EPE Online, Febuary 1999 - www.epemag.com - XXX

Volume 2 Issue 5May 2000

Unit 14 Sunningdale, BISHOPS STORTFORD, Herts. CM23 2PA

ADD £2.00 P&P to all orders (or 1st Class Recorded £4, Next day(Insured £250) £7, Europe £4.00, Rest of World £6.00). We accept allmajor credit cards. Make cheques/PO's payable to Quasar Electronics.Prices include 17.5% VAT. MAIL ORDER ONLYFREE CATALOGUE with order or send 2 x 1st class stamps (refundable) for details of over 150 kits & publications.

FACTORPUBLICATIONS

SPEED CONTROLLER for any common DC motor upto 100V/5A. Pulse width modulation gives maximumtorque at all speeds. 5-15VDC. Box provided. 3067KT£10.95 3 x 8 CHANNEL IR RELAY BOARD Control eight 12V/1Arelays by Infra Red (IR) remote control over a 20m range insunlight. 6 relays turn on only, the other 2 toggle on/off. 3 oper-ation ranges determined by jumpers. Transmitter case & allcomponents provided. Receiver PCB 76x89mm. 3072KT£44.95 PC CONTROLLED RELAY BOARDConvert any 286 upward PC into a dedicatedautomatic controller to independently turn on/offup to eight lights, motors & other devices aroundthe home, office, laboratory or factory using 8240VAC/12A onboard relays. DOS utilities, sampletest program, full-featured Windows utility & allcomponents (except cable) provided. 12VDC. PCB70x200mm. 3074KT £29.95 2 CHANNEL UHF RELAY SWITCH Contains thesame transmitter/receiver pair as 30A15 below plusthe components and PCB to control two240VAC/10A relays (also supplied). Ultra brightLEDs used to indicate relay status. 3082KT £26.95TRANSMITTER RECEIVER PAIR 2-button keyfobstyle 300-375MHz Tx with 30m range. Receiverencoder module with matched decoder IC.Components must be built into a circuit like kit 3082above. 30A15 £13.95 TELEPHONE LINE RELAY SWITCH Turn on/off 4relays over your phone line from anywhere in the world. 4-digit security code. Line protection circuitry built-in (non-approved). PCB 78x105mm. 3086KT £39.95 PC DATA ACQUISITION/CONTROL UNIT Use yourPC to monitor physical variables (e.g. pressure, tem-perature, light, weight, switch state, movement, relays,etc.), process the information & use results to controlphysical devices like motors, sirens, relays, servo &stepper motors. Inputs: 16 digital & 11 analogue.Outputs: 8 digital & 1 analogue. Plastic case with print-ed front/rear panels, software utilities, programmingexamples & all components (except sensors & cable)provided. 12VDC. 3093KT £79.95 PIC 16C71 FOUR SERVO MOTOR DRIVERSimultaneously control up to 4 servo motors. Software &all components (except servos/control pots) supplied.5VDC. PCB 50x70mm. 3102KT £14.95

PC SERIAL PORT ISOLATED I/O BOARDProvides eight 240VAC/10A relay outputs & 4 opti-cally isolated inputs. Designed for use in various con-trol & sensing applications e.g. load switching, exter-nal switch input sensing, contact closure & externalvoltage sensing. Controlled via serial port & a termi-nal emulator program (built into Windows). Can beused with ANY computer/operating system. Plasticcase with printed front/rear panels & all components(except cable) provided. 3108KT £49.95 UNIPOLAR STEPPER MOTOR DRIVER for any5/6/8 lead motor. Fast/slow & single step rates.Direction control & on/off switch. Wave, 2-phase &half-wave step modes. 4 LED indicators. PCB50x65mm. 3109KT £14.95 PC CONTROLLED STEPPER MOTOR DRIVERControl two unipolar stepper motors (3A max. each)via PC printer port. Wave, 2-phase & half-wave stepmodes. Software accepts 4 digital inputs from exter-nal switches & will single step motors. PCB fits in D-shell case provided. 3113KT £16.95 12-BIT PC DATA ACQUISITION/CONTROL UNITSimilar to kit 3093 above but uses a 12 bit Analogue-to-Digital Converter (ADC) with internal analogue mul-tiplexor. Reads 8 single ended channels or 4 differen-tial inputs or a mixture of both. Analogue inputs read 0-4V. Four TTL/CMOS compatible digital input/outputs.ADC conversion time <10uS. Software (C, QB & Win),extended D shell case & all components (except sen-sors & cable) provided. 3118KT £47.95 LIQUID LEVEL SENSOR/RAIN ALARM Will indi-cate fluid levels or simply the presence of fluid. Relayoutput to control a pump to add/remove water when itreaches a certain level. 1080KT £6.95 UNIVERSAL TIMER Seven crystal controlled tim-ing operations in steps of 0.1s from 0.1-6553.6s or 1second steps from 0.1-65536s. Allows 4 signal inputtypes from push button to electrically isolated volt-age switching sources. On-board relay will switch240V/5A. Box, software & all components provided.PCB 56 x 97mm. 3054KT £24.95

STEREO VU METER shows peak music powerusing 2 rows of 10 LED’s (mixed green & red)moving bar display. 0-30db. 3089KT £10.95 AM RADIO KIT 1 Tuned Radio Frequency front-end, single chip AM radio IC & 2 stages of audioamplification. All components inc. speaker provid-ed. PCB 32x102mm. 3063KT £9.95DRILL SPEED CONTROLLER Adjust the speedof your electric drill according to the job at hand.Suitable for 240V AC mains powered drills up to700W power. PCB: 48mm x 65mm. Box provided.6074KT £17.903 INPUT MONO MIXER Independent level con-trol for each input and separate bass/treble controls.Input sensitivity: 240mV. 18V DC. PCB: 60mm x185mm 1052KT £16.95 ELECTRONIC SIREN 5 Watt. Impressive 5Wpower output. Suitable for alarm systems, car,motorbikes, etc. Output frequency 1·2kHz. 6-12VDC. PCB: 37mm x 71mm. Siren not provided1003KT £5.95 NEGATIVE\POSITIVE ION GENERATORStandard Cockcroft-Walton multiplier circuit. Mainsvoltage experience required. 3057KT £9.95

LED DICE Classic intro to electronics & circuitanalysis. 7 LED’s simulate dice roll, slow down & landon a number at random. 555 IC circuit. 3003KT £7.95 STAIRWAY TO HEAVEN Tests hand-eye co-ordination.Press switch when green segment of LED lights to climbthe stairway - miss & start again! Good intro to severalbasic circuits. 3005KT £7.95 ROULETTE LED ‘Ball’ spins round the wheel,slows down & drops into a slot. 10 LED’s. Good introto CMOS decade counters & Op-Amps. 3006KT£9.95 DUAL LED DICE PIC 16C54 circuit performs sim-ilar function to 3003KT above but two dice. Goodintro to micro-controllers. 3071KT £11.95 9V XENON TUBE FLASHER Transformer circuitsteps up 9V battery to flash a 25mm Xenon tube.Adjustable flash rate (0·25-2 Sec’s). 3022KT £10.95 LED FLASHER 1 5 ultra bright red LED’s flash in7 selectable patterns. 3037MKT £4.50 LED FLASHER 2 Similar to above but flash insequence or randomly. Ideal for model railways.3052MKT £4.50 INTRODUCTION TO PIC PROGRAMMING.Learn programming from scratch. Programminghardware, a P16F84 chip and a two-part, practical,hands-on tutorial series are provided. 3081KT£21.95 SERIAL PIC PROGRAMMER for all 8/18/28/40pin DIP serial programmed PICs. 3rd party softwaresupplied expires after 21 days (costs US$25 to reg-ister). 3096KT £14.95 ‘PICALL’ SERIAL & PARALLEL PIC PRO-GRAMMER for all 8/18/28/40 pin DIP parallel ANDserial PICs. Includes fully functional & registeredsoftware (DOS, W3.1, W95/8). 3117KT £54.95 ATMEL 89Cx051 PROGRAMMER Simple-to-useyet powerful programmer for the Atmel 89C1051,89C2051 & 89C4051 uC’s. Programmer does NOTrequire special software other than a terminal emu-lator program (built into Windows). Can be used withANY computer/operating system. 3121KT £34.95

3V/1·5V TO 9V BATTERY CONVERTER Replaceexpensive 9V batteries with economic 1.5V batter-ies. IC based circuit steps up 1 or 2 ‘AA’ batteries togive 9V/18mA. 3035KT £4.95 STABILISED POWER SUPPLY 3-30V/2.5A Idealfor hobbyist & professional laboratory. Very reliable& versatile design at an extremely reasonable price.Short circuit protection. Variable DC voltages (3-30V). Rated output 2.5 Amps. Large heatsink sup-plied. You just supply a 24VAC/3A transformer. PCB55x112mm. Mains operation. 1007KT £17.50.Custom Designed Box 2007 £34.95 STABILISED POWER SUPPLY 2-30V/5A As kit1007 above but rated at 5Amp. Requires a24VAC/5A transformer. 1096KT £29.95. CustomDesigned Box 2096 £34.95 RFI POWER SUPPLY Designed to power RFtransmitters/receivers. Blocks high frequencies &eliminates problems like noise, overheating, stand-ing waves, etc. Output: 12-14VDC/3A.Thermal/short circuit protection & electronic stabili-sation. You just supply a 18VAC/3A transformer.PCB 72x82mm. 1171KT £24.95MOTORBIKE ALARM Uses a reliable vibration sen-sor (adjustable sensitivity) to detect movement of thebike to trigger the alarm & switch the output relay towhich a siren, bikes horn, indicators or other warningdevice can be attached. Auto-reset. 6-12VDC. PCB57x64mm. 1011KT £11.95 Box £5.95 CAR ALARM SYSTEM Protect your car fromtheft. Features vibration sensor, courtesy/boot lightvoltage drop sensor and bonnet/boot earth switchsensor. Entry/exit delays, auto-reset and adjustablealarm duration. 6-12V DC. PCB: 47mm x 55mm1019KT £11.95 Box £6.50 LIGHT ALARM Protect your valuables. Alarmsounds if circuit detects smallest amount of light.Place in cash box etc. 3008KT £4.50 PIEZO SCREAMER 110dB of ear piercingnoise. Fits in box with 2 x 35mm piezo elementsbuilt into their own resonant cavity. Use as analarm siren or just for fun! 6-9VDC. 3015KT £9.95 COMBINATION LOCK Versatile electronic lockcomprising main circuit & separate keypad forremote opening of lock. Relay supplied. 3029KT£9.95 ULTRASONIC MOVEMENT DETECTOR Crystallocked detector frequency for stability & reliability. PCB75x40mm houses all components. 4-7m range.Adjustable sensitivity. Output will drive externalrelay/circuits. 9VDC. 3049KT £12.95 PIR DETECTOR MODULE 3-lead assembled unitjust 25x35mm as used in commercial burglar alarmsystems. 3076KT £7.95 INFRARED SECURITY BEAM When the invisi-ble IR beam is broken a relay is tripped that can beused to sound a bell or alarm. 25 metre range.Mains rated relays provided. 12VDC operation.3130KT £11.95 FUNCTION GENERATOR Quad Op-Amp oscilla-tor & wave shaper circuit generates audio rangesquare waves (6Hz-6KHz), triangle & pseudo sineoutputs. 9VDC. 3023KT £3.95 LOGIC PROBE tests CMOS & TTL circuits &detects fast pulses. Visual & audio indication oflogic state. Full instructions supplied. 3024KT£6.95 SQUARE WAVE OSCILLATOR Generatessquare waves at 6 preset frequencies in factors of10 from 1Hz-100KHz. Visual output indicator. 5-18VDC. Box provided. 3111KT £7.95 PC DRIVEN POCKET SAMPLER/DATA LOG-GER Analogue voltage sampler records voltagesup to 2V or 20V over periods from milli-seconds tomonths. Can also be used as a simple digitalscope to examine audio & other signals up toabout 5KHz. Software & D-shell case provided.3112KT £19.95 20 MHz FUNCTION GENERATOR Square, tri-angular and sine waveform up to 20MHz over 3ranges using ‘coarse’ and ‘fine’ frequency adjust-ment controls. Adjustable output from 0-2V p-p. ATTL output is also provided for connection to afrequency meter. Uses MAX038 IC. Plastic casewith printed front/rear panels & all componentsprovided. 7-12VAC. 3101KT £49.95

X

338 Everyday Practical Electronics, May 2000

SSUURRVVEEIILLLLAANNCCEEHigh performance surveillance bugs. Room transmitters supplied with sensitive electret microphone & battery holder/clip. All trans-mitters can be received on an ordinary VHF/FM radio between 88-108MHz. Available in Kit Form (KT) or Assembled & Tested (AS).

RROOOOMM SSUURRVVEEIILLLLAANNCCEE MTX - MINIATURE 3V TRANSMITTEREasy to build & guaranteed to transmit 300m @ 3V. Long bat-tery life. 3-5V operation. Only 45x18mm. 3007KT £5.95AS3007 £10.95MRTX - MINIATURE 9V TRANSMITTEROur best selling bug. Super sensitive, high power - 500m range@ 9V (over 1km with 18V supply and better aerial). 45x19mm.3018KT £6.95 AS3018 £11.95HPTX - HIGH POWER TRANSMITTERHigh performance, 2 stagetransmitter gives greaterstability & higher qualityreception. 1000m range 6-12V DC operation. Size70x15mm. 3032KT £8.95AS3032 £17.95 MMTX - MICRO-MINIATURE 9V TRANSMITTERThe ultimate bug for its size, performance and price. Just15x25mm. 500m range @ 9V. Good stability. 6-18V operation.3051KT £7.95 AS3051 £13.95 VTX - VOICE ACTIVATED TRANSMITTEROperates only when sounds detected. Low standby current.Variable trigger sensitivity. 500m range. Peaking circuit sup-plied for maximum RF output. On/off switch. 6V operation. Only63x38mm. 3028KT £9.95 AS3028 £22.95HARD-WIRED BUG/TWO STATION INTERCOMEach station has its own amplifier, speaker and mic. Can beset up as either a hard-wired bug or two-station intercom. 10mx 2-core cable supplied. 9V operation. 3021KT £13.95 (kitform only) TRVS - TAPE RECORDER VOX SWITCHUsed to automatically operate a tape recorder (not supplied)via its REMOTE socket when sounds are detected. All conver-sations recorded. Adjustable sensitivity & turn-off delay.115x19mm. 3013KT £7.95 AS3013 £19.95

TTEELLEEPPHHOONNEE SSUURRVVEEIILLLLAANNCCEE MTTX - MINIATURE TELEPHONE TRANSMITTERAttaches anywhere to phone line. Transmits only when phoneis used! Tune-in your radio and hear both parties. 300m range.Uses line as aerial & power source. 20x45mm. 3016KT £7.95AS3016 £13.95 TRI - TELEPHONE RECORDING INTERFACEAutomatically record all conversations. Connects betweenphone line & tape recorder (not supplied). Operates recorderswith 1.5-12V battery systems. Powered from line. 50x33mm.3033KT £7.95 AS3033 £16.95 TPA - TELEPHONE PICK-UP AMPLIFIER/WIRELESSPHONE BUGPlace pick-up coil on the phone line or near phone earpieceand hear both sides of the conversation. 3055KT £10.95AS3055 £19.95 1 WATT FM TRANSMITTER Easy to construct. Delivers acrisp, clear signal. Two-stage circuit. Kit includes microphoneand requires a simple open dipole aerial. 8-30VDC. PCB42x45mm. 1009KT £14.95 4 WATT FM TRANSMITTER Comprises three RFstages and an audio preamplifier stage. Piezoelectricmicrophone supplied or you can use a separate pream-plifier circuit. Antenna can be an open dipole or GroundPlane. Ideal project for those who wish to get started inthe fascinating world of FM broadcasting and want agood basic circuit to experiment with. 12-18VDC. PCB44x146mm. 1028KT. £23.95 15 WATT FM TRANSMITTER (PRE-ASSEMBLED &TESTED) Four transistor based stages with Philips BLY88 in final stage. 15 Watts RF power on the air. 88-108MHz. Accepts open dipole, Ground Plane, 5/8, J, orYAGI configuration antennas. 12-18VDC. PCB70x220mm. SWS meter needed for alignment. 1021KT£69.95 SIMILAR TO ABOVE BUT 25W Output. 1028KT £79.95

3300--iinn--OONNEEEElleeccttrroonniicc PPrroojjeeccttss LLaabbBBAARRGGAAIINN

BBUUYY!!!!Great introduction to electronics. Ideal for the budding elec-tronics expert! Build a radio, burglar alarm, water detector,morse code practice circuit, simple computer circuits, andmuch more! NO soldering, tools or previous electronicsknowledge required. Circuits can be built and unassembledrepeatedly. Comprehensive 68-page manual with explana-tions, schematics and assembly diagrams. Suitable for age10+. Excellent for schools. Requires 2 x AA batteries.ONLY £14.95 (phone for bulk discounts).

PPRROOJJEECCTT KKIITTSSOUR PROJECT KITS COME COMPLETE WITH ALL COMPONENTS,

HIGH QUALITY PCBs, DETAILED ASSEMBLY/OPERATING INSTRUCTIONS

2 x 25W CAR BOOSTER AMPLIFIER Connects tothe output of an existing car stereo cassette player,CD player or radio. Heatsinks provided. PCB76x75mm. 1046KT. £24.95 1W+1W STEREO AMPLIFIER MODULE UsesSamsung KA2209 IC (equivalent to the TDA2822)designed for portable cassette players & radios. 1.8-9VDC. PCB 35x50mm. 3087KT £4.25 10W+10W STEREO AMPLIFIER MODULE UsesTDA2009 class audio power amp IC designed for highquality stereo applications. 8-28VDC. PCB 45x80mm.3088KT £9.95 18W BTL AUDIO AMPLIFIER MODULE Low volt-age, high power mono 18W BTL amp using HA13118IC. Delivers 14W into 4 Ohm’s (1% THD) with 13.2Vsupply. Thermal/surge protection. 8-18VDC. Heatsinkprovided. PCB 57x55mm. 3105KT £8.95 3-CHANNEL WIRELESS LIGHT MODULATORNo electrical connection with amplifier. Light modula-tion achieved via a sensitive electret microphone.Separate sensitivity control per channel. Power hand-ing 400W/channel. PCB 54x112mm. Mains powered.Box provided. 6014KT £24.90 12 RUNNING LIGHT EFFECT Exciting 12 LEDlight effect ideal for parties, discos, shop-windows &eye-catching signs. PCB design allows replacementof LEDs with 220V bulbs by inserting 3 TRIACs.Adjustable rotation speed & direction. PCB54x112mm. 1026KT £16.95; BOX (for mains opera-tion) 2026KT £8.50 DISCO STROBE LIGHT Probably the most excit-ing of all light effects. Very bright strobe tube.Adjustable strobe frequency: 1-60Hz. Mains powered.PCB: 60x68mm. Box provided. 6037KT £29.90 SOUND EFFECTS GENERATOR Easy to build.Create an almost infinite variety of interesting/unusu-al sound effects from birds chirping to sirens. 9VDC.PCB 54x85mm. 1045KT £8.95 ROBOT VOICE EFFECT Make your voice soundsimilar to a robot or Darlek. Great fun for discos,school plays, theatre productions, radio stations &playing jokes on your friends when answering thephone! PCB 42x71mm. 1131KT £8.95 AUDIO TO LIGHT MODULATOR Controls intensi-ty of one or more lights in response to an audio input.Safe, modern opto-coupler design. Mains voltageexperience required. 3012KT £7.95 MUSIC BOX Activated by light. Plays 8 Christmassongs and 5 other tunes. 3104KT £6.95 20 SECOND VOICE RECORDER Uses non-volatile memory - no battery backup needed.Record/replay messages over & over. Playback asrequired to greet customers etc. Volume control &built-in mic. 6VDC. PCB 50x73mm.3131KT £11.95 TRAIN SOUNDS 4 selectable sounds : whistleblowing, level crossing bell, ‘clickety-clack’ & 4 insequence. SG01M £4.95 ANIMAL SOUNDS Cat, dog, chicken & cow. Idealfor kids farmyard toys & schools. SG10M £4.50 3 1/2 DIGIT LED PANEL METER Use for basicvoltage/current displays or customise to measuretemperature, light, weight, movement, sound lev-els, etc. with appropriate sensors (not supplied).Various input circuit designs provided. 3061KT£12.95 IR REMOTE TOGGLE SWITCH Use any TV/VCRremote control unit to switch onboard 12V/1A relayon/off. 3058KT £9.95

Full details of all X-FACTOR PUBLICATIONS can be found inour catalogue. N.B. Minimum order charge for reports andplans is £5.00 PLUS normal P.&P.

SUPER-EAR LISTENING DEVICE Complete plans tobuild your own parabolic dish microphone. Listen to distantvoices and sounds through open windows and even walls!Made from readily available parts. R002 £3.50 TELEPHONE BUG PLANS Build you own micro-beetletelephone bug. Suitable for any phone. Transmits over 250metres - more with good receiver. Made from easy toobtain, cheap components. R006 £2.50 LOCKS - How they work and how to pick them. This factfilled report will teach you more about locks and the art oflock picking than many books we have seen at 4 times theprice. Packed with information and illustrations. R008 £3.50 RADIO & TV JOKER PLANSWe show you how to build three different circuits for dis-rupting TV picture and sound plus FM radio! May upsetyour neighbours & the authorities!! DISCRETIONREQUIRED. R017 £3.50 INFINITY TRANSMITTER PLANS Complete plans forbuilding the famous Infinity Transmitter. Once installed onthe target phone, device acts like a room bug. Just call thetarget phone & activate the unit to hear all room sounds.Great for home/office security! R019 £3.50THE ETHER BOX CALL INTERCEPTOR PLANS Grabstelephone calls out of thin air! No need to wire-in a phonebug. Simply place this device near the phone lines to hearthe conversations taking place! R025 £3.00 CASH CREATOR BUSINESS REPORTS Need ideasfor making some cash? Well this could be just what youneed! You get 40 reports (approx. 800 pages) on floppydisk that give you information on setting up different busi-nesses. You also get valuable reproduction and duplicationrights so that you can sell the manuals as you like. R030£7.50

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EPE Online, February 1999 - www.epemag.com - XXXEPE Online, May 2000 - www.epemag.com - 335

PROJECTS AND CIRCUITS

TECHNOLOGY TIMELINES - Part 4 - by Clive Maxfield and Alvin Brown Who, what, where and when - the facinating story of how technology

developed in the last millennium373

CIRCUIT SURGERY - by Alan Winstanley and Ian BellOpamps - outputs and short-circuit protection; Battery Flattery.

390

REGULARS AND SERVICES

NEWS - Barry Fox highlights technology’s leading edge. Plus everydaynews from the world of electronics.

408

READOUT - John Becker addresses general points arising. 412

SHOPTALK - with David Barrington The essential guide to component buyingfor EPE Online projects.

417

EDITORIAL 336

SERIES AND FEATURES

INGENUITY UNLIMITED - hosted by Alan WinstanleySensitive Hall Effect Switch; Infra-red Remote Tester;Experimenter’s Power Supply

346

NET WORK - THE INTERNET PAGE surfed by Alan WinstanleyGoogle Box; Free for All; Under the Surf; Looking Ahead 371

PIR LIGHT CHECKER - by Terry de Vaux-Balbirnie Be trigger happy with your outdoor security light system!

364

VERSATILE MIC/AUDIO PREAMPLIFIER - by Raymond HaighNew chip SSM2166 offers AGC compression, limiting and noise reduction 338

MULTI_CHANNEL TRANSMISSION SYSTEMpart 1 - by Andy Flind

An 8 to 16-channel 2-wire signalling link with optional interface

355

LOW-COST CAPACITANCE METER - by Robert PenfoldAn easy-to-build Starter Project that adds a userful tool to your workshop

349

NEW TECHNOLOGY UPDATE - by Ian PooleLower operating voltages speed microprocessor rates, but heat dissipationbecomes more of a problem

387

PRACTICALLY SPEAKING - by Robert PenfoldA novice’s guide to using stripboard

TEACH-IN 2000 - Part 7 - by John BeckerEssential info for the electronics novice, with breadboard experimentsand interactive computer simulations

393

404

EPE Online, May 2000 - www.epemag.com - XXX

CHIPPEREvery so often a new chip comes along that looks like it will be very popular for a wide range

of applications. Just such a chip forms the basis of our main cover subject this month – theVersatile Microphone/Audio Preamplifier. The problem with specialized chips of this nature isthat sometimes they don’t stay in production for a long period. This can obviously causeheadaches for hobbyists, who seem to like to build projects years after they have beenpublished. This is why we advise readers to check that all components are still available beforecommencing any project in a back dated issue. Whilst it is sometimes possible to find old chips(particularly via the internet) they are often highly priced and obviously supplies do eventually getexhausted.

We do, however, have high hopes that this chip will be around for some time as it appears tohave been designed to cover a very wide range of applications, including use for microphoneinputs to PCs. This fact alone will ensure high demand and therefore longevity, should it betaken up by the computer manufacturers. Let’s hope it is. However, that in itself does not entirelyget us out of the woods – the PC makers will no doubt use a surface mount device which doesnot necessarily guarantee continuing availability of the DIL version. Once development has takenplace, the industrial requirement for DIL versions often falls dramatically so they can sometimesbe discontinued.

NO WAYWe suppose the answer is to build it now and hope for the best. There seems to be no way

of knowing which chips will hang around and also no way of knowing of all the chips that havebeen discontinued. We usually only find this out when readers ring us with buying problems.Thankfully we can often help them out, but we have an ever-increasing list of past projects thatare no longer viable because of obsolete components. Unfortunately it is not a problem weexpect will improve as time goes by.

EPE Online, May 2000 - www.epemag.com - 335

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EPE Online, May 2000 - www.epemag.com - 337

CANUTE TIDE PREDICTORFor several years the author experimented with writing computer software intended to produce results that

matched those given in published tide tables, and which could ultimately form the basis for a low-power micro-processor controlled tide predictor (long before PICs came along).

Eventually he became aware that official tide tables are compiled not just according to the geometries ofthe Earth-Moon-Sun system, but also in relation to local data compiled over generations. There was no hope,therefore, of developing a simple system that could match standard tide-table accuracy.

However, most people do not need the accuracy of official tide tables. All they might be interest in, for ex-ample, is whether it is better to go to the beach in the morning or afternoon in order to find the tide at the pre-ferred state of rise or fall.

That is what the Canute Tide Predictor is aimed at achieving – to show on a liquid crystal display(LDC) atide-state bar-graph and high-low tide times accurate to within about an hour. The use of a PIC16F876 micro-controller has allowed a very simple unit to be designed.

Anyone who loves the sea, sandy beaches, or rocky shore lines will find this design a useful guide whenconsidering a quick trip to the coast.

TECHNOLOGY TIMELINES – THE FUTUREIt’s been interesting and fun looking back over the last 100 years or so to see how we got where we are –

quite a staggered path, with all sorts of odd developments coming together to produce major forward steps intechnology. Finally we get to peer into the future, it’s not quite as exact an art as looking back, but Max andAlvin are taking a stab at it from their starting point at the forefront of technology in the USA. We may not needto wait long to see if what they predict actually happens with the rate of development of new technology.

ATMOSPHERIC ELECTRICITYThe ionized layers of the atmosphere extend from about 40km to 200km (25 to 125 miles) above the Earth.

This ionization is caused by the “Solar Wind” passing the Earth and leaves the upper atmosphere positivelycharged.

There is thus an electric field between the upper atmosphere and the Earth and, given suitable instruments,this field can be detected as it results in a miniscule current through the atmosphere.

A potential of around 100 volts is often present just one meter off the ground. In other words, there may bea potential of 200 volts or more between your nose and toes! Of course, nobody gets electrocuted because theresistance of the air is so high that only a very tiny current is present. And this is why the actual values are gen-erally considered to be so difficult to measure. But in fact they can be measured quite easily and we will showyou how.

MAIL ORDER ONLY • CALLERS BY APPOINTMENT

135 Hunter Street, Burton-on-Trent, Staffs. DE14 2STTel 01283 565435 Fax 546932http://www.magenta2000.co.ukE-mail: sales@magenta2000.co.ukAll Prices include V.A.T. Add £3.00 per order p&p. £6.99 next day

MOSFET MkII VARIABLE BENCHPOWER SUPPLY 0-25V 2·5A.Based on our Mk1 designand preserving all thefeatures, but now withswitching pre-regulator formuch higher efficiency. Panelmeters indicate Volts andAmps. Fully variable down tozero. Toroidal mains trans-former. Kit includes punchedand printed case and allparts. As featured in April1994 EPE. An essential pieceof equipment.

12V EPROM ERASERA safe low cost eraser for up to 4 EPROMS at a timein less than 20 minutes. Operates from a 12V supply(400mA). Used extensively for mobile work – up-dating equipment in the field etc. Also in educa-tional situations where mains supplies are not al-lowed. Safety interlock prevents contact with UV.

KIT 790............................£29.90

PORTABLE ULTRASONICPEsT SCARERA powerful 23kHz ultrasound generator ina compact hand-held case. MOSFET outputdrives a special sealed transducer with in-tense pulses via a special tuned transformer.Sweeping frequency output is designed togive maximum output without any specialsetting up.

KIT 842........................£22.56

TENS UNIT DUAL OUTPUT TENS UNITAs featured in March ’97 issue.Magenta have prepared a FULL KIT for thisexcellent new project. All components, PCB,hardware and electrodes are included.Designed for simple assembly and testing andproviding high level dual output drive.

KIT 866.... Full kit including four electrodes £32.90

SEE ICEBREAK ADPAGE 367

ULTRASONIC PEsT SCARERKeep pets/pests away fromnewly sown areas, fruit,vegetable and flower beds,children’s play areas, patiosetc. This project producesintense pulses of ultrasoundwhich deter visiting animals.• KIT INCLUDES ALL

COMPONENTS, PCB & CASE• EFFICIENT 100V

TRANSDUCER OUTPUT• COMPLETELY INAUDIBLE

TO HUMANSKIT 812...............................................£15.00

• UP TO 4 METRESRANGE

• LOW CURRENT DRAIN

1000V & 500V INSULATIONTESTER

Superb new design. Regulatedoutput, efficient circuit. Dual-scale meter, compact case.Reads up to 200 Megohms.Kit includes wound coil, cut-outcase, meter scale, PCB & ALLcomponents.

KIT 848...................£32.95

EPE MICROCONTROLLERP.I. TREASURE HUNTER

The latest MAGENTA DESIGN – highlystable & sensitive – with I.C. controlof all timing functions and advancedpulse separation techniques.• High stability

drift cancelling• Easy to build

& use• No ground

effect, worksin seawater

• Detects gold,silver, ferrous &non-ferrousmetals

• Efficient quartz controlledmicrocontroller pulse generation.

• Full kit with headphones & allhardware

KIT 847......................£63.95

SPACEWRITERAn innovative and excitingproject. Wave the wand throughthe air and your message appears.Programmable to hold any messageup to 16 digits long. Comes pre-loadedwith ‘‘MERRY XMAS’’. Kit includesPCB, all components & tube plusinstructions for message loading.

KIT 849.......................£16.99WINDICATORA novel wind speed indicator with LED readout. Kit comescomplete with sensor cups, and weatherproof sensinghead. Mains power unit £5.99 extra.

KIT 856...........................................£28.00

328 Everyday Practical Electronics, May 2000

SUPER BATDETECTOR

1 WATT O/P, BUILT INSPEAKER, COMPACT CASE

20kHz-140kHzNEW DESIGN WITH 40kHz MIC.A new circuit using a ‘full bridge’ audioamplifier i.c., internalspeaker, and head-phone/tape socket. Thelatest sensitive transducer,and ‘double balanced mixer’give a stable, high peformancesuperheterodyne design.

KIT 861.......................£24.99ALSO AVAILABLE Built & Tested ....£39.99

MICRO PEsTSCAREROur latest design – The ultimatescarer for the garden. Usesspecial microchip to give randomdelay and pulse time. Easy tobuild reliable circuit. Keeps pets/pests away from newly sown areas,play areas, etc. Uses power sourcefrom 9 to 24 volts.

• RANDOM PULSES• HIGH POWER• DUAL OPTIONKIT 8 6 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . £ 1 9 .9 9KIT+S LAVE UNIT . . . . . . . . . . . . . . . . . . . £ 3 2 .5 0

Plug-in power supply £4.99

EPEPROJECT

PICsProgrammed PICs for

all* EPE Projects16C84/16F84/16C71

All £5.90 eachPIC16F877 now in stock£10 inc. VAT & postage

(*some projects arecopyright)

PIC PIPE DESCALER• SIMPLE TO BUILD• HIGH POWER OUTPUT• AUDIO & VISUAL MONITORINGAn affordable circuit which sweepsthe incoming water supply withvariable frequency electromagneticsignals. May reduce scale formation,dissolve existing scale and improvelathering ability by altering the waysalts in the water behave.Kit includes case, P.C.B, couplingcoil and all components.High coil current ensures maximumeffect. L.E.D. monitor

KIT 868 .........£22.95 POWER UNIT.........£3.99

• SWEPTFREQUENCY

Kit No. 845.................£64.95

DC Motor/GearboxesOur Popular and Versatile DCmotor/Gearbox sets.Ideal for Models, Robots,Buggies etc. 1·5 to 4·5VMulti ratio gearboxgives wide range of speeds.

LARGE TYPE – MGL £6.95SMALL – MGS – £4.77

Stepping Motors

MD38...Mini 48 step...£8.65MD35...Std 48 step...£9.99MD200...200 step...£12.99MD24...Large 200 step...£22.95

Set of4 spare

electrodes£6.50

EPETEACH-IN2000Full set of top quality NEWcomponents for this educationalseries. All parts as specified byEPE. Kit includes breadboard,wire, croc clips, pins and allcomponents for experiments, aslisted in Introduction to Part 1.*Batteries and tools not included.

TEACH-IN 2000 –

KIT 879 £44.95MULTIMETER £14.45

EE213

Everyday Practical Electronics, May 2000 329

DEVELOPMENT ANDTRAINING KIT

Mini-Lab & Micro LabElectronics Teach-In 7As featured in EPE and nowpublished as Teach-In 7. Allpartsare supplied by Magenta.Teach-In 7 is £3.95 from us orEPEFull Mini Lab Kit – £119.95 –Power supply extra – £22.55Full Micro Lab Kit – £155.95Built Micro Lab – £189.95

SIMPLE PIC PROGRAMMER

Based on February ’96 EPE. Magenta designed PCB and kit.PCB with ‘Reset’ switch, Program switch, 5V regulator andtest L.E.D.s, and connection points for access to all A and Bport pins.

PIC16C84 LCD DISPLAY DRIVERINCLUDES 1-PIC16F84WITH DEMO PROGRAMSOFTWARE DISK, PCB,INSTRUCTIONS AND16-CHARACTER 2-LINE

LCD DISPLAYAnother super PIC project from Magenta. Supplied with PCB,industry standard 2-LINE x 16-character display, data, allcomponents, and software to include in your own programs.Ideal develpment base for meters, terminals, calculators,counters, timers – Just waiting for your application!

68000 NEW PCB DESIGN 8 MHz 68000 16-BIT BUS MANUAL AND SOFTWARE 2 SERIAL PORTS PIT AND I/O PORT OPTIONS I2C PORT OPTIONS

INCLUDES 1-PIC16F84 CHIPSOFTWARE DISK, LEADCONNECTOR, PROFESSIONALPC BOARD & INSTRUCTIONS

INCREDIBLE LOW PRICE! Kit 857 £12.99

Kit 863 £18.99

Kit 860 £19.99

Kit 862 £29.99Power Supply £3.99

DISASSEMBLERSOFTWARE £11.75

FULL SOURCE CODE SUPPLIED.ALSO USE FOR DRIVING OTHERPOWER DEVICES e.g. SOLENOIDS.

Power Supply £3.99FULL PROGRAM SOURCECODE SUPPLIED – DEVELOPYOUR OWN APPLICATION!

Power Supply £3.99EXTRA CHIPS:

PIC 16F84 £4.84

SUPER PIC PROGRAMMER READS, PROGRAMS, AND VERIFIES WINDOWS SOFTWARE PIC16C6X, 7X, AND 8X USES ANY PC PARALLEL PORT USES STANDARD MICROCHIP HEX FILES OPTIONAL DISASSEMBLER SOFTWARE (EXTRA) PCB, LEAD, ALL COMPONENTS, TURNED PIN

SOCKETS FOR 18, 28, AND 40 PIN ICs.

SEND FOR DETAILEDINFORMATION – ASUPERB PRODUCT AT ANUNBEATABLE LOW PRICE.

PIC STEPPING MOTOR DRIVER

Another NEW Magenta PIC project. Drives any 4-phase unipolar motor – upto 24V and 1A. Kit includes all components and 48 step motor. Chip ispre-programmed with demo software, then write your own, and re-programthe same chip! Circuit accepts inputs from switches etc and drives motor inresponse. Also runs standard demo sequence from memory.

INCLUDES: PCB,PIC16F84 WITHDEMO PROGRAM,SOFTWARE DISK,INSTRUCTIONSAND MOTOR.

PIC16F84 MAINS POWER 4-CHANNELCONTROLLER & LIGHT CHASER

WITH PROGRAMMED 16F84 AND DISK WITHSOURCE CODE IN MPASM

ZERO VOLT SWITCHINGMULTIPLE CHASE PATTERNS

OPTO ISOLATED5 AMP OUTPUTS

12 KEYPAD CONTROL SPEED/DIMMING POT. HARD FIRED TRIACS

Kit 855 £39.95

All pricesinclude VAT. Add £3.00 p&p. Next Day £6.99

Now features full 4-channelchaser software on DISKand pre-programmedPIC16F84 chip. Easilyre-programmed for yourown applications. Softwaresource code is fully‘commented’ so that it canbe followed easily.

LOTS OF OTHER APPLICATIONS

KIT 621£99.95

ON BOARD5V REGULATOR

PSU £6.99 SERIAL LEAD £3.99

EPE PIC TutorialAt Last! A Real, Practical, Hands-On Series Learn Programming from scratch using PIC16F84

Start by lighting l.e.d.s and do 30 tutorials to SoundGeneration, Data Display, and a Security System

PIC TUTOR Board with Switches, l.e.d.s, and on boardprogrammer

PIC TUTOR BOARD KITIncludes: PIC16F84 Chip, TOP Quality PCB printed withComponent Layout and all components* (*not ZIFSocket or Displays). Included with the Magenta Kit is adisk with Test and Demonstration routines.KIT 870 ...... £27.95, Built & Tested ...... £42.95Optional: Power Supply – £3.99, ZIF Socket – £9.99LCD Display ................£7.99 LED Display ..............£6.99Reprints Mar/Apr/May 98 – £3.00 set 3

Tel: 01283 565435 Fax: 01283 546932 E-mail: sales@magenta2000.co.uk

PIC TOOLKIT V1 PROGRAMS PIC16C84 and 16F84 ACCEPTS TASM AND MPASM CODEFull kit includes PIC16F84 chip, top quality p.c.b. printed with componentlayout, turned pin PIC socket, all components and software**Needs QBASIC or QUICKBASIC

KIT 871 . . . £13.99. Built and tested £21.99

PhizzyB ALL PARTS FOR SERIES INCLUDING PCBs,PROGRAMMED CHIP, CD-ROM AND DISPLAYS

MAIN BOARD – FULL KIT £131.95 BUILT .......... £149.95I/O PORT KIT ................... £16.99 BUILT ............ £24.99L.C.D. ............................... £12.49 POWER SUPPLY ..£3.998-BIT SWITCH/LATCH ..... £7.95 INT. MODULE .£10.45

8-CHANNEL DATA LOGGERAs featured in Aug./Sept. ’99 EPE. Full kit with Magentaredesigned PCB – LCD fits directly on board. Use as DataLogger or as a test bed for many other 16F877 projects. Kitincludes programmed chip, 8 EEPROMs, PCB, case and all components.

KIT 877 £49.95 inc. 8 x 256K EEPROMS

PIC TOOLKIT V2 SUPER UPGRADE FROM V1 18, 28 AND 40-PIN CHIPS READ, WRITE, ASSEMBLE & DISASSEMBLE PICS SIMPLE POWER SUPPLY OPTIONS 5-20V ALL SWITCHING UNDER SOFTWARE CONTROL MAGENTA DESIGNED PCB HAS TERMINAL PINS AND OSCILLATOR

CONNECTIONS FOR ALL CHIPS INCLUDES SOFTWARE AND PIC CHIPKIT 878 . . . £22.99 with 16F84 . . . £29.99 with 16F877

EPE Online, May 2000 - www.epemag.com - 338

Intended primarily as ameans of processing microphoneinputs to computers, theSSM2166P integrated circuit (IC)– manufactured by AnalogDevices – has a wider range ofpossible applications. Publicaddress and surveillance systemsimmediately spring to mind, andthe device will be of particularinterest to radio enthusiasts,especially now that the popularPlessey 6270 IC mic/pre-amp,with voice gain, is no longeravailable.

This article describes how thenew IC can be used for a varietyof signal inputs, and additionalcircuitry is given for readers who

AMPLIFIERSThe input impedance of

buffer amplifier, A, is 180kilohms (180k) and its gain canbe set, by external feedbackresistors, between 0dB and20dB. There is a standing DCvoltage on the input, and ablocking capacitor must beused.

The input and outputimpedances of the controlledamplifier, D, are 1k, and 75ohms, respectively. A standingDC voltage necessitates the useof a blocking capacitor at theoutput.

Use one of the latest chips on the block to produce an audio pre-amp with AGCcompression, limiting, and noise reduction.

VERSATILE MIC/AUDIO PREAMPLIFIERby RAYMOND HAIGH

require a signal-strength meter.THE CHIP

The various amplifying andcontrol stages built into theSSM2166 chip are shown inFig.1.

Signal inputs are bufferedby opamp A, internallyconnected to a rectifier stage,B, which produces a DC voltagewhich varies in proportion tosignal strength.

After processing by thecontrol circuit, C, the DCvoltage is used to fix the largeand small signal gain of asecond opamp, D.

BUFFERAMPLIFIER

BUFFERAMPLIFIEROUTPUT

CONTROLLEDAMPLIFIER

+ +

6

13

12

4

7

8

35

A D

B C

VOLTAGECONTROLLEDAMP INPUTS

TRUE R.M.S.LEVELDETECTOR

CONTROLCIRCUITRY

BUFFERAMPINPUTS(AUDIO IN)

PROCESSEDOUTPUT

POWERDOWN(STAND-BY)

SET A.G.C.TIMECONSTANT

2 SETGAIN

111091

GROUND (0V) SET SQUELCHTHRESHOLD

SETCOMPRESSIONRATIO

SETLIMITINGTHRESHOLD

14

+5V LIMITING IS IMPOSED IN THIS REGION INORDER TO HOLD THE OUTPUT BELOW APRE-DETERMINED LEVEL

THRESHOLD OFLIMITING SETBY VR4

DOWNWARDEXPANSIONOR SQUELCHTHRESHOLDSET BY VR2

SIGNAL OUTPUT

SIGNAL INPUT

IN THIS REGION GAIN REDUCES ASSIGNAL STRENGTH INCREASES INORDER TO COMPRESS THE DYNAMICRANGE. COMPRESSION IS SET BY VR3

IN THIS REGION GAIN REDUCES ASSIGNAL STRENGTH REDUCES INORDER TO PREVENT HIGH-LEVELAMPLIFICATION OF NOISE UNDERNO-SIGNAL CONDITIONS

Fig.1. Internal block schematic for theSSM2166P microphone preamplifier, withvariable compression and noise gating.

Fig.2. Relationship between limiting, com-pression, and downward expansion or

“squelch”.

EPE Online, May 2000 - www.epemag.com - 339

Provision is made forsetting the nominal gain of thecontrolled stage between 0dBand 20dB, but AGC action willincrease amplification, at thelowest signal levels, to as muchas 60dB. The output can bemuted.

Interestingly, the noisegenerated by the controlledstage is designed to be at aminimum when its gain is at amaximum, and this significantlyimproves the overall signal-to-noise ratio of the system.

RECTIFIERThe circuit of the rectifier, or

level-detector stage (B), hasbeen specially developed forthis application. It produces aDC control voltage, which isproportional to the log of thetrue RMS value of the inputsignal.

The speed at which thecontrol voltage responds tochanges in signal level, or the“attack time”, can be controlledby the user. Response to high-level changes is automatically

speeded up by the IC in order tominimize the duration of anyoverload.CONTROL CIRCUIT

The control circuit (C)enables the user to program theperformance of the IC in a verycomprehensive way, and theamount of signal compressioncan be set between zero and60dB.

Signal limiting can also beapplied to prevent theoccasional transient exceeding

the desired maximum output. Itcan be set at outputs rangingfrom 30mV to 1V. Above thisthreshold, the maximumcompression ratio of 15:1 isapplied.

The response of the systemto very low level inputs can bereduced in order to prevent theamplification of noise under no-signal conditions. The thresholdof this downward expansion (thelower the signal the less it isamplified), can be set at inputs ofbetween 250uV and 20mV.

Constructional Project

bc

e

VR522k

VR610k

VR710k

VR3100k

VR14k7

R61k

R310k

R11k

R210k

GAIN

OUTPUTSIGNALLEVEL

COMPRESSION

INPUTSIGNALLEVEL

C722µ

C41µ

C91µ

C64 7µ

C10470µ

C14 7µ

C54 7µ

IC1SSM2166P

VR21M

VR447k

R72M2

R915k

R41k

R51k

R8SEE TEXT(TABLE 2)

SQUELCH LIMIT

C8100n

C310n

C2100n

0V

0V

AUDIOINPUT 1

AUDIOOUTPUT

AUDIOINPUT 2

WIRELINK

TR1BC547

+

VR810k

ME150 A TO 1mASIGNALSTRENGTHMETER(SEE TEXT)

µ

R104k7

++8V TO

18VIC2LM78L05

INOUT

COM

AUDIO IN (1): ELECTRET MICROPHONES ANDINPUTS REQUIRING A D.C. BLOCKING CAPACITOR

AUDIO IN (2) DYNAMIC (MOVING COIL) MICROPHONES

5 3 9 11 14

4 8 10 1 2

6

7

12

13

LK1

SCREEN

SCREEN

+5V

Fig.3. Complete circuit diagram for the Versatile Mic/Audio Preamplifier.

EPE Online, May 2000 - www.epemag.com - 340

Provision is made for thedevice to be placed in a “power-down” or stand-by mode, andthis feature will be of particularinterest when it is used insophisticated surveillancesystems. In this state, currentconsumption is reduced toaround 10mA and the input andoutput ports assume a highimpedance.

User programmable controlcircuitry, coupled with thecomplex rectifier or level-detector, contributessignificantly to the chip’sperformance. The relationshipbetween the noise reduction,compression and limitingfunctions is displayed in Fig.2.

RATINGSNo doubt with computer

circuit compatibility in mind, theSSM2166 is designed for a 5Vsupply. The absolute maximumsupply voltage is 10V. Currentconsumption is approximately10mA.

The maximum input to thebuffer is 1V, and the maximumoutput from the controlledamplifier is 14V RMS for 1 percent total harmonic distortion.Frequency response extendswell into the RF spectrum.

Static discharges candamage the IC, and the usualprecautions (discharging thebody) should be taken whenhandling and connecting it intocircuit.

The SSM2166P is embeddedin a 14-pin, dual-in-line package,and the suffix “P” refers to thestandard-size version. This is thetype most likely to be stocked bysuppliers. However, surface-mount types are alsomanufactured: these carry thesuffix “S”.

CIRCUIT DETAILSThe full circuit diagram for

the Versatile Mic/AudioPreamplifier, incorporating asignal strength meter, is given inFig.3. Provision for controlling somany functions results in aplethora of preset potentiometercontrols. However, they doenable the signal processing tobe tailored to individualrequirements, and theiradjustment is not critical ordifficult. A summary of theirvarious functions is set out inTable 1.

Preset VR1 permitsadjustment of the input signallevel to prevent overload and tooptimize the performance of the

circuit. Its value is appropriatefor moving coil and electretmicrophones, and for audiosignals derived from mosttransistor circuits. Keeping the

Constructional Project

COMPONENTSResistors

R1, R4, R5, R6 1k (4 off)R2, R3 10k (2 off)R7 2M2R8 (see Table 2)R9 15kR10 4k7

See also theSHOP TALK Page!

All 0.25W 5% carbon film

CapacitorsC1, C5, C6 4u7 radial electrolytic, 10V (3 off)C2, C8 100n ceramic (2 off)C3 10n ceramicC4, C9 1u radial electrolytic 10V (2 off)C7 22u radial electrolyticC10 470u radial electrolytic

SemiconductorsTR1 BC547 (or similar, e.g. BC239, BC548) npn low-power transistorIC1 SSM2166 microphone preamp (Analog Devices)IC2 LM78L05ACZ +5V 100mA voltage regulator

MiscellaneousME1 50uA to 1mA FSD moving coil meter (see text)

Printed circuit board availablefrom the EPE Online Store, code7000260 (www.epemag.com);14-pin DIL socket; screened cable,solder pins, solder, multistrandconnecting wire, etc.

$27Approx. CostGuidance Only(Excluding meter)

PotentiometersVR1 4k7 enclosed carbon preset, horizontalVR2 1M enclosed carbon preset, horizontalVR3 100k enclosed carbon preset, horizontalVR4 47k enclosed carbon preset, horizontalVR5 22k enclosed carbon preset, horizontalVR6, VR7, VR8 10k enclosed carbon preset, horizontal (3 off)

Preset Value

VR1VR2

VR3VR4VR5

VR6VR7

VR8

4k71M

100k47k22k

10k10k

10k

FunctionSet input signal level: clockwise to increase.Set threshold of downward expansion (squelch):clockwise to lower.Set compression: clockwise to increase.Set threshold of signal limiting: clockwise to lower.Set gain of controlled amplifier: clockwise toincrease.Set output signal level: clockwise to increase.Set signal strength meter pointer at full scale(when strongest signal being processed):clockwise gives clockwise pointer movement.Set signal strength meter pointer at zero (underno-signal conditions): clockwise gives clockwisepointer movement.

Table 1: Preset Control Functions

EPE Online, May 2000 - www.epemag.com - 341

value below 5 kilohms increasesthe stability margin of IC1.

Power can be supplied to anelectret microphone’s integralFET (field-effect-transistor)buffer via resistor R1, and C1acts as a DC blocking capacitor.The input arrangements foralternative microphones andother signal sources arediscussed at greater lengthlater.

The input signal to IC1 isapplied to the non-inverting (+)input of the buffer amplifierstage (Pin 7 – see Fig.1) viablocking capacitor C2. This IChas an extended frequencyresponse and C3 introduces ameasure of roll-off above 20kHzor so, again in the interests ofstability.

BUFFER GAINThe gain of the buffer

amplifier is set at 6dB byresistors R2 and R3, and this islikely to be sufficient for mostpurposes. Gain can beincreased to a maximum of20dB by decreasing R3 to about12 kilohms. Adding this to thegain of the controlled amplifierresults in an overall systemgain, when signals are too smallto initiate compression, of 80dB.

This is a great deal ofamplification in a smallpackage, and particular caremust be taken with thescreening and routing of inputand output leads, and theconnections to a shared powersupply, if instability is to beavoided. Separate ground, or0V leads, from signal sourcecircuitry, the preamplifier, andthe power amplifier, should berun to a common point at thepower supply. The screeningbraid of signal cables should beconnected to ground at thepreamplifier end only.

Constructional ProjectIf desired, the gain of the

buffer can be set at unity bydeleting R3 and inserting a wirelink in place of resistor R2 (toconnect pin 5 and pin 6).Blocking capacitor C4 maintainsthe correct DC conditions.

CONTROLLEDAMPLIFIER

The output from the bufferstage (pin 5) is connected, viaDC blocking capacitor C5 to thenon-inverting (+) input (pin 3) ofthe controlled amplifier stage. Acapacitor of identical value, C6,at pin 4 connects the inverting(–) input to ground (0V). (Thisconnection makes any electricalnoise on the ground rail appearas a common mode signal tothe controlled amplifier and thedifferential input circuitry rejectsit).

The nominal gain of thecontrolled amplifier can be set,by preset VR5, between unityand 20dB. Resistor R6 ensuresthat the gain does not fall belowunity.

Switched muting can beachieved by grounding pin 2 viaa 330 ohm resistor (the switchshould be located at the groundor 0V rail end). Switch clickscan be suppressed byconnecting a 10nF capacitorbetween pin 2 and ground.

The IC can be put in stand-by mode by disconnecting pin12 from ground and connectingit, via a 100 kilohms resistor, tothe +5V rail. (Provision has notbeen made for muting orpowering-down on the PCB.)

The processed output istaken from pin 13 andconnected, via DC blockingcapacitor C9, to preset VR6.This enables the output signallevel to be adjusted to suit theinput sensitivity of the power

amplifier.

ATTACK TIMEThe response or “attack”

time of the AGC system can becontrolled by adjusting the valueof the rectifier reservoircapacitor C7. The ICmanufacturer suggests a valuewithin the range 22uF to 47uF,with smaller capacitors beingsuitable for music and the largerfor speech.

Too low a value will result in“pumping” effects, withbackground noise “rushing up”between bursts of speech. Thiswill become increasinglyapparent as the compressionratio is raised.

Conversely, too high avalue will excessively slow theresponse of the system tochanges in signal level. The22uF component specified forC7 has been found to work wellwith both speech and musicinputs.

The attack time is controlledmainly by the value of C7, butthe much longer “decay” time isdependant upon this capacitorand the internal control circuit.Fast attack and slow decay helpto reduce the pumping effect,which seems far lesspronounced with this IC thanwith simpler audio AGCsystems.

COMPRESSIONThe amount of compression

is determined by preset VR3,which connects pin 10 toground. There is nocompression with thepotentiometer set to zero. Whenits resistance is at maximum, a60dB change in input level(above the downward expansionor squelch threshold) changes

EPE Online, May 2000 - www.epemag.com - 342

the output by less than 6dB.The onset of limiting is

controlled by preset VR4.Setting this potentiometer tomaximum resistance fixes it at30mV. With VR4 at minimumresistance, it is around 1V RMS.Above the threshold of limiting,a 15:1 compression ratio isimposed, irrespective of thesetting of compression controlVR3.

NOISE REDUCTIONPreset potentiometer VR2

sets the threshold below whichdownward expansion (gainreduces as the signals becomeweaker) is applied. Withmaximum resistance, downwardexpansion starts at signal levelsin the region of 250mV. Turnedto zero resistance, the thresholdis raised to around 20mV.

Gain rises to a maximumunder no-signal conditions withall conventional AGC systems,and the amplification of externaland internally generated noiseproduces a loud and tiresomehiss in the speaker or ‘phones.The IC’s noise reduction facility,which operates as a “squelch”control, is very effective inovercoming this. It can reduceoutput noise below the level ofaudibility when signal levels fallto zero.

With any squelch system, aneed to resolve very weaksignals overlaid by noisecompromises the usefulness ofthe feature. Radio enthusiastswith a particular interest in itcould mount VR2 as a panelcontrol so that the thresholdcould be adjusted to suitreception conditions.

POWER SUPPLYThe maximum safe supply

voltage is 10V, and it should be

noted that, under a light load, afresh 9V alkaline battery willusually deliver a higher voltagethan this.

However, in order to ensurethe correct operation of thedevice, and provide a highdegree of isolation from otherequipment sharing the samepower supply, a 5V 100mAvoltage regulator, IC2, isincluded in the circuit. Thisenables supplies with outputsranging from 8V to 18V (ormore, depending on IC2 rating)to be used.

Bypass capacitors C8 andC10 shunt the noise in theregulator output to ground. Notethat C8 is essential to thestability of IC1 and it must belocated as close as possible topin 14, even when the unit isbattery powered.

SIGNAL STRENGTHMETER

Some readers, especiallythose wishing to incorporate theunit into a radio receiver, maywelcome the provision of asignal strength meter. This isincluded in the circuit diagramof Fig.1 and consists oftransistor TR1, meter ME1 andassociated components.

The AGC control voltageappears on pin 8 of IC1. Itranges from 290mV under no-signal conditions toapproximately 720mV with highlevel inputs.

Transistor TR1, configuredas a DC amplifier, ensures thatIC1’s AGC line is only lightlyloaded, even when a 1mAmeter is used. It forms one armof a bridge circuit, the otherthree being its collector load,R9, and the potential dividerchain comprising preset VR8and resistor R10. The bridge isbalanced, and the meter set atzero under no-signal conditions,by preset potentiometer VR8.

When a signal is beingprocessed, the rising AGCvoltage on the base (b) of TR1increases its collector currentand, hence, the voltage dropacross resistor R9. Thisunbalances the bridge anddrives the meter pointer over.Preset VR7 adjusts thesensitivity of the meter so thatthe pointer can be set just shortof full-scale deflection (FSD)when registering a strong signal.

The circuit can be made toaccommodate meters with full-scale deflections ranging from50mA to 1mA by adjusting thevalue of resistor R8. Thisresistor controls the flow ofcurrent through the base-emitterjunction of transistor TR1, andvalues to suit a range of meterFSDs are given in Table 2. Biasresistor R7 provides a measureof negative feedback whichhelps to stabilize the operationof the circuit.

Almost any small-signal npntransistor should prove suitablefor TR1, and a 2N5827 or2N5828 could be used inaddition to the types listed in theComponents list. These deviceshave different case styles andthe base connections must bechecked.

CONSTRUCTIONAll the components, with the

exception of the meter ME1, are

Constructional Project

Meter FSD R8

50uA100uA500uA1mA

1M470k100k47k

Table 2: Signal StrengthMeter ( Values of R8 for differ-ent meter sensitivities)

EPE Online, May 2000 - www.epemag.com - 343

assembled on a small, single-sided, printed circuit board(PCB). The topside componentlayout, together with an(approximately) full-sizeunderside copper foil masterpattern, is shown in Fig.4. Thisboard is available from the EPEOnline Store (code 7000260) atwww.epemag.com

Commence construction inthe usual way by mounting thesmallest components firstworking up to the largest, but fitIC1, IC2, and TR1 last (seeearlier comments about thestatic sensitive nature of IC1). Aholder for IC1 will facilitatesubstitution checking. Solderpins, inserted at the lead-outpoints, will ease the task of off-board wiring.

SPOT-CHECKSWhen all the components

have been soldered in positionon the PCB, double-check theorientation of electrolyticcapacitors, the ICs, and thetransistor. Also, check the PCBfor bridged tracks and poorsolder joints.

Next, with IC1 “out ofcircuit”, connect a supplyvoltage of between 7V and 9Vand check that the output fromregulator IC2 is producing 5V. Afault in this device, or its wrongconnection, could result in thedestruction of IC1 when highervoltages are applied.

Once all is well, place IC1in its socket (checkingorientation), connect, viascreened cable, a signal sourceand a power amplifier. Adjustthe various presetpotentiometers until theprocessing meets yourrequirements. All presetfunctions are summarized inTable 1 for ease of reference.

Constructional Project

BC547BC239BC548

LM78L05ACZ

e b c

C2

C8

+

+

+

+

+

+

+

C4

C6

C7

C9

C5

C10

C1

R8

R1

R10

R7

R3

R2

R4C

3

R5

R6

R9

VR6

VR1

VR5

VR4

VR3

VR2

VR8

VR7

AUDIOINPUT 1

AUDIOINPUT 2

INPUTSIGNALLEVEL

SIGNALSTRENGTHMETER

SQUELCHDOWNWARDEXPANSION

THRESHOLDOF LIMITING

COMPRESSION

OUTPUTSIGNALLEVEL

GAIN

INPUTGROUND(0V)

POWER SUPPLY

NEGATIVE + +8V TO 18V

INCOMOUT

IC2

TR1

e b c

INCOM

OUT

+

–

SET METERAT FULLSCALE

SET METERAT ZERO

SIGNAL OUT

LK1

UNDERSIDEVIEW

UNDERSIDEVIEW

Fig.4. Printed circuit board component layout, inter-wiring de-tails and (approximately) full-size underside copper foil master.

MICROPHONESThe unit works well with

dynamic (moving coil), electret,crystal, and ceramicmicrophones. Screened cable

EPE Online, May 2000 - www.epemag.com - 344

must, of course, be used toconnect any type of microphoneto the preamplifier.

Very high quality studiomicrophones can be insensitiveand require balanced feeders tominimize hum pick-up. The pre-amplifier described here isconfigured for unbalancedinputs, and is not likely to besuitable, as it stands, formicrophones of this kind.

A few words about thevarious types of signal inputmay prove helpful.

Dynamic Microphones aremanufactured with impedancesranging from 50 ohms to 600ohms. Output tends to begreatest with the higherimpedance units.

This type of microphoneshould be connected to Input 2(i.e., directly across presetVR1), and the wire link must beremoved to isolate resistor R1from the 5V rail.

Electret Microphones are amodern development of thecapacitor microphone (apermanently chargeddiaphragm, the electret,

eliminates the need for anexternal charging voltage). Theoutput from the actual unit islow and at a high impedance, sothese microphones have anintegral FET buffer. The drainload for the internal FET isprovided at the amplifier end ofthe cable (resistor R1 in Fig.3),to facilitate line powering.

Electret microphones mustbe connected to Input 1, and thewire link must be in place toconnect resistor R1 to thesupply rail. The 1 kilohm drainload (R1), fed from the 5Vsupply, should ensure theoptimum performance of mostmicrophones of this kind.

Crystal and CeramicMicrophones rely upon thepiezo-electric effect to producea signal voltage. The vibratingdiaphragm induces stresses in awafer of crystal, often Rochellesalt, or in a barium titanate

element in the case of ceramicunits.

These microphones shouldbe connected to Input 2. Theyhave a high impedance, andfeeding them into preset VR1will reduce their response to lowaudio frequencies. Lowfrequency roll-off is, however,desirable for communicationswork, and more is said aboutthis later.

The use of long connectingcables will attenuate the signalbut have little effect onfrequency response (cablecapacitance is modestcompared to the self-capacitance of thesemicrophones, which can be ashigh as 30nF).

If an extended frequencyresponse is required frommicrophones of this type, theuse of an external, line-powered, FET buffer, as builtinto electret microphones, is

Constructional Project

g

d

s

VR12M2

C1

0V

+5V

R1

SCREENED CABLE

2N3819HIGH IMPEDANCEMICROPHONE

MICROPHONE CASE

R1, C1 AND VR1 ARE LOCATED ONTHE PREAMPLIFIER P.C.B.

1k10µ

Fig.5. The line-powered buffer stage built intoelectret microphones can also be used for ce-ramic and crystal types. (Most FETs will func-tion in this circuit with the source grounded,eliminating the need for the source resistor

and bypass capacitor.)

Layout of components on the completed circuit board. The“Signal Strength Meter” components, except the meter, have

been included on the board (bottom right).

EPE Online, May 2000 - www.epemag.com - 345

Constructional Project

recommended. This will alsoprevent signal losses when longcables are used.

A suitable circuit diagram isgiven in Fig.5 and the circuitcan be built inside themicrophone case. When thisarrangement is adopted, resistorR1 must, of course, beconnected to the supply rail,and the signal must be fed toInput 1.RADIO RECEIVERS

Audio derived AGC is oftenincorporated into directconversion radio receivers.Even simple superhets canbenefit from this form of control(sometimes a conventional RFderived system is not veryeffective when amateur single-side-band signals are beingprocessed).

Directconversionandregenerativereceivers willrequire asingletransistoraudioamplifier,after theproductdetector orregenerativedetector, inorder to

ensure sufficient signal voltagefor the SSM2166P. The outputfrom the detector stage in mostsuperhets will be more thanadequate.

Radio receivers should beconnected to Input 1, and thewire link removed. Theorientation of electrolyticcapacitor C1 will usually becorrect when the receiver has anegative ground or 0V rail.

However, some diodedetectors in superhets areconfigured to provide an outputwhich is negative going withrespect to ground (to suit thereceiver’s AGC circuit). Thepolarity of C1 will need to bereversed when equipment ofthis kind is connected.

SIGNALPROCESSING

The preamplifier’sfrequency response isreasonably flat from below100Hz to more than 20kHz.Speech clarity, especially undernoisy conditions, can beimproved by “rolling off”frequencies below 300Hz andabove 3000Hz, and active orpassive band-pass filters areoften used for this purpose.

A big improvement can,however, be made by modifyingsome of the coupling andbypass components in thepreamplifier. Constructorswishing to limit the frequencyresponse in this way shouldreduce the value of capacitorC9, to 47nF (a Mylar or ceramiccapacitor is then suitable). Thiswill attenuate the lowerfrequencies.

Wiring a 220nF ceramiccapacitor across preset VR1 willattenuate the higherfrequencies. Although extremelysimple, these measures arequite effective.

EPE Online, May 2000 - www.epemag.com - 346

ROLL-UP, ROLL-UP!Ingenuity is our regular round-up of readers' own

circuits. We pay between $16 and $80 for all materialpublished, depending on length and technical merit.We're looking for novel applications and circuit tips, notsimply mechanical or electrical ideas. Ideas must be thereader's own work and must not have been submittedfor publication elsewhere. The circuits shown haveNOT been proven by us. Ingenuity Unlimited is open toALL abilities, but items for consideration in this columnshould preferably be typed or word-processed, with abrief circuit description (between 100 and 500 wordsmaximum) and full circuit diagram showing all relevantcomponent values. Please draw all circuit schematicsas clearly as possible.

Send your circuit ideas to: Alan Winstanley,Ingenuity Unlimited, Wimborne Publishing Ltd., AllenHouse, East Borough, Wimborne, Dorset BH21 1PF.They could earn you some real cash and a prize!

Win a Pico PC-Based Oscilloscope• 50MSPS Dual Channel Storage

Oscilloscope• 25MHz Spectrum Analyzer• Multimeter• Frequency Meter• Signal Generator

If you have a novel circuit idea whichwould be of use to other readers, then a PicoTechnology PC based oscilloscope could be

yours.

Using the UGN3503U linearHall Effect sensor with a dualopamp allows the constructionof a simple but extremely sensi-tive Hall Effect switch that couldprove useful in many applica-tions. A circuit diagram for justsuch a switch is shown in Fig. 1.The Hall Effect device is IC1,which has just three terminals,for positive and negative sup-plies and the output. A regu-lated 5V supply is suggested formost applications.

With a 5V supply and in theabsence of a magnetic field, theoutput from IC1 is about 25V.On the approach of a magnet,the output will rise or fall, de-pending on the magnetic field’spolarity. With a 5V supply, theAD8532 is an excellent choicefor the dual opamp IC2 as it isdesigned for this supply voltage,has rail-to-rail inputs and out-puts and, being a CMOS com-ponent, has very high input

Sensitive Hall Effect Switch – Feel the Field

Fig.1. Sensitive Hall Effect Switch circuit diagram.

In addition it can supply up to250mA of output current.

The first opamp, IC2a, isused as a non-inverting ampli-fier with a voltage gain of about20 to amplify the output of IC1to more useful levels. BecauseAC coupling via capacitor C1 isused for the input, resistor R3sets the working point to half the

supply. This is necessary toavoid drift in the Hall Effect sen-sor IC1. The high value of R3allows the circuit to operate atvery low frequencies, down toless than 1Hz. The secondopamp IC2b is connected as acomparator with hysteresis setby resistors R7 and R8 to about500mV to ensure a rapid,

C1470n

OUT

0V

R210k

R610k

R4100k

R710k

V+

R110k R3

4M7

R510k R8

100k

12

3

4

8+

+

5V

0V

IC1UGN

3503U

1

3

2 6

57

OUTPUT

IC2aIC2bAD8532

AD8532

12

3 O/P

+VE0V

UGN3503U

EPE Online, May 2000 - www.epemag.com - 347

bounce-free switching action.With the values shown this

circuit can detect the approachof a small bar magnet of thetype commonly used for operat-ing reed switches, at a range ofabout 25mm. Sensitivity couldbe adjusted by altering the gainof the amplifier stage or thehysteresis. Note that thestrength of a magnetic field fallsin proportion to the square ofthe distance from its source.

The polarity of the outputchange from IC1 depends onthe polarity of the magneticfield. When the face bearing thedevice markings is approachedby a North pole the output volt-age falls, whilst the approach ofa South pole will make it rise.The sensor is said to be capableof operating up to 23kHz, so formost practical applications theupper speed limit will not be anissue!

It is also possible to place amagnet behind the sensor sothat the flux passing through itwill change on the approach ofa ferrous, but not necessarilymagnetic, object. Applicationsof this type might include sens-ing passing steel gearwheel

Ingenuity UnlimitedInfra-Red Remote Tester – Sounds Good

B19V

D1I.R.

PHOTODIODE

C122m

11

R110M

R21M

8

C2100n

IC1a4049BE

141

15

a

k

9 10

IC1f4049BE

a k

IC1b4049BE D2

1N4148

R34k7

D3

12

7

a

k

X1PIEZO DISC

IC1c4049BE C3

22p

IC1e4049BE

R410M

R51M

6

3 2

5

IC1d4049BE

4

+

LOWCURRENT

L.E.D.

NOT USED

Fig.2. Circuit diagram for an Infrared Remote Tester.

An Infrared Remote Control Tester, which gives both an audioand visual indication that a remote control is functioning, is shown inFig.2. Its operation is as follows: D1 is a reverse biased photodiode,which forms an infrared detector. Its output is buffered by IC1a andthen enters a pulse stretcher comprising capacitor C2, resistor R2and IC1b.

The pulse stretcher enables even the shortest of pulse trains totrigger the following oscillator. Diode D3 is a low current red LED,which sinks into the output of IC1b and provides the visual indication.

An oscillator is formed of IC1c and IC1d, which drives a piezodisc element X1. This is connected across IC1e (rather than moreconventionally to 0V) to provide a louder output. Oscillation isstopped by the normally high output from IC1b via diode D2. Whenan IR pulse train is detected, IC1b goes low and the input to IC1dsees only a high impedance from the diode and oscillation starts.The input of gate IC1f is grounded for stability.

The circuit runs from a 9V PP3, and the standby current of a little

Experimenter’s Power Supply – Variable States

A circuit for a handy Vari-able Power Supply which willmeet the needs of many elec-tronics hobbyists is shown inFig.4. It provides 0V to 25V atup to 250mA. The error ampli-fier within a 723-type voltageregulator chip (IC3) cannotfunction with rail-to-rail inputand therefore a precision, shuntreference, TL431C (IC2) biasespin 5 of the 723 regulator to+2·50V.

The regulator’s inverting

tential from potentiometer VR1,which can swing either above orbelow this level. Hence the out-put voltage, attenuated by the10k and 100k resistors R5 andR6 is forced to extend from 0V(CCW) to +25V (CW) to trackthe +250V reference.

An external TIP29C powertransistor TR1 extends the cur-rent limit to 250mA and is offsetby a second programmableZener diode IC4 at pin 10 of the723 regulator. This voltage dif-

EPE Online, May 2000 - www.epemag.com - 348

ference plus two VBE dropsgives sufficient headroom at pin13 for the amplifier stage whenVout = 0.

The 723’s internal Zenerdiode at pin 9 was not used, as

Ingenuity Unlimited

the 62V requires a largersource from the transformer atmaximum output, 25V. Evenwith this precaution it is possibleto exceed the 723’s absolutemaximum voltage at pins 11

therefore, a three terminal regu-lator, 78L05ACZ (IC1) wasadded to maintain 30V.

As constructed, a singlemains transformer with two, iso-lated 12VA windings alloweddual, floating DC sources of 0Vto 25V magnitude, a conve-nience if both positive and neg-ative voltages or their sum isdesired.

John A. Haas

bc

e

bc

e

C12200m

35V

T114V

12V.A.

230VA.C. MAINS

INPUT

E

L

N

D21N4003

14V

0V

0V

230V

C22200m

35V

R13k92

VR11K

C3100n

IC2

IC4

TL431C

TL431CR31k

R4200W

S1

ON/OFF

D1IN4003

OUT

R21k62

IN IC178L05

COM.

FREQ.COMP 13

TR1TIP29C

R72 4W

VZ

CUR.SENSE

CUR.LIMIT

VOLTS

5

0V

R510k

9

3

2

4

R6100k

C41n

OUTPUT

0V TO 25V250mA

R81k1W

C51m

D31N4003

6 VREF

IC3723

12+V 11VC

+

+

+

a

a

k

k

ak

a

k

a

k

akVOUT 10

N.C.

N.C.

N.C.

N.C.

7

REF

REF

INV ( )INNON INV

( ) IN

14

1

8

REFa

k

OUTCOM

IN

TL431C

78L05ACZ

bc

e

TIP29C

2 75V

2 50V

Fig.4. Experimenter’s Power Supply circuit using the 723 variable voltage regulator.

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A typical multimeter canmeasure voltage, current andresistance over a wide range ofvalues, and usually has a few“tricks up its sleeve” such ascontinuity tester and transistorchecker facilities. Somemultimeters have capacitancemeasuring ranges, but thisfeature remains something of ararity. This is a pity, becauseanyone undertaking electronicfaultfinding will soon need tocheck suspect capacitors and aready-made capacitance meter isan expensive item of equipment.

The unit featured here offersa low-cost solution to the problemof testing capacitors. It is ananalog capacitance meter thathas five switched ranges with full-scale values of 1nF; 10nF;100nF; 1uF; and 10uF. It cannotmeasure very high or low valuecomponents, but it is suitable for

under test.The duration of the output

pulse is proportional to thevalues of both components inthe CR network. If a 1nFcapacitor produces an outputpulse of one millisecond induration, components havingvalues of 22nF and 47nF wouldrespectively produce pulselengths of 22ms and 47ms.

Each output pulse must beconverted into a voltage that isproportional to the pulseduration. A moving coil panelmeter can then read thisvoltage, and with everything setup correctly it will provideaccurate capacitance readings.

If we extend the examplegiven previously, with apotential of one volt permillisecond being produced, ameter having a full-scale valueof 10V would actually read 0 to10nF. This time-to-voltageconversion is actually quitesimple to achieve, and isprovided by a constant currentgenerator and a charge storage

A simple starter project that will let you get the measure of most capacitors. Five switched ranges: 1nF to 10uF.

LOW-COST CAPACITANCE METERby ROBERT PENFOLD

testing the vast majority ofcapacitors used in everydayelectronics.

SYSTEM OPERATIONThe block diagram for the

Low-Cost Capacitance Meter isshown in Fig.1. Like mostsimple capacitance meterdesigns, this unit is based on amonostable circuit. Whentriggered by an input pulse amonostable produces an outputpulse having a duration that iscontrolled by a CR network. Inthis case the monostable istriggered manually using apushbutton switch each time areading is required.

The resistor in the timingnetwork is one of five resistorsselected via a switch, and theseresistors provide the unit with itsfive ranges. The capacitor in theCR network is the capacitor

MONOSTABLE CURRENTGENERATOR

BUFFERAMPLIFIER

METER

CHARGESTORAGE

CAPACITORRESETTEST

CAPACITOR

0V RAIL

MEASURE

Fig.1. Schematic block diagram for theLow-Cost Capacitance Meter.

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capacitor.When charged via a resistor

the potential across thecapacitor does not rise in alinear fashion. As the chargepotential increases, the voltageacross the resistor falls, giving asteadily reducing chargecurrent. The voltage thereforeincreases at an ever-decreasingrate (inverse exponentially).

A current regulator avoidsthis problem by ensuring thatthe charge current does notvary with time, giving a linearrise in the charge voltage. Thecircuit therefore provides therequired conversion fromcapacitance to voltage, but it isimportant that loading on thestorage capacitor is kept to aminimum.

Tapping off a significantcurrent could adversely affectthe linearity of the circuit andwould also result in readingsrapidly decaying to zero. Themeter is, therefore, driven via abuffer amplifier that has a veryhigh input impedance. Once areading has been taken andnoted, operating the Resetswitch discharges the storagecapacitor and returns thereading to zero so that a newreading can be taken.

CIRCUIT OPERATIONThe complete circuit

diagram for the Low-CostCapacitance Meter projectappears in Fig.2. Themonostable is based on a low-power 555 timer (IC1) used in

the standard monostableconfiguration.

Apart from the fact it givesmuch longer battery life, a low-power 555 is a better choice forthis type of circuit due its lowerself-capacitance. This producesmuch better accuracy on the 1nFrange, and a standard 555 is

Constructional Project

bc

e

b

c

e

B19V

TR1BC549

D21N4148

D11N4148

TESTCAP.C1

100n

R64k7

R110M1%

R21M1%

R3100k1%

1nS1

100n

10n

6

R710M

S2

C2150p

2

MEASURE

THRES.

TRIG.

GND

R815k

1

R410k1%

R51k1%

7RANGE

1m

10m

IC1TS555CN

DISCH.OUT

RST +V4 8

3

SENS

ON/OFF

RESET

R1210W

S3

C3220n

R1339k

VR147k

4

R94k7

R1110k

TR2BC559

R105k6

6

IC2CA3140E7

3

2

ME1100 Am

CAPACITANCE

S4

a

a

k

k

+

0V

SK1

SK2

POLE

Fig.2. Complete circuit diagram for the Low-Cost Capacitance Meter.

COMPONENTSResistors

R1 10M 1% metal filmR2 1M 1% metal filmR3 100k 1% metal filmR4 10k 1% metal filmR5 1k 1% metal filmR6, R9 4k7 (2 off)R7 10MR8 15kR10 5k6R11 10kR12 10 ohmsR13 39k

All 0.25W 5% carbon film, exceptwhere otherwise specified

CapacitorsC1 100n ceramicC2 150p ceramic plateC3 220n polyester

MiscellaneousME1 100uA moving coil panel meterSK1 2mm socket, redSK2 2mm socket, blackS1 12-way single-pole rotary switch (set for 5-way operation) (see text)S2, S3 pushbutton switch, push-to-make (2 off)S4 s.p.s.t. miniature toggle switchB1 battery (PP3 size), with connector leads

Metal insrument case (or type tochoice), size 150mm x 100mm x 75mm;stripboard 0.1-inch matrix, size 34 holesby 21 copper strips; 8-pin DIL socket(2 off); control knob; calibration capacitor(see text); test leads (see text); solderpins; multistrand connecting wire;solder, etc.

See also theSHOP TALK Page!

$19Approx. CostGuidance Only (Excluding batteries, case, & meter)

PotentiometerVR1 47k miniature enclosed or skeleton preset, horizontal

SemiconductorsD1, D2 1N4148 signal diode (2 off)TR1 BC549 npn transistorTR2 BC559 pnp transistorIC1 TS555CN low power timerIC2 CA3140E PMOS opamp

EPE Online, May 2000 - www.epemag.com - 351

therefore not recommended foruse in this circuit.

Switch S1 sets the Rangeand R1 to R5 are the five timingresistors. Resistors R1 to R5respectively provide the 1nF,10nF, 100nF, 1uF, and 10uFranges.

One slight flaw in the 555for this application is that it willonly act as a pulse stretcher andnot as a pulse shortener. Inother words, the output pulsewill not end at the appropriatetime if the input pulse is stillpresent.

If it were used to directlytrigger IC1, the input pulse frompushbutton switch S2 wouldinvariably be far too long. Asimple CR circuit is therefore

used to ensure that IC1 willalways receive a very shorttrigger pulse, regardless of howlong Measure switch S2 ispressed.

Resistor R6 holds the triggerinput of IC1 (pin 2) high understandby conditions, but it is brieflypulsed low when S2 is operatedand capacitor C2 charges via R6.When S2 is released, resistor R7discharges C2 so that the unit isready to trigger again the nexttime S2 is operated. Resistor R7has been given a very high valueso that the discharge time of C2is long enough to preventspurious triggering if S2 does notoperate “cleanly”. Mostmechanical switches suffer fromcontact bounce, and without thisdebouncing it is likely that re-

triggering would occurpractically every time S2 wasreleased.

Under standby conditionsthe output at pin 3 of IC1 is low,and both transistor TR1 andTR2 are switched off.Consequently, only insignificantleakage currents flow into thecharge storage capacitor C3. Anoutput pulse from IC1 switcheson TR1, which in turn activatesTR2.

Transistor TR2 is connectedas a conventional constantcurrent generator, and the valueof resistor R10 controls the

Constructional Project1

1

5

5

10

10

15

15

20

20

25

25

30

30

34

34

A

DEFGHIJK

M

OPQR

TU

B

N

L

C

S

A

DE

GHIJK

M

OPQR

T

U

B

N

L

C

SME1+

R5

R3

R2

D1

D2

IC1

TR2

TR1

R7

C2

C1

R9

R8

C3

R12

R6

R11

IC2 R13

VR1

SK1 SK2 S2

TEST CAP MEASURE

S3

RESET

CAPACITANCE

RED

BLACK

S4 ON/OFF

TO B1

+

e

b

c

eb

c

ak

a

kP

S1

RANGE

R10

R1

R 4

Fig.3. Stripboard component layout, interwiring, and details ofbreaks required in underside copper tracks.

Mount the Range resistorsdirectly on the switch tags

before the switch is fitted inthe case.

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output current. This is around115uA with the specified value.Transistors TR1 and TR2 switchoff again at the end of the pulsefrom IC1, and the chargevoltage on C3 is then read bythe voltmeter circuit based onpanel meter ME1.

METER CIRCUITOperational amplifier

(opamp) IC2 is used as thebuffer amplifier, and the PMOSinput stage of this deviceensures that there is nosignificant loading on the“charge” capacitor C3. The inputresistance of IC2 is actuallyover one million megohms.

However, the voltage on C3will gradually leak away throughvarious paths, including C3’sown leakage resistance. Thereading should remain accuratefor at least a minute or two, andin most cases it will not changenoticeably for several minutes.There will certainly be plenty oftime for a reading to be takenbefore any significant driftoccurs.

Briefly operating Resetswitch S3 discharges C3 andzeros the meter so that anotherreading can be taken. ResistorR12 limits the discharge currentto a level that ensures thecontacts of S3 have a longoperating life. The rate ofdischarge is still so high that itappears to be instant.

Preset VR1 enables thesensitivity of the voltmeter ME1to be adjusted, and in practicethis is adjusted so that therequired full-scale values areobtained. In order to ensuregood accuracy on all fiveranges it is essential for rangeresistors R1 to R5 to be closetolerance (one or two percent)components.

There is no overload

Constructional Projectprotection circuit for the meter,but this protection is effectivelybuilt into the design. The circuitdriving the meter is onlycapable of producing minoroverloads, and is incapable ofinflicting any damage. Thecurrent consumption of thecircuit is only about 3mA, and aPP3 size battery is adequate topower the unit.

CONSTRUCTIONThe Low-Cost Capacitance

Meter is built up on a smallpiece of stripboard having 34holes by 21 copper strips. Thetopside component layout,underside details and interwiringto off-board components isshown in Fig.3.

As this board is not of astandard size, a piece will haveto be cut from a large boardusing a small hacksaw. Cutalong rows of holes rather thanbetween them, and smooth anyrough edges produced using afile. Then drill the two 3mmdiameter mounting holes in theboard and make the 17 breaksin the copper strips. There is aspecial tool for making thebreaks in the copper strips, buta handheld twist drill bit ofaround 5mm diameter does thejob very well.

The circuit board is nowready for the components, linkwires, and solder pins to beadded. The CA3140E used forIC2 has a PMOS input stagethat is vulnerable to damagefrom static charges, and theappropriate handlingprecautions must therefore betaken when dealing with this IC.

It should be fitted to theboard via a holder, but it shouldnot be plugged into place untilthe unit is otherwise finished,and the board and wiringdouble-checked for any errors.

It should be left in its anti-staticpacking until then. Try to handlethe device as little as possiblewhen fitting it in its IC socket,and keep well away from anylikely sources of static electricitysuch as televisions sets andcomputer monitors.

Although the TS555CNtimer used for IC1 is not static-sensitive it is still a good idea tofit it in an IC socket. Be carefulto fit IC1 the right way aroundbecause it has the oppositeorientation to normal, with pinone at the bottom. This chipcould easily be destroyed if it isfitted the wrong way around.

In all other respectsconstruction of the board isfairly straightforward. The linkwires can be made from thetrimmings from resistor leadoutsor 22 s.w.g. tinned copper wire.In order to fit into this layoutproperly capacitor C3 should bea printed circuit mountingcomponent having 75mm (03-inch) lead spacing. Be careful tofit the diodes and transistorswith the correct orientation.Note that transistors TR1 andTR2 have opposite orientations.

RANGE RESISTORSThe five range resistors (R1

to R5) are mounted directly onthe Range rotary switch S1,which helps to minimize straycapacitance and pick up ofelectrical noise. This aids goodaccuracy, especially on the 1nFrange. It is best to mount theresistors on S1 before thisswitch is fitted in the case.

Fitting the resistors is mademuch easier if the switch isstuck to the workbench usingPlasticine, or Bostik Blu-Tack.Provided the tags and the endsof the leadouts are tinned withsolder it should then be quiteeasy to build this sub-assembly.

EPE Online, May 2000 - www.epemag.com - 353

Try to complete thesoldered joints reasonablyswiftly so that the resistors donot overheat. It takes quite a lotof heat to destroy resistors, butrelatively small amounts canimpair their accuracy.

CASING UPA medium size metal

instrument case is probably thebest choice for a project of thistype, but a plastic box is alsosuitable. The exact layout is notcritical, but mount SK1 and SK2close together.

Many capacitors will thenconnect directly into the socketswithout too much difficulty, buta set of test leads will also beneeded to accommodate somecapacitors. All that is requiredare two insulated leads about

100mm long. Each lead is fittedwith a 2mm plug at one end anda small crocodile clip at theother.

Fitting the meter on thefront panel is potentiallyawkward because a large roundcutout is required. For mostmeters a cutout of 38mmdiameter is required, but it isadvisable to check this point byactually measuring the diameterof the meter’s rear section. DIYsuperstores sell adjustable holecutters that will do the jobquickly and easily, or the cutoutcan be made using a copingsaw, Abrafile, etc.

Four 3mm diameter holesare required for the meter’sthreaded mounting rods.Marking the positions of these isquite easy as they are usually atthe corners of a square having

32mm sides, and the samecenter as the main cutout. Onceagain though, it would beprudent to check this by makingmeasurements on the meterprior to drilling the holes.

The circuit board ismounted on the base panel ofthe case towards the left-handside of the unit, leavingsufficient space for the batteryto the right of the board. Thecomponent panel is mountedusing either 6BA or metric M25bolts, and spacers or nuts areused to ensure that theunderside of the board is heldwell clear of the case bottom.To complete the unit the hardwiring is added. This offersnothing out of the ordinary, butbe careful to connect the batteryclip and meter ME1 with thecorrect polarity.

Constructional Project

Layout of components inside the metal case of the completed Low-Cost Capacitance Meter.The circuit board is mounted to one side to leave space for the battery

EPE Online, May 2000 - www.epemag.com - 354

CALIBRATIONPreset potentiometer (wired

as a “variable resistor”) VR1must be given the correctsetting in order to obtain goodaccuracy from the unit, and aclose tolerance capacitor isneeded for calibration. Foroptimum accuracy this capacitorshould have a value equal tothe full-scale value of the rangeused during calibration.

In theory it does not matterwhich range is used whencalibrating the unit, but inpractice either the 1nF or 10nFrange has to be used. Suitablecapacitors for the other rangesare either unavailable orextremely expensive.

The 10nF range is thebetter choice as the small self-capacitance of IC1 is less

significant on this range,although this factor seems tohave very little affect onaccuracy. Probably the bestoption is to calibrate the unit onthe 10nF range using a 10nFpolystyrene capacitor having atolerance of one percent.

It is possible that a largereading will be produced on themeter when the unit is firstswitched on, but pressing Resetswitch S3 should reset themeter to zero. If it is notpossible to zero the meterproperly, switch off at once andrecheck the entire wiring, etc.

If all is well, set preset VR1at maximum resistance(adjusted full clockwise). Thenwith the unit set to the correctrange and the calibrationcapacitor connected to SK1 andSK2, operate pushswitch S2.This should produce a strong

deflection of the meter, andVR1 is then adjusted forprecisely full-scale reading onmeter ME1. The unit shouldthen provide accurate readingson all five ranges.

IN USEThe Meter is suitable for

use with polarized capacitorssuch as electrolytic andtantalum types. However, it isessential that they areconnected to SK1 and SK2 withthe correct polarity. The positive(+) lead connects to SK1 andthe negative lead connects toSK2.

Especially when using the1nF and 10nF ranges, avoidtouching the lead that connectsto SK1 when a reading is beingtaken. Otherwise electricalnoise might be introduced intothe system producinginaccurate results.

Avoid connecting a chargedcapacitor to this or any othercapacitance meter, since doingso could result in damage to thesemiconductors in the metercircuit. If in doubt alwaysdischarge a capacitor beforetesting it.

Constructional Project

EPE Online, May 2000 - www.epemag.com - 355

This project provides up tosixteen channels of on-offsignaling communication throughjust a single pair of wires, in onedirection or in both directionssimultaneously. In a one-waysystem the Transmitter may bepowered through the same pair ofwires, which allows the monitoringof up to sixteen inputs fromlocations having no local powersupplies. An interfacing option(next month) enables operationthrough audio circuits, such asprivate internal telephone andintercom systems.

Although ideal for remotesignaling and alarm systemmonitoring, other possibleapplications could include suchthings as environmentalmonitoring, model railway controlsand switching for advancedlighting or display systems. Theversatility of using circuit modules,and the ways in which they can beconnected together, means thatpossible applications are limitedonly by the constructor’s ownimagination.

HOSPITAL CALLLike many designs, this one

began with a request from afriend, who on this occasion is thevolunteer engineer for the local“Hospital Radio”. Althoughoperated by amateurs, thisservice manages to maintain

microphone is in use. Securitymonitoring channels are alsoneeded since the original studiois housed in a “Portacabin” andhas suffered from attemptedbreak-ins.

The request, then, was forthe provision of sixteen “on-off”signaling channels to operatethrough a single circuit from thehospital’s internal telephonesystem. Plus, the icing on the

A PIC-based 8 to 16-Channel 2-wire on-off signaling communication link. An add-on Interface (next month) will extend possible options to internal private telephoneand intercom systems.

MULTI-CHANNEL TRANSMISSIONSYSTEM by ANDY FLIND

impressively high operatingstandards.

At present a new studio isbeing constructed at somedistance from their existing oneand for a while they will beoperating these simultaneously,often with a disk jockey. workingin both. To make this possible, anumber of signaling channelsare required for functions suchas indicating when a

The three modules: Receiver board; Interface (next month)and, foreground, Transmitter board.

EPE Online, May 2000 - www.epemag.com - 356

designer’s cake, it was requiredto operate simultaneously inboth directions.

TAKE YOUR PICInitial thoughts were that the

task could be carried out easilywith a suitably programmed PIC.Whilst the programming provedfar from easy, it eventuallyresulted in the extremelyversatile system described here.

It can operate to the originalspecification with sixteenchannels in each directionthrough a circuit capable only ofhandling low-level audio signals,but, as described, it can also beused in several other ways tosuit less demandingapplications. It can have eithereight or sixteen channels, in oneor both directions, and in somecases the Transmitter may bepowered through the signalingwires which can sometimes bevery useful.

Later upgrading of a systemis also simple, as the secondeight channels can be added bysimply plugging in extra PICs.This is a project offering lots ofpossible options for tailoring theconfiguration to suit the

individual constructor’s needs.

SENDING A SIGNALThe method of signal

transmission used is relativelysimple. A total of sixteen “clock”pulses are sent and for eachthere is a following “signal” pulseif the associated input is active.Part of the resulting waveform isshown in Fig.1.

It can be seen that thepulses are negative-going, witha positive quiescent state whichallows the signaling line to serveas the transmitter power supplyif required. The basic timing ofeach pulse is 05ms low, 05mshigh, so that if all the switchesare active the sequencebecomes a burst of 1kHz tone, asuitable frequency fortransmission through an audiocircuit.

Squarewaves with a peak-to-peak amplitude of 5V are notsuitable for telephone circuitshowever, as stray coupling intoadjacent circuits in the cables islikely to cause interference toother users. The originalintention was to “smooth” andattenuate the waveform withpassive low-pass filtering andrestore it at the far end with acomparator, but this idea failedsince telephone circuits usuallycarry only AC signals due tocoupling transformers andcapacitors.

The average DC content ofthe waveform produced by thisproject varies with the number ofactive inputs and the resultingvariation of the average level atthe far end of an AC coupledcircuit made it impossible toadjust the comparator forreliable operation. A solutionwas eventually found for thisproblem, the principle of whichis shown in Fig.2.

Two outputs from the PIC(RA2 and RA3) are connected

through a pair of 1 kilohm (1k)resistors and the output is takenfrom their junction. The quiescentstate consists of one output high(positive) and one low (negative)so that the output is half thesupply voltage. A “signal pulse”consists of making both outputslow, followed by a return to thequiescent state, then both outputshigh, then back to one high, onelow.

This results in the waveformshown at Fig.2a, which is muchbetter for transmission through an

Constructional Project

INPUTACTIVE

INPUTOFF

DATAPULSE

2ms

CLOCKPULSE

INPUTOFF

0 5ms

Fig.1. Transmission method

OUTPUTRA2

RA3

1k

1k

(B)

(A)

Fig.2. Waveform generation.

REPEAT FOR REMAININGFIFTEEN INPUT STATES

CANCEL RECEIVER MUTING

GET IC2s INPUT STATESAND STORE IN REGISTER SW2

TRANSMIT CLOCK PULSE

TRANSMIT PULSE

HAS LINE BEENINACTIVE FOR MORE

THAN 1 8ms?

MUTE THE LOCAL RECEIVER

GET OWN INPUT STATES ANDSTORE IN REGISTER SW1

IS FIRST INPUTSTATE LOW?

DELAY - 5ms OR 10ms

INITIALISE:ALL PORT B AS INPUT

WITH WEAK PULL-UPS ON

YES

YES

START

DELAY FORDURATIONOF PULSE

NO

NO

Fig.3. Flow diagram for thefirst Transmitter PIC, IC1.

EPE Online, May 2000 - www.epemag.com - 357

AC circuit. Furthermore, if the“low” and “high” states occupyaround 61 per cent of the totalperiod the energy content will besimilar to that of a cycle ofsinewave. When passedthrough a suitable low-pass filterthis produces a very goodapproximation of a sinewave asshown in Fig.2b, far more suitedto telephone circuits.

In passing, it’s worthmentioning that with a 5V supply

the current “wasted” by the tworesistors in the quiescent state isonly 25mA as they present aseries resistance of 2 kilohms,whilst the output impedance isonly 500 ohms as for this they areeffectively in parallel.

BI-DIRECTIONALOPERATION

Achieving bi-directionaloperation was more difficult. Intelephony there are “two-to-four-wire” converter circuits which splitthe conventional two wires intoseparate transmit and receivepairs. They work by coupling thecircuit to the receiver through animpedance of some kind, oftenjust a resistor, and injecting aninverted form of the locallytransmitted signal into thereceiver to cancel the bit of it thatcomes through this impedance.

Success with this type ofcircuit assumes that thetransmission path will have aknown and constant impedance,both resistive and reactive, andattempts to use it with theproposed telephone circuit failedmiserably. Eventually a softwaresolution was found in which eachtransmitter checks the line forsilence before transmitting andmutes the local receiver beforedoing so. Two such transmitterscan be made to synchronize toeach other and take turns totransmit.

The PIC16F84 can haveinternal “weak pull-up” resistorsapplied to the eight bits of port Bwhen these are configured asinputs, removing the necessity toprovide them externally. Eachinput can then be as simple asjust a switch pulling it to ground ifrequired.

A single PIC can only provideeight such inputs however, andthis project required sixteen.Since these ICs are now availableat a cost of less than 2 UK

pounds from some suppliers,the quickest and cheapest wayto obtain a further eight inputs isfrom a second PIC whichtransmits its inputs serially to thefirst upon request.

SOFTWAREOPERATION

An outline of the softwareoperation for the first PIC, IC1,in the Transmitter circuit isshown in the flow diagram Fig.3.The initial setting up includesconfiguring all of port B asinputs with active weak pull-ups.

This is followed by a briefdelay. It is unlikely but quitepossible that both transmitters ina bi-directional system mightcheck the line, find it inactiveand transmit together in perfectsynchronization. The use of aslightly different delay in eachtransmitter will quickly breaksuch a pattern to ensure correctoperation. Five and tenmilliseconds are the values usedfor this.

Following the delay the PICmonitors the line for a period ofinactivity greater than 18ms,after which it mutes the input tothe local receiver, collects theinput states from the secondPIC, IC2, and stores them in aregister named SW2, and thenstores its own input states inregister SW1. It then transmitsthe first clock “pulse” asdescribed earlier and checks thefirst bit of SW1. If this is clear,corresponding to an active input,a second pulse is transmitted. Ifit is set, the input was inactiveso a delay lasting the period of apulse is called.

This action is repeated forthe remaining seven bits of SW1followed by the eight bits ofSW2, the whole process takingprecisely 32ms. After this theprogram returns to the start andthe entire sequence is repeated.

Constructional Project

MAKE PORT A BIT 2 AN INPUT

MAKE PORT A BIT 2 OUTPUT

SET PORT A BIT 2 HIGH

IS PORT ABIT 2 HIGH?

GET PORT B STATESSTORE IN FILE SW1

IS PORT ABIT 2 LOW?

REPEAT FOR REMAININGSEVEN BITS OF SW1

SET PORT A BIT 2 LOW

DELAY FOR 100 sm

IS FIRST BITOF SW1 LOW?

INITIALISE:PORT A BIT 2 INPUT

ALL PORT B AS INPUT WITH WEAK PULL-UPS ON

START

YES

YES

YES

NO

NO

NO

Fig.4. Flow diagram for thesecond Transmitter PIC, IC2.

EPE Online, May 2000 - www.epemag.com - 358

A flow diagram of theTransmitter software for IC2 isshown in Fig.4.

PIC-TO-PICFrom time to time readers

have asked how communicationbetween PICs can be achievedso a detailed description of themethod used may be helpful. Inthis circuit two PIC connections(RA1 and RA2) are linked asshown in Fig.5. A 1k resistor isused in case both pins becomeoutputs simultaneously,although this should never bethe case.

Initially, both connectionsare configured as inputs and the10k resistor pulls them bothhigh. When IC1 requires datafrom IC2, it’s pin becomes anoutput and is pulsed low forabout 400us before returning tothe input state.

Meanwhile, IC2 has beenwaiting for the low pulse. Onseeing this it stores its inputstates in a register and waits forthe input to return to the highstate. When this happens itmakes its pin an output andsends the eight input statesserially at intervals of 100us.Following this the pin returns tothe input state and the programreturns to the start to wait for thenext pulse from IC1.

In the meanatime, 50usafter restoring its connection toinput, IC1 commences takingeight readings from it at 100usintervals and storing the resultsin register SW2. The whole

Constructional Project

process takes just over amillisecond and is easy toimplement, both in hardwareand software. This is serialcommunication at its simplest

and more sophisticated methodsare obviously possible but itprovides a starting point foranyone wanting to connect twoor more PICs together.

One advantage of thismethod is that for eight-channeloperation IC2 can be omitted.IC1 will still request theinformation but will “see” eightinactive inputs as each time itreads the pin it will see a highstate set by the 10k resistor.

RECEIVERSOFTWARE

Continuing with theReceiver, the flow diagram forthis is shown in Fig.6. Theprogram begins by looking for afalling edge in the input signalfrom the line. When it locatesone it clears the two inputregisters named SW1 and SW2which will contain the sixteenswitch states.

It then waits for 100us,which should take it into the lowportion of a pulse if this was theorigin of the edge. It checks theinput is still low, if not it returnsto the program start. Otherwise,it waits for 500us and checksthat the input is now high, as itwill be if a pulse is present.Again, if it isn’t the programreturns to the start.

After another 500us, whichtakes it to the point where theinput will be low or highdepending on the input statebeing transmitted, it samples thestate of the line and stores it inthe first bit of register SW1. Afurther delay of 1ms takes it tothe next clock pulse, where theprocess is repeated until allsixteen pulses have beenchecked and their associateddata bits read.

Both low and high states ofall sixteen clock pulses arechecked and if any are missing

1kRA1

IC1 10k

RA2

+5V

IC2

Fig.5. Communication|between two PICs

REPEAT FOR REMAINING SEVEN BITSOF SW1 AND EIGHT BITS OF SW2

COPY SW1 TO PORT B

COPY SW2 TO PORT B

WAIT 1ms

IS PORT ABIT 4 LOW?

NO

YES

CLEAR INPUT FILESSW1 AND SW2

INITIALISE:ALL PORT B AS OUTPUT

SET FIRST BIT OF SW1

WAIT 500 sm

IS INPUT TOPORT A

BIT 2 HIGH?

YES

NO

YES

IS INPUT TOPORT A

BIT 2 LOW?

WAIT 500 sm

IS INPUT TOPORT A

BIT 2 HIGH?

YES

WAIT 100 sm

IS INPUT TOPORT A

BIT 2 LOW?

YES

START

NO

NO

NO

Fig.6. Flow diagram for theReceiver.

EPE Online, May 2000 - www.epemag.com - 359

the program immediately returnsto the start. This provides rapidsynchronization to thetransmitter and good protectionagainst data corruption as acomplete valid sequence must

be received before output takesplace.

Assuming a completesequence is received, theprogram now checks the input to

port A bit 4. This is wired “high”for IC1 and “low” for IC2, so thePIC knows which socket it is inand sends the appropriate eightbits of data to port B, SW1 in thecase of IC1 and SW2 for IC2.

Constructional Project

bc

e

R71k

R31k

INPUTS INPUTS

32

4

7

5

8

R41k

R522k

COMMON

R122k

TR1BC184L

16

15

18

1

OSC OUT

OSC IN

5

RB2RB1

RB0VSS VSS

6

7

RB38

9

C122p

6

PIC16F84

12RB6

RB4

RB510

11

RB713

IC1

RA21

2RA3

RA1

X14MHz

C222p

1110

9

12

C3100n

15

14

13

16

OUTPUT 1

OUTPUT 2

OUTPUT 3

SENSE/MUTE

R2220k

MCLR

14

VDD VDDRA0

17

R64k7

4

R810k

C6100n

C410m

OSC IN

5

RB2

RB0

RB16

7

RB38

9

16

12 RB6

RB4

RB510

11

RB713

PIC16F84

RA21

IC2

C5100n

0V (GND)

C7100m

R94k7

MCLR

14

4

78L05

OUT

COM

IC3 IN

+12V

+5V

D11N4148

a k

+ +

**

*

*

*

*

*

*

**

*OPTIONAL- SEE TEXT

OPTIONAL- SEE TEXT

PL1 PL1

RA43

5V+

Fig.7. Full circuit diagram for the Transmitter section. Note the items markedwith an asterisk are optional – see text.

R9 R104k7 220W

0V (GND)

14

1

D1 TO D82mA RED

R1 TO R8680W 680W

COMMON

2345678

OUTPUTS

RB0

5

C122p

RB5

RB1RB2RB3RB4

RB6RB7

9876

13121110

OSC IN

15

16

OSC OUT

3RA4

RA21

MCLR4

RA017

IC1PIC16F84

+5V OUTPUT

INPUT 14R194k7

OUTPUTS

9X1

4MHz

C222p

D9 TO D162mA RED

R11 TO R18

C3100n

1213141516

1110

RB0

5

9876

13121110

1

RB5

RB1RB2RB3RB4

RB6RB7

RA2

OSC IN16

MCLR4

RA43

IC2PIC16F84

C5

10mC4

100n

C7470m

C6100n

7805IC3

COM

OUT IN

+12V

++a a

k k

*

*

VSS VSS

VDD VDD

OPTIONAL- SEE TEXT

PL1 PL1

Fig.8. Complete circuit diagram for the Receiver section of the Multi-ChannelTransmission System.

EPE Online, May 2000 - www.epemag.com - 360

Constructional Pro-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

PL1

IC1 IC2X1

IC3

IN

IN

COM

COM

OUT+

+

+12V

0V (GND)

+5V

C3

R9

R10C1

C2

R19

C4

C5

C C76

R1

R2

R3

R4

R5

R6

R7

R8

R11

R12

R13

R14

R15

R16

R17

R18

k kk kk kk kk kk kk kk ka aa aa aa aa aa aa aa a

Fig.9. Receiver printed circuit board componentlayout and (approximately) full-size copper

foil master pattern.

Complete Receiver module, includingthe LEDs. The LEDs, together with

their associated resistors, can be omit-ted if you wish – see text.

COMPONENTS

ResistorsR1 to R8, R11 to R18 680 ohms (16 off)R9, R19*R10

See also theSHOP TALK Page!

All 0.6W 1% metal film

Printed circuit board available from the EPE Online Store,code 7000265 (Receiver) at www.epemag.com; 18-pinDIL socket (2 off); small heatsink for IC3; multistrandconnecting wire; solder pins, solder, etc.*Note: Resistor R10 is optional (see text).

$32Approx. CostGuidance Only

4k7 (2 off)220 ohms

CapacitorsC1, C2C3, C5, C6C4C7

22p resin-dipped ceramic (2 off)100n resin-dipped ceramic (3 off)10u radial electrolytic, 63V470u radial electrolytic, 25V

SemiconductorsD1 to D16IC1, IC2

IC3

red LEDs, 2mA type (16 off)PIC16F84 pre-programmed microcontroller (2 off)7805 5V 1A voltage regulator

MiscellaneousX1PL1

4MHz crystal20-way IDC header plug

RECEIVER

EPE Online, May 2000 - www.epemag.com - 361

Constructional Project

In contrast to theTransmitter there is nocommunication between the twoICs which both simply checkand store all sixteen bits andoutput the appropriate set. Thisallows them to use identicalsoftware and, as with theTransmitter, if just eightchannels are required thesecond IC can be simplyomitted.

An examination of thesoftware of this project willreveal that it is written instraightforward “top-down” stylewith most repetitive operations

simply repeated the appropriatenumber of times in preference tousing loop techniques. Thistends to improve reliability and iseasy to follow, even though it ismore tedious to write.

TRANSMITTERCIRCUIT

As with many PIC projects,the circuits are relatively simpleas so much of the work is doneby the software. The onlycomplexity is in the Transmitter

where the various methods ofuse make some of thecomponents optional.

These options will beexplained in more detail nextmonth. For now the simplestmethod will be described so thatconstruction and testing can becarried out.

The full circuit diagram ofthe Transmitter is shown inFig.7. The two 16F84 PICs, IC1and IC2, share a common clockusing the oscillator of IC1 with a

COMPONENTS

Resistors*R1, R5 22k (2 off)R2 220k*R3, *R4, R7 1k (3 off)R6, R9 4k7 (2 off)R8 10k

See also theSHOP TALK Page!

All 0.6W 1% metal filmCapacitors

C1, C2 22p resin-dipped ceramic (2 off)C3, C5, *C6 100n resin-dipped ceramic (3 off)C4 10u radial electrolytic, 63V*C7 100u radial electrolytic, 25V

Semiconductors*D1 1N4148 signal diode*TR1 BC184L npn transistorIC1, IC2 PIC16F84 pre-prog- rammed microcontroller (2 off)IC3 78L05 5V 100mA voltage regulator

MiscellaneousX1 4MHz crystalPL1 20-way IDC header plug

PCB available from the EPEOnlineStore, code 7000264 (Transmitter)www.epemag.com; 18-pin DILsocket (2 off); solder pins, solder,multistrand connecting wire, etc.Note, All components marked withan asterisk are optional (see text).

$26Approx. CostGuidance Only

TRANSMITTERR3

R4

R6

R5

D1

C3 IC1

R1

R2R7R8

X1

C1 C2

IC2

R9

C5

C4

C7

C6

IC3

TR1 PL1e

bc

INCOM

OUT

+

+

COMMON

COMMON

OUTPUT 1

OUTPUT 2

OUTPUT 3

SENSE/MUTE

1 16INPUTS

+5 VOLTS

+12 VOLTS

GND (0V)

k

a

Fig.10. Printed circuit board topside component layout and(approximately) full-size under-side copper foil master pattern

for the Transmitter.

EPE Online, May 2000 - www.epemag.com - 362

Constructional Project

4MHz crystal X1 and capacitorsC1 and C2.

Both IC1 and IC2 have alleight inputs of port B pulled highinternally so these are simplybrought out to pins to whichexternal connections can bemade. The communicationbetween them is throughresistor R7 with pull-up resistorR8. A digital output is taken fromIC1 port A bit 2 (at pin 1), whichis normally high and goes lowfor clock and data pulses.

The sensing and mutingfunction, only required forsynchronized bi-directional use,is performed with port A bit 1 (atpin 18) and operates as follows.When used in this way thesignal is coupled to the localreceiver through a 10k resistor,and the sense/mute pin is alsoconnected to the receiver side ofthis resistor.

Initially it is an input, andlistens for a continuous “high”signal to confirm that the othertransmitter is not sending. Oncethis is detected it is converted toan output and set high for theduration of transmission, so thelocal receiver effectively sees acontinuous inactive line. Wherethis facility is not required,resistor R2 holds this pin high sothat transmission will take placeanyway.

Other optional bits areresistors R3 and R4 which areonly required if the unit is usedwith the Interface circuit to bedescribed next month, andresistors R1, R5, transistor TR1and diode D1, are needed if it isto be powered through a 2-wireconnection from the distantReceiver. The principle here isthat one of the two wires is acommon ground (0V), ornegative, whilst the other isenergized from +5V through a220 ohm resistor (an option inthe Receiver) and chargescapacitor C4 via diode D1 whilstthe line is high. Then C4supplies the circuit whilst the lineis pulled low for pulses bytransistor TR1.

Finally, there is an optionalon-board 5V supply regulator,IC3. In most cases theTransmitter will be supplied with+5V from a Receiver, eitherlocal for a bi-directional systemor remote. However, if anapplication requires that itshould be self-powered for anyreason, regulator IC3 can befitted together with inputdecoupling capacitors C6 andC7. In most cases these threecomponents will not be needed.Also, of course, where only eightchannels are needed IC2 maybe omitted.

RECEIVER CIRCUITThe Receiver circuit

diagram shown in Fig.8 is evensimpler. As with the Transmitter,the two PIC16F84s, IC1 andIC2, share a common 4MHzcrystal clock. However, there isno communication betweenthem. Instead the input signal isconnected to RA2 (at pin 1) ofboth PICs. Each of the sixteenoutputs is provided with aresistor supplying a LED (light-emitting diode). These can beomitted if not required althoughthey are useful when testing. Forclarity only one resistor and oneLED are shown for each IC inFig.8, since the others areidentical. The supply regulatorIC3 is a robust 1A type mountedon a small heatsink as is has tosupply the LEDs and probablyalso some output circuits and aTransmitter. The only optionalcomponent is resistor R10 whichis needed if 2-wire operationwith the Transmitter poweredfrom the line is intended.

CONSTRUCTIONConstruction of this project

is straightforward. TheTransmitter and Receivercircuits, that make up the Multi-channel Transmission System,are both built up on single-sidedprinted circuit boards (PCBs).These boards are available fromthe EPE Online Store (codes7000264 (Transmitter) and7000265 (Receiver)). TheInterface PCB (next month) isalso available (code 7000266),all from the EPE Online Store atwww.epemag.com

Starting with the Receiver,all the components exceptresistor R10, just above IC1,should be fitted as shown inFig.9. The use of DIL sockets isrecommended for the two PICs,IC1 and IC2.

OUTPUT 3

OUTPUT 1OUTPUT 2

SENSE/MUTE

COMMON

COM

IN

+5V+5V

0V

0V+12VSUPPLY

TRANSMITTER

INPUTS

OUTPUTS

RECEIVER

Fig.11. Test set-up for checking out the two PCBs.

EPE Online, May 2000 - www.epemag.com - 363

Constructional ProjectSolder pins are suggested

for the external connections, asthese will then be more robustand can be made from thecomponent side of the board. Adegree of force is sometimesrequired to insert such pins so itmay be best to fit them first.

The LEDs, which should be2mA types, and their associatedresistors are optional. Wherefitted it is not too difficult to bendtheir leads in the requiredmanner, and a little “Blu-Tack”or “Play-Do” may be helpful forholding them in position duringsoldering.

Not mentioned so far is theplug PL1. A requirement for theoriginal application was a meansof rapid connection and removalfor testing and service purposesso 20-way IDC header plugswere included in the design.These are retained in thisproject but can be omitted if notrequired.

The two PICs should not beinserted yet. An initial test is tosupply the completed Receiverboard with +9V to +12V whichshould result in a supply currentof about 48mA whilst theregulated output of +5V shouldbe available from the solder pin,marked +5V, just above IC1.

TRANSMITTERIf the above test is

satisfactory construction cancontinue with the TransmitterPCB, the component layout,together with an approximatelyfull-size copper foil master, isshown in Fig.10. All the optionalcomponents should be omittedat this stage and the two PICsshould not be inserted.

Once the board has beencompleted, it is worth checkinginitially by powering it with a 5Vsupply taken from the Receiver.It should draw virtually no

current at all since the onlycomponents bridging the supplyare the three decouplingcapacitors. However, shortcircuits do occasionally occur inconstruction and electrolyticcapacitors have been known tobe fitted the wrong way round!

TESTINGIf all seems well IC1,

programmed with TXIC1_5(5ms delay) or TXIC1_10 (10msdelay) software, can be inserted.This should raise the supplycurrent to about 2mA and theaverage voltage measured witha meter at Output 2 should beabout 4V, indicating that IC1 isoperating and transmitting anappropriate pulse sequence.

Next, a PIC programmedwith receiver RX softwareshould be inserted into theReceiver board at IC1 positionand a connection made fromOutput 2 of the Transmitter to“IN” of the Receiver as shown inFig.11. Connecting any of thefirst eight inputs (1 to 8) toground (0V) should nowilluminate the correspondingoutput LEDs on the Receiver ortake the appropriate outputshigh if the LEDs are not fitted.

Finally, if all sixteenchannels are required, a secondPIC with RX software can befitted to the Receiver and onewith TXIC2 software to thetransmitter, after which theremaining eight channels (9 to16) can be tested. The twoboards are now operational andready for use.

RESOURCESSoftware for the Multi-

Channel Transmission SystemTransmitter and Receivermodules is available for freedownload from the EPE OnlineLibrary at www.epemag.com

Ready-programmed PICsare also available and fulldetails, including the aboveoptions, can be found in theShoptalk page in this issue.

Next Month: Details of thevarious ways in which theseunits can be used will be given,together with the construction ofan Interface board for use withinternal telephone circuits orsimilar long lines. This iseffective in reducing oreliminating the radiatedinterference sometimes causedby high-level digital signals intransmission circuits.

EPE Online, May 2000 - www.epemag.com - 364

Passive infrared (PIR) lamps,of the type that may be bought inany DIY store, are now verypopular with householders.Mounted on an outside wall, theymay be used to improve securityor simply to illuminate dark areaswhen a member of the familypasses by.

JUST PASSINGTHROUGH

These lamps are designed toswitch on for a certain time whensomeone walks in the detectionfield. This extends fan-shapedfrom a “window” in the front of thedetector. In simple units, theoperating time is fixed atmanufacture. However, it is moreusual to provide a control, whichmay be used to adjust it over a

on a filament bulb. In the largersecurity-type lamp, the bulb willbe a halogen unit of some 150Wto 500W rating. Smaller PIRlamps use an ordinary 60Whousehold bulb.

BLOWING IN THEWIND

When the PIR unit isproperly installed, the lamp doesits job well and rarely causesproblems. However, when it isnot properly set up it may beactivated by animals such asdogs and cats passing by.

Any warm object moving in(and especially across) thedetection field is likely to causethe unit to trigger – even warmair from a nearby central heatingflue. Tree branches and otherobjects moving in the windsometimes activate it –presumably because they reflectinfrared from somewhere else.

Any cause of false triggeringmay be difficult to track down. Itcan occur even when the userhas taken every precautiondetailed in the installation guide.After supposedly “correct”setting-up, there is often atendency towards occasionalfalse triggering. This will requirefurther adjustment on a “trial anderror” basis to eliminate itcompletely.

Most PIR lamps have a“test” facility, which enablesthem to operate in daylight andthis helps with the initialadjustment process. However, itwill miss any false triggering

Be trigger happy with your outdoor security lightsystem

PIR LIGHT CHECKERby TERRY DE VAUX-BALBIRNIE

certain range.Normally, the lamps operate

only when the ambient light fallsbelow a certain level so that theywill not switch on during daylighthours. Again, the point at whichthis happens is often adjustableusing a control on the unit.

The working part of a PIRlamp is a sensor, which detectsthe infrared radiation that isnaturally emitted by a warmbody. The detector may becontained in a separate unitconnected remotely to the lamp.In most DIY units, however, it isattached to the lamp itselfbecause this makes for simplerinstallation.

When a warm object movesin the sensitive zone, a signal isgiven which, after processing,operates a relay and switches

EPE Online, May 2000 - www.epemag.com - 365

which happens only occasionally. There could be considerable difficultywhen the lamp is mounted in a position that cannot be seen from thehouse.

Normally, the only way to check for correct operation would be tostand outside and watch it for a long period of time! Unnecessaryoperation of the lamp can be a nuisance to neighbors as well aswasting electricity and reducing the life of the bulb. With this PIR LightChecker, however, you can leave the monitoring to automaticelectronics!

CLOCKEDThis self-contained battery-operated unit will automatically monitor

a PIR lamp over aperiod of severalhours overnight. ALED (light-emittingdiode) displayregisters the numberof times it has beentriggered, up to nine.If the count exceedsthis, the display willreturn to zero but thedecimal point will lightup. This shows the“overflow” – that is, anumber greater thannine. When the unit isswitched off then onagain, the count isreset to zero, readyfor a further test.

By adjusting theaim and sensitivitycontrol on the lamp (ifone exists), re-sitingand cutting awayfoliage as necessary,any improvement canbe easily monitored.Multiple causes offalse triggering maythen be eliminatedone by one over aperiod of a few days.Note that if the unit isused to monitor thelamp overnight, it willrecord an extra countat dawn and this willneed to be subtractedfrom the total.

This circuit is onlysuitable for use at

Constructional Project

af

bg

ec

dX2

b

c e

B1 6V

R6

1M

R5

3M

3

+ IC1

ICL7

611

3 2

4

78

6

R7

1M

R8

2M

2R10

1M

R19

2M

2

R11

1M

R9

1M

R3

470k

R1

56k

R4

470k

R2

L.D

.R.

(2M

DA

RK)

VR1

2M

2

C1

47

n C2

100n

C4

100n

C3

100

n

C7

100n

C5

1µ

C8

220µ

IC2

a556

IC2

b5

56

IC3

40110

69

410

5 113

212

76

84

14

16

+VE

+VE

GN

DG

ND

TRIG

TRIG

RES

ET

RESE

T

OUT

OUT

DIS

DIS

THRES

THRES

9 5

CLO

CK U

P

RESE

T

TAG

EN

LATC

HEN

1 2 3 12

13

14

15

10

C6

100n

R1

2 T

O R

18

270

Ω

R20

270

Ω

X1

7

10 9 1 2 4 6

a g f e d c b

5D

P

8

S1

GN

D

DIS

PLA

Y

ON

/OFF

D1

1N

400

1k

aS2

TR1

2N

3903

3

Fig.1. Complete circuitdiagram for the PIR Light

COMPONENTSResistors

R1R2

R3, R4R5R6, R7, R9 to R11R8, R19R12 to R18, R20

See also theSHOP TALK Page!

All 0.25W 5% carbon film

PCB available from the EPE Online Store, code 7000263(www.epemag.com); 8-pin DIL socket; 14-pin DILsocket;16-pin DIL socket; 1.5V AA-size alkaline cell (4 off) andholder; plastic case, size 138mm x 76mm x 38mminternal;PCB supports (2 off); connecting wire; solder, etc.

$32Approx. CostGuidance Only (excl. battery pack)

56ksub-miniature light-dependent resistor (LDR), dark resistance 5M ohms approximately (see text)470k (2 off)3M31M (5 off)2M2 (2 off)270 ohms (8 off)

PotentiometerVR1 2M2 miniature enclosed carbon preset,

horizontalCapacitors

C1C2C3, C4, C6, C7

C5C8

47n metallised polyester, 2.5mm pitch100n metalised polyester, 2.5mm pitch100n metalised polyester, 5mm pitch (4 off)1u radial electrolytic, 63V220u radial electrolytic, 10V

SemiconductorsD1TR1IC1IC2IC3

1N4001 1A 50V rectifier diode2N3903 npn transistorICL7611 micropower opampICM75561PD dual CMOS timer40110B decade up/down counter

MiscellaneousX1

S1

S2

7-segment, common cathode LED display, 12.7mmminiature s.p.s.t. push-to-make or biased toggle switchminiature s.p.s.t. toggle switch

EPE Online, May 2000 - www.epemag.com - 366

night with the PIR lamp in“normal” mode. There must beno other bright sources of lightnearby which could result infalse counts.

BATTERY SAVINGThe circuit is housed in a

small plastic box. This has aseven-segment LED displayshowing through a hole in thelid. There are also two switches(see photograph). One of theseis simply an on-off switch whilethe other activates the display.This latter switch is operatedonly when a reading needs to betaken and so saves batterypower. A hole in the side of thebox allows light from the lampbeing monitored to reach asensor on the printed circuitboard (PCB) inside.

The unit draws power froma 6V battery pack consisting offour AA-size alkaline cells.Under standby conditions, thecurrent requirement of theprototype unit is some 400uA.

When the display isoperated, the current rises to avalue that depends on thenumber being displayed. This isbecause each digit is formed bylighting up the appropriatesegments in the display. Themost current-hungry case iswhen the number “8” is involved(since this uses all sevensegments) together with the“over-flow” decimal point.

Since each segment andthe decimal point require 12mAapproximately, the total currentwill be about 100mA. However,this will only be needed for a fewseconds during each test and,as stated earlier, it is the “worst”case. In practice, the batterypack should last for at least ayear under normal conditions.

HOW IT WORKSThe full circuit diagram for the

PIR Light Checker is shown inFig.1. Power is derived from a 6Vbattery pack (4 x 15V cell) B1 viaon-off switch S2 and diode D1.The diode prevents possibledamage if the supply were to beconnected in the opposite sense.If this were done, the diode wouldbe reverse-biased so no currentwould flow.

It will be found that the actualnominal supply voltage is 53Vtaking into account the forwardvoltage drop of the diode (07Vapproximately). Capacitor C8charges up almost instantly andhelps to provide a smooth andstable supply.

The light-sensing section ofthe circuit is centered on IC1 andassociated components. The lightdetector itself is a light-dependentresistor (LDR), R2. The resistanceof this device rises as theillumination of its sensitive surfacefalls.

The LDR works in conjunctionwith fixed resistor R1 and presetpotentiometer VR1 to form apotential divider connected acrossthe supply. Thus, as theresistance of the LDR increases,the voltage across it will rise. Thisvoltage will therefore be greaterwhen the LDR is dark than when itis illuminated. The actual “dark”and “light” voltages can be variedwithin certain limits by adjustingVR1. The voltage appearingacross the LDR is applied to theinverting input (pin 2) ofoperational amplifier (opamp) IC1.The non-inverting input (pin 3) isconnected to the mid-point of afurther potential divider consistingof fixed resistors R3 and R4.Since these have the same value,the voltage here will be equal toone-half that of the supply – thatis, 26V approximately.

A BIT DIMWith preset VR1 suitably

adjusted, under dim conditionsthe voltage at the opampinverting input will exceed that atthe non-inverting one, so thedevice will be off with the output(pin 6) low. When the LDR issufficiently illuminated, theconditions will reverse with theinverting input voltage fallingbelow the non-inverting one.The opamp will then switch onand output pin 6 will go high.Resistor R5 applies somepositive feedback to the system,which sharpens the switchingaction at the critical light level.

Transistor TR1 inverts theoutput state of the opamp.When the output is high, currentflows into TR1 base (b) throughcurrent-limiting resistor R6. Thisswitches the transistor on andits collector (c) goes low. Whenthe opamp output is low, nocurrent will enter the base andthe transistor will remain off. Thecollector will then take on a highlogic state via load resistor R7.The state of the collector istherefore in the opposite senseto that of the opamp output.

MONOSTABLETransistor TR1’s collector is

connected to the trigger input(pin 6) of a monostable basedon IC2a, which is one half ofdual integrated circuit timer, IC2.

When TR1 collector goesfrom high to low (that is, theLDR is illuminated), the triggerinput receives a low pulsethrough capacitor C1. Themonostable output (pin 5) thengoes high for a time dependenton the values of resistor R10and capacitor C4. With thevalues specified, the timedperiod is 01s, approximately.

While the opamp output

Constructional Project

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remains low (the LDR dimlyilluminated), the high state ofTR1’s collector has no effect. Infact, in the absence of a lowpulse, IC2a trigger input is kepthigh through resistor R9 and thisprevents possible falsetriggering. Capacitor C3decouples this section of thecircuit.

At the instant of powering-up, capacitor C2 keeps IC2areset input (pin 4) low and thisdisables the monostable. Thecapacitor soon charges upthrough resistor R8, pin 4 goeshigh and the monostable thenfunctions normally. The purposeof this is to allow time for thepower supply to settle down to asteady state since, otherwise,the monostable could self-trigger and a false count wouldbe registered.

COUNTING PULSESWhen the monostable

outputs a pulse, this istransferred to the “clock up”input (pin 9) of counter and 7-segment driver IC3. Thisregisters the number of pulsesreceived and decodes the resultinto a form which will directlydrive the 7-segment LED displayX1.

Constructional ProjectThe seven segments of the

LED display are identified byletters a to g as shown in Fig.2.Note that the unit used in thiscircuit is a common cathodetype. In this, all the LEDcathodes (including that of thedecimal point) are connectedtogether and taken to pin 3(GND).

Each segment requires acurrent-limiting resistor (R12 toR18) as with a conventionalLED. With the value specified,each one will draw 12mAapproximately when using a newbattery.

The display, however, willdo nothing until push-to-make“Display” switch S1 is operated.This allows current to flowthrough any active segmentsand complete the circuit via pin3 to the 0V line. With S1 in the

off state, no current is drawn bythe display.

At the instant of switchingon, IC3’s reset input (pin 5) ismaintained in a high state whilecapacitor C5 charges upthrough R11. During this time,the counter is reset so thedisplay will always begin at zero.After a short time, C5 will chargesufficiently, pin 5 will go low andthe counter will functionnormally. Capacitor C6decouples this section of thecircuit.

When the count passesfrom 9 to 0, IC3’s carry output,pin 10, goes low momentarily.This would normally be used tofeed the clock input of a secondcounter/driver IC and a furtherdisplay would provide a readoutup to 99.

b

a

f b

e c

g

d

Fig.2. The seven segmentsof the display are identified

by letters (a to g)

The PCB removed from the case lid to show wiring to the dis-play and on/off toggle switches. The display switch needs to be

a “biased” off type. Note the 7-segment display chip must bethe highest component on the PCB.

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To save costs only onecounter and display are used inthis circuit. However, the lowpulse provides the “overflow”indication by operating thedecimal point. This uses IC2b(the second section of dual timerIC2). It is configured as a formof latch by making the thresholdand discharge inputs (pin 12 andpin 13) low. Thus, oncetriggered by making pin 8 low foran instant, the output (pin 9) willgo high and remain high until thesupply is interrupted. The outputfeeds the decimal point viacurrent-limiting resistor R20.

CONSTRUCTIONThe PIR Light Checker

circuit is built on a single-sidedprinted circuit board (PCB). Thetopside component layout and(approximately) full-sizeunderside copper foil masterpattern are shown in Fig.3. Thisboard is available from the EPEOnline Store (code 7000263)www.epemag.com

In the prototype, one cornerof the PCB had to be cut off toavoid a bush in the box, seephotograph. Begin constructionby drilling the two fixing holesand soldering the three ICsockets and four link wires intoplace.

Omit display X1 for themoment. Note that it must endup as the highest component onthe PCB. Checks should bemade at intervals during theother assembly by inserting itinto its holes in the PCB (but donot solder it yet).

Follow with all resistors(including preset VR1 but notLDR R2). Note that many of theresistors are mounted vertically(see photo).

Solder electrolyticcapacitors C5 and C8 in positiontaking care over their polarity. Ifthey are not of the sub-miniaturetype, it may be necessary to

mount them flat on the PCB sothat they will not be higher thanthe display.

Cut the LDR leads to alength of about 10mm andsolder them to the R2 positionon the PCB. Bend them throughright-angles so that the “window”points to the left. Note that thespecified LDR is a sub-miniaturetype having a body diameter of5mm approximately. If one ofthese is not readily available, itwould be possible to use astandard ORP12 device, butsome adjustment may beneeded to the end leads toprevent the body getting in the

Constructional Project

DISPLAYON/OFF

S2

af bge cdX2

X1

C2

C4

C1

C3

C6

C7

+

+

C8

C5

R14

R12

R20

R2R1 R9

R4

R3

R5

R6

R7

R8

R11

R18R17R16R15

R10

VR1

cbe

TR1

R19

R13

D1

a

k

S1(SEETEXT)

BATTERY VE+

BATTERY VE–

Fig.3. Printed circuit board component layout,(approximately) full-size underside copper foil master

pattern, and wiring to the two off-board switches.

Close-up of circuit boardshowing the leads of the LDRcarefully bent at right-anglesto the PCB to align with “light

window” in side of case.

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way of anything else.Add the diode and transistor

to the PCB, taking care overtheir orientation. The flat face oftransistor TR1 should face to theright as viewed in Fig.3.

Solder the display to thePCB (with the decimal point atbottom right, as shown in thephoto) using minimum heat fromthe soldering iron to preventpossible damage.

Solder 10cm pieces of light-duty stranded connecting wire tothe points labeled +6V and S1.Solder the negative (black)battery connector lead to the 0Vpoint. Adjust VR1 toapproximately mid-trackposition.

TESTINGImmediately before handling

the pins of IC1, IC2 and IC3,touch something which is“earthed” (such as a metal watertap). This will remove any staticcharge, which may be presenton the body. Insert the ICs intheir sockets with the correctorientation.

Before mounting the PCB inits case, perform a basic checkso that any minor problems may

be resolved more easily.To do this, bare the end few

millimeters of the wires fordisplay switch S1 and connectthem together. Similarly, barethe end of the +6V wire. Insertthe cells for battery B1 into theirholder and apply the connector.Twist the battery connectorpositive (red) wire to the +6Vwire from the PCB.

The display should light upand read zero. The decimal

point should also be off. If itshows some other number, orthe decimal point is on, theconnection was probably notdone “cleanly”, so disconnectthe battery, wait for 30 secondsand try again!

Cover the LDR with thehand then remove it to allowlight to reach its window. Thedisplay should advance to acount of 1. If this does not work,it is likely that the LDR has notbeen properly covered. Tryworking in a dark cupboard andopen the door slightly to give theflash of light. If this still does notwork, re-adjust preset VR1 andtry again.

By allowing repeated flashesof light to reach the LDR, thecounter should increment to 9and the next flash should returnit to zero. However, the decimalpoint should now be seen to belit up. If you wish to reset thedisplay, you will need to wait forup to thirty seconds betweendisconnecting the battery andre-connecting it again.

Constructional Project

Layout of components on the completed circuit board.Note, one corner of the board has been trimmed off so it will fit

in the case.

Completed PIR Light Checker front panel layout.The display cutout has been backed with a piece of translucent

filter material.

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ENCLOSUREIf all is well, the PCB may

now be mounted in the box.Note that when using thespecified unit, everything maybe attached to the lid section.This method places least strainon the battery connecting wires.

First, disconnect the positivesupply wire and detach thebattery connector. Decide onpositions for the PCB, batterypack and switches, checkingthat there is sufficient space foreverything to fit. Arrange for theLDR to lie between 5mm and10mm from the side of the box.

In the prototype, a miniaturetoggle switch with “make”contacts was used for the on-offswitch and a matching biasedtoggle switch was used for thedisplay. A biased switch is onethat springs back to the offposition when pressure isremoved from the actuatinglever. It is best to use either abiased toggle switch or a push-to-make switch to activate thedisplay so that it cannot be lefton accidentally.

Mark through the PCB fixingholes. Measure the position ofthe display and mark around itsoutline. Mark also the positiondirectly in line with the LDR

window and VR1 on the top.Mark the position of theswitches. Remove the PCB anddrill all these holes.

The hole for the LDR shouldhave a diameter ofapproximately 4mm (about twiceas large if the ORP12 type LDRis used). The hole above thepreset VR1 position should belarge enough to allow it to beadjusted using a thinscrewdriver or trimming tool.

The easiest way to makethe hole for the display is to drillsmall holes within its outlinethen remove the plastic using asmall hacksaw blade, or sharpchisel. Finally, smooth the edgesup to the line using a small file.

Attach the PCB temporarilyusing nylon fixings and withshort plastic stand-off insulatorson the bolt shanks. Adjust thelength of the stand-off insulatorsso that the display will end up1mm approximately below theinside face of the box. Whensatisfied, re-attach the PCB.

Check that the LDR windowlies directly in line with the holedrilled for it. If not, adjust theposition of its end leads so that itis. Attach the switches. Securethe battery pack using a smallbracket or adhesive pads. Referto Fig.3 and complete the

internal wiring.In the prototype, a piece of

red plastic filter was glued overthe display hole on the inside ofthe box. This gives aprofessional appearance andalso improves the contrast of thedisplay. If a piece of real filter isnot available, perhaps suitablematerial could be obtained froma sweet wrapper or somethingsimilar.

INTO SERVICEWith switch S2 off, connect

the battery and attach the lid.Find a suitable place for the unitso that light from the PIR lampwill reach the LDR directlythrough the hole in the side ofthe box. The fact that the LDR issome distance behind the holemakes the response directional.This is useful because it tendsto discriminate against othersources of light, which couldresult in false counting.

Make some tests at night.For initial trials, you may find ithelpful to use an elastic band orPVC tape to hold the displayswitch (S1) on, so that the countmay be observed over a periodof time. Remember that thiswastes the batteries so don’t doit for too long.

Adjust preset VR1 for besteffect. Remember to protect theunit against rain entering if thisis a possibility.

No more disturbedneighbors with this “TriggerHappy” circuit!

Constructional Project

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GOOGLE BOXEagle-eyed readers will

have noticed that I recentlyplaced a Google search enginein the 100 percent revalidatedNet Work A-Z page on our website, which contains many of theexisting links I have highlightedin the past. Google is hugely fastand easy to use. Instead of try-ing to index every known website, Google actually indexes onthe basis of all the other linksmade to those same web sites.The search engine makes thereasonable assumption that thebetter a web site is, the greaterthe number of links pointing tothat site. More importantly,Google keeps a cache of storedweb pages, so that even if aweb site is taken down there isstill a possibility that you can re-trieve the content from Google’scache. Give it a try.

In March 2000 Net Work Ioutlined the evolution of AltaVista, Digital Equipment’s lead-ing search engine and portal sitewhich was acquired by computermanufacturer Compaq in early1998. There is no doubt that AltaVista is a first-rate search en-gine, offering further options tonon-English users courtesy of itsBabel Fish language translator.Alta Vista continues to roll outacross Europe, starting out inGermany almost a year ago, fol-lowed by Sweden and more re-cently the UK in December1999. The Californian-basedcompany then launched intoFrance and the Netherlands lastmonth.

FREE FOR ALLIn early March Alta Vista UK

took the wind out of the sails ofcable operator NTL(www.askntl.com) as well asBritish Telecom, by announcingits new free Internet access ser-vice for UK users. In fact it isn’tentirely free – there will be aone-off set-up charge of any-thing up to 50 UK pounds beingreported, and an annual cost ofsay 20 UK pounds. The newservice, to be called Al-taVista0800, will be rolled out ata rate of 90,000 users permonth starting in June 2000.

In the USA and Canada, aservice called AltaVista Free Ac-cess has been available sinceAugust 1999 (seewww.microav.com), offeringcompletely free Internet accessto its users. There are no set-upor subscription charges at all.Instead, AltaVista Free Accessemploys a “Micro Portal” – awindow on the user’s computerscreen which contains rotatingadverts and other customizablecontent. The technology behindthis is provided by 1stUp.com, aUS developer specializing inadvert-supported dial-up ac-counts. The advertising windowmust always remain open to en-joy free Internet access, which isa powerful incentive for manyconsumers already conditionedto banner adverts to remainloyal.

In the UK, by using an 0800access number, subscribers arerelieved of the worry of the cost

of the phone call, though obvi-ously they still have to pay linerental charges. Alta Vista UK’snew service will not allow a per-manent 24 x 7 connection to theInternet, because it will time outafter five minutes of inactivity.Furthermore, any attempt to“ping” an open connection with akeep-alive utility such asWakeUp will be treated as anabuse, presumably leading towithdrawal of the service.

UNDER THE SURFThe new service announced

by Alta Vista UK wrong-footedBritish Telecom into declaring itsown revised plan for un-meteredaccess. BT previously sug-gested its SurfTime package(see Net Work Feb ’00) couldcost anything up to 35 UKpounds a month for always-onaccess. I showed how this wasfive times more than a user inDallas, Texas who pays just $12(7 UK pounds a month) afterloyalty discounts, with local In-ternet and voice calls thrown infor free.

In light of Alta Vista UK’snew 0800 package, BT wasforced into firming up its ownposition. They make much of thefact that their SurfTime optionwill be available to businessesas well as home users, and theyare attempting to cater for users’differing habits, given that manyusers are obviously at work dur-ing the day and only access theInternet during evenings andweekends. The cheapest option

By Alan Winstanley

SURFING THE INTERNET

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that BT now proposes is for oc-casional users, paying 1 penceper minute daytimes, 06 penceevenings and 05 pence week-ends, on top of line rental at9.26 UK pounds per month.

As usual, BT’s press re-lease is not entirely straightfor-ward, partly because they hint atan all-inclusive cost for Internetaccess by bundling in rental fig-ures plus an estimate of monthlyISP charges. For a service feeof 5.99 uk pounds a month ex-cluding rental, BT customerscan choose the evening andweekend package which allowsfor un-metered access plus upto 80 minutes’ voice calls. BT’salways-on package is likely tocost 19.99 UK pounds permonth plus rental for homeusers and 29.74 UK pounds ex-cluding VAT (including rental)for business users.

Realizing that the rates willbe scrutinized by an increasinglyimpatient audience, BT hasgone to extraordinary lengths toemphasize how competitive theysay their dial-up Internet pack-ages are in comparison withsimilar ones in the USA.

The rates won’t be availableto end-users until June 2000,and there is a further complica-tion: BT SurfTime will requireusers to access their preferredISP by using an 0844 04 num-ber. If your preferred ISP doesn’toffer one, then you can’t use theSurfTime package. More prob-lems in store include the factthat no wholesale pricing hadbeen offered, therefore no otherservice provider (e.g. Freeserve)

would be able to compete by re-selling BT SurfTime.

As if BT’s convoluted phonetariffs aren’t enough, don’t forgetthe offerings over at BT Internet(www.btinternet.com), thetelco’s Internet Service Providerarm. Un-metered 0800 eveningand weekend access is nowavailable at a new lower rate of9.99 UK pounds a month or109.98 UK pounds a year, andas a sign of their eagerness tohelp novices getting to grips withthe Internet, BT have actuallyincreased the cost of supportcalls to 50 pence a minute upfrom local rate.

LOOKING AHEADThe UK Internet market re-

mains as volatile as ever, andfurther sweeping changes areprobable over the next 12 to 18months before the market finallysettles down. For Freeserve, the18-month old pioneer of the freeISP model, interesting times areahead. As with all free ISPs,Freeserve makes its revenuefrom that all-important slice ofthe cost of the BT 0845 phonecall, plus advertising and thecost of providing technical sup-port.

Consumers tend not to havemuch loyalty towards their ISPand if they suddenly decide tojump ship from the free ISPsand move to a service such asAltaVista0800, preferring to payan annual fee for free unlimitedcalls, this is bound to have aprofound impact on Freeservewhich charges nothing as an

ISP but makes you pay for thecalls instead. It is hard to knowwhat will happen to those freeISPs that also bundle your do-main name and technical sup-port in with the deal.

It is not as though any ofthese free ISPs can levy even asmall monthly fee, as they don’thave any billing mechanism inplace. Freeserve’s latest moveinvolves offering free Internetaccess, provided customersmake 10 UK pounds of calls permonth routed through Energis,its parent telco. However, BT isnow spending the interveningmonths rolling out the intercon-nect components, and ISPswhich adopt the 0844 SurfTimetariff are expected to be able tocharge their subscription fees totheir customer using the user’sBT phone bill. When there arenew offers springing up all thetime, it makes sense not to com-mit to a long-term agreementuntil all the players have madetheir moves.

You can E-mail me atalan@epemag.demon.co.uk.My web site is at http://home-pages.tcp.co.uk/~alanwin

Net Work

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The purpose of this series isto review how we came to bewhere we are today (technology-wise), and where we look likeending up tomorrow. In Part 1 wecast our gaze into the depths oftime to consider the state-of-the-art in electronics,communications, and computingleading up to 11:59pm on 31December 1899, as the world waspoised to enter the 20th Century.

Parts 2 and 3 coveredfundamental electronics andcommunications in the 20thCentury, respectively. Now, inPart 4 we consider some of thekey discoveries in computing thatoccurred during the 20th Century.These developments have set thescene for what is to come as weplunge forth into the thirdmillennium. But before we start,let’s first consider logic diagramsand logic machines, which usuallyreceive little mention …

LOGIC AND LOGICDIAGRAMS

With the exception of CharlesBabbage’s proposal for amechanical computer called theAnalytical Engine in 1832, verylittle thought was given tocomputing prior to 1900. Instead,effort was focused on simplemechanical calculators, and alsoon variations of anothermechanism put forward in 1882

University called Allan Marquandinvented a graphical techniqueof representing logical problemsusing squares and rectangles.Marquand’s efforts set a numberof people to pondering, includingthe Reverend Charles LutwidgeDodgson, who published hisown diagrammatic technique ina book called The Game ofLogic in 1886. (The Reverend isbetter known to most of us asLewis Carroll, the author ofAlice’s Adventures inWonderland.)

In the early 1890s, yetanother approach was putforward by the English logicianJohn Venn, who was extremelyimpressed by Boole’s work.Unlike earlier graphicaltechniques, Venn’s diagramswere based on the use of circlesand ellipses, which could beemployed to represent Booleanequations.

The rectangles and squaresof Marquand and Carrolleventually led to MauriceKarnaugh inventing a graphicaltechnique for both representingand minimizing Booleanexpressions in the 1950s. Thesetechniques were to becometremendously useful todesigners of digital logic, andKarnaugh maps and VennDiagrams are both still taughtand used to this day.

LOGIC MACHINESIn addition to speculating

about logic, it should come as

Part 4 – COMPUTERS 1900-1999by Clive “Max” Maxfield and Alvin Brown

by Babbage called a DifferenceEngine, which could be used togenerate certain mathematicaltables.

However, this is not to saythat nothing of interest(computing-wise) was takingplace, because there were anumber of developments thatwould prove to be extremelyinteresting to computerscientists in the 20th Century.

First of all, the self-taughtBritish mathematician GeorgeBoole published two key papersin 1847 and 1854. These papersdescribed how logicalexpressions could berepresented in a mathematicalform that is now known asBoolean Algebra. What Boolewas trying to do was to create amathematical technique thatcould be used to represent andrigorously test logical andphilosophical arguments.

We can only imagine whatBoole would have thought hadhe realized that his newmathematics would findapplication in designing digitalcomputers 100 years in hisfuture. But we digress …

LOGICWONDERLAND

In 1881, a lecturer in logicand ethics at John Hopkins

Boldly going behind the beyond, behind which no onehas boldly gone behind, beyond, before!

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no surprise to learn that peoplehave been experimenting withso-called “logic machines” forquite some time. Perhaps theearliest example is a set ofconcentric, nested discsrevolving around a central axisas proposed by the Spanishtheologian Ramon Lull in 1274.Each disc contained a numberof different words or symbols,which could be combined indifferent ways by rotating thedisks.

Lull’s disks were followed bya variety of other techniquesover the centuries, most ofwhich we would now consider to

be “half-baked” on agood day.

The world’s firstreal logic machine(that is, one thatcould actually beused to solve simplelogic problems, asopposed to Lull’swhich tended tocreate moreproblems than itsolved) was inventedin the late 1700s bythe British scientist

and statesmanCharles Stanhope(third Earl ofStanhope).

This device, the StanhopeDemonstrator, was a small boxwith a window in the top, alongwith two different colored slidesthat the user pushed into slots inthe sides. Although this doesn’tsound like much it was a start,but Stanhope wouldn’t publishany details and instructed hisfriends not to say anything aboutwhat he was doing.

In fact, it wasn’t until aroundsixty years after his death thatthe Earl’s notes and one of hisdevices fell into the hands of theReverend Robert Harley, whosubsequently published an

Special Feature

Stanhope Square Demonstrator, late18th century. Courtesy Science Museum/

Science and Society Picture Library.

TIMELINES1274: Spain. Theologian Ra-

mon Lull proposes a“logic machine” consist-ing of a set of concentric,nested disks.

1777: Charles Stanhope in-vents a mechanical cal-culating machine.

Late 1700s: Charles Stanhopeinvents the StanhopeDemonstrator.

1822: England. Charles Bab-bage starts to build a me-chanical calculating ma-chine – the DifferenceEngine.

1832: England. Charles Bab-bage conceives the firstmechanical computer –the Analytical Engine.

1847: England. George Boolepublishes his first ideason symbolic logic.

1869: William Stanley Jevonsinvents the Logic Piano.

1881: Alan Marquand invents agraphical technique ofrepresenting logic prob-lems.

1886: Reverend CharlesLutwidge Dodgson(Lewis Carroll) publishesa diagramatic techniquefor logic representation inThe Game of Logic.

1890s:John Venn proposeslogic representations us-ing circles and ellipses.

1925: America. Scientist, engi-neer, and politician Van-nevar Bush designs ananalog computer calledthe Product Intergraph.

1930: America. Vannevar Bushdesigns an analog com-puter called a DifferentialAnalyzer.

1936: America. Efficiency ex-pert August Dvorakpatents his layout for

Stanhope’s calculating machine, 1777. Courtesy Science Mu-seum/Science and Society Picture Library.

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article on the StanhopeDemonstrator in 1879.

Stanhope also invented acircular demonstrator and amechanical calculating machine.

LOGIC PIANOSWorking on a somewhat

different approach was theBritish logician and economistWilliam Stanley Jevons, who, in1869, produced the earliestmodel of his famous Jevons’Logic Machine. This device isnotable because it was the firstmachine that could solve alogical problem faster than thatproblem could be solved withoutusing the machine!

Jevons was an aficionado ofBoolean logic, and his solutionwas something of a crossbetween a logical abacus and apiano (in fact it was sometimesreferred to as a “Logic Piano”).This device, which was about ameter (three feet) tall, consistedof keys, levers and pulleys,along with letters that could beeither visible or hidden. Whenthe operator pressed keysrepresenting logical operations,

the appropriate letters appearedto reveal the result.

The next real advance inlogic machines was made byAllan Marquand, whom wepreviously met in connectionwith his work on logic diagrams.In 1881, by means of theingenious use of rods, levers,and springs, Marquandextended Jevons’ work toproduce the Marquand LogicMachine. Like Jevons’ device,Marquand’s machine could onlyhandle four variables, but it wassmaller and significantly moreintuitive to use.

ROCKET-POWEREDFRISBEES

Things continued to developapace. In 1936, the Americanpsychologist Benjamin Burackfrom Chicago constructed whatwas probably the world’s firstelectrical logic machine.Burack’s device used light bulbsto display the logicalrelationships between acollection of switches, but forsome reason he didn’t publishanything about his work until

1949.In fact, the

connectionbetweenBoolean algebraand circuitsbased onswitches hadbeen recognizedas early as 1886by an educatorcalled CharlesPierce.However,nothingsubstantialhappened in thisarea until 1938,at which time theAmericanengineer ClaudeE. Shannon

Special Featurekeys on a typewritercalled the Dvorak Key-board.

1936: America. PsychologistBenjamin Burack con-structs the first electricallogic machine (but hedidn’t publish anythingabout it until 1949).

1937: America. George RobertStibitz, a scientist at BellLabs, builds a simple dig-ital calculator machinebased on relays calledthe Model K.

1937: England. Graduate stu-dent Alan Turing (ofColossus fame) writeshis ground-breaking pa-per: On ComputableNumbers with an Appli-cation to the Entschei-dungsproblem.

1937: England. Alan Turing in-vents a theoretical(thought experiment)computer called the Tur-ing Machine.

1938: America. Claude E.Shannon publishes anarticle (based on hismaster’s thesis at MIT)that showed howBoolean algebra could beused to design digital cir-cuits.

1938: Germany. Konrad Zusefinishes the constructionof the first working me-chanical digital computer(the Z1).

1939: America. George RobertStibitz builds a digital cal-culator called the Com-plex Number Calculator.

1939: America. John VincentAtanasoff (and CliffordBerry) may or may nothave constructed the firsttruly electronic special-purpose digital computercalled the ABC (but it didnot work until 1942).

Monroe’s “Full Automatic” calculating ma-chine, 1922, the first machine to offer fully au-tomatic multiplication and division. CourtesyScience Museum/Science and SocietyPicture Library.

EPE Online, May 2000 - www.epemag.com - 376

published an article based onhis master’s thesis at MIT.

Shannon’s thesis has beendescribed as: “Possibly the mostimportant Master’s thesis of thetwentieth century.” In his paper,which was widely circulated,Shannon showed how Boole’sconcepts of TRUE and FALSEcould be used to represent thefunctions of switches inelectronic circuits. (Shannon isalso credited with the inventionof the rocket-powered Frisbee,and is famous for riding downthe corridors at Bell Laboratorieson a unicycle whilesimultaneously juggling fourballs.)

Following Shannon’s paper,a substantial amount of attentionwas focused on developingelectronic logic machines.Unfortunately, interest inspecial-purpose logic machineswaned in the 1940s with theadvent of general-purposecomputers, which proved to bemuch more powerful and forwhich programs could be writtento handle formal logic.

ELECTROMECHANICAL COMPUTERS

In 1927, with the assistanceof two colleagues at MIT, theAmerican scientist, engineerand politician Vannevar Bushdesigned an analog computerthat could solve simpleequations. This device, whichBush dubbed a ProductIntergraph, was subsequentlybuilt by one of his students.

Bush continued to develophis ideas and, in 1930, built abigger version, which he called aDifferential Analyzer. TheDifferential Analyzer was basedon the use of mechanicalintegrators that could beinterconnected in any desiredmanner. To provideamplification, Bush employedtorque amplifiers, which werebased on the same principle asa ship’s capstan. The finaldevice used its integrators,torque amplifiers, drive belts,shafts, and gears to measuremovements and distances (notdissimilar in concept to anautomatic slide rule).

Special Feature

1940: America. George RobertStibitz performs first ex-ample of remote comput-ing between New Yorkand New Hampshire.

1941: Germany. Konrad Zusefinishes the first truerelay-based general-purpose digital computer(the Z3).

1942: Germany. Between 1942and 1943 Konrad Zusebuilds the Z1 and Z2computers for the Hen-schel aircraft company.

1942: Germany, Between 1942and 1945/6 Konrad Zusedevelops the ideas for ahigh-level computer pro-gramming languagecalled Plankakul.

1943: England. Alan Turing andteam build a special-purpose electronic(vacuum tube) computercalled Colossus.

1944: America. Howard Aikenand team finish buildingan electromechanicalcomputer called the Har-vard Mark I (also knownas the IBM ASCC).

1945: America. Hungarian/American mathematicianJohann (John) Von Neu-mann publishes a paperentitled First Draft of areport on the EDVAC.

1946: America. John WilliamMauchly, J. Presper Eck-ert and team finish build-ing a general-purposeelectronic computercalled ENIAC.

1948: America. Work starts onthe first commercial com-puter, UNIVAC 1.

1948: America. First commer-cial computer, UNIVAC1, is completed.

1949: England, Cambridge Uni-versity. Small experimen-

Hartree Differential Analyzer, 1935, based on that invented byVannevar Bush. Courtesy Science Museum/Science and Soci-

ety Picture Library.

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Although Bush’s firstDifferential Analyzer was drivenby electric motors, its internaloperations were purelymechanical. In 1935 Bushdeveloped a second version, inwhich the gears were shiftedelectro-mechanically and whichemployed paper tapes to carryinstructions and to set up thegears.

In our age, when computerscan be constructed the size ofpostage stamps, it is difficult tovisualize the scale of theproblems that these earlypioneers faced. To providesome sense of perspective,Bush’s second DifferentialAnalyzer weighed in at awhopping 100 tons! In additionto all of the mechanicalelements, it contained 2000vacuum tubes, thousands ofrelays, 150 motors, andapproximately 200 miles of wire.

As well as being a majorachievement in its own right, theDifferential Analyzer was alsosignificant because it focusedattention on analogue computingtechniques, and thereforedetracted from the investigationand development of digitalsolutions for quite some time.

FLASHLIGHT BULBSAND TIN CANS

However, not everyone wasenamoured by analogcomputing. In 1937, GeorgeRobert Stibitz, a scientist at BellLaboratories built a digitalmachine based on relays,flashlight bulbs and metal stripscut from tin-cans, which hecalled the Model K (becausemost of it was constructed onhis kitchen table).

Stibitz’s machine worked onthe principle that if two relayswere activated they caused athird relay to become active,where this third relayrepresented the sum of theoperation. For example, if thetwo relays representing thenumbers 3 and 6 wereactivated, this would activateanother relay representing thenumber 9. (A replica of theModel K is on display at theSmithsonian.)

Stibitz went on to create amachine called the ComplexNumber Calculator, which,although not tremendouslysophisticated by today’sstandards, was an importantstep along the way. In 1940,Stibitz performed a spectacular

Special Featuretal computer called ED-SAC performs its firstcalculation.

1949: England. EDSAC com-puter uses first assem-bler called Initial Orders.

1949: America. MIT’s first real-time computer, Whirl-wind.

1950: America. Jay Forrester atMIT invents magneticcore store.

1951: America. Computers aresold commercially.

1952: America. John WilliamMauchly, J. Presper Eck-ert and team finish build-ing a general-purpose(stored program) elec-tronic computer calledEDVAC.

1956: America. John Backusand team at IBM intro-duce the first widely usedhigh-level computer lan-guage, FORTRAN.

1956: America. John McCarthydevelops a computer lan-guage called LISP forartificial intelligence ap-plications.

1956: America. MANIAC 1 isthe first computer pro-gram to beat a human ina game (a simplified ver-sion of chess).

1957: America. IBM 610 Auto-Point computer is intro-duced.

1958: America. Computer datais transmitted over regu-lar telephone circuits.

1959: America. COBOL com-puter language is intro-duced for business appli-cations.

1961: Time-sharing computingis developed.

1963: In America, the LINCcomputer was designedat MIT.

Harvard Mark I, the first large-scale automatic digital computer.Courtesy of IBM.

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demonstration at a meeting inNew Hampshire.

Leaving his computer inNew York City, he took ateleprinter to the meeting andproceeded to connect it to hiscomputer via telephone. In thefirst example of remotecomputing, Stibitz astounded theattendees by allowing them topose problems, which wereentered on the teleprinter; withina short time the teleprinterpresented the answersgenerated by the computer.

HARVARD MARK IMany consider that the

modern computer eracommenced with the first large-scale automatic digitalcomputer, which was developedbetween 1939 and 1944. Thisdevice, the brainchild of aHarvard graduate, Howard H.Aiken, was officially known asthe IBM automatic sequencecontrolled calculator (ASCC),but is more commonly referredto as the Harvard Mark I.

The Mark I was constructedout of switches, relays, rotatingshafts, and clutches, and wasdescribed as sounding like a“roomful of ladies knitting.” Themachine contained more than750,000 components, was 50feet long, 8 feet tall (152m x24m), and weighedapproximately five tons(5080kg)!

Although the Mark I isconsidered to be the first digitalcomputer, its architecture wassignificantly different frommodern machines. The deviceconsisted of many calculators,which worked on parts of thesame problem under theguidance of a single control unit.

Instructions were read in onpaper tape, data was providedseparately on punched cards,

and the device could onlyperform operations in thesequence in which they werereceived. This machine wasbased on numbers that were 23digits wide – it could add orsubtract two of these numbers inthree-tenths of a second,multiply them in four seconds,and divide them in ten seconds.

KONRAD ZUSEIn the aftermath of World

War II, it was discovered that aprogram controlled calculatorcalled the Z3 had beencompleted in Germany in 1941,which means that the Z3 pre-dated the Harvard Mark I. TheZ3’s architect was a Germanengineer called Konrad Zuse,who developed his firstmachine, the Z1, in his parents’living room in Berlin in 1938.

Although based on relays,the Z3 was very sophisticatedfor its time; for example, itutilized the binary numbersystem and could handlefloating-point arithmetic. (Zusehad considered employingvacuum tubes, but he decided touse relays because they weremore readily available and alsobecause he feared that tubeswere unreliable).

In 1943, Zuse started workon a general-purpose relaycomputer called the Z4. Sadly,the original Z3 was destroyed bybombing in 1944 and thereforedidn’t survive the war (althougha new Z3 was reconstructed inthe 1960s). However, the Z4 didsurvive – in a cave in theBavarian Alps – and by 1950 itwas up and running in a Zurichbank.

It is interesting to note thatpaper was in short supply inGermany during the war, soinstead of using paper tape,Zuse was obliged to punchholes in old movie film to store

Special Feature1965: John Kemeny and

Thomas Kurtz developthe BASIC computer pro-gramming language.

1968: First Static RAM ICreaches the market.

1970: First floppy disk (8.5inch) is used for storingcomputer data.

1970: America. Ethernet devel-oped at Palo Alto Re-search center by BobMetcalfe and DavidBoggs.

1971: America. Datapoint 2200computer introduced byCTC.

1971: CTC’s Kenbak-1 Com-puter is introduced.

1971: America. Ted Hoff de-signs (and Intel releases)the first computer-on-a-chip, the 4004 micropro-cessor.

1971: Niklaus Wirth developsPASCAL computer lan-guage (named afterBlaise Pascal).

1972: November, America. Intelintroduce the 8008 mi-croprocessor.

1973: America. Xerox AltoComputer is introduced.

1973: May, France. 8008-based Micral microcom-puter is introduced.

1973: June, the term micro-computer first appears inprint in reference to the8008-based Micral micro-computer.

1973: America. Scelbi Com-puter Consulting Com-pany introduce theScelbi-8Hmicrocomputer-baseddo-it-yourself computerkit.

1973: PDP-8 becomes the firstpopular microcomputer.

1974: America. Intel introduce

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his programs and data. We mayonly speculate as to the filmsZuse used for his hole-punchingactivities; for example, were anyfirst-edition Marlene Dietrichclassics on the list? (MarleneDietrich fell out of favor with theHitler regime when sheemigrated to America in theearly 1930s, but copies of herfilms would still have beenaround during the war.)

Zuse was an amazing manwho was well ahead of his time.In fact there isn’t enough spaceto do him justice in this article,but you can find a “world-exclusive” feature article onZuse at the EPE Online web siteat www.epemag.com. Thisarticle, which was written byKonrad’s eldest son, HorstZuse, contains over 100photographs from Horst’sprivate collection, many of whichhave never been publishedbefore!

FIRST ELECTRONICCOMPUTERS

We now turn our attention toan American mathematician andphysicist, John VincentAtanasoff, who has the dubioushonor of being known as theman who either did or did notconstruct the first truly electronicspecial-purpose digitalcomputer.

A lecturer at Iowa StateCollege (now Iowa StateUniversity), Atanasoff wasdisgruntled with thecumbersome and time-consuming process of solvingcomplex equations by hand.Working alongside one of hisgraduate students (the brilliantClifford Berry), Atanasoffcommenced work on anelectronic computer in early1939, and had a prototypemachine by the autumn of thatyear.

In the process of creatingthe device, Atanasoff and Berryevolved a number of ingeniousand unique features. Forexample, one of the biggestproblems for computerdesigners of the time was to beable to store numbers for use inthe machine’s calculations.

Atanasoff’s design utilizedcapacitors to store electricalcharge that could representnumbers in the form of logic 0sand logic 1s. The capacitorswere mounted in rotatingBakelite cylinders, which hadmetal bands on their outersurface. These cylinders, eachapproximately 12 inches tall and8 inches in diameter (30cm x20cm), could store thirty binarynumbers, which could be readoff the metal bands as thecylinders rotated.

Input data was presented to

Special Featurethe 8080 microprocessor.

1974: August, America. Mo-torola introduce the 6800microprocessor.

1974: June, America. RadioElectronics magazinepublishes an article byJonathan (Jon) Titus onbuilding an 8008-basedmicrocomputer called theMark-8.

1975: America. MOS Technol-ogy introduce the 6502microprocessor.

1975: January, America, EdRoberts and his MITscompany introduce the8080-based Altair 8800microcomputer.

1975: April, America. Bill Gatesand Paul Allen found Mi-crosoft.

1975: July, America. Microsoftrelease BASIC 2.0 for theAltair 8800 microcom-puter.

1975: America. MOS Technol-ogy introduce the 6502-based KIM-1 microcom-puter.

1975: America. Sphere Corpo-ration introduce the6800-based Sphere 1microcomputer.

1975: America. Microcomputerin kit form reaches UShome market.

1976: America. Zilog introducethe Z80 microprocessor.

1976: March, America. SteveWozniak and Steve Jobsintroduce the 6502-basedApple 1: microcomputer.

1976: April 1st, America. SteveWozniak and Steve Jobsform the Apple computercompany.

1977: April, America. Apple in-troduces the Apple II mi-crocomputer.

1977: April, America. Com-

Rebuilt versionof Konrad

Zuse’s ZI com-puter. The

original wasbuilt in his par-

ent’s livingroom in Berlinin 1938. Cour-tesy of Horst

Zuse.

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GENIUSES ANDECCENTRICS

Many of the people whodesigned the early computerswere both geniuses andeccentrics of the first order, andthe English mathematician AlanTuring was “first amongstequals.” In 1937, while agraduate student, Turing wrotehis ground-breaking paper OnComputable Numbers with anApplication to theEntscheidungsproblem.

Since Turing did not haveaccess to a real computer (notunreasonably, because theydidn’t exist at the time), heinvented his own as an abstract“paper exercise”. Thistheoretical model, whichbecame known as a TuringMachine, was both simple andelegant, and subsequentlyinspired many “thoughtexperiments”.

During World War II, Turingworked as a cryptographer,decoding codes and ciphers at

Special Feature

the machine in the form ofpunched cards, while intermediateresults could be stored on othercards. Once again, Atanasoff’ssolution to storing intermediateresults was quite interesting – heused sparks to burn small spotsonto the cards. The presence orabsence of these spots could beautomatically determined by themachine later, because theelectrical resistance of acarbonized spot varied from thatof the blank card.

Some references report thatAtanasoff and Berry had a fullyworking model of their machine by1942. However, while someobservers agreed that themachine was completed and didwork, others reported that it wasalmost completed and would haveworked, while still others statedthat it was just a collection ofparts that never worked. Sounless more definitive evidencecomes to light, it’s a case of: “Youpays your money and you takesyour choice”.

modore Business Ma-chines present their6502-based CommodorePET microcomputer.

1977: August, America. Tandy/Radio Shack announcetheir Z80-based TRS-80microcomputer.

1978: America. Apple introducethe first hard disk drivefor use with personalcomputers.

1979: America, the first truecommercial microcom-puter program, the Visi-Calc spreadsheet, isavailable for the Apple II.

1979: ADA programming lan-guage is named after Au-gusta Ada Lovelace (nowcredited as being the firstcomputer programmer).

1981: America. First IBM PC islaunched.

1981: America. First mousepointing device is intro-duced.

1981: First laptop computer isintroduced.

1983: Apple’s Lisa is the firstpersonal computer to usea mouse and pull-downmenus.

1983: Time magazine namesthe computer as Man ofthe year.

1984: 1 MB memory chips in-troduced.

1985: CD-ROMs are used tostore computer data forthe first time.

The Colossus computer, Bletchley Park, 1943. Used to de-cypher the German “ENIGMA” codes during WWII. Courtesy

Science Museum/Science and Society Picture library.

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general-purpose electroniccomputer was the electronicnumerical integrator andcomputer (ENIAC), which wasconstructed at the University ofPennsylvania between 1943 and1946.

ENIAC, which was thebrainchild of John WilliamMauchly and J. Presper EckertJr., was a monster – it was 10feet (3m) tall, occupied 1,000square feet (300m2) of floor-space, weighed in atapproximately 30 tons(30480kg), and used more than70,000 resistors, 10,000capacitors, 6,000 switches, and18,000 vacuum tubes. The finalmachine required 150 kilowattsof power, which was enough tolight a small town.

One of the greatestproblems with computers builtfrom vacuum tubes wasreliability; 90 percent of ENIAC’sdown-time was attributed tolocating and replacing burnt-outtubes. Records from 1952 show

that approximately 19,000vacuum tubes had to be replacedin that year alone, which averagesout to about 50 tubes a day!

In August 1944, Mauchly andEckert proposed the building ofanother machine called theelectronic discrete variableautomatic computer (EDVAC).This new machine was intendedto feature many improvementsover ENIAC, including a new formof memory based on pulses ofsound racing through mercurydelay lines.

FIRST DRAFTIn June 1944, the Hungarian-

American mathematician Johann(John) von Neumann first becameaware of ENIAC. Von Neumann,who was a consultant on theManhattan Project, immediatelyrecognized the role that could beplayed by a computer like ENIACin solving the vast arrays ofcomplex equations involved indesigning atomic weapons.

Special Featureone of the British government’stop-secret establishmentslocated at Bletchley Park. Duringthis time Turing was a keyplayer in the breaking of theGerman’s now-famous codegenerated by their ENIGMAmachine. However, in addition toENIGMA, the Germans hadanother cipher that wasemployed for their ultra-top-secret communications. Thiscipher, which was vastly morecomplicated that ENIGMA, wasgenerated by a machine called aGeheimfernschreiber (secrettelegraph), which the alliesreferred to as the “Fish”.

In January 1943, along witha number of colleagues, Turingbegan to construct an electronicmachine to decode theGeheimfernschreiber cipher.This machine, which theydubbed Colossus, comprised1,800 vacuum tubes and wascompleted and working byDecember of the same year!

By any standards Colossuswas one of the world’s earliestworking programmableelectronic digital computers. Butit was a special-purposemachine that was really onlysuited to a narrow range oftasks (for example, it was notcapable of performing decimalmultiplications). Having said this,although Colossus was built asa special-purpose computer, itdid prove flexible enough to beprogrammed to execute avariety of different routines.

ENIAC AND EDVACBy the mid-1940s, the

majority of computers werebeing built using vacuum tubesrather than switches and relays.Although vacuum tubes werefragile, expensive and used a lotof power, they were much fasterthan relays (and much quieter).If we ignore Atanasoff’s machineand Colossus, then the first true

One small sec-tion of the re-ceiver unit for

ENIAC. Anotherphoto of ENIACwas shown in

Part 2. Courtesy

Science Museum/Science and Society Picture Library.

Electronic vacuum tubes, 1946, which replaced electric re-lays and made operational programs that could multiply two10-digit numbers 40 times per second. Courtesy of IBM.

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Von Neumann wastremendously excited by ENIACand quickly became aconsultant to both the ENIACand EDVAC projects. In June1945, he published a paperentitled First Draft of a report onthe EDVAC, in which hepresented all of the basicelements of a stored-programcomputer:

o) A memory containing bothdata and instructions. Alsoto allow both data andinstruction memorylocations to be read from,and written to, in anydesired order.

o) A calculating unit capableof performing botharithmetic and logicaloperations on the data.

o) A control unit, which couldinterpret an instructionretrieved from the memoryand select alternativecourses of action based onthe results of previousoperations.

Special Feature

The key point made by the paperwas that the computer couldmodify its own programs, inmuch the same way as wasoriginally suggested by CharlesBabbage in the 1830s. Thecomputer structure resultingfrom the criteria presented inthis paper is popularly known asa von Neumann Machine, andvirtually all digital computersfrom that time forward havebeen based on this architecture.

Unfortunately, although theconceptual design for EDVACwas completed by 1946, severalkey members left the project topursue their own careers, andthe machine did not becomefully operational until 1952.When it was finally completed,EDVAC containedapproximately 4,000 vacuumtubes and 10,000 crystal diodes.A 1956 report shows thatEDVAC’s average error-free up-time was approximately eighthours.

EDSAC TO UNIVACIn light of its late completion,

some would dispute EDVAC’sclaim-to-fame as the first stored-program computer. A smallexperimental machine based onthe EDVAC concept consistingof 32 words of memory and a 5-instruction command set wasoperating at ManchesterUniversity, England, by June1948.

Another machine calledEDSAC (Electronic DelayStorage Automatic Calculator)performed its first calculation atCambridge University, England,in May 1949. EDSAC contained3,000 vacuum tubes and usedmercury delay lines for memory.Programs were input usingpaper tape and output resultswere passed to a teleprinter.

Additionally, EDSAC iscredited as using one of the firstassemblers called Initial Orders,which allowed it to beprogrammed symbolicallyinstead of using machine code.Last but not least, the firstcommercially availablecomputer, UNIVAC I (UniversalAutomatic Computer), was alsobased on the EDVAC design.Work started on UNIVAC I in1948, and the first unit wasdelivered in 1951, whichtherefore predates EDVAC’sbecoming fully operational.

MAGNETIC CORESTORES

One of the biggest problemsfaced by early computerdesigners was the lack of small,efficient memories. In GermanyKonrad Zuse experimented withpurely mechanical memories(which were surprisinglyreliable), whilst other engineersworked with a variety of esoterictechniques, including thephosphorescent effect in

Magnetic core memory store. Logic 0 and 1 dependedon the polarity of the magnetized field for each bead.

Courtesy of IBM.

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storage oscilloscopes, andmercury delay lines asdiscussed earlier.

Around 1950, Jay Forresterat MIT came up with the idea ofusing ferromagnetic beads(“cores”) threaded onto wires tostore logic 0s and 1s (dependingon which way they weremagnetized). Although unwieldyby today’s standards, these corestores were incredibly useful atthat time, and they paved theway for bigger and bettercomputers.

Of course, the advent of thetransistor was revolutionary incomputing circles, becauseeach transistor could replace avacuum tube that was 100s oftimes larger, consumed 100s oftimes more power, and was100s of times less reliable.

However, transistors bythemselves did not oust corestores. This was due to the factthat storing one bit of datarequired a single core only 1mmor less in diameter, but storingthe same bit would requirebetween four and six transistors.Thus, it was not until theinvention of the integrated circuitthat useful semiconductormemory devices started toappear in the early 1970s. (Thedevelopment of vacuum tubes,transistors, and integratedcircuits were discussed in Part2.)

FIRSTMICROPROCESSORS

In 1970, the Japanesecalculator company Busicomapproached Intel with a requestto design a set of twelveintegrated circuits for use in anew calculator. The task waspresented to one Marcian “Ted”Hoff, a man who could foresee asomewhat bleak and never-ending role for himself designingsets of special-purpose

Special Featureintegrated circuits for one-of-a-kind tasks.

However, during his earlyruminations on the project, Hoffrealized that rather than designthe special-purpose devicesrequested by Busicom, he couldcreate a single integrated circuitwith the attributes of a simple-minded, stripped-down, general-purpose computer processor.

The result of Hoff’sinspiration was the world’s firstmicroprocessor, the 4004,where the ‘4’s were used toindicate that the device had a 4-bit data path (there is a photo inPart 2). The 4004 was part of afour-chip system which alsoconsisted of a 256-byte ROM, a32-bit RAM, and a 10-bit shiftregister.

The 4004 itself containedapproximately 2,300 transistorsand could execute 60,000operations per second. Theadvantage (as far as Hoff wasconcerned) was that by simplychanging the external program,the same device could be usedfor a multitude of future projects.

In November 1972, Intelintroduced the 8008, which wasessentially an 8-bit version ofthe 4004. The 8008 containedapproximately 3,300 transistorsand was the first microprocessorto be supported by a high-levellanguage compiler called PL/M.The 8008 was followed by the4040, which extended the4004’s capabilities by addinglogical and compareinstructions, and by supportingsubroutine nesting using a smallinternal stack.

However, the 4004, 4040,and 8008 were all designed forspecific applications, and it wasnot until April 1974 that Intelpresented the first true general-purpose microprocessor, the8080. This 8-bit device, whichcontained around 4,500transistors and could perform

200,000 operations per second,was destined for fame as thecentral processor of many of theearly home computers.

Following the 8080, themicroprocessor field explodedwith devices such as the 6800from Motorola in August 1974,the 6502 from MOS Technologyin 1975, and the Z80 from Zilogin 1976 (to name but a few).

Unfortunately, documentingall of the differentmicroprocessors would require abook in its own right, so wewon’t even attempt the taskhere. Instead, we’ll create acunning diversion that will allowus to leap gracefully into thenext topic … Good grief! Did yousee what just flew past yourwindow?

FIRST PERSONALCOMPUTERS (PCS)

Given that the 8008 was notintroduced until November 1972,the resulting flurry of activity wasquite impressive. Only sixmonths later, in May 1973, thefirst computer based on amicroprocessor was designedand built in France.

Unfortunately the 8008-based Micral, as this device wasknown, did not provetremendously successful inAmerica. However, in June ofthat year, the term“microcomputer” first appearedin print in reference to theMicral.

In the same mid-1973 time-frame, the Scelbi ComputerConsulting Company presentedthe 8008-based Scelbi-8Hmicrocomputer, which was thefirst microprocessor-basedcomputer kit to hit the market(the Micral wasn’t a kit – it wasonly available in fully assembledform). The Scelbi-8H wasadvertised at $565 and came

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equipped with 1Kbyte of RAM.In June 1974, Radio

Electronics magazine publishedan article by Jonathan Titus onbuilding a microcomputer calledthe Mark-8, which, like theMicral and the Scelbi-8H, wasbased on the 8008microprocessor. The Mark-8received a lot of attention fromhobbyists, and a number of usergroups sprang up around theUS to share hints and tips anddisseminate information.

LAUNDROMATS INALBUQUERQUE

Around the same time that

Special Feature

Jonathan Titus was penning hisarticle on the Mark-8, a mancalled Ed Roberts waspondering the future of hisfailing calculator companyknown as MITS (which was nextdoor to a laundromat inAlbuquerque, New Mexico).Roberts decided to take agamble with what little fundsremained available to him, andhe started to design a computercalled the Altair 8800 (the name“Altair” originated in one of theearly episodes of Star Trek).

Roberts based his systemon the newly-released 8080microprocessor, and theresulting do-it-yourself kit was

advertised in PopularElectronics magazine in January1975 for the then unheard-ofprice of $439 US. In fact, whenthe first unit shipped in April ofthat year, the price had fallen toan amazingly low $375 US.

Even though it onlycontained a miserly 256 bytes ofRAM and the only way toprogram it was by means of aswitch panel, the Altair 8800proved to be a tremendoussuccess. (These kits weresupplied with a steel cabinetsufficient to withstand mostnatural disasters, which is why aremarkable number of themcontinue to lurk in their owner’sgarages to this day.)

BASIC GATESAlso in April 1975, Bill Gates

and Paul Allen foundedMicrosoft (which was to achievea certain notoriety over thecoming years), and in July ofthat year, MITS announced theavailability of BASIC 2.0 on theAltair 8800. This BASICinterpreter, which was written byGates and Allen, was the firstreasonably high-level computerlanguage program to be madeavailable on a home computer –

Altair 8800b microcomputer, 1975. Courtesy ScienceMuseum/Science and Society Picture Library.

A Commodore PET 32K computer, 1979. Cas-sette recorders provided external data stor-

age. Courtesy John Becker

An early IBM PC. Its architecture became thefoundation that all PC-compatibles must emu-

late. Courtesy of IBM.

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MITS sold 2,000 systems thatyear, which certainly made EdRoberts a happy camper, whileMicrosoft had taken its firsttentative step on the path towardworld domination.

In June 1975, MOSTechnology introduced their6502 microprocessor for only$25 US (an Intel 8080 woulddeplete your bank account byabout $150 US at that time). Ashort time later, MOSTechnology announced their6502-based KIM-1microcomputer, which boasted2K bytes of ROM (for themonitor program), 1K byte ofRAM, an octal keypad, aflashing LED display, and acassette recorder for storingprograms. This unit, which wasonly available in fully-assembledform, was initially priced at $245US, but this soon fell to anastoundingly low $170 US.

The introduction of newmicrocomputers proceededapace. Sometime after the KIM-1 became available, the SphereCorporation introduced itsSphere 1 kit, which comprised a6800 microprocessor, 4K bytesof RAM, a QWERTY keyboard,and a video interface (but nomonitor) for $650 US.

JOBS AND WOZNIAKIn March 1976, two guys

called Steve Wozniak and SteveJobs (who had been fired withenthusiasm by the Altair 8800)finished work on a home-grown6502-based computer whichthey called the Apple 1 (a fewweeks later they formed theApple Computer Company onApril Fools day).

Although it was nottremendously sophisticated, theApple 1 attracted sufficientinterest for them to create theApple II, which many believe tobe the first personal computer

Special Feature

that was both affordable andusable. The Apple II, whichbecame available in April 1977for $1,300 US, comprised 16Kbytes of ROM, 4K bytes of RAM,a keyboard and a color display.

Apple was one of the greatearly success stories – in 1977they had an income of $700,000US (which was quite a lot ofmoney in those days), and justone year later this had soaredtenfold to $7 million US! (whichwas a great deal of money inthose days).

Also in April 1977,Commodore Business Machinespresented their 6502-basedCommodore PET, whichcontained 14K bytes of ROM,4K bytes of RAM, a keyboard, adisplay and a cassette tapedrive for only $600. Similarly, inAugust of that year, Tandy/Radio Shack announced theirZ80-based TRS-80, comprising4K bytes of ROM, 4K bytes ofRAM, a keyboard and a cassettetape drive for $600.

WOT! NOSOFTWARE?

One aspect of computingthat may seem strange today isthat there were practically noprograms available for theseearly machines (apart from theprograms written by the users

themselves). In fact, it wasn’tuntil late in 1978 thatcommercial software began toappear.

Possibly the most significanttool of that time was the VisiCalcspreadsheet program, whichwas written for the Apple II by astudent at the Harvard BusinessSchool and which appeared in1979. It is difficult to overstatethe impact of this program, but itis estimated that over a quarterof the Apple machines sold in1979 were purchased bybusinesses solely for thepurpose of running VisiCalc. Inaddition to making Apple veryhappy, the success of VisiCalcspurred the development ofother applications such as wordprocessors.

When home computers firstbegan to appear, existingmanufacturers of largecomputers tended to regardthem with disdain (“It’s just a fad. .. it will never catch on”).However, it wasn’t too longbefore the sound of moneychanging hands began toawaken their interest. In 1981,IBM launched their first PC for$1,365 US, which, if nothingelse, sent a very powerful signalto the world that personalcomputers were here to stay.

The advent of the general-purpose microprocessor

Two of the bal-listic track mon-itors of SAGE,

the massive USmilitary com-

puter thathelped defendthe WesternWorld duringthe Cold War.Courtesy of

IBM.

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heralded a new era in computing– microcomputer systems smallenough to fit on a desk could beendowed with more processingpower than monsters weighingtens of tons only a decadebefore. The effects of thesedevelopments are still unfolding,but it is not excessive to say that

Special Featuredigital computing and thepersonal computer havechanged the world moresignificantly than almost anyother human invention, andmany observers believe thatwe’ve only just begun ourjourney into the unknown!

EVER SO HUMBLEWe crave your indulgence

and ask you to accept ourhumblest apologies for all of thethings we had to leave out.Surely computer languages likeFORTRAN, COBOL, BASIC, C,LISP, FORTH and … (the listgoes on) deserve a mention?How could we neglectmicrocomputers such as thePDP and VAX from DigitalEquipment Corporation (DEC)that had such an impact on theindustry? Are operating systemslike VMS, UNIX, and Windowsto be ignored? What aboutbehemoths like SAGE (whichconsumed a million watts ofpower) and CRAYSupercomputers?

The problem is that onecould go on forever, so wechose to restrict ourselves onlyto those topics that we felt wereparticularly germane to thisseries. As usual you may ofcourse disagree (or you maysimply crave more once thisseries is finished), in which caseplease feel free to vent yourfeelings by inundating the Editorwith your letters and emails.

ACKNOWLEDGEMENTPortions of this article were

abstracted from our book,Bebop BYTES Back (AnUnconventional Guide toComputers), with the kindpermission of its publisher,Doone Publications. (BebopBYTES Back is available fromthe EPE Online Store atwww.epemag.com)

NEXT MONTHIn the fifth and final

installment of this series weshall gird up our loins andpontificate on the future. Wheredo you think the technologyroller-coaster will take us in thenext 10, 100 or 1000 years?Start pondering now and see ifyou agree with us in nextmonth’s exciting issue – sametime ... same place ... samechannel!

QUOTABLE QUOTES“Computers in the future

may weigh no more than 1·5tons.” Popular Mechanics, fore-casting the relentless march ofscience, 1949.

“I think there is a world mar-ket for about five computers.”Thomas Watson, Chairman ofIBM, 1943.

“I have traveled the lengthand breadth of this country andtalked with the best people, andI can assure you that data pro-cessing is a fad that won’t lastout the year.” The editor incharge of business books forPrentice Hall, 1957.

“There is no reason for anyindividual to have a computer intheir home.” Ken Olson, Presi-dent of Digital Equipment Cor-poration (DEC), 1977

“So we went to Atari andsaid, ‘Hey, we’ve got this amaz-ing thing, even built with some ofyour parts, and what do youthink about funding us? Or we’llgive it to you. We just want to doit. Pay our salary, we’ll comework for you.’ And they said,‘No.’ So then we went toHewlett-Packard, and they said,‘Hey, we don’t need you. Youhaven’t got through college yet.’’Apple Computer Inc. founderSteve Jobs on attempts to getAtari and HP interested in hisand Steve Wozniak’s personalcomputer.

“640K of memory ought tobe enough for anybody.” BillGates, CEO of Microsoft, 1981.

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It is a commonly known factthat microprocessor clockspeeds are increasing all thetime. Only a few years ago,clock speeds of 1GHz werethought to be many years away.Now a number of manufacturershave offerings with speedsaround 1GHz that will shortly hitthe marketplace. IBM have a 64-bit Power PC chip. Compaq,have their 1GHz Alpha, and Intela version of a Pentium III. Thesedevices have been able toachieve their speed as a resultof a number of developmentsthat have been undertaken inmany research institutes anddevelopment areas.

All of the devices havegeometries that are less than 02microns, and this means that theoperating voltages are low. Forexample the Alpha operates ona voltage of 165 volts. Not onlyis this low voltage requiredbecause of the low breakdownvoltages associated with theminute geometries, but it alsoreduces the power consumption.

HEAT PROBLEMSPower consumption is an

increasing problem asdemonstrated by the fact thateven modest Pentium chipsrequire cooling. However when itis realized that IBM’s 64-bitPowerPC uses 19 milliontransistors, it is hardly surprisingthat very significant amounts ofheat are dissipated. Some of thenew chips now underdevelopment dissipate levels ofheat well in excess of 50 watts,

and the trend of increasinglevels of power dissipation islikely to continue. With theincreasing levels of powerdissipation, thermal control ofchips is an integral part of thedesign and it is every bit asimportant and challenging as theelectrical performance.

It is interesting to note thatwhen the first bipolar integratedcircuits were introduced, limits ofaround 20 transistors werethought to be the limit ofintegration as a result of thermalconsiderations. The introductionof CMOS techniques enabled aquantum leap to be made in thelevels of integration and thetrend towards ever-larger ICshas increased since then.

More recently, the reductionof supply voltage has been ofassistance as the thermalboundaries have beenapproached, because powerlevels are proportional to thesquare of the voltage. Even soother effects prevent the picturefrom being quite so rosy. Thedesign of the transistors in thechip has to be altered to enablethem to operate at low voltages.One of the results of this is thatthey become far more leaky andthis effect means that theyconsume power even when theyare switched off.

TEMPERATURERISES

To ensure the highestspeed of operation, devicesshould be operated at a lowtemperature. The unwanted

additional power consumptionfrom the leaky transistors raisesthe temperature and thisreduces the electron mobilitybecause of the increasednumber of collisions that occuras the electrons move aroundthe crystal lattice.

Accordingly, it makes themethods and techniques usedfor heat extraction from the IC apoint of major importance ifspeeds are to increase at thecurrent rate. A company namedKryotech is already marketing achip that is cooled, increasing itsperformance by a half again. Tothe same end, IBM areoptimizing the performance oftheir basic silicon designs forlow temperature operation with aview to this being one of theways forward for the future.

However, even thoughspeed generally increases withcooling, it also increases thethreshold voltage for theindividual devices, and this inturn increases the level ofleakage and partially offsets anygains that are made. This is oneof the factors that makesoptimizing the design for low-temperature operation soimportant. By choosing theoptimum level of thresholdvoltage, the maximum use canbe made of any cooling that isused.

EXTRACTING HEATThere are a number of ways

in which heat can be extractedfrom the chips. One method isto use a system that iseffectively a small-scale version

WHILST LOWER OPERATING VOLTAGES ENABLE MICROPROCESSORS TO RUNFASTER, THE PROBLEM OF HEAT DISSIPATION BECOMES MORE SIGNIFICANT.IAN POOLE REPORTS.

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of a domestic refrigerator.These systems are verysuccessful, being alreadyemployed in a number of highend products and they are ableto cool chips down to atemperature of around –50°C.

Whilst this can givesignificant advantages inperformance, lowertemperatures can provide evengreater improvements. Toachieve this there are a numberof methods that can be adopted.The most popular idea is that ofthermo-electric heat pumps.These do not involve the samelevel of mechanical hardwareand are accordingly lessexpensive. They can also beinterfaced to the basic chip moreeasily, and can actually be madeas part of the same assembly.

However, the basic Peltierdevices, although attractive atfirst sight, leak too much heatback into the chip, and as aresult they are not as efficient asthey are required to be for thisfunction. Fortunately, new workundertaken at theMassachusetts Institute ofTechnology has resulted in theproduction of new materials andstructures that give far moreeffective and efficient solutions.

The requirement is to beable to remove a considerableamount of heat from a smallarea. One of the new solutionsusing a thin film semiconductorheat pump can extract as muchas 100 watts per squarecentimeter. With further work itis expected that these devices

could be built into the basic chippackage, providing a veryconvenient, efficient and reliablemethod of extracting heat fromthe devices.

PACKAGESWhilst heat is a major

problem that is being overcome,new package technology is alsopart of the solution. Long goneare the days when dual-in-linepackages were able to meetmost requirements. Even thequad flat packs are not suitable,and in addition to this,equipment manufacturers dislikethem because they are easilydamaged. Flip chip packageswhere the silicon is directlybonded to the package are ableto give performanceimprovements. This gives aspeed increase as a result of itslower resistance and RC delaysas well as giving a physicallyshorter connection.

Further improvements havebeen made by adopting asystem that enables the criticalleads to be kept as short aspossible. Although thistechnique requires the additionof an extra layer of metalisationand a complete re-layout of thechip, it provides an increase thatalthough small still helps toincrease the overall speed ofoperation.

A further increase inperformance is achieved byusing a dielectric with a lowvalue. In turn this reduces thelevels of capacitance and cross

talk, which were large enough toslow down the speed ofoperation. The material chosenfor this is silicon oxyfluoride(SiOF).

SUMMARYAlthough different

manufacturers use differenttechniques to give the newhigher clock speeds, the overallpattern is clear, and it is likelythat in a few years time they willall be used as standard. Many ofthem give minor improvementson their own, but when usedwith the other techniques theyenable a significantimprovement in performance tobe achieved in the chip as awhole. This demonstrates thefact that is commonly true intechnology that a variety ofimprovements are required togive the overall improvement inperformance.

New technology Updates

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This month we round off ourexploration of the opamp bylooking at the level shiftercircuits which are used to getthe DC bias levels correct indifferent stages of an opamp.We outline typical short-circuitprotection techniques and alsooutput stages, which are poweramplifiers that share some ofthe features of basic audiopower amplifier output stages.Some of the principles we’lloutline also apply to designs,which use discrete transistorsinstead of integrated circuits.

SHIFTY CIRCUITSNo coupling capacitors can

be used between stages withinopamps – they have to workwith DC and very low frequencyinputs. Biasing is easy inmultistage capacitively-coupledamplifiers, because the biasingof each stage is isolated by thecoupling capacitor.

In an opamp, life is not sosimple. We might, for example,have one stage with an outputwhose signal varies around abias point of half the positivesupply, which has to beconnected to a stage that needsa signal which varies around 0V(ground) instead. Thereforewhat we would need to do is“shift” the DC bias level of asignal.

Ideally, a circuit for thispurpose should provide a stableshift in DC level without

introducing noise, it should notattenuate the signal, andshould allow the designer toselect any level shift required(within reason). We couldachieve a shift using a two-resistor potential divider, butthis attenuates the signal. Wecould use a Zener diode toprovide a voltage drop, butthese are noisy. We could usediode voltage drops, but theseonly come in steps of about06V per diode used.

The circuit in Fig.1a actsas a level shifter, changing theDC level from Vin to Voutwithout significant attenuationof the signal. The currentsource (see Circuit Surgery,May and June ’99) provides acurrent I that flows through Rto give a fixed voltage drop ofIR. The voltage drop is fixed (itdoes not depend on the signal)because the current sourceproduces the same currenteven if the voltage across itvaries due to the signal.

The total fixed voltagedrop from Vin to Vout alsoincludes the VBE voltage of thetransistor, which will also notvary a great deal as the signalvaries. Thus the circuit shiftsthe DC level of the signal downby (IR + VBE) volts.

OUT OF THEOPAMP

An opamp output stagemust be capable of supplying

sufficient current to the externalload (i.e. out of the chip onwhich the opamp is fabricated).In order to do this it must havelow output resistance and

Our circuit surgeons provide more advice to help with reader’s problems andconclude their mini-series investigating the inner workings of operationalamplifiers, looking at output stages and short-circuit

by ALAN WINSTANLEY

Fig.1a. A level shifter circuit.

Fig.1b. Emitter followercircuit.

Fig.1c. Basic push-pullamplifier.

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provide power gain. It does nothave to provide voltage gain asthis is done by earlier stages.

The output stage is a poweramplifier – a term that conjuresimages of circuits which delivermany watts of power. This doesnot have to be the case – it isthe fact that power gain isprovided rather than the amountof power available that matters.However, there are, of course,high power opamps and theopamp output circuits sharefeatures with some types ofaudio power amplifier.

The well-known emitterfollower circuit (Fig.1b) has avoltage gain of just less thanunity, high input resistance andlow output resistance. So it candeliver a relatively high-currentversion of a “weak” voltagesignal. The circuit is called anemitter follower because thesignal voltage at the emitter,which is where the load isconnected, “follows” (is thesame as) the voltage at thebase. The absolute voltage atthe emitter is one VBE drop(about 06 to 07V) lower thanthe base voltage, but this is justa shift in DC level.

Our emitter follower circuitof Fig.1b has the right kind ofproperties for an opamp outputstage, but is not suitable as itstands because we require theload to be connected to groundand the output signal to be both

amplified, which leads todistortion of the output. Fig.2shows a sinewave input to abasic push-pull amplifier (dottedline) and the resulting distortedoutput (solid line). Although thiscircuit is not suitable foropamps, it may be of use inother applications where thedistortion does not matter, forexample in a basic motor controlcircuit.

CROSSOVERDISTORTION

To overcome crossoverdistortion, the output transistorsare biased so that with no signalpresent they are both just on thepoint of conduction. Then whenVIN = 0 both transistors are justconducting, when VIN > 0 TR1conducts and TR2 is off, andwhen Vin < 0 TR2 conducts andTR1 is off.

This can be achieved usingtwo diodes, or two transistorsconnected as diodes, to providethe 2 x VBE difference in biasvoltage required between the

positive and negative. In thiscircuit the transistor would turnoff with negative input voltages,so we would only amplify halfthe signal!

To overcome this, twoemitter-followers are used inwhat is known as a push-pullamplifier (see Fig.1c). This typeof circuit is also referred to as aclass-B amplifier because eachoutput transistor conducts forone-half of the waveform cycle.Transistors in class-A amplifiersconduct for the whole cycle, andin class-C for less than half.

The basic push-pull outputstage suffers from a problemcalled crossover-distortion. Onlyone transistor can be on at anytime, that is: if Vin > VBE thenTR1 is conducting, and if Vin < –VBE then TR2 is conductinginstead. However, this meansthat for small inputs, neithertransistor is on: if –VBE < Vin <VBE then TR1 and TR2 are bothoff.

So signals, or parts ofsignals, in this range are not

Circuit Surgery

Fig.2. Sinewave withcrossover distortion.

Fig.3. Output transistors biased to prevent crossover dis-tortion. The diodes may be implemented using transistor

base-emitter junctions.

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constant 2 x VBE difference between the twobase voltages. The actual base voltages willvary with the signal, but the differencebetween them is fixed by this biasingarrangement.

The diodes are ideally at the sametemperature as the output transistors so thatchanges in their voltage drop withtemperature tracks those of the outputtransistor. This applies on opamp ICs andfor discrete component power amplifiersusing this type of circuit.

SHORT CIRCUITSThe push-pull amplifier is likely to be

damaged if its output is short-circuited toground, due to excessive collector current inthe conducting transistor. A short-circuitprotection arrangement may be added toovercome this problem. The protectioncircuit monitors the current flowing in theoutput and turns off the output transistor ifthe current exceeds some pre-defined limit.The current detection is usually achieved byusing a small resistor in the output signalpath, and a transistor to switch off the output(see Fig.4); the output current causes avoltage drop across the resistor.

A protection transistor switches on whenthe resistor voltage reaches about 06V to07V. The protection transistor is connectedso that when it is on, it effectively short-circuits the input to the power transistors, sothey have no signal to amplify. Theprotection resistor values, Rp1 and Rp2 maybe chosen using Rp1 = Rp2 = VbeTRP1 / Imaxwhere VbeTRP1 is the turn-on voltage of theprotection transistor (typically 06V to 07V)and Imax is the maximum output current, i.e.the current at which the protection kicks in.This kind of protection circuit is whatenables opamps to have the “infinite outputshort circuit duration” quoted on many datasheets.

AUDIO POWER AMPThe circuit shown in Fig.5 is a discrete

component version of the circuit in Fig.4,which could form the basis of an audiopower amplifier output stage. Resistors areused instead of the current sourcesubiquitous in IC circuits.

Circuit Surgery

Fig.4. Output stage with protection circuit (protectioncomponents are shown using bold lines).

Fig.5. Audio power amplifier using similar configura-tion to opamp output stage.

transistors’ bases (see Fig.3). The diodes are biased withthe current required to give the correct VBE value by meansof a current source.

As the input signal varies the diodes maintain a

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Circuit SurgeryBiasing is achieved using

what is known as a VBE multiplierand is manually adjustable usingpreset VR1 to give the requiredquiescent current for the circuit(the degree to which the outputtransistors are “just on” with nosignal). The VBE multiplier circuitconsists of TRa, preset VR1 andresistor R2. The voltage Vbias iseffectively fixed by virtue of thefact that TRa’s VBE voltage doesnot vary much, resulting in afixed voltage across R2 andhence a fixed current through it.

If the preset VR1 andresistor R2 are chosen so thattheir current is much larger thanTRa’s base current then we canassume all of the current in R2also flows in VR1. Thus the totaldropped across VR1 and R2(i.e. Vbias) is equal to VBEmultiplied by the ratio of the totalresistance of VR1 and R2 to thevalue of R2, i.e.

Vbias = VBE (VR1 + R2) / R2

We hope that our discussionon the opamp over the past fewmonths has given you someinsight into what is inside thesechips that get used in so manyconstructor’s projects. Ofcourse, there is a lot more to thecircuitry of modern opamps thanwe have space to discuss in thisseries – as a browse throughthe schematics inmanufacturers’ data sheets willreveal.

Hopefully, however, youwould also be able to recognizeat least some of the basic sub-circuits (e.g. differentialamplifier) in these schematics,even if there are one or twoextra transistors present. Wealso hope that some of youmight find other uses for thecircuits we have shown in yourown designs – let us know if youdo. Ian Bell.

BATTERY FLATTERYBriefly on the subject of

troubleshooting lead acidchargers, Mr. Alister Bottomleywrote: “Am I correct in assumingthat in order to charge a 12V leadacid battery it must receive avoltage greater than 12V across it– the greater the voltage thegreater the charging current?

My battery charger hassuddenly reduced its output to117V as measured on its ‘high’tapping. I can’t find any losses orproblems in the circuit.”

A lead acid battery requiressomething like 22V per cell orhigher constant voltage to charge.A higher voltage could be usedbut the battery life will beshortened, and it is true that thegreater the applied voltage, thegreater the charge current will be.Current gradually reduces to atrickle as the battery charges up.

The reason you aremeasuring a strange DC voltageis because it isn’t a smooth levelDC voltage you are actuallytesting. Ordinary car batterychargers have a rectified DCoutput, which is unsmoothed.However, your multimeter willwant to read a pure DC voltage,or it will read an RMS voltage onits AC range instead. Anoscilloscope would highlight theproblem.

An electronics mains powersupply uses a smoothing or“reservoir” capacitor to iron outthe ripple, to produce a higherpeak value and much smootherDC voltage. The capacitor thencharges to the peak value of therectified DC sinewave. In fact, it’sthe car battery itself that acts as agiant smoothing capacitor acrossthe supply. Hooking this acrossthe battery charger means thatthe voltage seen across thebattery will then increase.

Also, your test equipmentmay actually cause you tomisinterpret the result, andsometimes the very use of testequipment can affect theoperation of the circuit as well.Constructors gradually learn tocompensate for this withexperience.

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Probably the most importantelectronic component in theanalog designer’s armory is theoperational amplifier. Betterknown, perhaps, by itsabbreviated name of opamp (orop-amp, or even op.amp) thisfamily of devices seemingly hasmore applications than there aredesigners who use it!

In this month’s Tutorial weillustrate some of the opamp’smajor features as an amplifier. Innext month’s Tutorial we follow onby going into a bit more simpleexperimental detail, discussingwhat else opamps can do, andgetting you to try it.

FIRSTDEMONSTRATION

From your stock ofcomponents (as described in Part1), select one of the 8-pin dual-in-line (DIL) devices labeled LM358,call it IC4. Now assemble yourbreadboard according to Fig.7.1.Any previous components in thearea illustrated should beremoved (all your counting andlogic gate experiments from lastmonth have already served their

PART PART 7 – Op.ampsby John Becker

purpose, we hope!). Leave theoscillator components intact fornow.

The equivalent circuitdiagram and the componentvalues are shown in Fig.7.2.

Connect the oscillatorwaveform from the junction of

Over the previous six parts of Teach-In2000, which we know you have been greatlyenjoying, we have covered passivecomponents and several digital logic circuits.Via the interactive computer programs and thesimple interface you assembled, you have alsobeen able to observe the various waveformsgenerated by the experimental breadboardcircuits, showing how a few electroniccomponents can be connected to achieveinteresting results.

We now move on from the “interesting” tothe “practical”, in terms of describing activecomponents which can be used to amplify andotherwise modify the waveforms generated. itis opamps we now examine, those simplerobust components that feature so frequentlyin audio and other analog circuits. This monthwe demonstrate their basic nature, next monthwe get you experimenting with some usefulapplications.

capacitor C1 and IC1a pin 1(see Fig.4.3 of Part 4) to thepoint marked “DC INPUT”. Usea crocodile-clipped link.

Ignore the points labeled“Buffer Input” and “BufferOutput”, their purpose will bediscussed later on in theTutorial.

The oscillator should havediodes D2 and D3 included; its

Breadboard showing thecomponents for the first

opamp experiments, part ofIC1 is just visible at the left.

161615

15 1717

1818

1919

2020

2121

2222

2323

2424

2525

2626

2727

2828

2929

3030

3131

A.C.INPUT

D.C.INPUT

(SEE TEXT)

(SEE TEXT) BUFFERINPUT

BUFFEROUTPUT

OUTPUT

IC4VR2 VR3

VR4

C2+

TO R.H.SIDE OFBOARD

TO R.H.SIDE OFBOARD

Fig.7.1 Breadboard layout forthe first opamp experiments.

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capacitor C1 value should be100uF. Set the frequency controlVR1’s wiper to midway, so thatthe generated waveform will beroughly triangular.

Set each of the Fig.7.1presets (VR2 to VR4) so thattheir wipers (moving contacts)are also in a midway position,providing approximately equalresistance to either side of thewiper.

Referring to Fig.5.6 of Part5, connect IC4 pin 1 to the inputto the analog-to-digital converter(IC2 pin 2), and then connectIC2’s output to IN1 of thecomputer interface section. Runthe Analog Input WaveformDisplay program.

Connect up your battery tothe breadboard (as you’ve donea good few times before – is thebattery power still OK?) andobserve the computer screendisplaying the triangularwaveform being generated byIC1a and associatedcomponents.

VARIABLEAMPLITUDE

We are now going to askyou to make variousadjustments on the opamp’sthree presets and observe thescreen responses. We shalldiscuss what you observe in duecourse. First, carefully adjust thewiper position of VR4 until thewaveform is roughly central onthe screen.

Next, slowly adjust the wiper

of VR3 in a clockwise direction(to the “right”) while observingthe screen. This actionincreases the resistancebetween the wiper and the endconnected to IC4 pin 1, theopamp’s output.

It will no doubt interest youto see that the vertical size ofthe waveform increases, in otherwords, its amplitude increasesthe further you adjust VR3. Thelimit will be reached when youcannot turn the wiper anyfurther. The amplitude will nowbe about twice that you startedwith.

Note, though, how thewaveform’s relative position onthe screen probably changes as

you rotate VR3. Carefully adjustthe wiper of VR4 to set thewaveform back to a mid-screenposition if it does.

Now rotate VR3’s wiperanticlockwise. The waveformamplitude will be seen todecrease, and once the wipergoes beyond the midway position,the waveform amplitude will beginto fall below that at which itstarted. The waveform is now saidto have been attenuated.

Towards the far end of theanticlockwise rotation thewaveform should be seen just asa straight-ish horizontal line.

Set VR3’s wiper to its fullyclockwise position and leave itthere.

PEAK FLATTENINGTurn your attention now to

preset VR2. First rotate itclockwise, to increase theeffective resistance between itswiper and IC4 pin 2, one of theopamp’s two inputs. Note how an

TEACH-IN 2000

+

C222m

A.C. INPUT

0V

+6V

D.C. INPUT

VR2100k VR4

100k3

2

VR3100k

IC4aLM358

4

8

1OUTPUT

+

Fig.7.2.Circuit dia-gram asso-ciated with

Fig 7.1

PANEL 7.1 – OPAMP MANUFACTURING CON-STRUCTION

The opamp type (LM358) used in this Teach-In is manufacturedusing a structure called bipolar. In essence, this uses many transis-tors internally interconnected on the opamp chip and which requirecurrent to flow through them. In most cases the current is not great,but can still place a load on the circuit, which is being fed into theopamp inputs.

Another type of opamp manufacturing process uses the field ef-fect transistor (FET) technique. FET devices operate on a differentprinciple to those used in bipolar devices and respond to the voltage(field) on their inputs rather than through their inputs. These devices,therefore, do not draw current from the circuit feeding into them andso place no load on them.

All the circuits discussed in this Teach-In part could probably haveused FET opamps instead of the bipolar LM358. Such devices in-clude TL062, TL072 and TL082.

Component suppliers’ catalogs and manufacturers’ data sheetsshould be consulted for information about the different opamp typesavailable. Internet access to various manufacturers’ web sites anddata sheets can be gained via the EPE web site atwww.epemag.wimborne.co.uk

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increase in resistance herecauses a decrease in thewaveform amplitude.

Next adjust VR2’s wiperanticlockwise. Once it goesbeyond the original midwayposition, note how rapidly thewaveform amplitude increases.You may also notice a change inthe waveform’s frequency (fewercycles per screen-full!). Notealso that its top and bottompeaks become flattened themore that VR2’s resistance isdecreased. The peaks areunlikely to be evenly flattened,however. Carefully adjust VR4until they become more equal.

As you further rotate VR2’swiper, you will eventually see awaveform somewhat resemblinga square wave instead of atriangle. Adjusting VR4 willchange the waveform’s mark-space ratio (discussed in Part4). Before VR2 reaches itsminimum resistance, and withVR4 set too much to either sideof midway, the oscillator mightstop functioning.

WAVEFORMINVERSION

That’s the first set ofobservations – on to the next.Return all three opamp wipers(VR2 to VR4) to a midwayposition. If the oscillator hadindeed ceased functioning, thisaction should restart it. If itdoesn’t, briefly disconnect thepower and then reconnect it.

Adjust IC1’s preset VR1 sothat a rising-ramp waveform isseen (ramps were discussed inPart 5). Leave all presets asthey are and connect the ADC

input to the input of IC1a (pin 1).Look back at the screen.Whereas you had a rising rampa few moments ago, you shouldnow see a falling ramp –curiouser and curiouser!

Time, then, to discuss yourfindings in relation to anopamp’s basic nature.

BASIC OPAMPNATURE

In essence, an opamp is atwo-input single-output device,which has the capability ofgreatly amplifying a voltagedifference between its twoinputs. For this reason, it can becalled a differential amplifier.Within limits, the amplification isaccording to a linearrelationship. (You will recall thatwe discussed linearrelationships when wediscussed potentiometers inPart 3.) For each unit of changeat the input, an equivalent butlinearly amplified increase willresult at the output.

For some purposes (but notall) the amount of amplification(gain) available is far too greatto be of use – it can be severalhundred thousand times forsome opamps. However, thereis a simple technique that canbe used to restrict theamplification to a moremanageable level.

In Fig.7.3 is shown the basicsymbol for an opamp (no longercluttered as it is Fig.7.2). Thesymbol shows that one input ismarked as inverting (–), and theother as non-inverting (+). Thedifferent input modes have greatsignificance to the way in whichthe opamp can be used andcontrolled.

Note that the “–“ and “+”symbols have nothing to do withpower supply connections, theymerely symbolize the inverting

and non-inverting nature of therespective input.

INPUT TO OUTPUTFirst, suppose that both

inputs have the same voltagelevel applied to them. Becausethere is no difference betweenthe voltages, the output will beheld at the same voltage. If thevoltage on the non-invertinginput rises fractionally abovethat on the inverting input, theoutput voltage will try to rise bythe same amount amplified(multiplied) by the gain factor(of, say, 100,000).

Conversely, should thevoltage on the non-invertedinput fall below that on theinverted input, so the output willtry to fall by an equivalentlyamplified amount.

Obviously, a similar effectwill occur, but in the oppositedirection, if it is the invertinginput voltage that changes whilethe non-inverting input voltageremains constant.

NEGATIVEFEEDBACK

Consider, though, whathappens if part of the outputvoltage is fed back to theinverting input, see Fig.7.4.Feedback into this input (viaresistor R2 in this case) isknown as negative feedback.The output will try to swing in thedirection prompted by therelative voltage difference

TEACH-IN 2000

+

INVERTING INPUT ( )

NON-INVERTING INPUT ( )OUTPUT

+

Fig.7.3. Opamp symbol andconnection names.

R2

INPUT

BIAS

0V

R1

+V

OUTPUT+

Fig.7.4. Feeding back part ofthe output voltage to the in-

verting input.

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TEACH-IN 2000

connected so that it feeds backpart of the output voltage to theinverting input. Jointly, the effectof both resistances, VR2 andVR3, determines the amount ofnegative feedback that occurs.Respectively, they are theequivalent of R1 and R2 inFig.7.4.

If both resistances areequal, then the negativefeedback amount is the same asthe basic input amount, butinverted. The result is that thevoltage actually “seen” at theinverting input is the inputvoltage minus the feedbackvoltage, i.e. nil! At least, that

across the two inputs, but theeffect will be diminishedaccording to the amount of thatchange which is fed back tocounteract it.

The output might be tryingto change by 100,000 times, butthe feedback might be set for99,990 times. The net differenceis thus only 10. Therefore, theeffective amplification is only 10times that of the differenceoriginally fed into the two inputs.The gain is thus said to have avalue of 10.

More strictly, when thesignal is being applied to theinverting input, the gain shouldbe said to be –10 (minus 10)because of the inversion. Forthe most part here, though, weshall just refer to the gain as apositive value.

Low gain values are of much better use if you want to justslightly raise the amplitude of awaveform, as you have justdone with the triangle waveform.In fact, when you first adjustedpreset VR3 to its maximumresistance, the gain you gave tothe waveform was about 2, i.e.you doubled the waveformamplitude. So let’s explain themechanism that is used in thecircuit of Fig.7.2 to control thesignal gain.

CONTROLLING GAINFirst, assume that the

waveform being sent to theinverting input of IC4 via presetVR2 is alternating about amidway voltage level. Let’s saythe midway level is at 3V (halfthe voltage applied to the fullcircuit, as supplied by your 6Vbattery).

Your initial adjustment ofpreset VR4 applied just aboutthe same midway voltage (whichwe refer to as the bias) to thenon-inverting input (you will

recall that you were asked tooriginally set its wiper to amidway position). This actionroughly balanced the two inputsat the midway voltage. Thechanges in voltage caused bythe triangle waveform’s swingthus became evenly balancedas seen across the two opampinputs.

The voltage being fed intothe inverting input, however,passes through the resistanceoffered by preset VR2. On itsown that resistance has noappreciable affect on the voltageactually reaching the input.Preset VR3, though, is

PANEL 7.2 – OPAMP PACKAGESOpamps are manufactured with encapsulations (packages) con-

taining one, two or four individual opamps – these packages beingknown respectively as single, dual and quad.

The vast majority of opamps have the same pinout configurationper packaging type, as shown here. Afew of the opamp types available arelisted below their packages. There arehundreds of other type numbers manu-factured. Note that some specialistopamps do not conform to thesepinouts.

With single opamp types, there arevarious functions that can be per-formed via pins 1, 5 and 8. These aretoo numerous to discuss here and formost simple amplification-type circuits

they can be ignored. They can

control such matters as, forinstance, the fine adjustment of theDC output level (offset) to match the basic zero-differential input volt-age. Manufacturers’ data sheets should be consulted for more infor-mation.

2 7

8

4 5

63

1

++INPUT ( )

INPUT ( )

OUTPUT

+ VE

VE

SINGLE

EXAMPLES

741 (VARIOUS PREFIXES) BIPOLAR

TL061 F.E.T.TL071 F.E.T.

TL081 F.E.T.

NE531 BIPOLAR

2 7

8

4 5

63

1

INPUT A

+( )INPUT A

+( )INPUT B

( )

INPUT B ( )

OUTPUT B

OUTPUT A + VE

VE

+

+

DUAL

EXAMPLESLM358 BIPOLAR

TL062 F.E.T.

TL072 F.E.T.

TL082 F.E.T.

NE5532 BIPOLAR

1458 (VARIOUS PREFIXES) BIPOLAR

VE

2

7 8

4

5

6

3

10

9

13

11

12

141

+

+

+

+

INPUT A INPUT D

+( )INPUT A +( )INPUT D

+( )INPUT B +( )INPUT C

( ) ( )

INPUT B ( ) INPUT C ( )

OUTPUT B OUTPUT C

OUTPUT A OUTPUT D

+ VE

QUAD

EXAMPLES324 (VARIOUS PREFIXES) BIPOLARTL064 F.E.T.TL074 F.E.T.TL084 F.E.T.

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TEACH-IN 2000would be the case if you ignoredthe non-inverting input.

But you can’t and don’t. Theinternal circuitry of the opampeffectively adds this input’svoltage (3V in this case) to thevoltage on the other input. Thusboth inputs end up with the samevoltage on them! Confirm thispoint using your meter to monitorthe voltage on opamp pins 2 and3, and that from IC1a pin 1.

What’s the good of that? youmight ask. Well, there’s a lot! Inorder to achieve that balancebetween inputs, the output hashad to change its voltage level.And that’s what we are interestedin – the change in output level inresponse to a change in inputlevel.

BALANCING ACTIn the above equal resistance

example (VR2 = VR3), the outputchanges by the same amount asthe input, but in the oppositedirection. An input change of 1Vupwards, for instance, causes anoutput voltage change of 1Vdownwards.

If, though, VR3 resistance istwice that of VR2 resistance, twicethe amount of feedback isrequired in order to achieve thebalance at the opamp inputs.Consequently, a 1V input changeresults in a 2V output change inthe opposite direction.

Similarly, if VR3 resistance ishalf that of VR2 resistance, thenthe output change required toachieve balance is only half thatfed into VR1. Thus, a 1V inputrise will cause a 05V output fall.

Indeed you have alreadyproved the truth of thesestatements when observing theaffect of changing the values ofVR2 and VR3. You have provedboth signal gain (amplification)and signal reduction (attenuation).

INVERTING GAINFORMULA

There is a simple formulawhich defines the gain in relationto the value of the inverting inputresistance (call it R1, as inFig.7.4) and the negativefeedback resistance (call it R2):

Gain = R2 / R1

Thus if R2 is 100k and R1is 10k the gain is 100k/10k =10. The output signal will be tentimes greater than that comingin through R1 (within certainlimits, as discussed in amoment).

Similarly, if R2 is 10k andR1 is 100k then the gain willbe 10k/100k = 01. The outputsignal will then be 01 (onetenth) of the input signal.

VIRTUAL EARTHThere is a term used to

describe the effect seen at theinverting input when there is abalance of voltage directly at thetwo inputs when feedback isemployed – “virtual earth”. Thisis not a “true” earth in the sensethat applies when referring tothe common or 0V line of acircuit, but nonetheless it is oneinto which voltages can be fedvia resistances from manysources without causing achange in the virtual earthvoltage level at the invertinginput. Under feedbackconditions, only adjusting thevoltage level on the non-inverting input will cause achange in level on the invertinginput, this occurring due to theopamp’s internal circuitry, assaid earlier.

Note that a virtual earthcondition does not exist ifnegative feedback is notemployed.

WAVEFORMCLIPPING

What we have not yetaccounted for is the “flattening”of the waveform peaks as thegain is increased beyond acertain point. The term given tothis effect is “clipping”.

The clipping has twopossible causes. First, theopamp is powered at aparticular voltage, 6V (orthereabouts) in yourexperiments. Reason must tellyou that the opamp cannotoutput a voltage greater than itspower supply.

In a perfect opamp, theoutput voltage would be capableof swinging fully between thetwo power line levels, 0V and 6Vin this case. The flatteningoccurs when the swing canincrease no further, irrespectiveof the amount of amplificationavailable.

There are, indeed, somemore-specialized opampsmanufactured whose outputscan swing almost completelybetween the power line levels.The term given is that they havea “rail-to-rail output capability”,where rail means power-rail(power-line).

Most general purposeopamps, though, do not haverail-to-rail output. Most will onlyswing within limits somewhatless than the power rail range.The actual range depends onthe opamp type, the voltage thatit is being powered at, and theamount of current that is beingdrawn from its output by the loadinto which it is feeding. Underno-load conditions, the LM358you are using here has a typicalswing of about 05V to 38V for a5V power supply.

EPE Online, May 2000 - www.epemag.com - 398

them whilst allowing alternatingcurrent (AC) to pass through. Weshall discuss this ability more fullyin a future Teach-In part, but forthe moment let’s accept this as afact. But bear in mind that futurediscussions will point out that thisability is governed by thecapacitance value and the loadresistance into which the “output”side of the capacitor is fed. Twoother terms come into use in thatdiscussion, differentiation andintegration.

AC COUPLINGSo let’s prove the point even

though we don’t explain it.Remove the oscillator connectionfrom the “DC Input” point on yourbreadboard and connect it to the“AC Input” point. This nowprovides the input path with whatis known as AC coupling (asopposed to DC coupling, whichhas been the situation so far).

Run the same group ofadjustments using opamp presetsVR2 to VR4 as you did earlier andobserve the screen results.

You should find that onceVR4 has been set midway, thereshould be no vertical shift of the

waveform as you adjust gain,just the amplitude change.

There is, though, a muchmore pronounced effect that youmay observe, that of a reductionin oscillator frequency with theadditional capacitor in circuit.Furthermore, the frequency andshape of the waveform are likelyto vary when adjusting opamppreset VR2.

The effect is due to theoscillator now seeing twocapacitors in parallel, its ownC1, and C2 of the opamp circuit.The effective value of the latteris also changed by the amountof resistance it sees from VR2.

We have a cure for this aswell!

SIGNAL BUFFERINGThere is a scenario that we

have not yet explored, that offeeding a voltage into the non-inverting input, and merelyfeeding the output straight backinto the inverting input, butwithout any additional voltage orcurrent being fed into that input.Such a circuit is shown inFig.7.5.

The interesting thing aboutthis circuit is that the totalnegative feedback ensures thatthe voltage applied to the non-inverting input receives neitheramplification nor attenuation atthe output. Whatever changethere is on this input is exactlyfollowed by the output, and inthe same direction. In thisconfiguration, the circuit isknown as a unity gain amplifier,i.e. the gain is 1. The circuit isalso said to function as a buffer.

What is now worth noting isthat inputs to opamps draw verylittle current (some types drawnone at all, see Panel 7.1). Theyare said to have high impedanceinputs, where impedance canloosely be described as

TEACH-IN 2000

INPUT CAPACITOREarlier, we drew your

attention to the likelihood that,when the gain was beingadjusted, the waveform positionseen on screen would changeas well as amplitude. This is inpart due to the fact that thetriangular waveform tapped fromIC1a pin 1 may not be swingingabout an exact midway voltagelevel.

Consequently, any DCvoltage difference between thewaveform’s midway level andthat set by VR4 is amplified bythe opamp, causing the verticalshift observed.

There is a very easy cure forthis – to stop the DC level fromthe oscillator reaching theopamp, just allowing the ACvoltage change through.

In Teach-In 2000 Part 2 wediscussed capacitors in terms oftheir ability to be charged anddischarged through aresistance. We displayed it viaone of the computer demos andgave formulae for it.

Capacitors have anotherattribute, the ability to stop directcurrent (DC) passing through

PANEL 7.3 – OPAMP POWER LINESMost opamps are designed to be run from a dual-rail power sup-

ply, i.e. one having positive, negative and zero (ground) output voltagerails, typically 15V but may not be as high as this with some devices.

As we deliberately demonstrate in this Teach-In, opamps can alsobe powered from a power supply having only positive and 0V (ground)connections. In this case the middle rail voltage is provided by usingthe voltage at the center of an equally-divided potential divider (asused in the demos).

Note that whilst in theory all opamp circuits shown in this Teach-Inpart can be powered from dual-rail supplies, on no account shoulddual-rail supplies be used if the circuit is to be connected to the ADCdevice or to a computer. Additional circuitry would be required in orderto permit computer connection. Failure to observe this could seriouslyharm the ADC and/or computer.

For optimum stability of an opamp circuit, the power supplyshould be regulated at fixed voltage levels. Power supplies will be dis-cussed in Part 9.

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resistance.In the earlier experiments,

capacitor C2 caused afrequency change at theoscillator because the current/voltage flowing through theresistance provided by VR1 wasbeing partly diverted into C2. Ifwe insert a unity gain opamp toisolate (buffer) the chargingprocess for C1 from the effect ofC2, then the oscillator frequencywill be unaffected by thepresence of C2.

The circuit diagram for thissimple improvement is shown inFig.7.6. Here we now use thesecond half of the dual opamp,IC4b, to provide the unity gainbuffer stage and then feed itsoutput into the amplifier stage

around IC4a, still via C2 andVR2.

Referring back to thebreadboard layout in Fig.7.1,connect the oscillator outputfrom IC1a pin 1 to the pointlabeled “Buffer Input” andconnect “Buffer Output” to “ACInput”. Ensure that the small linkbetween columns 25 and 26 ofrow G is inserted.

Having made the changes,observe that the frequency isnow unaffected by the presenceof the amplifying stage.

SIGNALNON-INVERSION

We have not fully discussedthe fact the waveform beingoutput from IC4a is an upsidedown (inverted) version of thatgenerated by the oscillator.There are occasions when thisinversion might be undesirable,in DC voltage amplification, forexample.

As another example, inaudio amplification involvingmany simultaneously processedsources, signals must oftenremain the “right-way-up” withrespect to each in order tomaintain their correctrelationships. Failure to do socould have severely detrimentaleffects on the overall soundquality. It could even result intwo signals canceling each other(a principle we shalldemonstrate next month).

One way that we canmaintain the correct phaserelationship, as it is known inwaveform processing, is to usethe amplifier stage in its non-inverting mode rather than itsinverting mode. We can stillprovide the same amount ofamplification.

NON-INVERSIONCIRCUIT

First we need to connect thesignal to the non-inverting inputof the opamp. We must thenprovide the correct midway biasvoltage to the inverting input. Itis, though, this input that is stillresponsible for partly controllingthe negative feedback, and thusthe gain. Consequently, theresistance of the bias setting

TEACH-IN 2000

0V

INPUT

D.C. INPUT

+6V

6

57

22m

VR4100k

VR2100k

C2

3

2

OUTPUT1

VR3100k

+

+

IC4aLM358

4

8

IC4bLM358

+

Interactive computer screen illustrating a non-invertingopamp circuit.

+INPUTOUTPUT

Fig.7.5. Unity gainopamp buffer.

Fig.7.6Adding abuffer to thecircuit ofFig.7.2

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control must also be taken intoaccount.

TEACH-IN 2000

breadboard layout to that shownin Fig.7.8.

The signal is still being fedin via capacitor C2 in order toisolate the amplification from theaffects of the DC bias that mightexist from the oscillator.However, we must provide theopamp side of C2 with adischarge path, as supplied viapreset VR4.

Note that in the “real world”,the discharge resistance valuein relation to the value ofcapacitor C2 affects thefrequency range that can becorrectly handled (to bediscussed when we examineintegration in a later part of theseries), and should be chosenaccordingly. For the sake of thisdemo, however, we’ll ignoresuch niceties – the relationshipis OK for what are trying toshow.

We retain presets VR2 andVR3, but have to provide asecond midway bias voltage intothe now “loose” wiper of VR2.This is provided by the voltagedivider formed by resistors R3and R4. At their junction isadded a capacitor (C3) to“smooth” the voltage here,which could otherwise varysignificantly with the changingsignal levels in the feedbackpath. As we discussed whenconsidering voltage dividersfeeding into another resistance(Part 1), the resistance of R3and R4 should be equal in orderto provide the midway voltage,but also of a value about tentimes less than the loadresistance (effectively VR2 inthis case). A value of 4k7 hasbeen chosen on the assumptionthat VR2 is set about midway(about 47k). Fine adjustmentof the midway level can bemade using preset VR4.

Note that the wrong figurewas published for Fig.1.12 of

OUTPUT

C322m

22mC2

0V

6

5INPUT

7

VR2100k

VR4100k

R44k7

3

2

4

1

+6V

R34k7

VR3100k

+

+

+

+

IC4bLM358

8

IC4aLM358

Fig.7.7. Non-inverting

opamp ampli-fier circuit.

161615

15 1717

1818

1919

2020

2121

2222

2323

2424

2525

2626 27 28

2929

3030

3131

TO R.H.SIDE OFBOARD

TO R.H.SIDE OFBOARD

INPUT

OUTPUT

IC4VR2 VR3

VR4

C2 C3+ +

R4

R3

Fig.7.8 and photo. Breadboard layout for Fig.7.7. Note that thesecond half of IC4 is now required to be connected into

the circuit.

There are severalapproaches to this problem. Weshall just take an option that iseasy to implement with thebreadboard layout we arealready using. The circuit isshown in Fig.7.7. Change the

Interactive computer display illustrating an inverting opamp circuit.

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Part 1. The correct figure, whichillustrates a voltage dividerfeeding into another resistance(Rm), is shown now as Fig.7.9.The circuit in Fig.7.7 does notinvert the signal output. Examinethe output at IC4 pin 1 as shownon your screen (via the ADC)and confirm that it is the sameway up as the original. Now thatyou have a buffer amp in circuit,you can monitor the basicoscillator waveform from itsoutput (IC4 pin 7).

NON-INVERSIONGAIN

Earlier we defined theformula for calculating inversiongain as:

R2 / R1

…where R1 and R2 representedthe inverting input and feedbackresistances respectively.

Using the inversionconfiguration, signal attenuationcan be achieved as well asamplification.

For the non-invertingamplifier, the lowest level ofsignal amplification is 1 (unity),attenuation can never beachieved. Consequently, the

TEACH-IN 2000

PANEL 7.4 – RESISTANCE VALUESThe question of what decade ranges the resistance values should

be chosen from for opamp designs is a slightly tricky one. You willrecognize that a uniform potential divider, for instance, can be madeup from any two equal resistance values. So, should the values eachbe 1, 10k or even 10M? We have to say first that only a textbook heavily dedicated to opamps can really give definitive answers.There are, though, some basic considerations that it is appropriate tomention here:

One consideration is that power economy should always be at theforefront of any designer’s mind. Since power consumption is lesswith higher value resistors, this does not favor very low resistances forthe divider. We have already said that a divider’s resistance shouldideally be at least ten times less than the load into which it feeds.When the divider is feeding into the non-inverting input simply as bias,its resistance can therefore be comparatively high. When it is provid-ing bias to the inverting input of a feedback circuit, the load is pro-vided by the effective input resistance and so the divider resistanceshould be chosen with respect to that.

The input resistance is chosen in relation to the gain required andthe resulting value of the feedback resistor. One matter to considerhere is that the gain of an amplifier (as opposed to a comparator) isusually best kept below about 200, and preferably below 100.

In DC amplification circuits, another factor to be considered is that(again, ideally) both opamp inputs should have equal current flow, andthe resistances should be chosen to meet this condition. In choosinggain setting values, and those of the dividers, it should be borne inmind that very high values of resistance are prone to causing the cir-cuit to pick up signals from external electrical sources, such as mains“hum”, motorbike ignition, frequency radiation from TV or computermonitors, etc.

Additionally, the relationship between signal capacitors and resis-tors affects the frequency response of the circuit as a whole. Some ofthis capacitance can be due to that which exists in the opamp itself,as well as the proximity of one printed circuit board track to another.

With some opamps a further problem is that if the load they feedinto has quite a low resistance, distortion of the output can occur if thefeedback resistance is too high.

As a very rough guide, however, the ranges of input and feedbackresistances are usually best kept between about 1k and 1M, but itdoes depend on the circumstances. It is then usually best if the valuesare chosen from the “commonplace” decade multiples, such as thosein the E12 range (see Part 1): 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7,5.6, 6.8 and 8.2.

To help you become more conversant with what resistance val-ues can be chosen for different opamp circuits, study the publishedcircuits of other electronics designers.

Finally, despite this list of considerations, for general experimen-tation opamps are really easy devices to use successfully! You canplay around with component values to your heart’s content with an ex-cellent chance of achieving results (even if they are not perfect). Fur-thermore, opamps are such hardy devices that you are highly unlikelyto ever kill one!

R1

R2 Rm

Fig.7.9. Why a meter(represented by resistance

Rm) affects the voltage read-ing at resistor junctions. Cor-

rected Fig.1.12 for Part 1.

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TEACH-IN 2000inputs seemingly remains thesame. Such a change is knownas temperature drift. It will be farmore pronounced at higheramplification settings. Typicaltemperature drift values areusually quoted in componentdata sheets, often in the form ofgraphs.

You should also be awarethat an opamp can heat upinternally when the current intoor out of its output increases,and that this will affect itscharacteristics.

For AC amplification(waveforms fed in and out viacapacitors), though, temperaturedrift is seldom of anysignificance.

COMPUTER DEMOSWe suggest that you now

run two of the opamp demoprograms, which interactivelyillustrate the functioning ofinverting and non-invertingopamp circuits.

From the main menu selectOpamps – Menu, this will bringup another screen from whichfive opamp demo programs canbe selected. The two wesuggest you play with now areSimple Inverting Amp andSimple Non-Inverting Amp. Weshall discuss the other threeoptions next month.

The inverting demoillustrates an opamp’s responseto an AC-coupled input trianglewaveform in respect of differentresistance values. Resistors R1and R2 control the gain, whileR3 and R4 change the bias onthe non-inverting input.

The non-inverting demoalso inputs an AC-coupledtriangle waveform. Resistors R1and R2 control the gain. R4 hasa fixed unspecified value; it isthe “bleed” resistor requiredfollowing input capacitor C3. Thebias voltage applied via R4 can

gain calculation becomesfeedback resistance (R2)divided by inverting inputresistance (R1), as before, but avalue of 1 is then added:

Gain = (R2 / R1) + 1

You will probably spot that ifR2 has a value of zero, then theunity gain condition exists,irrespective of the value of R1.

DC AMPLIFICATIONFor the last several

paragraphs we haveconcentrated heavily on anopamp’s ability to amplifywaveforms (i.e. AC signals).You may not have recognized it,but you have already proved thatthe opamp is equally suited toamplifying DC levels.

When you were initiallyexperimenting with therelationships of the threepresets in Fig.7.2, you neededto adjust VR4 in order tocompensate for the DC biascoming from the oscillator. Youmay have noticed that theadjustment became moresensitive when the gain was sethigh by either VR2 or VR3.

This was entirely due to theDC bias from the oscillator andVR4 being amplified. At thattime the DC amplification wasundesirable. There are, though,many occasions in which a DClevel needs to be amplified inorder to be useful.

One example that comes tomind is the way in which thethermistor (heat sensor) andlight dependent resistor (LDR)you were encouraged to try aspart of various oscillatorexperiments could be used (Part3). We’ll examine how voltagechange with temperature or lightintensity can be monitored onyour multimeter and computerscreen as part of next month’sTutorial.

DC STABILITYWe must comment now,

however, that amplifying DCvoltage is not necessarily aseasy as one might like. We havesaid that the values of electroniccomponents can change withtemperature, indeed thethermistor is an exaggeratedexample of this (but intended todo so beneficially).Unfortunately, opamp circuitsare no exception to this rule. Notonly can the components thatare used in conjunction withopamps have their valueschanged according totemperature, but so too can thecharacteristics of the opampitself.

The subject is actually toocomplicated to fully discuss ordemonstrate as part of Teach-In, but it is something of whichyou should be aware. There aremany circuit techniques bywhich the problem can beovercome, and moresophisticated (and expensive)opamps in which the situation isless pronounced.

TEMPERATUREDRIFT

A particular example of anopamp’s temperaturedependency affects the DCoutput relative to the input. In aDC amplification circuit you canset up a bias level on one inputin order to bring it closer to theDC level on the other input (i.e.narrow the differential voltagebetween the two). The resultingDC output level, however, mayonly hold true at the temperatureat which the adjustment ismade.

As the ambient(surrounding) temperaturechanges, so the DC output levelcan change, even though thevoltage difference on the two

EPE Online, May 2000 - www.epemag.com - 403

TEACH-IN 2000be changed.

Signal input amplitude andfrequency rate (Cycle Count)can also be changed. Thepower supply voltage is fixed at+5V/0V. Note that the demoopamp has been given outputmin/max limits of 1V and 4V.

The controls are stated onscreen, press the appropriatekeys to activate the function tobe changed. Note the varyingconditions under which the

output signal can becomeclipped. You can press the<PAUSE> key to stop thewaveform scrolling, then pressany other key to restart scrolling.

NEXT MONTHThis seems a convenient

point at which to end thismonth’s Tutorial. We do nothave room for an Experimentalsection as such, this has been

moved forward to next monthand becomes Tutorial Part 8.

In reality, Part 7 and Part 8are both a mixture of Tutorialand Experimental. What wediscuss in Part 8, though, is allbased on the characteristics wehave been describing here inPart 7, and illustrates someinteresting ways in whichopamps can be used.

EPE Online, May 2000 - www.epemag.com - 404

Robert Penfold looks at the Techniques of Actually Doing It!

It is said that the simplestinventions are the best ones,and, for the electronics hobbyist,stripboard possibly ranks along-side sliced bread and the wheelin the “best inventions” stakes.

Stripboard is a proprietaryprinted circuit board that is notedfor its versatility. For most pro-jects it represents the only prac-tical alternative to using a cus-tom printed circuit board (PCB).

A project based on a cus-tom board is actually the bestchoice for a complete beginnerdue to the relatively foolproofnature of these boards. How-ever, many small and mediumsized projects are based onstripboards, and newcomers tothe hobby soon find themselvesusing this method of construc-tion.

Although stripboard is notquite as straightforward to useas custom PCBs, it is not reallythat difficult to use either. Thereare a few traps waiting for theunwary, but once you are awareof the pitfalls it is not too difficultto obtain perfect results almostevery time.

RIGHT PITCHSo what exactly is strip-

board? It is based on a boardabout 16mm thick that is madefrom an insulating material. Pre-sumably the color of the boardvaries from one manufacturer toanother, but it seems to be sup-plied in a variety of yellow-browncolors from almost white to virtu-ally black.

The board is drilled withone-millimeter diameter holeson a regular matrix. In the past it

was possible to obtain strip-boards with the holes spaced at01-inch, 015-inch, or 02-inchintervals, but these days only01-inch boards are readily avail-able. It is only 01-inch pitchboards that are of any real usewith modern projects, becausemany components will not fitonto 015-inch or 02-inchboards.

One side of stripboard isplain, while the other side hascopper strips running alongthe rows of holes. It is, ofcourse, from these copperstrips that the stripboardname is derived. Many peo-ple still refer to this materialby the old proprietary nameof “Veroboard”.

Like an ordinary single-sided printed circuit board,the components aremounted on the plain side.The leadout wires aretrimmed short on the otherside and soldered to thecopper strips, which thencarry the connections fromone component to another.Fig.1 shows the plain and

copper sides of two scraps ofstripboard.

BRITTLE EXPERIENCEWith a custom PCB there is

usually no preparation required.When you are ready to startconstruction you simply beginfitting the component to theboard. With stripboard a smallamount of work is needed be-fore the board is ready to accept

Fig.1. Stripboard only has the copper strips on one side.

Fig.2. The underside view of theboard will clearly show the positions

of any breaks required in thecopper strips.

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the components, and the normalfirst step is to cut out a board ofthe required size.

As pointed out previously,stripboard comes in a rangeyellow-brown colors, reflecting arange of materials used in theboard. Some of these materialsare tougher than others, butmost stripboards seem to beslightly brittle. It is best to err onthe side of caution and assumethat all these boards are brittle,

duces some very rough edges,but these are easily filed to aneat finish using a flat file.

The finished board mightslot into place in the case, but itis more likely that it will bebolted in place. Any mountingholes should be drilled at thisstage using an ordinary HSStwist drill. Use a piece of scraptimber, chipboard, etc. under-neath the board, and use onlymoderate pressure. This shouldgive good “clean” holes andavoid any cracking around them.

When drilling any form ofcopper laminate board it is bestto drill the board with the copperside uppermost, as there is oth-erwise a risk of the copper beingtorn away from the board.

BIG BREAKSWith anything but the most

simple of projects it is necessaryto make some breaks in thecopper strips. Without any cutseach strip can only carry one setof interconnections, but bybreaking a strip into (say) threepieces, it can carry three sets ofconnections.

The article describing theproject should include a diagramthat clearly shows the positionsof the breaks, as in the exampleof Fig.2. Double-check the posi-tion of each break before actu-ally making it. If a mistakeshould be made it is possible tosolder a small piece of wire over

the break, but more than the oc-casional repair will give scrappylooking results and poor reliabil-ity.

A special strip-cutting tool isavailable, and it is often referredto as a “spot face cutter” in com-ponent catalogs. This is basi-cally just a drill style cutting toolfitted in a handle. In order to cuta strip the point of the tool isplaced in position and the han-dle is given a couple of rotationswhile applying moderate pres-sure (Fig.3).

If you will be producing any-thing more than the occasionalstripboard project, it is certainlyworthwhile buying this tool. Ini-tially you may prefer to use ahandheld twist drill bit of about5mm in diameter, which will dothe job quite well.

Either way, make quite surethat the strips are cut rightacross their full width. Very fineresidual tracks of copper can bedifficult to see with the nakedeye, so it is worth checking theboard with the aid of a magni-fier.

Although you need to makesure that the strips are cut prop-erly, do not go to the other ex-treme and practically drillthrough the board. With a largenumber of breaks this would se-riously weaken it. Brush awayany copper shavings as thesecould otherwise cause short cir-cuits.

Practically Speaking

Fig.3. Using the special toolprovides the easiest way of

making breaks in thecopper tracks.

and exercise due care whenworking on them. Use the“hammer and tongs” approachand you may well end up withthree or four small boards in-stead of one large one!

Over the years various sug-gestions have been made forquick and easy ways of cuttingstripboard to size using imple-ments such as glass and tilecutters. The problem with thesemethods is that they work wellwith some makes of stripboard,but can produce disastrous re-sults with others.

The only truly reliablemethod found so far is to care-fully cut along rows of holes us-ing a hacksaw. Due to the closespacing of the holes and widthof the blade it is not practical tocut between rows. Cutting alongrows of holes inevitably pro-

Fig.4. An example stripboardlayout diagram.

Fig.5. Removing some ex-cess flux rendered this solder

bridge fairly obvious.

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MOVING INAt this stage the board is

ready for the components to beadded. This is one respect inwhich stripboard is rather moreawkward than a custom printedcircuit board. With a PCB thereis one hole per leadout wire orpin, but with stripboard less than10 percent of the holes are nor-mally used.

Mistakes with componentplacement are more easilymade, and when they do occurthey can be difficult to spot. Tocompensate for this it is neces-sary to proceed more carefullyand to double-check the posi-tioning before fitting and solder-ing each component in place.

Having to remove and refit asmall component occasionally isnot a major disaster, but gettinga multi-pin component, such asan integrated circuit (IC), in thewrong place can be more diffi-cult to deal with. Removing thistype of component requiresproper desoldering equipmentand risks damaging the board.

Getting a large number ofcomponents shifted out of posi-tion is time consuming to cor-rect, and all the soldering anddesoldering could take its toll onthe board. It is much better toproceed carefully and get thingsright first time.

ON YOUR MARKSStripboard layout diagrams

often have letters to identify thecopper strips and numbers toidentify the columns of holes, asin the dummy layout diagram ofFig.4. Many constructors find ituseful to mark the board itselfwith these letters and numbers,so that they can quickly andeasily match any point on theboard with its equivalent pointon the diagram.

A fine point fiber-tip pen isrequired, as there is not a greatdeal of space available for thelabels. Also, it needs to be atype that is capable of writing onglass and other non-porous sur-faces. Otherwise it will not markthe board properly, or the labelswill rub off the first time you han-dle the board.

It is difficult to mark num-bers for all the columns of holes,but navigating your way aroundthe board should still be easy ifonly every fifth or tenth columnis labeled. Similarly, it is onlynecessary to label every othercopper strip, or even everyfourth or fifth strip.

Do not make the classicmistake of getting the orientationof the board wrong so that allthe components are fitted in thewrong places. There are usuallymounting holes that make thecorrect orientation obvious, butthe diagrams for the two sides ofthe board normally have amarker that indicates the samecorner of the board in bothviews. This is included in Fig.2and Fig.4, and leaves no excusefor getting it wrong.

MISSING LINKWith the preliminaries out of

the way, assembling a compo-nent panel is much the samewhether it is based on stripboardor a PCB. There are a couple ofdifferences though, one of whichis the higher number of link-wires encountered when build-ing projects based on strip-board.

The copper tracks of a cus-tom PCB can weave all roundthe board if necessary, but thisis clearly not possible with strip-board. Link-wires provide ameans of compensating for thelack of versatility in the trackpattern, and enable connections

to run from any given point onthe board to any other point. Vir-tually every stripboard layouthas at least a few link-wires, andthe larger boards can havedozens of them.

The usual way of fitting link-wires is to pre-form a piece ofwire to fit into the layout in muchthe same way as resistors andaxial capacitors are fitted. Theends of the wires are thentrimmed to length and solderedto the board in the usual way.

An alternative method is tocut a piece of wire that is slightlyover-length and then solder oneend of it to one of the holes inthe board. Next thread the otherend of the wire through the sec-ond hole and pull it tight using asmall pair of pliers. Finally trimthe wire and solder it to theboard.

The trimmings from resistorleadout wires are ideal for shortlink-wires, but for longer wires22s.w.g. or 24s.w.g. (or about06mm diameter) tinned copperwire is needed. Where a layouthas a lot of link-wires be sure tometiculously check that everylink has been fitted to the board.

There is no need to insulateshort links, but with wires ofaround 25mm or more in lengththere is a slight risk of short cir-cuits occurring, particularlywhere there are several wiresrunning side by side. In thiscase, it is advisable to fit thelonger link-wires with pieces ofPVC sleeving.

BUILDING BRIDGESThe second potential prob-

lem with stripboard is accidentalshort circuits due to soldersplashes and excess solder onjoints. This can be a problemwith any form of printed circuitboard, but it tends to be moreproblematic with stripboard due

Practically Speaking

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to the very narrow gap betweenone track and the next. There isactually only about 03mm be-tween adjacent tracks.

Usually it is fairly obviouswhen excess solder bridges twotracks, and remedial action canbe taken straight away. Smallamounts of excess solder canusually be wiped away with thebit of the soldering iron, butlarge amounts should be re-moved using a desolderingpump. The affected joint orjoints can then be carefully re-made.

The real problems arecaused by the tiny trails of sol-der that are barely visible. Infact, they are sometimes buriedunder excess flux and can onlybe seen if the board is cleaned(Fig.5).

Having completed a strip-board it is definitely advisable to

clean the copper side of theboard and thoroughly check itfor solder bridges. Cleaning flu-ids for removing excess flux areavailable, but simply scrubbingthe board vigorously with some-thing like an old toothbrushseems to do an equally goodjob.

Any reasonably powerfulmagnifying glass will provide agood close-up view of the board,but something like an 8x or 10xloupe is best. These are primar-ily intended for viewing slidesand they are available frommost camera shops. The cheap-est of loupes is adequate for thisapplication.

If a stripboard refuses towork for no apparent reason it isworthwhile making some checksusing a continuity tester. Checkthat there are no short circuitsbetween adjacent copper strips

or across any breaks in thestrips. Usually, once you know ashort circuit is there it will mirac-ulously appear when the boardis given another visual check!

Practically Speaking

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A team of researchers fromIBM’s laboratory in Zurich re-cently revealed their road mapfor the future of data storage.One surprise is that punchedcards are due for a comeback,but around a million timessmaller than they were the firsttime round.

The real density of hard diskrecorders is now increasing at120 percent per year, thanks toIBM’s 1988 discovery of GMR,the giant magneto-resistive ma-terial which was ready for com-mercial exploitation two yearsago and is now used in 70 per-cent of all hard drive read heads– either made by IBM or underlicense.

GMR is a multi-layer sand-wich of magnetic and non-magnetic materials, which showdramatic changes of resistancein a changing magnetic field.This makes the read head tentimes more sensitive, and solets it detect smaller magneticdomains.

Hard Drive DensityCurrently hard drives can

record around 20 Gigabits ofdata for every square inch(6.45cm2) of surface area and35 Gigabit per square inchdrives are already working in thelab. The likely practical limitlooks like being 100 Gigabits. Atthis density, the individual mag-netic domains are so small andclose that they affect each other,making storage unreliable. Be-fore this limit is reached, the cur-

rent method of recording a sig-nal, with a miniature inductivehead, will have become unwork-able.

IBM’s Magneto-electronicsteam leader, Dr Stuart Parkin,believes that in two years timehard drives will have to use ver-tical recirding (VR) instead of

horizontal recording (HR). ForVR the domains are switched bya field that aligns the magneticparticles through the disk coat-ing.

IBM’s Microdrive, the harddrive no bigger than a largepostage stamp, currently stores340 Megabytes, or nearly 900

BACK TO THE FUTURE WITH MILLIPEDESMillipedes may help bring back punched card technology

for PC data storage. Barry Fox reports

A ROUNDUP OF THE LATEST EVERYDAY NEWSFROM THE WORLD OF ELECTRONICS

Microchip Handbook

Many of you will already have found how useful Microchip’sEmbedded Control Handbook can be when writing PIC microcon-troller software. The latest update to this book has been releasedand provides a comprehensive reference tool for anyone usingPICs and related products.

The 848-page updated handbook provides current applicationnotes, technical briefs and reference designs which have beenwritten and published since the previous edition. The book can beobtained through any authorized Microchip representative. Thematerial is also available individually on Microchip’s website atwww.microchip.com

It is interesting to note from another Microchip press releasethat the organization is offering a comprehensive University Pro-gram within the UK, designed to help university professors givetheir engineering students a hands-on understanding of PICs andrelated products. Details can be found via www.microchip.com/university

Microchip’s UK HQ is at Microchip House, 505 Eskdale Road,Winnersh Triangle, Wokingham, Berks RG41 5TU, UK.Tel: +44 (0) 118-921-5858Fax: +44 (0) 118-921-5835

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The fundamental science onAtomic Force Microscopy wasdone in 1980, and won a NobelPrize for IBM’s Dr Gerd Binnig in1986. The first laboratorydemonstrations are now nearlyready. “It’s back for the future”,says Gerd Binnig, alluding to theoriginal punch card, developedin the last century for censusdata processing and then usedby early computers.

But whereas original punchcards had permanent perfora-tions, the new Millipede record-ing material can be erased, byheating the plastic to re-flow de-pressed areas. The chip movesacross the surface, in a scan-ning raster, writing 1GB of datain a 3mm x 3mm area.

The write/read system mustbe kept surgically clean, but thetechniques used to keep harddrives clean are applicable. Airis continually blown over the sur-face, through very fine filters.

Tunneling RamThere are also new develop-

ments coming in Random Ac-cess Memory. Current RAMneeds power to retain data. TMJ(tunneling magnetic junction)-RAM retains data even when the

power is switched off. So itworks like flash memory, butwith much higher capacity, andmuch lower power needed forwriting. With Magnetic RAM aPC could boot up within sec-onds.

The system relies on theability to detect and control thespin parameters of electrons inferromagnetic materials. Switch-ing at low power is possible be-cause of the quantum physicsphenomenon known as tunnel-ing, whereby if enough electronsconfront a barrier, some willpass through, even though theirenergy state is theoretically toolow to allow it.

NEWS. ....still pictures from a video cam-era. The next Microdrives willhold 1GB. By comparison an8MB Compact Flash card holdsaround 20 pictures.

So what happens when diskdrives run out of space? DrBernard Meyerson, IBM’s Direc-tor of TelecommunicationsTechnology, dismisses the ideathat optical disk can take overfrom magnetic hard disks.

“IBM has worked for yearson optical storage, includingdisks with several layers to in-crease density. But there is abasic limiting factor – the wave-length of light”.

Millipede TipsBig Blue’s Blue Sky re-

search efforts are now concen-trated on an extraordinary newtechnology called Millipede. Asilicon chip, the size of a finger-nail, is made with a matrix of1024 tiny cantilever arms, eachwith a sharp tip, around 1nm insize. When the tips are moved,by feeding current to activatedrive coils, they press down on aspinning plastic film disk to cre-ate tiny indentations. These canthen be read by sensor tips.

GreenweldCatalog

You will be glad to hear thatGreenweld have reopened.Chris Knight tells us that aftermany months of work they havefinally got the business goingagain. You will probably recallthat the original Greenweld Elec-tronics ceased trading last year,but the remaining stock was as-tutely bought by Chris and TimKnight and Geoffrey Carter.

The trio have a scientificbackground and wide business

experience, along with interestsranging from computers to carsand robots to recycling.

We are pleased to have re-ceived the new Greenweld cata-log, the first under the new own-ership and management team.The catalog lists many ranges ofthe types of component thatanyone involved in electronics islikely to need, not only resistors,capacitors, potentiometers, logicICs, LCSs and so on, but alsoitems such as meters and tools,hardware and surplus stocks,and there is a selection of booksas well.

Good wishes to Greenweldfrom us all at EPE and EPE On-line.

For more information, con-tact Greenweld Ltd., Dept EPE,PO Box 144, Hoddesdon, Herts,EN11 0ZG, UK.Tel: +44 (0) 1277-811042Fax: +44 (0) 1277-812419E-mail: ad-min@greenweld.co.ukWeb: www.greenweld.co.uk

WINDPOWER

A book called WindpowerWorkshop: Building Your OwnWind Turbine, has been writtenby Hugh Piggott, the technicalconsultant to the BBC 1 docu-soap Castaway 2000, in whichthe castaways rely on renewableenergy for their power needs.

Hugh has lived for 25 yearson a similar island and runs hisown wind turbine company. Hisbook is based on his knowledgeof wind power harnessing and isaimed at helping budding sur-

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sent to the project throughoutthe year 2000. The site will besealed in 2001 and shall remainburied for 200 years. A trust isbeing established to ensure thesite is excavated in 2200. Any-one can take part by filling atime capsule pack and sendingit back to the project. Each packcontains a time capsule measur-ing 340mm x 250mm x 80mm,along with specialized foldersand envelopes which help topreserve the items within it.

Records of the project, in-cluding the participants and lo-cation of the site shall be keptwith The International TimeCapsule Society in Atlanta, keyregional record repositories inBritain and within the records ofthe project itself.

The location of the site willbe announced later this year.

Everyone who takes partreceives a certificate givingthem legal title to their time cap-sule and its contents, allowingthem to leave their legacy to fu-ture generations.

Packs can be ordered bysending a check for 41 UKPounds made payable to: TheMillennium Time Capsule Pro-ject and sent with the partici-pants name and address to: TheMillennium Time Capsule Pro-ject, PO Box 736, Newcastleupon Tyne NE99 1LQ, UK. Al-ternatively, packs can be or-dered through the websiteshown below.Tel: +44 (0) 191-261-6784Fax: +44 (0) 191-232-1274E-mail: editorial@millennium-timecapsule.comWeb: www.millennium-timecapsule.com

Why not tell us, for possibleinclusion in Readout, what youare sending or would like to

have sent if the capsules hadbeen large enough? Repliesmay be serious or humorous(but at least loosely connectedwith electronics)!

NEWS. ....vivalists, hobby engineers, self-sufficiency hopefuls and stu-dents of renewable energy tolearn more about wind power.

The book is priced 10 UKpounds (plus 1.75 pounds ship-ping and handling), and is avail-able by mail order from the Cen-ter for Alternative Technology,Machynlleth, Powys SY20 9AZ,UK.Tel: +44 (0) 1654-702400Fax: +44 (0) 1654-702782Email: me-dia@catinfo.demon.co.ukWeb: www.cat.org.uk

Three Counties Ra-dio

& Computer RallySunday 21 May 2000. By pop-

ular demand following the 1999rally, the Three Counties annualradio and computer rally is to bestaged a week before the SpringBank holiday. It will be held at thePerdiswell Leisure Centre, BilfordRoad, Worcester, UK.

For those not familiar with thevenue, the following facilities areavailable: full restaurant servicesfrom 7 AM, licensed bar from 11AM, all traders in two adjacenthalls, easy access to the hallswhich are at ground level, freeparking for 900 cars and coaches.

The organizers point out that,being close to the City Center,wives and children can spend apleasant day in Historic Worces-ter, sight seeing, shopping or aboat trip on the river Severn (we’dlike to ask why wives and childrenshould be shuttled off – we aresure they are just as interested inwhat the rally has to offer as therest of us are!).

For more details contactWilliam E. Cotton G4PQZ, Tel/Fax: +44 (0) 1905-773181 (for faxplease ring first).

Farnell’s CatalogNo longer need you complain

that “the Farnell catalog is greatbut it’s just too big”! Thisrenowned supplier is separatingits catalog into six books, split byproduct. It will be available fromApril.

Farnell say that this allowsemphasis to be placed on their

MILLENNIUMTIME CAPSULE

The Millennium Time Cap-sule is a major national UK pro-ject in celebration of the newmillennium. Businesses,schools, clubs and the generalpublic are invited to take part inthis prestigious event and tosend a personal message to fu-ture generations. The Millen-nium time capsule will be buriedfor 200 years. Inside, contribu-tors will have their own box con-taining whatever they wish toinclude. Photographs, videos,books, tapes, letters – the list isas long as your imagination.

The time capsule will be thelargest ever constructed, provid-ing a snapshot of the UK at theend of the millennium. But it willalso be a very personal record.Every contributor will be sentcertification, identifying their boxwithin the capsule. In this wayindividual boxes can be passeddown from generation to gener-ation and inherited by our de-scendants in the year 2200.

The massive site will housethousands of time capsules frompeople from all over Britain andabroad. Time capsules can be

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NEWS. ....core product strengths, marketthe full range, provide the“ultimate one stop shop” and tofocus on new products threetimes a year instead of twice, asat present.

From the summer editiononwards, color pages will alsobe made available for suppliersto advertise their products inthese books.

Tel: +44 (0) 113-263-6311Fax: +44 (0) 113-263-3411Web: www.farnell.com

Farnell’s catalog always hasbeen a “must” to have in yourelectronics workshop. Thischange of binding will surely bewelcomed. Don’t forget, also,that Farnell have product dataon CD-ROM as well.

For more information con-tact Farnell, Canal Road, LeedsLS12 2TU, UK.

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WIN A DIGITALMULTIMETER

The DMT-1010 is a 3 1/2digit pocket-sized LCD multi-meter which measures a.c.and d.c. voltage, d.c. current,and resistance. It can alsotest diodes and bipolar tran-sistors.Every month we will give aDMT-1010 Digital Multimeterto the author of the bestReadout letter.

* LETTER OF THEMONTH *

REGENERATIVE RECEIVERDear EPE,

Regarding the RegenerativeReceiver (March ’00 issue):

The use of positive feed-back to increase the Q of tunedcircuits was widely used in com-mercial wireless receivers in the1920s and early 1930s; ama-teurs continued to use the tech-nique until the 1950s. The termcommonly used was “reaction”and controls with this label ap-pear on many receivers. Thefeedback in early sets was in-ductive with a pivotally mounted“reaction” coil that could beturned towards a static “tuning”coil. In later sets the feedbackwas fed through a variable ca-pacitor, usually with solid dielec-tric, to a supplementary reactionwinding on the coil used to form

the main tuned circuit.There were disadvantages

with using reaction to increasesensitivity. If the feedback ex-ceeded a critical limit the circuitoscillated and, as it was usuallycoupled to the aerial, became atransmitter. This had disastrousconsequences as many re-ceivers in the neighborhoodwould be swamped by the radia-tion and those with critically ad-justed reaction controls alsoburst into oscillation.

The BBC Handbook for1929 has warnings about theproblem, leaflets were issued bythe Post Office (then the wire-less licensing authority) and car-toons on the subject appearedin Punch. (In 1929 the BBCHandbook carried circuit dia-grams to assist potential listen-ers to construct receivers!).People were branded as badneighbors if they were sus-pected of oscillating. The addi-tion of an RF amplifier stage al-leviated the problem as the os-cillating circuit was no longerdirectly connected to the aerial.

Increasing the Q of a tunedcircuit also limits bandwidth. Inthe early days of broadcastingthe signals were Morse (CW)and bandwidth limitation was apositive advantage. By the1950s, AM broadcast soundquality had reached a standardwhere this bandwidth limitationwas significant.

The oscillation caused bypositive feedback in circuits us-ing “leaky grid” detectors wasrectified by the grid driving thevalve into cut-off until the chargeleaked away, so called

“squegging”, generating pulsesof radiation. This circuit system,as the self-quenching super-regenerative receiver, was alsopatented by the brilliant EdwinArmstrong. The ratio of signalfrequency to squegging fre-quency needed to be between100 and 1000 so that audio re-ception was only possible atShort Wave and higher -frequencies.

The circuit was used in theshort range “B” set of WirelessSet 19 by the Army, in IFF re-sponders (MkIII) by the Navyand Air Force. The Lichtensteinnight fighter radar used by theLuftwaffe also relied on the cir-cuit. Super-regenerative re-ceivers, if mistuned, could re-ceive FM, and Hazeltine“Fremodyne” used a double tri-ode to receive FM and providean audio output.

PS In an act of generosity,possibly misplaced, I presentedmy copy of the BBC Handbook1929 to the East Midland RadioMuseum in upstate New York.The place is worth a visit if onlyto see the spark gaps operatedand the poster advertising thereturn trip of the Titanic. Possi-bly there is a copy at Amberley.

A book worth reading isSuper-Regenerative Receiversby J.R. Whitehead, CambridgeUniversity Press, 1950. (DrWhitehead was a TRE man sothe book is biased towards air-craft equipment.)

Guy Selby-LowndesPatent Attorney

Plaistow, Billingshurst

John Becker addresses some of the general points readers have raised. Haveyou anything interesting to say? Email us at editor@epemag.com!

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Thank you Guy, most inter-esting information. We assumeyou refer to Amberley Chalk PitsMuseum, near Arundel, WestSussex, UK – it has many his-toric items relating to radio andelectron

PIC LOCATIONS 0 TO 3Dear EPE,

PICToolkit Mk 2 (May-June’99) forces the addition of a JMPto location 0005 at the ResetVector and loads any HEX filefrom location $0004 onwards,despite the HEX file address in-formation. Similarly, the decoderoutine used during disassemblytotally ignores ANY CODE in lo-cations $0000 to $0003 inclu-sive.

Note that it is common foran interrupt routine code blockto start at 0004 and continuethrough 0005 until the end of theroutine. It is also common forother code such as Subroutinesto start at 0005, the Reset vec-tor being a JMP to the relevantlocation.

Peter Balcombevia the Net

The reason for Toolkit notallowing programmed access tolocations 0 to 3 is historical, hav-ing adopted the convention usedin the Simple PIC16C84 Pro-grammer of Feb ’96, as de-signed by Derren Crome. Thesame question was subse-quently raised by other readersand we have replied in variousways since then. The followingis Derren’s original reply:

When the PIC is put intoprogramming mode, it sets itsinternal counter to $0000, whichis the reset vector address. Itwas decided to have the pro-gram set this to $0005, which isthe beginning of the program

memory address space.Addresses $0001 to $0003

are not of use to the user. Ad-dress $0004 is the interrupt vec-tor which is set by the first line inthe assembly code. The userprogram starts at $0005.

PIC16F87x PROBLEMSDear EPE,

I have been experimentingwith the new PICs (’876 and’877) and Toolkit Mk2 hardwareand software, mostly using a32768MHz crystal clock rate.

I have problems with Port Cbits 0 and 1 as outputs at rela-tively fast state changes. With“slow” switching, e.g. using yourTKTEST4 program, all seemsOK. However, if I halve theCOUNTs to speed up switching,the bit C0 square wave be-comes erratic and the bit C1square wave gets somewhatnoisy. If I output $00 and $FF toall ports in a rapid loop, thesetwo bits are a mess, while allother bits, on Port C (and PortsA, B, D and E) are OK.

I am setting PAGE0 andPAGE1 OK using both bits RP0and RP1, and I have even triedensuring that TIMER1 is off bysetting file T1 CON to all zeros. Iassume the problem is some-thing to do with the multi-functional nature of these twobits, and most likely to do withTIMER1. I have examined thePIC datasheet and recent issuesof EPE but I am foxed. I note inthe 8-Channel Data Logger(Aug-Sep ’99), that you usethese two bits for push-buttoninput, which is slow (and not out-put), so maybe you have notseen this problem yet. Can youoffer any help, please?

TK2 is great! Thanks foradding recognition of TABs forspaces. You asked for sugges-tions for other facilities: are you

able to add two useful functionsto the assembler (functionswhich TASM supports), I thinkmany people would find theymake for clearer programs:

(a)Simple addition in state-ments, e.g.:HIGHBIT EQU %10000000RETLW ‘P’RETLW ‘I’RETLW ‘C’ + HIGHBIT;end of string ‘x’ of total ‘n’strings

I use this for a useful LCD textdriver, with top bit set for end ofeach string, filtered off beforedisplaying. It currently causes anerror in TK2.

(b)Multiple components to a#DEFINE statement, e.g.:EQUs for STATUS, RPO,RP1, then:#DEFINE SETRPO BSFSTATUS,RP0#DEFINE CLRRP1 BCFSTATUS,RP1#DEFINE PAGE1SETRPO\ CLRRP1 etc. (atthe moment this only codesone statement.)

Could you please pass onmy appreciation to all your stafffor an excellent magazine and avery useful web-site. I find all thePIC stuff extremely clear andvery useful.

Incidentally, my first intro-duction to electronics was ETImagazine, back in the mid-seventies. Since then I havebuilt RAM and I/O boards anddata loggers for the NASCOM(Z80), the North Star Horizon(Z80 and S100 bus), the dearold BBC (6520) and all the IBMPCs from 1981 to date, withinboth my professional work andmy hobby.

Roger D. Redmanvia the Net

Readout

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I’ve not come up against yourPort C problem. Are you sure thatyour PIC is fully healthy? I haveseen erratic waveforms on otherdevices when part of them hasdied for some reason. You mightwant to buy another PIC and trythe program on it.

Both your TK suggestionsseem useful. Thank you Roger.Perhaps one day …!

PATENTLY DIFFICULTDear EPE,

I have designed amicroprocessor-based PC Cardfor the ISA bus, but no one wantsto know. I have tried over 40 UKmanufacturers that were specifi-cally selected to do the job of buy-ing the manufacturing rights of mycard, and have contacted a tele-coms company here as my carduses Distinctive Ring Patternsfrom them, but not one of themhas had the decency to reply.

Are all UK-based inventors ofelectronic devices treated this pa-thetic way all the time? Or is theresomeone you know out there whowould be interested in manufac-turing my card? I am made to feelthat I am insignificant, a has-been, a nobody.

You would not believe theamount of E-mail and snail-mail Ihave sent out, but nothing what-soever has come back. I am sodepressed with it all. I think thequestion is, after the Patent, whatnow ?

I would be ever so grateful toyou and your colleagues if youcould publish this and your reply,as I feel it would benefit other UKElectronics Inventors in the samesituation.

Jim DelaneySheffield, UK

Jim’s plea was E-mailed toour Online Editor, Alan, who of-fered the following extremelypractical reply:

I’ve worked on both sides ofthe desk, both designing a widevariety of products as a develop-ment manager and also receiv-ing proposals from outside de-signers hoping that my companywould take on their idea. It hap-pens all the time, and it’s sur-prising how badly presented theapplicant’s case can be. Not ev-ery manufacturer would be assympathetic to the aspiring de-signer as I was, so first impres-sions count, and a crispbusiness-like approach can onlyhelp as well as setting youabove all the other (amateur)applications vying for the samespot.

However, I was one of thefew who would always spare alittle time to look at an externalidea, but my next problem wouldbe selling the merits of the ideato the senior management. If adesigner couldn’t sell the con-cept to me, I had no hope sellingit up the ladder.

Often designers had no ideaof the investments needed intooling, production and distribu-tion either. No basic market re-search, no sales projections, nobudgeting – a design withoutsuch initial marketing researchreally would need to set theworld alight. James Dyson foundout the hard way and perse-vered, now everyone buys hisvacuum cleaners (even me!).The Black & Decker Lawnrakerand their WorkMate bench aretwo more examples of productsdesigned by amateur designers.The Bayliss wind-up radio is an-other.

In my career in product de-

velopment, I have only comeacross one really switched-onprofessional-looking designer (areal “ideas man”) who presenteda very powerful case, with goodquality prototypes and lots ofideas that forced us to sit up andlisten. He was immensely enthu-siastic and positive, and had re-ally thought of everything andhad come up with some verycute answers.

A personal meeting sold uson the concepts. He had all theideas and we were prepared todevelop the product and tool upfor mass production, which iswhat we did (for a special typeof tool kit). It would also be truethat, sometimes, manufacturersjust don’t know what they’relooking at and are being veryshortsighted, so you have nohope with such firms.

Manufacturers usually havetheir own agenda with their ownproducts currently on the draw-ing board (CAD screen anyway),so to take on an external designcould mean dumping one oftheir own in-house designs.There would need to be a verygood reason to do that.

In the case of electronics,there is also the developmentcost involved with making theproduct EMC compatible andgaining CE-approval. Maybe thefact it’s an ISA card rather than,say, PCI might also detract fromit, I couldn’t say. I believe thatISA is being phased out in thePC2000 specification, althoughof course ISA slots will bearound on legacy systems forsome time to come.

It’s only worth patenting ifyou can afford the legal costs offighting an infringement. A pend-ing patent enables designers togo with an NDA instead andthen try to sell All Rights. I think

Readout

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it’s not just the treatment of inven-tors that is the problem, morelikely it’s the pressure the manu-facturers are already under withtheir own product lines. They gettoo wrapped up in their own prob-lems to want to go looking formore! But a forward-thinking andprogressive company (e.g. Black& Decker) will listen to outsideideas, if only to get the feel for anidea that may subsequently beproposed to their competitors.

You should ask yourselfwhether it’s worth sub-contractingthe production yourself, andmaybe get a small batch madeand sell direct if you have to (e.g.on a web site). CE approval isyour next hurdle, then give a fewsamples away and get the markettalking about it. If you can make abig enough nuisance of yourself inthe marketplace, it could then bethat someone will buy the rights.Just make sure you are fully pro-tected with design rights. Thereare plenty of good electronic engi-neers who can CAD up a boardand polish off its development.

Try looking at James Dyson’sweb site (www.dyson.com/co.uk), there used to be an inven-tor’s resource there. There is alsoan alt. newsgroup for inventorswhere I’m sure you’ll get morehelp. Also you could try a localUniversity – an example in mycase would be the new ProductDesign & Development Centrebased at the University of Hull,with whom I’m working.

I’m sorry I can’t be of moreassistance, but hope the abovehelps – good luck!

Alan Winstanley

REGEN RECEIVER AND FETsDear EPE,

I am writing to you as a sub-scriber to EPE and an electronicsconstruction enthusiast for some35 years. First, thank you for the

superb series of articles on RFDesign by Raymond Haigh, cul-minating in the High Perfor-mance Receiver (March ’00),which I have decided to con-struct.

I am not “new” to construc-tion, having built over 23 re-ceivers for short waves over theyears, designing and producingmy own PCBs for these re-ceivers.

It is the power gain of the2N3819 FETs used in RaymondHaigh’s design that I wish toquery. The problem is that manycomponent suppliers use thesame manufacturer for 2N3819sand the spread in characteristicsof these devices may affect thereceiver’s performance. In myown designs I use BF244 andBF245. Your comments are re-quested.

John B. DickinsonTamworth, Staffs, UK

John Dickinson gave a lotmore information in his letterand sent an example 2N3819.We forwarded everything toRaymond Haigh, who replied:

Thank you for letting meread Mr. Dickinson’s interestingand helpful letter. I should begrateful if you would thank himfor having taken so much trou-ble and for his very kind remarksabout my recent series of arti-cles. I would offer the followingobservations:

He is, of course, quite cor-rect in pointing out the widespread in FET characteristics.He is also correct when he saysthat the specifications for theBF244 and BF245 are tighterthan the specification for the2N3819. Referring to the tablespublished in Farnell’s catalog,the transconductance spread for

the BF244 and BF245 is 3 to65mA/V, whilst the spread forthe 2N3819 is slightly greater at2 to 65mA/V.

Unfortunately, the BF244and BF245 may not be readilyavailable outside Europe, andregard must be had to the world-wide circulation of EPE. Be-cause of this, I will have to con-tinue specifying the ubiquitous2N3819.

I have built about six ver-sions of the circuit using thistransistor as a drain bend detec-tor. They all worked well with thecomponent values quoted andwithout any selection of the2N3819s.

I have, however, exploredthis question. The outcome ofthe trial was as follows:

(a) Thirty-three 2N3819transistors were connected intocircuit and provision made formonitoring the audio output volt-age. Of these, 23 performed in acompletely satisfactory way,three were marginally betterthan the rest, and seven per-formed badly, or would not workat all, unless the detector sourceresistor was increased in value.The transistor kindly supplied byMr. Dickinson was one of thosethat would not work at all.

(b) When the source biasresistor (R5) was increased to15k, all 33 specimens of the2N3819 performed in a com-pletely satisfactory way. Withthe source bias resistor in-creased, several specimens of2SK168, MPF102, TIS14 andJ310 (about twenty transistors intotal) all worked well in the cir-cuit also.

(c) The few transistors thatwere marginally better than therest gave a very slightly higheroutput with the specified 4k7source resistor. It would seemmy earlier endeavors to milk the

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last drop of performance from thecircuit had revealed this. It is un-fortunate that I made a chanceselection of transistors that wouldwork with this source bias resis-tor. Had I not done so I wouldhave discovered that the circuitwould not suit devices at the otherextreme of the characteristicspread.

(d) Working and non-workingdevices in the test were dis-tributed across a random selec-tion of the products of differentmanufacturers. Presumably,therefore, the problem is primarilyone of characteristic spreadsrather than manufacturing differ-ences. However, as suggested byMr. Dickinson, there could well bea tendency for some manufac-turer’s transistors to drift towardsa particular extreme of the toler-ance range.

(e) I suggest that the value ofTR2 source bias resistor, R5, beincreased to 15k to ensure that allspecimens of 2N3819 are oper-ated in the non-linear region oftheir characteristic curve. Withthis value for the source resistor,most other jFETs, including the2SK88, MPF102 and J310 shouldalso work well.

Raymond HaighDoncaster, S. Yorks, UK

BINARY CONVERSIONDear EPE,

Your Teach-In 2000 Part 6(Logic gates, Binary and Hex)brought to mind a system ofconverting decimal to binary Ilearnt many years ago.

Dredging through my per-sonal memory-bank and withmany false starts, the system isto divide the number to be con-verted by 2 continuously, ignor-ing any remainder. Every timethe number is odd, put a dash (–) beside it. If it is even put “o’’.The top “–’’or “o’’ is the least sig-nificant figure and the bottomone most significant. For exam-ple, decimal 3353 is convertedas follows:

3353 –1676 o838 o419 –209 –104 o52 o26 o13 –6 o3 –1 –

Turn the paper through 90degrees clockwise so that themost significant figure is to theleft then read off the binarycode. Thus decimal 3353 equals110100011001 binary.

This method is probablywell-known in the “Trade” but itmight be new to some readers.

Harry NairnAshtead, Surrey, UK

It’s certainly new to me aswell Harry. It’s a form of long di-vision, of course. How obviouswhen it’s pointed out! Manythanks.

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EPE Online, May 2000 - www.epemag.com - 417

Some Component Suppliersfor EPE Online ConstructionalArticlesAntex

Web: www.antex.co.ukBull Electrical (UK)

Tel: +44 (0) 1273-203500Email: sales@bull-electrical.comWeb: www.bullnet.co.uk

CPC Preston (UK)Tel: +44 (0) 1772-654455

EPE Online Store and LibraryWeb: www.epemag.com

Electromail (UK)Tel: +44 (0) 1536-204555

ESR (UK)Tel: +44 (0) 191-2514363Fax: +44 (0) 191-2522296Email: sales@esr.co.ukWeb: www.esr.co.uk

Farnell (UK)Tel: +44 (0) 113-263-6311Web: www.farnell.com

Gothic Crellon (UK)Tel: +44 (0) 1743-788878

Greenweld (UK)Fax: +44 (0) 1992-613020Email: greenweld@aol.comWeb:www.greenweld.co.uk

Maplin (UK)Web: www.maplin.co.uk

Magenta Electronics (UK)Tel: +44 (0) 1283-565435Email:sales@magenta2000.co.ukWeb:www.magenta2000.co.uk

MicrochipWeb: www.microchip.com

Rapid Electronics (UK)Tel: +44 (0) 1206-751166

RF Solutions (UK)Tel: +44 (0) 1273-488880Web: www.rfsolution.co.uk

RS (Radio Spares) (UK)Web: www.rswww.com

Speak & Co. Ltd.Tel: +44 (0) 1873-811281

Versatile Mic/AudioPreamplifier

To date, we have onlytraced two sources for theSSM2166P microphonepreamplifier IC used in theVersatile Mic/Audio Preamplifierproject. It is currently listed byMaplin (code GS39N) and isalso carried by Farnell (code114-7249).

If you are going to includethe Signal Strength Meteroption, the actual selection ofthe moving coil meter is left toindividual choice, hence thesmall table giving resistor valuesfor meter movements rangingfrom 50mA up to 1mA. Manycomponent suppliers should beable to offer a suitable smallpanel meter.

Low-Cost Capacitance MeterThe only item that needs

highlighting when buying partsfor the Low-Cost CapacitanceMeter – this month’s StarterProject – is the timer IC.

A low-power version of the555 timer must be used in thisproject as, due to its low self-capacitance, it gives better

accuracy on the 1nF range.Therefore, use the TS555 timerinstead of the standard NEversion. The low-power versionshould be widely stocked andreadily available.

Once again, the meter is leftto individual choice as pricesseem to vary quite considerably.The model uses a 100uAmovement obtained from Maplin(code RW92A).

The 12-way single-polerotary range switch is a Lorlintype, which has an adjustablerotation limiting “end-stop” thatshould be set to 5-ways. Thiswas also purchased from theabove, code FF73Q.

Multi-Channel TransmissionSystem

Most of the componentscalled up for the Multi-ChannelTransmission System should bestock items, evenunprogrammed PIC16F84s arenow widely available.

The author is able to supplyready-programmed PIC16F84s.You will need to order at leasttwo microcontrollers, oneTransmitter (Tx) and oneReceiver (Rx). We understandthat the first two will cost 6 UKpounds each and any additionalPICs 5 UK pounds each,inclusive of postage (overseasreaders add 1 UK pound perorder for postage). Ordersshould be sent to: Andy Flind,22 Holway Hill, Taunton,somerset, TA1 2HB, UK.Payments should be made outto A. Flind. For those who wishto program their own PICs, thesoftware is available for freedownload from the EPE OnlineLibrary at www.epemag.com

with DAVID BARRINGTON

Some Component Suppliers for EPE Online Construc-tional Articles

EPE Online, May 2000 - www.epemag.com - 418

PIR Light CheckerNearly all the components

needed to build the PIRLightChecker project should beobtainable from your usual localsupplier. The miniature light-dependent resistor (LDR) andthe 7-segment, commoncathode, display both came fromMaplin (codes AZ83E andFR41U) respectively. (You can,of course, use the good oldORP12 LDR.)

Details and prices for all ofthis month’s printed circuitboards can be found at the EPE

Online Store atwww.epemag.com

Teach-In 2000No additional components

are called for in this month'sinstallment of the Teach-In 2000series. For details of specialpacks readers should contact:

ESR ElectronicComponents – Hardware/Toolsand Components Pack.

Magenta Electronics –Multimeter and components, Kit879.

FML Electronics (Tel: +44(0) 1677-425840) – Basiccomponent sets.

N. R. Bardwell (Tel: +44 (0)114 255-2886) – DigitalMultimeter special offer.

PLEASE TAKE NOTE: Micro-PICscope April '00

Unfortunately a digit was missedfrom the order code for theorange box, which should be281-6841. We apologize for thiserror

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