibratoraction of IC1 is disabled, output of IC2does not change further. Hence, search-

jeetu97


ing through the channels continues un-til it receives an audio signal exceedingthe preset threshold value. The skip

switch S1 is used to skip a channel evenif audio is present in the selected chan-nel. The number of channels can be eas-

ily extended up to ten, by using addi-tional 4066 ICs.

jeetu98

ELECTRONICS FOR YOU JULY 2001


S.C. DWIVED

I

T his small circuit, based on popu-lar CMOS NAND chip CD4093,can be effectively used for protect-ing your expensive car audio systemagainst theft.

When 12V DC from the car battery is

applied to the gadget (as indicated byLED1) through switch S1, the circuit goesinto standby mode. LED insideoptocoupler IC1 is lit as its cathode ter-minal is grounded via the car audio (am-plifier) body. As a result, the output atpin 3 of gate N1 goes low and disablesthe rest of the circuit.

Whenever an attempt is made to re-move the car audio from its mounting bycutting its connecting wires, theoptocoupler immediately turns off, as itsLED cathode terminal is hanging. As aresult, the oscillator circuit built around

gates N2 and N3 is enabled and it con-trols the on/off timings of the relay viatransistor T2. (Relay contacts can be usedto energise an emergency beeper, indica-tor, car horns, etc, as desired.)

Different values of capacitor C2 givedifferent on/off timings for relay RL1 tobe on/off. With 100F we get approxi-

mately 5 seconds as on and 5 seconds asoff time.

Gate N4, with its associated compo-nents, forms a self-testing circuit. Nor-mally, both of its inputs are in high state.However, when one switches off the igni-tion key, the supply to the car audio isalso disconnected. Thus the output of gateN4 jumps to a high state and it providesa differentiated short pulse to forward biastransistor T1 for a short duration. (Thecombination of capacitor C1 and resistorR5 acts as the differentiating circuit.)

As a result, buzzer in the collectorterminal of T1 beeps for a short duration

to announce thatthe security cir-cuit is intact.This on periodof buzzer can bevaried by chang-ing the values ofcapacitor C1and/or resistorR5.

After con-struction, fix theLED and buzzerin dashboard asper your re-quirement andhide switch S1

in a suitable location. Then connect leadA to the body of car stereo (not to thebody of vehicle) and lead B to its positivelead terminal. Take power supply for thecircuit from the car battery directly.

Caution. This design is meant for caraudios with negative ground only.

T.K. HAREENDRAN


Audio-VisuAl ExtrA ringEr for PhonE

Many a times one needs an extra telephone ringer in an ad- joining room to know if there is an incoming call. For example, if the telephone is installed in the drawing room you may need an extra ringer in the bedroom. All that needs to be done is to connect the given circuit in parallel with the existing telephone lines using twin flexible wires.

This circuit does not require any ex-ternal power source for its operation. The section comprising resistor R1 and diodes D5 and LED1 provides a visual indication of the ring. Remaining part of the circuit is the audio ringer based on IC1 (BA8204 or ML8204). This integrated circuit, specially designed for telec- om application as bell sound generator, requires very few external parts. It is readily available in

of the resistors and capacitors may be carried out to obtain desired pleasing tone.

Working of the circuit is quite simple. The bell signal, approximately 75V AC, passes through capacitor C1 and resistor R2 and appears across the diode bridge comprising diodes D1 to D4. The rectified DC output is smoothed by capacitor C2. The dual-tone ring signal is output from pin 8 of IC1 and its volume is adjusted by volume control VR1. Thereafter, it is impressed on the piezo-ceramic sound generator.

8-pin mini DIP pack.Resistor R3 is used for bell sensitiv-

ity adjustment. The bell frequency is controlled by resistor R5 and capacitor C4, and the repetition rate is controlled by resistor R4 and capacitor C3. A little experimentation with the various values

CIRCUIT IDEAS

ELECTRONICS FOR YOUAUGUST '99

Dual-Channel DigitalVolume ControlSHEENA K.

This circuit could be used for re-placing your manual volume con-trol in a stereo amplifier. In thiscircuit, push-to-on switch S1 controls theforward (volume increase) operation ofboth channels while a similar switchS2 controls reverse (volume decrease)operation of both channels.

Here IC1 timer 555 is configured asan astable flip-flop to provide low-fre-

quency pulses to up/down clock inputpins of pre-setable up/down counter74LS193 (IC2) via push-to-on switchesS1 and S2. To vary the pulse width ofpulses from IC1, one may replace tim-ing resistor R1 with a variable resistor.

Operation of switch S1 (up) causesthe binary output to increment whileoperation of S2 (down) causes the bi-nary output to decrement. The maxi-

mum count being 15 (all outputs logic1) and minimum count being 0 (all out-puts logic 0), it results in maximum andminimum volume respectively.

The active high outputs A, B, C andD of the counter are used for control-ling two quad bi-polar analogue switchesin each of the two CD4066 ICs (IC3 andIC4). Each of the output bits, when high,short a part of the resistor network com-prising series resistors R6 through R9for one channel and R10 through R13for the other channel,and thereby control the output of theaudio signals being fed to the inputs ofstereo amplifier. Push-to-on switch S3is used for resetting the output ofcounter to 0000, and thereby turningthe volume of both channels to the mini-mum level.

A.P.S. DHILLO

N

CIRCUIT IDEAS

ELECTRONICS FOR YOUAUGUST '99

Using this low-cost project onecan reproduce audio from TVwithout disturbing others. Itdoes not use any wire connection be-tween TV and headphones. In place ofa pair of wires, it uses invisible infra-red light to transmit audio signals from

TV to headphones. Without using anylens, a range of up to 6 metres is

possible. Range can be extended byusing lenses and reflectors with IR sen-sors comprising transmitters and re-ceivers.

IR transmitter uses two-stage tran-sistor amplifier to drive two series-con-nected IR LEDs. An

audio out-put trans-former isused (in re-verse) tocouple audiooutput fromTV to the IRtransmitter.TransistorsT1 and T2amplify theaudio sig-nals re-ceived from

TV through the audio transformer. Low-impedance output windings (lower

gauge or thicker wires) are used for con-nection to TV side while high-imped-ance windings are connected to IR trans-mitter. This IR transmitter can be pow-ered from a 9-volt mains adapter or bat-tery. Red LED1 in transmitter circuitfunctions as a zener diode (0.65V) aswell as supply-on indicator.

IR receiver uses 3-stage transistoramplifier. The first two transistors (T4and T5) form audio signal amplifierwhile the third transistor T6 is used todrive a headphone. Adjust potmeter VR2for max. clarity.

Direct photo-transistor towards IRLEDs of transmitter for max. range. A

9-volt battery can be used with receiverfor portable operation.

Infrared CordlessHeadphonePRADEEP G.

G.S. SAGOO


7 is therefore off. The out-put (at pin 3) reverses (goes low) when pin 2 is taken more positive than 1/3 Vcc. At the same time pin 7 goes low (as Q output of in-ternal flip-flop is high) and the ED con-nected to pin 7 is lit. Both tim-ers (IC1 and IC2) are config-

ured to function in the same fashion.Preset VR1 is adjusted for under

voltage (say 160 volts) cut-out by ob-serving that LED1 just lights up when mains voltage is slightly greater than 160V AC. At this setting the output at pin 3 of IC1 is low and transistor T1 is in cut-off state. As a result RESET pin 4 of IC2 is held high since it is connect-ed to Vcc via 100 kilo-ohm resistor R4.

Preset VR2 is adjusted for over voltage (say 270V AC) cut-out by ob-

serving that LED2 just extinguishes when the mains voltage is slightly less than 270V AC. With RESET pin 4 of IC2 high, the output pin 3 is also high. As a result transistor T2 conducts and energises relay RL1, connecting load to power supply via its N/O contacts. This is the situation as long as mains voltage is greater than 160V AC but less than 270V AC.

When mains voltage goes beyond 270V AC, it causes output pin 3 of IC2 to go low and cut-off transistor T2 and de-energise relay RL1, in spite of RESET pin 4 still being high. When mains voltage goes below 160V AC, IC1s pin 3 goes high and LED1 is extinguished. The high output at pin 3 results in conduction of transistor T1. As a result collector of transistor T1 as also RESET pin 4 of IC2 are pulled low. Thus output of IC2 goes low and transistor T2 does not conduct. As a result relay RL1 is de-energised, which causes load to be disconnected from the supply. When mains volt-age again goes beyond 160V AC (but less than 270V AC) the relay again energises to connect the load to power supply.

Auto Reset oveR/undeR voltAge Cut-out

J. Gopalakrishnan

This over/under voltage cut-out will save your costly electrical and electronic appliances from the adverse effects of very high and

very low mains voltages.The circuit features auto reset and

utilises easily available components. It makes use of the comparators available

inside 555 timer ICs. Supply is tapped from different points of the power sup-ply circuit for relay and control circuit operation to achieve reliability.

The circuit utilises comparator 2 for control while comparator 1 output (connected to reset pin R) is kept low by shorting pins 5 and 6 of 555 IC. The positive input pin of comparator 2 is at 1/3rd of Vcc voltage. Thus as long as negative input pin 2 is less positive than 1/3 Vcc, comparator 2 output is high and the internal flip-flop is set, i.e. its Q output (pin 3) is high. At the same time pin 7 is in high imped-ance state and LED connected to pin

ELECTRONICS FOR YOU APRIL 2001


H ere are two simple, low-cost cir-cuits that can be used to shutoff the mains supply to any au-dio or video equipment (such as tape re-corder, CD player, and amplifier). Thesecircuits are helpful to those in the habitof falling asleep with their music systemon.

The circuits will also protect the equip-ment from getting damaged due to high-voltage spikes whenever there is a re-sumption of power after a break. This ispossible because the equipment will getswitched off automatically under such con-ditions but will not get switched on auto-matically on resumption of mains supply.

The circuit in Fig. 1 can be used toshut off any cassette player that has areliable auto-stop mechanism. Wheneverswitch S1 is pressed momentarily, it ex-tends the supply to the step-down trans-former of the tape recorder and chargescapacitor C1 through diode D1. This, inturn, makes transistor T1 conduct andenergise relay RL1 to provide a parallelpath to switch S1, so that supply to thestep-down transformer continues evenwhen switch S1 is released.

When any button on the cassetteplayer is pressed, the capacitor chargesthrough diode D2. This ensures conduc-tion of transistor T1 and thus the conti-nuity of operation of cassette player. How-ever, whenever the auto-stop mechanismfunctions at the end of a tape, the leafswitch gets opened. This cuts the charg-ing path for the capacitor and it startsdischarging slowly. After about oneminute, the relay opens and interruptsmain power to the transformer. The timedelay can be increased by increasing thevalue of capacitor C1.

If the appliance used is a two-in-onetype (e.g. cassette player-cum-radio), justconnect another diode in parallel with di-odes D1 and D2 to provide an additionalpath for charging capacitor C1 via thetape-to-radio changeover switch, so thatwhen radio is played the relay does not

interrupt the power supply.The other circuit, shown in Fig. 2,

functions on the basis of the signal re-ceived from preamp of the appliance used.In this circuit, opamp A741 is wired ininverting opamp configuration. It ampli-fies the signal received from the preamp.Timer NE555 is used to provide the nec-essary time delay of about one minute.

Preset VR1 is used to control the sen-sitivity of the circuit to differentiate be-

tween the noise and the signal. ResistorR4 offers feedback resistance to controlthe gain of the opamp. By increasing ordecreasing the value of resistor R4, thegain can be increased or decreased, re-spectively. The preset time delay of timerNE555 (which is about one minute) canbe increased by increasing the valueof C4.

Initial energisation of relay RL2 andcharging of capacitor C4 take place ondepression of switch S3 in the same man-ner as charging of capacitor C1 (refer Fig.1) on depression of switch S1. As a re-sult, pins 2 and 6 of NE555 go high andthe output of timer goes low to switch offmains supply from the relay to step-downtransformer X2 of the appliance. Bleederresistor R6 is used to discharge capacitorC4. Now if signals are received from the

SUNIL KUMAR

ARTHUR LOUIS

ELECTRONICS FOR YOU APRIL 2001


preamplifier, these are amplified by 741and fed to the base of transistor T2, whichkeeps capacitor C4 charged through re-sistor R5. When there is no signal, T2will not conduct and the capacitor slowlydischarges through R6. The output of 555goes high to switch off the relay and thus

the mains supply to transformer X2.Switch S2 can be depressed momentarilyif the device needs to be manuallyswitched off.

Note. The 12V supply should be pro-vided to the circuit from the equipmentspower supply. Opamp 741 should be

driven from the preamplifier of the gad-get used, and not from its power ampli-fier output. Switches S1 and S2 are 2-pole push-to-on switches. These can alsobe fabricated from 2-pole on-off switches,which are widely used in cassette play-ers, by removing the latch pin from them.


This terminal count output from pin 7, after inversion by gate N3, is con-nected to clock pin 14 of decade counter IC3 (CD4017) which is configured here as a toggle flip-flop by returning its Q2 output at pin 4 to reset pin 15. Thus output at pin 3 of IC3 goes to logic 1 and logic 0 state alternately at each terminal count of IC2.

Initially, pin 3 (Q0) of IC3 is high and the counter is in count-up state. On reaching ninth count, pin 3 of IC3 goes low and as a result IC2 starts counting down. When the counter reaches 0 count, Q2 output of IC3 momentarily goes high to reset it, thus taking pin 3 to logic 1 state, and the cycle repeats.

The BCD outputs of IC2 are con-

nected to 1-of-10 decoder CD4028 (IC4). During count-up operation of IC2, the outputs of IC4 go logic high sequentially from Q0 to Q9 and thus trigger the tri-acs and lighting bulbs 1 through 10, one after the other. Thereafter, during count-down operation of IC2, the bulbs light in the reverse order, presenting a wonderful visual effect.

AutomAtic DuAl- output DisplAy

This circuit lights up ten bulbs sequentially, first in one direction and then in the opposite direction, thus presenting a nice visual effect.

In this circuit, gates N1 and N2 form

an oscillator. The output of this oscillator is used as a clock for BCD up/down coun-ter CD4510 (IC2).

Depending on the logic state at its pin 10, the counter counts up or down.

During count up operation, pin 7 of IC2 outputs an active low pulse on reaching the ninth count. Similarly, during count-down operation, you again get a low-going pulse at pin 7.

CIRCUIT

IDEAS

1 1 4 M A R C H 2 0 0 7 E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

PRIYANK MUDGAL

AUTOMATIC EMERGENCY LIGHT

T his emergency light has thefollowing two advantages:1. It turns on automatically

when the mains power fails, so youneed not search it in the dark.

2. Its battery starts charging as soonas the mains resumes.

Operation of the circuit is quitestraightforward. Mains supply is

stepped down by transformer X1, rec-tified by a full-wave rectifier compris-ing diodes D1 and D2, filtered by ca-pacitor C1 and fed to relay coil RL1.The relay energises to connect the bat-

tery to the charging circuit through itsnormally-opened (N/O) contacts. Free-wheeling diode D3 acts as a spikebuster for the relay.

The charging circuit is built aroundnpn transistor BD139 (T1). The trans-

former output is fed to the collector oftransistor T1, which provides a fixedbias voltage of 6.8V to charge the bat-tery. When the battery is fully charged,the battery voltage becomes equal to

the breakdown voltageof the zener diode(ZD1). Zener diode ZD1conducts to provide analternative path for thecurrent to ground andbattery charging stops.

When mains fails,relay RL1 de-energises.The battery now getsconnected to the whiteLED array (comprisingLED1 through LED6)through current-limit-ing resistor R2. The

LEDs glow to light up the room. Toincrease the brightness in your room,you can increase the number of whiteLEDs after reducing the value of resis-tor R2 and also use a reflector assem-bly.

S.C. DWIVE

DI


AUTOMATIC EMERGENCY TORCH

Just dont think that this is yet another addition to other emergency light circuits published in EFY earlier. This circuit is a hit different. Its main features are:

1. Very reliable operation.2. As transformer is not used, it is

compact and cost-effective.3. The torch bulb glows automatically

at power off and goes out on restoration of power.

4. Since Ni-Cd battery is used, no maintenance is required. Also, battery life is very long, nearly 4-5 years (though this depends on frequency of usage and also on ampere-hour rating of the battery used).

Sounds interesting, doesnt it? Read on then. The circuit is very simple, compris-ing just a handful of components. This implies that the circuit operation also is very simple. The circuit consists of two parts:

1. Power supply for charging the Ni-Cd battery.

2. Switchover circuit which detects mains failure and switches the bulb on.

In the power supply section, capacitors C1 and C2 function as non-dissipating, re-active impedances which limit the current to a safe value. With the values of capaci-tors as shown, the maximum current that can be drawn is limited to about 70 mA at 230V AC. Resistor R2 limits the initial surge current and resistor R1 assists in discharging the capacitors after switch off. Diodes D1 through D4 form a conventional bridge rectifier while capacitor C3 is the filter capacitor. Fuse F1 is for protection and is very helpful in the event of any component giving up the ghost. This sup-ply charges the battery as long as mains is present.

In the switchover section, transistor T1 is used as switch. Normally, when AC mains supply is present, the rectifier output

charges the battery through resistor R4 and LED D5 combination at about 50mA rate. The glowing LED (D5) also gives an indica-tion of mains presence. Further, due to the LED (D5), base of transistor T1 is about 1.6V (drop across D5) more positive than its emitter. This voltage is more than sufficient to keep the transistor at cut-off.

As soon as the mains voltage fails, the base of transistor T1 is pulled low through resistor R3 which drives transistor T1 to saturation thereby turning the bulb on. Since the transistor is in its saturated state, the voltage drop across it is very low. Hence the bulb glows with full bril-liance. The bulb can be switched off by the ON/OFF switch, when not required. With this bulb (2.2V, 250mA) the torch can work continuously for about two hours. The batteries should be charged for about 14 hours after they are discharged.

You can verify following voltages in the circuit:

1. Base voltage of the transistor must be 1.8V to 2.0V, i.e. about 0.6V less than the battery voltage.

2. Emitter voltage must be equal to the battery voltage.

3. Collector voltage must be 2.0V to 2.2V, i.e. nearly equal to the battery voltage.

All above voltages should be checked with AC mains off. If any of the above-mentioned voltages is absent it indicates that the transistor is bad and it should be replaced by a good one.

Here is a word of caution now. Since the circuit is not isolated from AC mains. it may be hazardous to touch any component when the mains supply is on, especially if the supply wires (live and neutral) get interchanged. It is strongly recommended to use an all-plastic enclosure (including the reflector for the bulb) for the circuit. Also the ON/OFF switch used should have a plastic lever. Take proper care and pre-cautions while building, testing and using the circuit, and never ever allow the supply wires to interchange. It is advisable to pro-vide a plug for the mains input on the box itself so that it can be plugged directly into a mains outlet. This reduces the chances of mains supply wires getting interchanged.

With proper precautions and a little care, it is hoped that this small circuit will help make life a bit more comfortable.

ELECTRONICS FOR YOU MARCH 2001


S.C. DWIVEDI

sound from UM3561. Resistor R4 in se-ries with a 3V zener is used to providethe 3V supply to UM3561 when the re-

SUKANT KUMAR BEHARA

lay is in energised state. LED1, con-nected in series with 68-ohm resistorR1 across resistor R4, glows when thesiren is on.

To test the working of the cir-cuit, bring a burning matchstickclose to transistor T1 (BC109),which causes the resistance of itsemitter-collector junction to go lowdue to a rise in temperature and itstarts conducting. Simultaneously,

transistor T2also conducts be-cause its base isconnected to thecollector of tran-sistor T1. As aresult, relay RL1energises andswitches on thesiren circuit toproduce loudsound of a fire-brigade siren.

Lab note.We have added a

table to enable readers to obtain all pos-sible sound effects by returning pins 1and 2 as suggested in the table.

Pin Designation Sound EffectSEL1 SEL2No Connection No Connection Police Siren+3V No Connection Fire Engine SirenGround No Connection Ambulance SirenDo not care +3V Machine Gun

This circuit uses a complementarypair comprising npn metallictransistor T1 (BC109) and pnpgermanium transistor T2 (AC188) to de-tect heat (due to outbreak of fire, etc)in the vicinity and energise a siren. Thecollector of transistor T1 is connectedto the base of transistor T2, while thecollector of transistor T2 is connectedto relay RL1.

The second part of the circuit com-prises popular IC UM3561 (a siren andmachine-gun sound generator IC), whichcan produce the sound of a fire-brigadesiren. Pin numbers 5 and 6 of the ICare connected to the +3V supply whenthe relay is in energised state, whereaspin 2 is grounded. A resistor (R2) con-nected across pins 7 and 8 is used to fixthe frequency of the inbuilt oscillator.The output is available from pin 3.

Two transistors BC147 (T3) andBEL187 (T4) are connected inDarlington configuration to amplify the


AutomAtic Room PoweR contRol

An ordinary automatic room power control circuit has only one light sensor. So when a person enters the room it gets one pulse and the lights come on. When the person goes out it gets another pulse and the lights go off. But what happens when two persons enter the room, one after the other? It gets two pulses and the lights remain in off state.

The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to auto-matically switch on and switch off the lights in a desired fashion.

The circuit uses two LDRs which are placed one after another (separated by a distance of say half a metre) so that they may separately sense a person going into the room or coming out of the room.

Outputs of the two LDR sensors, after processing, are used in conjunction with a bicolour LED in such a fashion that when a person gets into the room it emits green light and when a person goes out of the room it emits red light, and vice versa. These outputs are simultaneously applied to two counters.

One of the counters will count as +1, +2, +3 etc when persons are coming into the room and the other will count as -1, -2, -3 etc when persons are going out of the room. These counters make use of Johnson decade counter CD4017 ICs. The next stage comprises two logic ICs which can combine the outputs of the two counters and determine if there is any person still left in the room or not.

Since in the circuit LDRs have been used, care should be taken to protect them from ambient light. If desired, one may use readily available IR sensor modules to replace the LDRs. The sensors are in-stalled in such a way that when a person enters or leaves the room, he intercepts the light falling on them sequentiallyone after the other.

When a person enters the room, first he would obstruct the light falling on LDR1, followed by that falling on LDR2. When a person leaves the room it will be the other way round.

In the normal case light keeps fall-ing on both the LDRs, and as such their resistance is low (about 5 kilo-ohms). As a


result, pin 2 of both timers (IC1 and IC2), which have been configured as monostable flip-flops, are held near the supply voltage (+9V).

When the light falling on the LDRs is obstructed, their resistance becomes very high and pin 2 voltages drop to near ground potential, thereby triggering the flip-flops. Capacitors across pin 2 and ground have been added to avoid false triggering due to electrical noise.

When a person enters the room, LDR1 is triggered first and it results in triggering of monostable IC1. The short output pulse immediately charges up capacitor C5, forward biasing transis-tor pair T1-T2. But at this instant the collectors of transistors T1 and T2 are in high impedance state as IC2 pin 3 is at low potential and diode D4 is not conducting.

But when the same person pass-es LDR2, IC2 monostable flip-flop is triggered. Its pin 3 goes high and this potential is coupled to transistor pair T1-T2 via diode D4. As a result transistor

pair T1-T2 conducts because capacitor C5 retains the charge for some time as its discharge time is controlled by resistor R5 (and R7 to an extent). Thus green LED portion of bi-colour LED is lit momentar-ily.

The same output is also coupled to IC3 for which it acts as a clock. With entry of each person IC3 output (high state) keeps advancing. At this stage transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is no longer positive as its output pulse duration is quite short and hence transistor collectors are in high imped-ance state.

When persons leave the room, LDR2 is triggered first, followed by LDR1. Since the bottom half portion of circuit is identical to top half, this time, with the departure of each person, red portion of bi-colour LED is lit momentarily and output of IC4 advances in the same fashion as in case of IC3.

The outputs of IC3 and those of IC4 (after inversion by inverter gates N1

through N4) are ANDed by AND gates (A1 through A4) and then wire ORed (using diodes D5 through D8). The net effect is that when persons are entering, the output of at least one of the AND gates is high, causing transistor T5 to conduct and energise relay RL1. The bulb connected to the supply via N/O contact of relay RL1 also lights up.

When persons are leaving the room, and till all the persons who entered the room have left, the wired OR output continues to remain high, i.e. the bulb continues to remains on, until all persons who entered the room have left.

The maximum number of persons that this circuit can handle is limited to four since on receipt of fifth clock pulse the counters are reset. The capacity of the circuit can be easily extended to handle up to nine persons by removing the connection of pin 1 from reset pin (15) and utilising Q1 to Q9 outputs of CD4017 counters. Additional inverters, AND gates and diodes will, however, be required.


AUTOMATIC SPEED-CONTROLLER

FOR FANS AND COOLERS

During summer nights, the tem-perature is initially quite high.As time passes, the temperaturestarts dropping. Also, after a person fallsasleep, the metabolic rate of ones bodydecreases. Thus, initially the fan/coolerneeds to be run at full speed. As timepasses, one has to get up again and againto adjust the speed of the fan or the cooler.

The device presented here makes thefan run at full speedfor a predeterminedtime. The speed isdecreased to mediumafter some time, andto slow later on. Af-ter a period of abouteight hours, the fan/cooler is switched off.

Fig. 1 shows thecircuit diagram ofthe system. IC1 (555)is used as an astablemultivibrator togenerate clockpulses. The pulsesare fed to decadedividers/countersformed by IC2 andIC3. These ICs act as

divide-by-10 and divide-by-9 counters,respectively. The values of capacitor C1and resistors R1 and R2 are so adjustedthat the final output of IC3 goes highafter about eight hours.

The first two outputs of IC3 (Q0 andQ1) are connected (ORed) via diodes D1and D2 to the base of transistor T1. Ini-tially output Q0 is high and thereforerelay RL1 is energised. It remainsenergised when Q1 becomes high. Themethod of connecting the gadget to thefan/cooler is given in Figs 3 and 4.

It can be seen that initially the fan

shall get AC supply directly, and so it shallrun at top speed. When output Q2 becomeshigh and Q1 becomes low, relay RL1 isturned off and relay RL2 is switched on.The fan gets AC through a resistance andits speed drops to medium value. This con-tinues until output Q4 is high. When Q4goes low and Q5 goes high, relay RL2 isswitched off and relay RL3 is activated.The fan now runs at low speed.

Throughout the process, pin11 of the IC3 is low, so T4 is cutoff, thus keeping T5 in satura-tion and RL4 on. At the end ofthe cycle, when pin 11 (Q9) be-comes high, T4 gets saturatedand T5 is cut off. RL4 is switchedoff, thus switching off the fan/cooler.

Using the circuit described above, thefan shall run at high speed for a com-paratively lesser time when either of Q0or Q1 output is high. At medium speed, itwill run for a moderate time period whenany of three outputs Q2 through Q4 is

high, while at low speed, it will run for amuch longer time period when any of thefour outputs Q5 through Q8 is high.

If one wishes, one can make the fanrun at the three speeds for an equal amountof time by connecting three decimaldecoded outputs of IC3 to each of thetransistors T1 to T3. One can also getmore than three speeds by using anadditional relay, transistor, and associated

components, and connecting one or moreoutputs of IC3 to it.

In the motors used in certain coolersthere are separate windings for separatespeeds. Such coolers do not use a rheostattype speed regulator. The method ofconnection of this device to such coolers isgiven in Fig. 4.

The resistors in Figs 2 and 3 are thetapped resistors, similar to those used inmanually controlled fan-speed regulators.Alternatively wire-wound resistors ofsuitable wattage and resistance can beused.


Here is a circuit through which the speed of a fan can be linearly controlled automatically, depending on the room temperature. The circuit is highly efficient as it uses thyristors for power control. Alternatively, the same circuit can be used for automatic tempera-ture controlled AC power control.

In this circuit, the temperature sensor used is an NTC thermistor, i.e. one having a negative temperature coefficient. The value of thermistor resistance at 25C is about 1 kilo-ohm.

Op-amp A1 essentially works as I to V (current-to-voltage) converter and converts temperature variations into voltage variations. To amplify the change in voltage due to change in temperature, instrumentation ampli-fier formed by op-amps A2, A3 and A4 is used. Resistor R2 and zener diode

D1 combination is used for generating reference voltage as we want to am-plify only change in voltage due to the change in temperature.

Op-amp A741 (IC2) works as a comparator. One input to the compara-tor is the output from the instrumen-tation amplifier while the other input is the stepped down, rectified and suitably attenuated sample of AC volt-age. This is a negative going pulsating DC voltage. It will be observed that with increase in temperature, pin 2 of IC2 goes more and more negative and hence the width of the positive going output pulses (at pin 6) increases lin-early with the temperature. Thus IC2 functions as a pulse width modulator in this circuit. The output from the comparator is coupled to an optocou-pler, which in turn controls the AC

power delivered to fan (load).The circuit has a high sensitivity and

the output RMS voltage (across load) can be varied from 120V to 230V (for a temp. range of 22C to 36C), and hence wide variations in speed are available. Also note that speed varies linearly and not in steps. Besides, since an optocoupler is used, the control circuit is fully isolated from power circuit, thus providing added safety. Note that for any given tempera-ture the speed of fan (i.e. voltage across load) can be adjusted to a desired value by adjusting potmeters VR1 and VR2 appropriately.

Potmeter VR1 should he initially kept in its mid position to realise a gain of ap-proximately 40 from the instrumentation amplifier. It may be subsequently trimmed slightly to obtain linear variation of the fan speed.

AUTOMATIC TEMPERATURE CONTROLLED FAN


blown fuse indicatorGenerally, when an equipment indicates no power, the cause may be just a blown fuse. Here is a circuit that shows the condition of fuse through LEDs. This compact circuit is very useful and reliable. It uses very few components, which makes it inexpensive too.

Under normal conditions (when fuse is alright), voltage drop in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in

second arm it is only 2V. So current flows through the second arm, i.e. through the green LED, causing it to glow; whereas the red LED remains off.

When the fuse blows off, the supply to green LED gets blocked, and because only one LED is in the circuit, the red LED glows. In case of power failure, both LEDs remain off.

This circuit can be easily modified to produce a siren in fuse-blown condition

(see Fig. 2). An optocoupler is used to trigger the siren. When the fuse blows, red LED glows. Simultaneously it switches on the siren.

In place of a bicolour LED, two LEDs of red and green colour can be used. Similarly, only one diode in place of D1 and D2 may be used. Two diodes are used to increase the voltage drop, since the two LEDs may produce different voltage drops.


CD-ROM DRive as Digital -auDiO CD-PlayeR

since it has self-contained power supply circuit inside.

While there may be minor differ-ences amongst the available CD-ROM drives external controls, a typical

drives controls are shown in the figure here. Please ensure that a proper power supply con-nector available from computer spare parts vendor is used for connection to CD-ROM drive. To identify +5V and +12V pins on the drive connector, please note that in the computer +12V

is routed using a yellow wire and for +5V a red wir is used, while for ground black wires are used with the sup-ply connector.

Once the power supply has been connected correctly, you will notice that LED indicator on the drive starts flashing. Now the digital audio CD can be loaded after pushing the eject but-ton. A second push of the same button causes retraction of CD carriage into the drive. One can change the track (song) on the CD using play switch on the CD-ROM drive.

ACD-ROM drive can be used as a stand-alone unit for playing dig-ital audio CDs without interfacing with a computer. The stereo output of CD player available at the audio jack can be amplified using audio input fa-cility which is normally available on a tape-deck/tape-recorder or a stereo amplifier. Audio socket on front/rear of the CD-ROM drive is capable of driving headphones or speakers of less than 500 mW. Proper stereo jacks for interconnec-tion between CD-ROM drive and tape deck are available from computer/tape recorder spares vendors. The principle of operation is illustrated here with the help of block diagram.

The 4-pin power supply socket avail-able at the rear of a CD-ROM player is meant for +5V, ground (two middle pins) and +12V inputs. The power supply can be easily derived using a conventional power supply circuit as shown in the figure. If you have an external CD-ROM drive, it can be simply plugged into the mains

circuitideas

w w w . e f y m a g . c o m electronics for you mar ch 2008 105

Dr C.H. VitHalani

Drinking Water alarm s.c. dwivedi

The State Jal Boards supply water for limited duration in a day. Time of water supply is decided by the management and the public does not know the same. In such a situation, this water alarm circuit will save the people from long wait as it will inform them as soon as the water supply starts.

At the heart of this circuit is a small water sensor. For fabricating this water sensor, you need two foilsan aluminium foil and a plastic foil. You can assemble the sensor by rolling aluminium and plastic foils in the shape of a concentric cylinder. Connect one end of the insulated flexible wire on the aluminium foil and the other end to resistor R2. Now mount this sensor inside the water tap such that water can flow through it uninterrupted. To complete the circuit, connect another wire from the junction of pins 2 and 6 of IC1 to the water pipeline or the water tap itself.

The working of the circuit is sim-ple. Timer 555 is wired as an astable multivibrator. The multivibrator will

work only when water flows through the water tap and completes the cir-cuit connection. It oscillates at about 1 kHz. The output of the timer at pin 3 is connected to loudspeaker LS1 via capacitor C3. As soon as water starts flowing through the tap, the speaker starts sounding, which indicates re-sumption of water supply. It remains on until you switch off the circuit with switch S1 or remove the sensor

from the tap. The circuit works off a 9V battery supply.

Assemble the circuit on any gen-eral-purpose PCB and house in a suitable cabinet. The water sensor is inserted into the water tap. Connect the lead coming out from the junc-tion of 555 pins 2 and 6 to the body of the water tap. Use on/off switch S1 to power the circuit with the 9V PP3 battery.


ELECTRONICS FOR YOUNOVEMBER 2002

ASHOK K. DOCTOR

A flashing beacon has many uses. Itcan be employed as a distress sig-nal on highways or as a directionpointer for parking lots, hospitals, hotels,etc. Here we present a flashing beaconthat uses well-known regulator IC LM317T.As LM317T regulator can deliver more than1 amp. A small 12V, 10W bulb with ahigh-quality reflector can serve as a goodvisible blinker.

A 12-15V, 1A DC supply is connectedto the input pin of the IC. A 12V, 10Wbulb and a combination of resistors andcapacitors are connected between the out-put pin and ADJ pin of the IC as shown in

S.C. DWIVED

I

the figure. The IC is pro-vided with an aluminiumheat-sink to dissipate theheat generated while deliv-ering full current. Since theIC has an inbuilt switch-oncurrent limiter, it extendsthe bulb life.

For the shown valuesof resistors and capacitors,the bulb flashes at approxi-mately 4 cycles per second.The number of flashes de-pends on the charge-dis-charge time of the capaci-tors. Different values of resistors and ca-pacitors can be used to increase or de-

FLASHING BEACON

crease the number of flashes.This circuit costs around Rs 50.

circuitideas

w w w . e f y m a g . c o m104 mar ch 2008 electronics for you

Fig. 1: Glow plug controller

Fig. 2: Pin configurations of bs170 and bc548

T.A. BABu

GLOW PLuG CONTROLLER s.c. dwived

i

In diesel engines, the air in the cylinders is not hot enough to ignite the fuel under cold condi-tions. Therefore each cylinder of these

engines is fitted with an electric heater known as glow plug. A control circuit is necessary to optimise the functioning of glow plugs. It raises the air tem-perature inside the engine cylinder for quick and reliable starting, extended battery life and reduced diesel con-sumption.

The glow plug controller (Fig. 1) uses a simple timer circuit built around MOSFET T1 for reliability and simplic-ity. Momentary pushing of switch S2

charges capacitor C1 rapidly via resis-tor R1. When the voltage on capacitor C1 exceeds the threshold voltage of the gate (G) of MOSFET T1, it starts charging reservoir capacitor C2 and simultaneously energises relay RL1.

MOSFET T1 remains conducting as long as the voltage on C1 is greater than the threshold voltage of the MOS-FET gate.

The on time period depends on the value of capacitor C1 and resistor R2, which govern the discharge current of capacitor C2. The component values given here will produce on time of around 25 seconds. In effect, when you press switch S2 momentarily, the relay energises for about 25 seconds and

the glow plug gets the power supply through its contact.

The red LED (LED1) indicates that the heating process of glow plugs is

on. When the on time is over, the green LED (LED2) turns on for a while, followed by a short beep from the buzzer, which in-dicates that the engine is ready for starting. Glow plugs draw a heavy current, hence high-cur-

rent-rating contacts of an automotive relay are required.

Assemble the circuit on any gen-eral-purpose PCB and house in a suitable case. Connect the glow plug wire to the relay contact. 12V battery already available with the vehicle is used to power the circuit. Connect the piezobuzzer and LED1 and LED2 through an external connection and place it at a convenient location for the driver to operate.


ELECTRONICS FOR YOU NOVEMBER 2002

This circuit (Fig. 1), used in conjunc-tion with a thin piezoelectric plate,senses the vibration generated onknocking a surface (such as a door or atable) to activate the alarm. It uses readily-available, low-cost components and canalso be used to safeguard motor vehicles.The piezoelectric plate is used as the sen-sor. It is the same as used in ordinary

piezobuzzers and is easily available in themarket.

The piezoelectric plate can convert anymechanical vibration into electrical varia-tion. As it doesnt sense sound from a dis-tance like a microphone, it avoids falsetriggering. The plate can be fixed on a door,cash box, cupboard, etc using adhesive. A 1-1.5m long, shielded wire is connected be-tween the sensor plate and the input of the

KNOCK ALARM

PRADEEP G.

SUNIL KUMA

R

circuit. When someone knocks on thedoor, the piezoelectric sensor gener-ates an electrical signal, which is am-plified by transistors T1 through T3.

The amplified signal is rectifiedand filtered to produce a low-levelDC voltage, which is further ampli-fied by the remaining transistors. Thefinal output from the collector of pnptransistor T6 is applied to reset pin 4of 555 (IC1) that is wired as an

astable multivibrator. Whenever the col-lector of transistor T6 goes high, the astablemultivibrator activates to sound an alarmthrough the speaker. The value of resistorR12 is chosen between 220 and 680 ohmssuch that IC1 remains inactive in the ab-sence of any perceptible knock.

When the circuit receives an input sig-nal due to knocking, the alarm gets acti-vated for about 10 seconds. This is the

Fig. 1: The circuit of knock alarm

Fig. 2: Proposed installation of knock alarm

time that capacitor C5 connected betweenthe emitter of transistor T4 and groundtakes to discharge after a knock. The timedelay can be changed by changing thevalue of capacitor C5. After about 10 sec-onds, the alarm is automatically reset.

The circuit operates off a 9V or a 12Vbattery eliminator. The proposed installa-tion of the knock alarm is shown in Fig. 2.

This circuit costs around Rs 75.

CIRCUITIDEAS

8 0 A U G U S T 2 0 0 5 E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

CMYK

PRADEEP G.

MEDIUM-POWERFM TRANSMITTER

T he range of this FM transmi-tter is around 100 metres at 9VDC supply.The circuit comprises three stages.

The first stage is a microphone pream-plifier built around BC548 transistor.The next stage is a VHF oscillator wiredaround another BC548. (BC series tran-sistors are generally used in low-fre-quency stages. But these also work fine

S.C. DWIVEDI

Fig. 1: FM transmitter

Fig. 2: Pinconfigurations oftransistors BC548and C2570

Fig. 3: Walkie-talkie arrangement

in RF stages as oscillator.) The thirdstage is a class-A tuned amplifier thatboosts signals from the oscillator. Useof the additional RF amplifier increasesthe range of the transmitter.

Coil L1 comprises four turns of20SWG enamelled copper wire woundto 1.5cm length of a 4mm dia. air core.Coil L2 comprises six turns of 20SWGenamelled copper wire wound on a4mm dia. air core.

Use a 75cm long wire as the an-

tenna. For themaximum range,use a sensitive re-ceiver. VC1 is afrequency-adjust-ing trimpot. VC2should be adjustedfor the maximumrange. The trans-mitter unit is pow-

ered by a 9V PP3 battery. It can becombined with a readily available FMreceiver kit to make a walkie-talkie setas shown in Fig. 3. z

CIRCUIT

IDEAS

9 2 A U G U S T 2 0 0 7 E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

D. MOHAN KUMAR

MOBILE SHIELD MALAYAPPASAM

Y

P rotect your mobile phone fromunauthorised use or theft usingthis simple circuit. It can gener-ate a loud chirping sound when some-body attempts to take away the mo-bile handset. The added feature is thatthe circuit also works as a mobilecharger.

The circuit is powered by astep-down transformer X1 with recti-fier diodes D1 and D2 and filter ca-

pacitor C1. Regulator IC 7812 (IC1)along with noise filter capacitors C2and C3 provides regulated power sup-ply.

The circuit utilises two NE555timer ICs: One as a simple astablemultivibrator (IC2) and the second asa monostable (IC3). The astablemultivibrator has timing resistors R1and R2 but no timing capacitor as itworks with stray capacitance. Its pins6 and 2 are directly connected to a pro-tecting shield made up of 10cm10cmcopper-clad board.

The inherent stray capacitance ofthe circuit is sufficient to given an out-put frequency of about 25 kHz withR1 and R2. This arrangement provides

greater sensitivity and enables the cir-cuit with hand capacitance effect. Out-put pulses from the oscillator are di-rectly given to trigger pin 2 of themonostable. The monostable uses alow-value capacitor C6, resistors R3and preset VR1 for timing.

The output frequency of themonostable is adjusted using presetVR1 such that it is slightly less thanthat of the astable. This makes thecircuit standby, when there is nohand capacitance present. So in the

standby mode, the astables outputwill be low. This makes the triggerinput of monostable low and outputhigh.

The warning LED1 and buzzer areconnected such that they become ac-tive only when the output of themonostable sinks current. In thestandby state, the LED1 remains offand the buzzer is silent. As somebodytries to take the mobile phone fromthe protecting shield, his hand comesnear the shield or makes contact withthe shield, which introduces hand ca-pacitance in the circuit. As a result, theastables frequency changes, whichmakes the trigger pin of themonostable low and its output oscil-

lates. This produces chirping soundfrom the buzzer and also makes theLED1 blink.

The circuit can also be used as amobile charger. It provides output of6V at 180 mA through regulator IC7806 (IC4) and resistor R5 for charg-ing the mobile phone. Diode D3 pro-tects the output from polarity rever-sal.

The circuit can be wired on acommon PCB. Enclose it in a suitablecase with provision for charger out-

put leads. Make the protective shieldusing 10cm10cm copper-clad boardor aluminium sheet. Connect it tothe circuit using a 15cm plastic wire.Leads of all capacitors should beshort.

Adjust VR1 slowly using a plasticscrewdriver until the buzzer stopssounding. Bring the hand close to theshield and adjust VR1 until the buzzersounds. With trial-and-error proce-dure, set it for the maximum sensitiv-ity such that as soon the hand comesnear the shield, the buzzer startschirpring and the LED blinks. Insteadof using the copper cladding for shield,a metallic mobile phone holder can beused as the shield.

circuitideas

88 august 2008 electronics for you w w w . e f y m a g . c o m

R.G. ThiaGaRaj KumaR and P. Kasi Rajan

ThRee-Phase aPPliance PRoTecToR

s.c. dwivedi

Many of our costly appliances require three-phase AC sup-ply for operation. Failure of any of the phases makes the appli-ance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to moni-tor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.

The complete circuit of a three-phase appliance protector is described here. It requires three-phase supply, three 12V relays and a timer IC NE555 along with 230V coil contactor having

four poles.Relays RL1 and RL2 act as a

sensing devices for phases Y and B, respectively. These relays are connected such that each acts as an enabling device for the subsequent relay. Therefore the combination of the relays forms a logical AND gate connected serially.

The availability of phase R en-ergises relay RL1 and its normally-opened (N/O) contacts close to connect phase Y to the input of transformer X2. The availability of phase Y energises relay RL2 and its N/O contacts close to connect phase B to the input of transformer X3, thus applying a triggering input to timer IC NE555 (IC1).

Therefore the delay timer built around NE555 triggers only when all the phases (R, Y and B) are avail-able. It provides a delay of approxi-

mately four seconds, which energises relay RL3 and its N/O contact closes to connect the line to the energising coil of four-pole contactor relay RL4. Contactor RL4 closes to ensure the availability of the three-phase sup-ply to the appliance.

The rating of contactor RL4 can be selected according to the full-load cur-rent rating of the appliances. Here the contact current rating of the four-pole contactor is up to 32A. The availability of phases R, Y and B is monitored by appropriate LEDs connected across the secondary windings of transform-ers X1, X2 and X3, respectively. Hence this circuit does not require a separate

circuitideas

electronics for you august 2008 89w w w . e f y m a g . c o m

indicator lamp for monitoring the availability of the three phases. When phase R is available, LED1 glows. When phase Y is available, LED2 glows. When phase B is available, LED3 glows.

The main advantage of this protec-tor circuit is that it protects three-phase appliances from failure of any of the

mounted on the backside of cabinet. Connect the appliance through exter-nal wires.

Caution. To avoid the risk of elec-tric shock, ensure that AC mains is disconnected during assembly of the circuit and double check everything before connecting your circuit to the mains.

phases by disconnecting the power supply through the contactor and automatically restores the three-phase supply to the appliance (with reason-able time delay) when all the phases are available.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a cabinet with the relays and contactor

circuitideas

114 August 2009 electronics for you w w w . e f y m A g . c o m

There are many ways of battery charging but constant-current charging, in particular, is a popular method for lead-acid and Ni-Cd batteries. In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours. So for a 4.5Ah battery, constant charging cur-rent would be 450 mA.

This battery charger has the follow-ing features:

1. It can charge 6V, 9V and 12V bat-teries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2.

2. Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery.

3. Once the battery is fully charged, it will attain certain voltage level (e.g.

13.5-14.2V in the case of a 12V battery), give indication and the charger will switch off automatically. You need not remove the battery from the circuit.

4. If the battery is discharged be-low a limit, it will give deep-discharge indication.

5. Quiescent current is less than 5 mA and mostly due to zeners.

6. DC source voltage (VCC) ranges from 9V to 24V.

7. The charger is short-circuit pro-tected.

D1 is a low-forward-drop schottky diode SB560 having peak reverse volt-age (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Nor-mally, the minimum DC source volt-age should be D1 drop+Full charged battery voltage+VDSS+ R2 drop, which is approximately Full charged battery voltage+5V. For example, if we take full-charge voltage as 14V for a 12V battery, the source voltage should be

14+5=19V. For the sake of simplicity, this con-

stant-current battery charger circuit is divided into three sections: constant-current source, overcharge protection and deep-discharge protection sec-tions.

The constant-current source is built around MOSFET T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter VR1. Diode D2 is a low-temperature-coefficient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to +70C. Gate-source voltage (VGS) of T5 is set by adjusting VR1 slightly above 4V. By setting VGS, charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the batterys Ah capacity) and then calculate the nearest standard value of R2 as follows:

R2 = 0.7/Safe fault current

Monoj Das

Constant-Current Battery Charger

s.c. dwivedi

circuitideas

electronics for you August 2009 115w w w . e f y m A g . c o m

R2 and T1 limit the charging cur-rent if something fails or battery termi-nals get short-circuited accidentally.

To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its required value.

Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All com-ponents in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influ-ence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the bat-tery.

In overcharge protection, zener

diode ZD1 starts conducting after its breakdown voltage is reached, i.e., it conducts when the battery voltage goes beyond a prefixed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor con-ducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops.

Normally, zener diode ZD2 con-ducts to drive transistor T3 into con-duction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V bat-tery, adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts.

LED2 will glow to indicate that the bat-tery voltage is low.

Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current pro-vided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 pack-age of IRF540 can handle up to 50W.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box.

circuitideas

84 DECEmb Er 2008 electronics for you w w w . E f y m a g . C o m

Aniruddh K.S.

BAttery-level indicAtors.c. dwived

i

Normally, in mobile phones, the battery level is shown in dot or bar form. This lets you easily recognise the battery level. Here we present a circuit that lets you know the battery level of a device from the number of LEDs that are glowing. It uses ten LEDs in all. So if three LEDs glow, it indicates battery capacity of

30 per cent. Unlike in mobile phones where the battery-level indicator func-tion is integrated with other functions, here o

350 electronics for you projects

Documents

c i r c u i t i d e

output of counter ic2

measured output

simple circuit

circuit com

output voltage

r11 r12 r13 r14

ohms fixedand r2