look! here's how · complete with insulated probes. leads, battery, circuit diagram and...

LOOK! Here's how youmaster electronics.....the practical way.

1 Buildanoscilloscope.

Assn-rrststageof youactually build your own Cathode rayoscilloscope! This is no toy, but a testinstrument that you will need not onlyfor the course's practical experiments, butalso later if you decide to develop yourknowledge and enter the profession. Itremains your property and represents avery large saving over buying a similarpiece of essential equipment.

This new style course will enable anyone tohave a real understanding of electronics by amodern, practical and visual method. Noprevious knowledge is required, no maths, andan absolute minimum of theory.

You learn the practical way in easy stepsmastering all the'essentials of your hobby or tofurther your career in electronics or as a self-employed electronics engineer.

All the training can be carried out in thecomfort of your own home and at your ownpace. A tutor is available to whom you canwrite, at any time, for advice or help duringyour work. A Certificate is given at the end ofevery course.

T

2Read, draw andunderstandcircuit diagrams.

InInd snort ',Mu you w br abic toread and draw circuit diagrams, under-stand the very fundamentals of television,radio, computors and countless otherelectronic devices and their servicingprocedures.

3)

w=g).400ts-7oo -1

40133F

3 Carry out over40 experimentson basic circuits.

Ive snow you how to conductexperiments on a wide variety of differentcircuits and turn the information gainedinto a working knowledge of testing,servicing and maintaining all types ofelectronic equipment, radio, t.v etc.

All students enrolling in our courses receivea free circuit board originating from acomputer and containing many differentcomponents that can be used in experimentsand provide an excellent example of currentelectronic practice.

SIM 1 EMI 11111O MEI MN MI= MEM NMITo find out more about how to learn electronics in a new, exciting and absorbing way, just clip thecoupon for a free colour brochure and full details of enrolment.

British National Radio & Electronic SchoolP.O. Box 156, Jersey, Channel Islands.

NAMEADDRESS

Block caps pleaseIn= =MI UM =NM =MI NEM MEI MIN Mill 111111 NM MB a=

ROOM THERMOSTATFamous Satchwell, elegant Ninon, intended forwall mounting. Will switch up to 20 amps at mainsvoltage, covers the range 0.30 C. Special snip thismonth E3 N.

WINDSCREENWIPER CONTROLVary speed of your wiper to suitconditions. All pans and instruc-tions to make £3.7S.

MICRO SWITCH BARGAINSRated at 5 amps 2S0V. ideal to make aswitch panel for a calculator and for dozensof other applications. Parcel of 10 (twotypes) for Lt.

RADIO STETHOSCOPEEasiest way to fault find, traces signalfrom aerial to speaker when signal slopsYou've found the fault Use it on Radio.TV, amplifier, anything. Kit comprisestransistors and pans including probetube, twin stetho-set. L3 IS

MULTISPEED MOTORSSix speeds are available 500, NO and 1,000r.p.m. anti 7,000, 9,000 and 11,000 r.p.m. ShaftIs I In diameter and approximately I in long2301240V. Its speed may IN further controlledwith the use of our Thyristor controller. VeryPowerful and useful motor size approx. 2 india. 5 In. long. Price £2.

THIS MONTH'S SNIP -is a miniature sealed relay 12v dc operated with twosets of change over contacts. The unique feature ofthis relay is Its heavy lead out wires; these provideadequate support and therefore the relay needs nofixing; on the other hand there is a fixing bolt protrudingthrough one side so if you wish you can flx the relay/bid use its very strong lead outs to secure circuit com-ponents -an expensive relay. but we are offering it foronly tip each. Don't miss this exceptional bargain!

EXTRACTOR FANEx computers -made by Woodsof Colchester. ideal for fixingthrough panel -reasonably quietrunning -vary powerful 2500rpm. Choice of two sizes 5" or6r dia. fAl 43.

MAINS RELAYSWith triple 10 amp changeover contacts -operating colt wound for 230V a.c. Chassismounting one screw fixing, ex unusedequipment Pp each, post and VAT paid.

MICRO AMPLIFIEREx behind the ear hearingaids complete with volumecontrol. Et 16. Case notsupplied

MERCURY BATTERIESBank of 7 Mercury cells type 625 wry am,which are appro. fin diameter ' Ntlisby (in. thick in plastic tubeBiving a total of 10.7V.

eing in a plastic tube It Is very easy tobreak up the battery into separate cellsand use these for radio control andsimilar equipment. Carton of 25 batteriesE1 N.

PP3/PP9 REPLACEMENTJapanese made in plastic container withleadsize 2in 11in. 1iin . this is ideal toPower a calculator or radio, it has a full waverectifier and smoothed output of 9V suitablefor loading of up to 100mA 12 S3.

SWITCH TRIGGER MATSSo thin is undetectable under carpet but willswitch on with slightest pressure. For burglaralarms shop doors. etc. 24in. 18in. £2.51.13in 10in. M.N.

MAINS TRANSISTOR PACKDesigned to operate transistor sets and amplifiers Adjustabl eoutput 6V. 9V. 12V for up to 500mA (Class D working). Takesthe place of any of the following batteries. PP1. PP3.PP$, PP7. PP9 and other, Kit comprises. main translormerrectifier, smoothing and load resistor condensers and in-structions Real snip at only U.N.

DRILL CONTROLLERElectronically changes speed from ay -proximately 10 revs to maximum FullPower at all speeds by finger-tip con-trol Kit includes all parts, case. everything and lull instruction E3.45Made-up model Et entre

8 POWERFULBATTERY MOTORSFor models, Meccanos, drills, remote controlplanes, boats. etc. N.

ROTARY PUMPSelf priming. portable, fits drill or elec-tric motor, pumps up to 200 gallons perhour depending upon revs. Virtuallyuncorrodable. use to suck water oil.

----- Petrol, fertiliser, chemicals, anything',fluid Hose connectors each end E2.

TANGENTIAL HEATER UNITA most efficient and quiet runningblower -heater by Solatron-sametype as is fitted to many famous name

N heaters -Comprises mains inductionmotor -long turbo fan -split 2 kwheating element and thermostaticsafety trip -simply connect to the

mains for immediate heat -mount in a simple wooden or metalcase or mount d.rect onto base of say kitchen unit -price£4.95 post £1 50 control switch to give 2kw, 1inv, cold blow oroff available 60p extra. 3kw Model LS IPS + LI SO P. & P.

MULLARD UNILEXA mains operated 4 + 4stereo system. Rated oneof the finest performers inthe stereo field this wouldmake a wonderfu' gift for almostanyone in easy -to -assemblemodular form and completewith a pair of Plessey speakersthis should sell at about L311 -but due to a special bulk buyand as an incentive for you to buy this month we offer thesystem complete at only LIS N including VAT and postage.

HUMIDITY SWITCHAmerican made by Rain*. their type No.J1I. The action of this device dependsupon the dampness coining a membraneto stretch and trigger a sensitive micro -switch adjustable by a screw, quitesensitive -breathing on it for Instancewill switch it on Micro 3 amp at 250V a.cOverall size of the device approx. 36inlong. I in. wide and 191n. deep. 75p.

DELAY SWITCHMains operated -delay can be accu-rately set with pointers knob for periodsof up to 21 hrs. 2 cortacts suitable toswitch 10 amps -second contact opensfew minutes alter 1st contact 95p.

CENTRAL HEATING HEARTSRandal (illustrated) replacement in3060 etc ES 75. HORTSMAN Et M.SMITHS Cowl/oiler 10/100 completein wall mounting case E7 M.

TEACH INStart right away. Order thepart for this series -send £3deposit and £2 monthly for5 months or £12 cash. Past

lessons available.

PRINTED CIRCUIT KIT PROJECTSCONTENTS: (I) 2 CopperLaminate Boards 41in. 2f in(2)1 Board for Matchbox Radio

:- Miz (3) 1 Booed for WristwatchRadio, etc. (4) Resist. (5)

11, Resist Solvent (6) Etchanl!' Cleanser/Caw (8) 16 -

page Booklet Printed Circuitsfor Amateurs. (9) 2 MiniatureRadio Dials SW/PAW/LWAlso free with each kit. (10)Essential Design Data Circults. Chassis Plans, etc forPROJECTS. E1 Si.

SOUND TO LIGHT UNITAdd colour or white light to your amplifierWill operate 1, 2 or 3 lamps (maximum450W) Unit in box all ready to work ELM.

MINI -MULTI TESTERAmazing, deluxe pocket size pre-cmion moving coil instrument -jewelled bearings -1000 OP. -Mir-rored scale.11 Instant ranges measure. -DC volts 10, 50. 250, 1000AC colts 10. SO, 250, 1000DC amps 0-1 mA and 0-100 mAContinuity and resistance 0-1501(ohmsComplete with insulated probes.leads, battery, circuit diagram andinstructions.

Unbelievable value only ES Np + 511p oost and insurance.FREE Amps ranges kit enable you to 'lad AC current from0,10 amps directly on the 0.10 scale It's free if you purchasequickly but if you already own a mini tester and would like onesend LI 50p.

TERMS: Cash with order -but orders under Lt must add 500to offset packing etcBULK ENQUIRIES INVITED PHONE C1-6881833

J.BU LL (ELECTRICAL) LTD(Dept. EE), 103 TAMWORTH RD.

CROYDON CR9 ISG

IT'S FREEOur monthly Ad Advertising Bargains List givesdetails of bargains arriving *thief arrived -often bargainswhich sell out before our advertisement can appear -it'san interesting list and It's free -lust send S.A.E. Beloware a few of the Bargains still available from previouslines.

Special *nip. Japanese made FM Tuner and a matchingdecoder. We are offering the pair at less than the price mostfirms are asking for a similar tuner only, namely £11If you wish you can buy the items separately, a brief descrip-tion is as follows:

FM Tuner (A 111511), the tuner is very compact but hasquite a smart dial with cord drive and pointer, it covers therange 118-106 mhz, has an input sensitivity of 10uV. uses 6transistors and 3 diodes and needs s 9 volt gm A power supplyIt is an excellent performer and when tested here have resultsquite equal to a tuner costing over £40. Price only E7 32.

Decoder INSMS. This uses 5 transistors and 6 diodes, hasprovision for stereo indicator lamp, we can supply this as anextra at Sip with wiring instructions, needs 9v power supplyor battery and is partner to the above tuner, price E7:51 + gap.

RIBolshoi. This is a multi band radio, stereo amplifierand record player. an impressive console having manyfeatures not found in the average radio. This originally soldfor well over L100. The radio side has motorized auto tuning,4 short were bands, long, medium and F.PA It has turntablefor playing records al 33, 45 or 78, and facilities for taperecording, tape playing back, microphone amplificationthrough stereo amplifier which has a peak output of 10 wattsPer channel. The equipment weighs approximately 150Ibsand is in a console type cabinet size approximately 3' ',-wide, 2' 8" 14" deep on 11" legs. We have only a few ofthese, they are new but may be slightly second for somereason or other. We offer these to callers only, you mustattend here, test the equipment and satisfy yourself that youare getting an acceptable bargain because we would sell asseen anci accepted but without guarantee because the pricewe are asking, namely E35 is a giveaway. VAT of course at12i% must be added to this price.

Still Available the Rigonda Calypso 10 + 10 stereo recordplayers. Price E12 N (note these are repairable items).

Multi Tester Bargain. An instrument you will be proud toown, no: a pocket model, it measures 6" 43" 2" and it hasmany features which make it very desirable so if you arecontemplating buying a multitester you must consider thisone. Jaoanese made this has a large clear scale with mirrorrenecto. It has a total of 24 ranges (26 if you count the twoextra ranges which we added to it by our free gift) as follows.AC volts: -0 to 6. 30. 120, 300, 600, 1200DC volts: -0 to 6, 30, 121).600. 1200, 3000. 6000DC current. -0 to 60 ua, 6 ma. 60 ma, 600 maResistanctr-0 to 6K, 600K. 6 meg, 30 meg. 60 megDecibe-c--20 db to +63 db. Site: -6" 43" 2"Price of this £16 75. Special offer -with this instrument wewill give free of charge our shunt hit enabling you to measureDC amps in 2 ranges 0-5 amps, 0 -IS amps.

Fluor Tube Invader. For camping -car repairing -emergency lighting from a 12v battery you can't beat fluores-cent lighting. It will offer plenty of well distributed light andis economical We offer rnvertor for 21" 13 watt miniaturetube for only E3 75 with tube and tube holders as well.

4 Changeover 12 volt Relay. This may be just what you arelookini for, for your complicated circuit the changeovercontacts are 10 amp rated, limited quantity only Et 62.

Mains Operated 3 Changeover (10 amp) type Relay. Thisis single screw fixing type very reliable and popular ex equip-ment but guaranteed oh. Price Nip.

Ditto but new in maker's boa Et N.

Relays, plug in types with two 10 amp changeover contactsall with dust covers and available in the following coil voltages6. 12, 50, 110 volts. All priced Np each. octal base to suitthese 27p each.

3 Changeover 110 amp Contacts e available withthe following coil voltages6, 12, 24. 48, 115 volts AC and110 volts DC, all £1 N each. II pin base

Soil Heating. We do not have to stress the advantage youget from soil warming but where there is the slightest chanceof damage by cultivation tools then low voltage must be used.Our very efficient 250w mains transformer will operate 100metres of soil warming cable very effectively. The price isLa 2.0 for the transformer and L8 N for the warming cable.

Bench Warming for fast germination. mist propagation, etc.The warming cables are buried in sand on the bench somairs voltage cables may be used We can supply a 6000 watt"big" grower pack which will warm 200ft of 311 wide benchat a concentration of lOw per square loot. price is E24 N.This big grower pack contains 3 100 metres twin warmingcables and 3 connection boxes. Medium growers pack 1200

atts.

Noon Novelties. One of the most versatile of electronicdevices is the neon, you can make it do all sorts of things toregulate voltage, imitate "Ernie" by lighting numbers atrandom. etc .

etc. We have miniature neon at 211p each alsoneon valves 4 pin type, 10 for LI + 8p Post 50p + 4p.

Ex.O.P.O, Telephones. Normal desk type with internal belland dial, and handset of course. Tested, with diagramsnowing how to connect as extensions or in pairs or multiwaycircuits, price Et 311.

Ditto but less dial E2 95.

Accessories for Telephones. Extra earphones. Veryuseful in noisy situations or for old people who suffer withdeafness. pricell1p. Extension bells (in wall box) often saveyot, missing vital calls. price E3 + 25p Wall -type phones.no dials or bells but hook switch E2 75

lull and oft Switch. This is a 4 pole changeover switcheach of 10 amp 230v rating, panel mounting toggle opercitionUseful where 2 similar lamps or banks of lamps, heaters.motors, etc. are required to be run in series or parallelEsecielly useful in photographic studios when setting up,sacs expensive lamps. Supplied complete with circuitdiagram. Price Et N + 8P

Everyday Electronics, March 1978 305

CRESCENT RADIO LTD.MAIL ORDER DEPT. 1 St Michaels Terrace, Wood Green

London N22 4SJ Phone 8118-4474

MULTIMETER BARGAINSA special bulk purchase of EagleMultimeters enables us to offer threefrom their range at up to 15% offEagle's own recommended retail price.

KEW 71,000 opv pocket multi -meter with 'off' damp-ing.All basic ranges in-cluded. Leads, batteryand instructions sup-plied.Spec. as follows:-DC volts: 0 to 10, 50,250, 1,000 volts. ACvolts: 0 to 10, 50, 250,1,000 volts. DC 0 to 100mA. Resistance: 0 to 150 K ohms(mid -scale): 3 K ohms. Decibels: -10 to t 22dB. Dimensions: 90x60x27 mm. PRICE: f.5 97 8% VAT

POWER SUPPLY UNITTYPE 'PPI'

Switched 3, 41, 6, 71, 9 and 12 volts at500mA. With on/off switch and pilotlight.Size: 130mm 55mm . 75mm approx.OUR PRICE:Only £600 8% VATC1095 20,000 opv multimeter withhinged 4 -position scale for easy -readingbench use. Anti -parallax mirror, 3 -

colour scale, overload protection.Ranges: DC volts: 5, 25, 50, 250. 500,2,500 (20,000 opv). AC volts: 10, 50,100, 500, 1,000 (10,000 opv). DC

. 50 uA, 2.5 mA, 250 mA.Resistance: 50 K. 5 meg. Decibels:-20 to -22 dB. Dimensions: 14085 x 35 mm.PRICE: E14-31 8% VAT

EM 1200100,000 opv multimeter. Comprehen-sive meter with 3 -colour scale, anti -parallax mirror, taut band movement,electronic protection, reversible polar-ity, 15 amps AC current range. Suppliedwith leather carrying case, shoulderstrap and probes.DC Voltage: 0-0 15, 0 .5, I 5,5. 15, 50,150, 500, 1500 volts (100,000 opv). ACvoltage: 0-1.5, 5, 15, 150, 250. 500,1500 volts (30,000 opv). DC . 10µA, 15µA, 500 µA. 15 mA, 500 mA, 1 -

amps, IS amps. AC C : 15 amps.Resistance: 0-20 K ohms, 200 K ohms,2 megohms, 20 megohms, 200 megohms.(Mid -scale): 100 ohms, I K ohm, 10K ohms, 100 K ohms, 1 megohm.A y: DC 2%, AC -3%.Dimensions (inc case): 210 17880 mm.PRICE: (40-23 : 8% VAT.H2009 STEREO HEADPHONES

Eagle headphones available at newreduced price.Spec. - Frequency range: 20-20.000Hz 5 dB. Second harmonic distor-tion: 0.7% maximum. Third har-monic distortion: 1-0% maximum.Matching impedance: 8-16 ohms.Weight: 360 grm. Switching: +5 dBr'm 100 Hz (Popular), Flat 100 Hz(Classical), -5 dB (a 100 Hz (Vocal).These headphones have a bass cut/liftcontrol on each earpiecePRICE: EIS -50+121% VAT.

'BARGAIN TRANSFORMERS'240v PRIMARY 12-0-12v, 500m/ASEC. Approx. size: 60 40 50mm.Fixing centres: 75mm.

PRICE: El .80 f 8% VAT.Also available MAINS TRANS-FORMER with I8v 500mA SEC.Price and size same as above.

ACCESS AND BARCLAYCARD ACCEPTED-PHONE ORDERS WELCOMED,ALL PRICES INCLUDE POSTAGE-PLEASE ADD VAT AS SHOWN-S.A.E. WITH ALL ENQUIRIES PLEASE.Personal Callers Welcome at: 21 Green Lanes, Palmers Green London, N. 13.Phone: 888-3206 and 13 SOUTH MALL, EDMONTON, N9. Phone: 803-1685.

SOLDERING IRONfor H 0 BBYorTRADE

MAIL ORDER PRICES:(including VAT and P and PI

Irons C370each

Bits 41p each

(Bit type 20 fittedas standard).

4.0

40`

fS3t.a9ndOeasch

&Solder 28.,5

/. 8 watts of powerat 240 v. straight to the bit

BIT SIZES:19 11.5mml 20 13.0 mm)21 145 mml 22 16.0 mmi

Drcule (tfruiries Welcome

From your Local Dealer or Direct from the Manufacturers :

S.& R. BREWSTER Ltd.86-88 UNION ST PLYMOUTH Tel 0752 65011

Over 200 kits in the free Heathkit Catalogue

NEW Digital Clockwith repeater alarm

Freezer Alarm Gives audible signalif freezer temperature rises to 6 Cfor any reason

NEW 4 FunctionSolid State Multimeter

One of a whole rangeof test equipment

Car Intrusion Alarm- Gives a distinctiveyelping' sound signal

the moment your caris tampered with

Ni

CM('

The Heathkit catalogue ispacked with top quality kits-hundreds of practical andfascinating items which youcan build yourself.

I Send forI your copy now!I To Heath (Gloucester) Ltd.I Department EE -38.I Bristol Road, Gloucester, GL2 6EE.I Please send a copy of the Heathkit Catalogue.

I enclose I1 p in stamps to cover postage only.Name

I Address

IIIII

Solderingiron offerFREE

When you receiveyour catalogueyou'll get details ofthis free offer worthapproximately £4.75

sessr-

The world'sbiggest producersof electronic kits.

gm WIThere are Heathkit Electronics Centres at 233 Tottenham Court Road,

London (01-636 7349) and at Bristol Road, Gloucester (Gloucester 29451).

306 Everyday Electronics, March 1978

TheWiest kitinovation:from S UNA digthe quickest fitting \ \CLIP ON \ , \...,capacitive discharge,,-----electronic ignitionin KIT FORMIntroductory .-- .

SPECIAL OFFER£2 OFF Kit orReady Built Unit /2 Smoother running // Instant all-weather starting Continual peak performance Longer coil/battery/plug life Improved acceleration /top speeds Optimum fuel consumptionSparkrite X4 is a high performance, high quality capacitive discharge, electrc nitignition system in kit form. Tried, tested, proven, reliable and complete. It can beassembled in two or three hours and fitted in 15, 30 mina.Because of the superb design of the Sparkrite circuit it completely eliminate*problems of the contact breaker. There is no misfire due to contact breakerbounce which is eliminated electronically by a pulse suppression circuit whichprevents the unit firing if the points bounce open at high R .P.M. Contact breakerburn is eliminated by reducing the current to about I ,50th of the norm. It witperform equally well with new, old, or even badly pitted points and is notdependent upon the dwell time of the contact breakers for recharging the system.Sparkrite incorporates a short circuit protected inverter which eliminates theproblems of SCR lock on and, therefore, eliminates the possibility of blowin; thetransistors or the SCR. (Most capacitive discharge ignitions are not completelyfoolproof in this respect I. The circuit incorporates a voltage regulated output forgreatly improved cold starting. The circuit includes built in static timing light,systems function light, and security changeover switch. All kits fit vehicles withcoil ,distributor ignition up to 8 cylinders.

THE KIT COMPRISES EVERYTHING NEEDEDDie pressed epoxy coated case. Ready drilled, aluminium extruded bass and heatsink, COX mounting clips, and accessories. Top quality 5 year guaranteedtransformer and components, cable*, Connectors, P.C.B., nuts, bolts and silicongrease. Full Instructions to assemble kit neg. or pos. earth and fully Illustratedinstallation Instructions.

NOTE -Vehicles with current impulse tachometers (Smiths code on dial Awl IWill require a tachometer pulse slave unit. Price C3.35 int. VAT poet Et petting.

e82lII th 8Ir et Wale II II

g

II

LTACNI NM SLAVE UNIT OS

Electronics Desin Associates, Dept. IIISeth Street, Walsall, WS1 30E. Phone: 10922) 33152

Name

Address

Phone your order with Access or Serolaycard

In VAT ore', QUANTITY Rao

M Eli CIA 51215

St INADY PUNT E36 atm

I endow atestemOr rot

£Cheque No

Ier SAE it brochurs Orif 'ewsti

Everyday Electronics, March 1978

E.E. PROJECTSAUDIO VISUAL METRONOME. Jan.I3 95 inc. case.PHOTOFLASH SLAVE. Dec. CU 65inc. case. Less lead.VHF RADIO. Nov. LIO 20 inc. case.TREASURE LOCATOR. Oct. L7.17.Case C126.TRANSISTOR TESTER. Oct. C6.40.Case a 95.SHORT WAVE RECEIVER. Aug.L8.65 Case LI .45.DICE. March. L4 24 inc. case.METRONOME. May. 0'130 inc. case.

Prices Inclusive. Articles extra 33p.Projects not listed, please write,

SOIL MOISTURE. June. 11.95 inc.case and probe.PHONE DOORBELL REPEATER. July.L4 70 inc. case.RAPID DIODE CHECK. Ian. 0.12 Inc.case.

FUZZTONE UNIT. July. C5 7S eeC,case.ADD ON CAPACITANCE UNIT.Sept. 04.50 inc. case.

Prices inclusive. Articles extra 15peach

1978 COMPONENTS CATALOGUE MAGENTA Magenta gives you a wide range of the things you need -components -tools -hardware -and lots more. First-class delivery. NO minimum order. Inclusiveprices. All products stock lines. III d catalogue with data. New products,Send 2 x 9p stamps.This Catalogue is a MUST kr Constructors, Write Today I

AMPLIFIER MODULE -STEREO 1; +l*W. S ohms. Tone Vol. control, plusFascia 9V D.C. C4 90.PRE - AMP -STEREO. Matches Mag-netic cartridge to above amplifier.9V D.C. L645.CAPACITANCE SUB. BOX. 9 valvesI 00OFF to 22.IAF 9S.DIODE TESTER. With leads LI .68.SIGNAL INJECTOR AF.RF proles andleads £4.95.MULTIMETER. TYPE I. 1,000 o.p.v.with probes. 2" x 34" x 1" L5.95MULTIMETER. TYPE 2. 20,000 o.p.v.with case and probes 5" x 34" x If"L10+95.ANTEX X25 SOLDERING IRON 2SW.Ideal for electronics L3 68.SOLDERING IRON STAND. ANTEX5T3 CI .55.DESOLDER BRAID. 62p.HEAT SINK TWEEZERS 124p.DRILL. 11 VOLT. 10,000 r.p.m. Handor stand use 00.0S.DRILL STAND. Adjustable high[.

INTERCOM. 2 station desk model.Cable and battery 45.95.HEADPHONES. STEREO. ohm.13-15 KHs. Padded L3 .85.HEADPHONES. MONO. 2K ohms.Sensitive. t2 SO.PANEL METERS. Nominal r matsblack Fascia. 0-50/aA, 0-100µA, 0.1mA[4.12.LEVEL MITER 0.200#A, 1200 ohmsLI .10.PHONE PICK-UP COIL. 3 '5rnmplus. 65p.EYEGLASS. Jewellers magnificationx 3. 93p.

TRIPLE MAGNIFIER. Each lens X5magnification. LI I5.DENTISTS MIRROR. Adjustable LI 70.SCREWDRIVER SET. 6 miniaturejewellers type, in case, a .48.NEEDLE FILES, 3mm square andround, 68p.AERIAL. 6 section telescopic, 70cm.95p.SPEAKERS. Miniature El ohm 59p.64 ohm 85p, SO ohm LI .15.

TRANSFORMER for 12V drill L1'37. EARPIECES. Crystal Sep, m c 150.POWER SUPPLY. Safe, economical Also capacitors, resistors, semi -plug type 6, 7.5, 9V D.C. 300mA, conductors, knobs, connectors, p.c.b.with lead L3 95. materials, potentiometers, variableNIGHT LIGHT. Safe. Built in 13A plug. capacitors, switches, cases, trans -Very cheap to run LI I8. formers, etc., etc.MORSE KEY. Use with 6V buzzer for All prices include VAT and FIRSTpractice set. Code included. LI 18. CLASS POST. Orders under 0.00BUZZER. 6V. 74p. please add !Sp.MAGENTA ELECTRONICS LTD., EB2, 61 Newton Lays, Burton -on -Trent. Staffs. DEIS ODW. C2113-63435.

TEACH -IN '78 COMPLETE KIT 113.95

SeanELECTRONIC

ENGINEERDo something PRACTICAL about your future.Firms all over Britain are crying out for qualified people. With

the right training, you could take your pick of these Jobs.Now, the British Institute of

Engineering Technology will trainYOU in your spare time to be anElectrical Engineer.

You risk nothing I We prom-ise to get you through yourchosen course -or, refund yourfeel

So, join the thousands whohave built a new future throughhome study Engineering courses.

Courses in

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Plus over 60 otherhome study courses.

POST COUPON FOR FREE 44 PAGE GUIDE

BRITISH INSTITUTE OFENGINEERING TECHNOLOGY

Aldermaston Court, Dept. TEE 31 Reading FIG 7 4PF.

NAME (Block csPitils alms)

ADDRESS

POSTCODE

Other SublectsL Accredited by CACC

El NM El NI MI El MI

AGE

MI IIIIMember of ABCC

307

THYRISTORS(Er No. THY1A/50 lAmp. 50 volt TO5.41 No. THYIA/400 !Amp. 400 volt 105

ckNo. THY3A/50 3Amp. 50 volt 1064No. THY3A/200 3Amp. 200 volt 1064

64, No. THY3A/400 3Amp. 400 volt TO64.0. No. THY5A/50 5Amp. 50 volt TO66

No. THYSA/400 5Amp. 400 volt TO66"IS No. THY5A/600 5Amp. 600 volt 1066figr No. CI06/4 6Amp. 400 volt 10220

TRIAC584 8Amp. 400 volt TO220 Plastic

(Non Isolated Tab)

.C4

< No. 16178

() Prj4:: ireNo. S19

..4 No. 520

ti) No. S21

CAPACITOR PAKS

DIACSBRIOO 15pD32 15p

im-PAKGRE AT SPACEWE NEED THE SPACE

SPEC/AL OFFER/I ets32p2Sp32p40p25p40p50p42p

110p

SWITCHES5 Mains Slide Switches5 x Miniature Slide Switches4 x Standard Slide Switches4 x Miniature Push to Makesingle hole mounting3 x Miniature Push to Breaksingle hole mountingPush button Switch Pak4 x Assorted types multibank and singlesLatching and non -latching

40p.40p*40p*

40p.

40p*

LI 006

TYPEAC 107ACI26AC 127AC 128ACI28KAC 176AC 176KACI87AC 187KACI88AC188KADI61/162 MPAFI 39AF239BC107BC108BC109BC118BC 147BC148BC 149BCIS4BC157BC 158BC 159BC 1 69CBC170BC 171BCI72BCI73

PRICE25p14p16p16p24p16p24p16p26p16p26p

80p30plOp6p6p6p

10pflp13pOp

16p9p.flp9pIflp6p6p6p7p

TRANSISTORSBRAND NEW -FULLY GUARANTEED

TYPE PRICE TYPE PRICE TYPE PRICEBC177 lip BF194 9p TIP32C 36pBC178 lip 8F195 9p TIP4IA 34pBC 179 12p BF 196 12p TIP4 I B 35pBC 182 9p BFI97 Ilp TIP4I C 36pBC182L 9p BF200 25p TIP42A 36pBCI83 9p BFX29 220 TIP42B 37pBC I 83L 9p BFX84 lap TIP42C 3Sp

C184 lip BFYSO 12p T1P2955 65p9 BFY5I 12p TIP3055 41p10p BFY52 lip10p MPSA05 22p ZTX10

ZTX1078 fop*Op10p MPSA06 22p ZTX109 7p10p MPSASS 22p ZTX300 7p

10p* MPSA56 2.2p10p 0C44 lip ZTX301 7p10p 0C45 lip ZTX302 Sp12p 0071 9p ZTX500 Op.Ilp 0072 12p ZTX501 10Ilp 0075 10p ZTX502 12p40p 0C131 14p 2N696 Iflp35p TIP29A 35p 2N697 10p37p TIP29B 36p 2N706 7p17p TIP29C 38p 2N706A IlisISp TIP30A 36p 2N708 Op20p TIP3011 37p 2N1302 Ilp25p TIP30C lip 2N1303 15p15p TIP3 I A 32p 2N I 304 15p15p TIP3 I B 33p 2N1307 lip25p TIP3I C 34p 2N I 308 22p15p TIP32A 34p 2N 1309 12p15p TIP32B 35p 2N1613 15p

BC! 84LBC2I2BC2 I2LBC2I 3BC2I3LBC2I 4BC2I4LBC251BCY70BCY7IBCY72BDI ISBD13100132BFI ISBFI67OF17313E180BFI81BF182BF183/3E184BF 185

TYPE PRICE2N 17112N 18932N22I82N2218A2N22I92N221 9A2N22212N222 IA2N22222N2222A2N23692N29042N2904A2N29052N2905A2N29062N2906A2N29072N2907A2N2926G2N2926Y2N30532N30552N 37022N37032N37042N39032N39042N39052N3906

SpalpspSpspOpSpOPSp6pOp4pSp4p5p1p4p2p3psp7pIlp35p7p7p6pIlpI IpIlpIlp

I

O

O(1

16201 18 Electrolytics 4.7oF-10oF16202 18 Electrolytics 10AF-1004F16203 18 Electrolytics 1000E-680oF

ALL 3 AT Special Price of LI-2016160 24 Ceramic Caps 22pF-82pF16161 24 Ceramic Caps 100pF-390pF16162 24 Ceramic Caps 470pF-3300pF16163 21 Ceramic Caps 4700pF-0-047oF

ALL 4 AT Special Price of LI 600

DIODESTYPE PRICE TYPE PRICE TYPE PRICE TYPE PRICE TYPE PRICEAAI 19 Sp BAXI6 BYZI6 30p OAKS 7p 1544 3pAAZ 13 4p 0A202 SP BYZ 17 lip 0A90 6p INS400 10pBA100 OP BY100 15p BYZ le lip 0A91 7p N5401 1 1 pBA115 5p BY127 10P BYZI9 2Bp 0A95 7p N5402 lipBA144 Sp BYZIO 32p 0A47 SP IN34 Sp N5404 13pBA 1413 10pBA 173 10p BYZII 32p 0A70 Sp 1N60 6p NS406 16pBAX13 BYZI2 32p 0A79 7p IN914 4p N5407 17p0A200 5p BYZI3 30p 0A8 I 7p IN4148 4p N5408 19p

UNTESTEDSEMICONDUCTOR PAKS

Code No's shown below are given as a guide tothe type of device. The devices themselves arenormally unmarked.No. 16130 100 Germ. Gold bonded

diodes like 0A47No. 16131 150 Germ. Point contact

diodes like 0A70/81No. 16132 100 200mA Sil. diodes like

0A200No. 16133 ISO 75mA Sil. Fast switching

diode like IN4148No. 16134 50 750mA Sil. top hat Recta.No. 16135 20 3amp. Sil. stud Rect.No. 16136 50 400mw. Zeners D.0.7

caseNo. 16137 30 NPN Plastic trans. like

BC107/8No. 16138 30 PNP Plastic trans. like

BC 177/8No. 16139 25 NPN Trans. like 2N697/

2N17I1 TO 39No. 16140 25 PNP Trans. like 2N2905

TO 39No. 16141 30 NPN Trans. like 2N706

TO 18No. 16143 30 NPN Plastic trans. like

2N3906No. 16144 30 PNP Plastic trans. like

2N3905No. 16145 30 PNP Germ. trans. like

0071No. 16147 10 NPN 103 Power trans.

like 2N3055

40p

40p

40p

40p40p40p

40p

40p40p.

40p

40p

40p

40p

40p

40p

80p

RESISTOR PAKSOrder No16213 60 On, I 00ohm-820ohm16214 60 kW. I K -8.2K16215 60 1VV 10K -82K16216 60 SW 100K -820K

ALL 4 AT Special Price of El MP16217 40 "W 100ohm-820ohm16218 40 IK-E12K16219 40 W 10K -82K16220 40 W 100K -820K

ALL 4 AT Special Price of [1-60.

LINEAR IC'sTBA800 2 pin QIL 75p UA703 7099(Plastic)IflpTBA810 1 pin QIL LI 00 741P 8 pin DIL lipTBA1320 4 pin QIL 1110p 72741 14 pin D1L 20pLM380 4 pin DIL 110pLM381 4 pin DIL LI .3SUA74IC 7099 20p72709 4 pin DIL 28p 72747 14 pin DIL 53pUA709 1099 18p 748P 8 pin DIL lipUA71 I C 7099 25p UA748 1099 2Sp

72558 (Dual 748)T099 45p

MCI 310P 14pin DILL1.1576115 14 pin QIL LI 15NESS5 El pin OIL lipNE556 14 pin DIL 60pSL4I4A 10 pin LI .80

NEW CONSIGNMENT ZN414 RADIO CHIP 75p*

IC SOCKET PAKSNo. S66 II 8 pin D.I.L. SocketsNo. S67 10 14 pin D.I.L. SocketsNo. S68 9 16 pin D.I.L. SocketsNo. S69 4 24 pin D.I.L. SocketsNo. S70 3 28 pin D.I.L. Sockets

TRANSISTOR SOCKETSNo. S71 15 TOI8 SocketsNo. S72 10 TO5 Sockets

LI 00LI

ggfl 00LI 00

00LI 00

MOUNTING PADSNo. 573 50 Mixed Transistor Pads TO18

and TO5 40p

TRANSISTOR FALL -OUTPACK GERM, SILICON,

POWER, NPN, PNP.ALL MIXED, YOURS TO

SORT AND TEST.APPROX. SOO PIECES

Order No 523 LI 25 per pack

VOLTAGE REGULATORSPositiveNo. MVR7805 oA7805 TO220No. MVR7812 µA7812 TO220No. MVR7815 sok7815 10220No. MVR78I8 µA7818 TO220No. MVR7824 p.A7824 TO220NegativeNo. MVR7905No. MVR7912

oA7905 TO220oA7912 TO220

No. MVR7915 01/47915 10220No. MV R7918 µA7918 TO220No. MVR7924 4,6,7924 TO220I4A723C TO99 38p 72723 14 pin

Lt1309K TO3 LI 20

85p85p85p85p85p

LI 10LI 10

LI 10LI 10LI 10

Dil 38p

MICROPHONESDYNAMIC DUAL IMPEDANCE UNIT

DIRECTIONAL CARDIOID MICROPHONEImpedance 600ohms. and 50K.

Response 50-14,000 Hz.Sensitivity 54db. at 50K. Size 16" Dia < 6r Long

Order No. 1328 0. SO.DYNAMIC CASSETTE MIC.

Fitted with On/Off switch. I meter of toughlead with floating 2.5 and 3.5 mm plugs.

Impedance 200 ohms. Sensitivity 90db.Frequency 90-10,000 Hz. Size 20mm Dia x120mm long.

Order No. 1326 1115

OPTOELECTRONICSDISPLAYS

No. 1510 707 LED Display 70p eachNo. 1511 747 LED Display LI SO eachS53 DL33 Triple 7 segment, LEDDisplay. Character height .1 I"Common cathode 12 pin DIL 30p ach

LED'sNo. SSI Red T1L209 (5 x I2.5") SOpNo. 552 Red FLVII7 (5 x .2") 50pNo. 1502 Green 125" 18p eachNo. 1505 Green 2" 18p eachNo. 1503 Yellow 125" IBp eachNo. 1506 Yellow 2" ISp eachNo. S82 Clear 2" (Illuminating

Red) Ilp

2nd QUALITY LED PAKS1507 10 it LED's Assorted 75p

5081508 12'

LED CLIPS 1252

3 for lip5 for 15p

SPECIAL REDUCTIONS1514 NORP 12 45p each576 OCP71 5 for LI -00583 5 NIXIE Tubes ITT 5870 ST. Ct 00

(Including Data)S77 Neon Indicator Lamps

230 V. A.C.State Colour (Red, Amberand Green) 25p ach

Transistor Heatsink Pak10 Assorted types TOI, T05, TOIL TO92Our Mix.

Order No. 575 60p

Transistor Insulating KitsMica washers and bushes assorted types, i.e.

TO220, 1066, TO3 etc.Approx 100 pieces (Approx 40 sets)

Order No. S74 SOp

Darlington Power Trans.70 watt 8 amp. NPN and PNP in plastic case 199High Voltage (Typ. 80Y) High gain.

10 pieces 5 NPN and S PNPData Sheet supplied.

Order No. S78 LI 00 per Pak

D.I.Y.PRINTED CIRCUIT KIT

Contains 6 pieces of copper laminateboard, box of etchant powder, measure,tweezers, marker pen, high qualitypump drill, Stanley knife and blades and6 in. metal rule.Full easy to follow instructions.Order No. 564 SALE PRICE f.350

MAMMOTH I.C. PAKApprox. 200 pieces.Assorted fall out i ad circuits,including: Logic, 74 series, Linear, Audioand D.T.L. Many coded devices, but someunmarked -you to identify.

Order No. 16223 LI 00

MATCHED PAIRS OF GERMANIUMMED. POWER TRANS.

2 amp. 750 reWVCE VCB HFE

NKT301 40 60 30-100 3Sp per pairNKT302 40 60 50-150 3Sp per pairNKT303 20 30 30-100 2Sp per pairNKT304 20 30 50 -ISO 25p per pair

P.C.B. BOARDS561 8 pieces 8" - 31" (Approx).

single sided paper 50pS62 4 pieces 8" - 3k" (Approx).

single sided fibreglass SOp563 3 pieces 7" 3r (Approx).

double sided fibreglass 50p

LOGIC PROBEA pocket size instrument capable of detectingT.T.L., D.T.L., Flip Flop and other pulsecircuits. It is easy to use and operates from the5V. D.C. supply of the circuit under test. Thelogic levels are indicated by 2 red L.E.D.'s onefor High and the other for Low. There is also agreen L.E.D. for the Pulse Mode of the unit.

No. 559 Our Special Price EIS 9S

ETCH RESIST PENSOrder No. 1609 50p each

SOLDER5 m. of 18 sw. Multi -core Solder

Order No. S60 50p

POWER SUPPLYSTABILIZER BOARD

Unused ax -equipment stabilizer board.input 30 V. D.C. output 20 V. Completewith circuit diagram.

Order No. 581 LI .25

P.O. RELAYSS85 2 Off Post Office relays 40p

ZENER PAKSNo. S55 20 mixed values 400mW Zener

diodes 3-10V. E1.00No. S56 20 mixed values 400mW Zener

dioes II -33V. II.00No. 557 10 mixed values IW Zener

dioes 3-10V. LI 00No. S58 10 mixed values I W Zener

dioes II -33V. LI 00

BATTERY HOLDERSto take 6 HP7's

Order No. 202 10p each

UNIJUNCTION TRANSISTORSUT46 - TIS43 20p

2N38192N5458

FET'S15p18p

EX. G.P.O. MICROSWITCHESOrder No. S84A 4 For 50p

I.C. INSERTIONEXTRACTION TOOL

Order No. 2015 30p

CABLE CLIPS565 50 2.5 mm round single pin

fixing 30p

PONY M/SS OUR SPEC/AL CASSETTE

No. 545No. 546No. S47

2 AMP. BRIDGE RECTIFIERSMetal Stud Mounting

50 V.100 V. (KBS 01)200 V. (KBS 02

28p30p34p

10 AMP. BRIDGE RECTIFIERS 200 V.ON HEATSINK

Order No. 522 SPECIAL CLEARANCE LI 00

OFFER/ LOOK AT OUR BOONS508 Everyday Electronics, March 1978

VING SALE BI-PAKYOU MAKE THE SAVING!

SILICON RECTIFIERSSIMILAR IN4000 SERIES G.E. I Amp.

No. S4I 25 Like IN4001 (IA 50 V)No. S42 20 Like IN4002 (1A/100V)No. 543 18 Like IN4003 (1A/200V)No. S44 IS Like IN4004 I IA1400V)

SILICON RECTIFIERS-{ Amp. G.E.No. 548 40 x 50 V.No. S49 30 x 200 V.No. 550 20 x 700 V.

60p60p1110p

609

60960p60p

G.E. HIGH VOLTAGE SILICON RECTIFIERSGR5S9 10inis. 14 KV (14000 V) 20p eachGA432 I AMP. 2 KV (2.000 V) 209 eachFD25 2 5 KV. Voltage Doubler 206 each

POTENTIOMETERSSlider 40 mm. TRAVELOrder No.16191 6 x 470 Ohm LIN Single 40p524 6 x IK LIN Single 40pS2S 6 x SK LIN Single 40p16192 6 x 10K LIN Single 40pS26 6 x 10K LOG Single 40p16193 6 x 22K LIN Single 40p16195 6 x 47K LOG Single 40p16194 6 x 47K LIN Single 40p527 6 x 100K LIN Single 40pS28 6 x 100K LOG Single 40pS29 6 x 500K LOG Single 40p60 mm. TRAVELS30 6 x 2.5K LOG Single 40pS3I 6 x 10K LIN Single 40pS32 6 x 50K LIN Single 40pS33 6 x 250K LOG Single 40pS34 4 x 5K LOG Dual 40pS35 4 x 10K LIN Dual 40pS36 4 x 100K LOG Dual 40pS37 4 x 1.3 MEG LOG Dual 40p538 20 MIXED SLIDER POTS. VARIOUS

VALUES & SIZES -OUR MIX 20 FORONLY 11.00

S39 6 x CHROME SLIDER KNOBS 40pWIREWOUND

A range of wirewound single gang pots. with lineartracks of I watt rating.Order No. Value Order No. Value1891 10 ohms 1896 470 ohms1893 47 ohms 1897 I K1894 100 ohms 1898 2K21895 220 ohms 1899 4K7

NOW ONLY 35p each

Type

740074017402740374047405740674077401374097410741174127413741674177420742274237425742674277428743074327433743774387440744174427443744474457446

74 SERIES TTL ICsType

744874507451745374547460747074727473747474757476748074817482748374847485748674897490749174927493749474957496741007410474105741077411074111

QuantityI 100CpCp7006$12 0.1012 0.10

12 0.10I2 0-1012 0-102402320 0 1926 0 2224 0 2344 0.4026 0 2545 0 4290 0 8875 0 7388 0 8285 0 8010 10028 0 2670 2 SO38 0 32

65 0 6243 0 3S38 0 3S70 0.6060 0 5870 0 6895 0 9040 0 3530 0 2530 0 2548 0 4S75 0 72

74118 85 0 8274119 I30 120

0-70 0.68 74121 0.28 0.26

Quantity1 100[pip

0-09 0.08O 11 0-10O II 0.100.11 0 100-11 0.100.11 0.100.28 0 2S02$ 0 250.12 0 11

0.12 0 110.09 0 000 22 0 200 22 0 200.26 0 250 28 0 250.26 0 250.11 0.100 19 0 180-21 0 200 25 0 230.25 0.230-25 0-230 36 0 340.12 0.100.20 0.19O 3$ 0 360 26 0.250.26 0.25O 12 0 100.60 0 570 00 0.700.95 0400.95 0.1100.80 0-750-00 0-75

Type

7412274123741417414574150741517415374154741557415674157741607416174162741537415474155741667417474175741767417774180741817418274184741907419174192741937419474 9574196741977419874199

QuantityI 100Op (co

0.45 0.420650620.6$ 0.65075072

1 10 1 050 65 0 60O 70 0 68

1 20 1 1070 0 6870 0 6870 0 6895 0 8595 095 095 0

1 20 1

1 2011 20 1

I 10 I

0 85 0I 10 I

1 10 1

I 10 I

1 90 1

O 80 0SO

40401005OS

858S851010100082Old

0000807848303000000)

0 80 0 75O 90 0 85O 90 0 85

1 90 1 801 80 1 70

Devices may be mixed to qualify for quanti y p -ice. Data is avaiable for the above se ies of IC's in booklet form Price 3Sp

SPEC/AL OFFER/COMPONENT PAKS

Order No. Quantity16164 200 approx. Resistors, mixed values

(count by weight) 40p16165 ISO approx. Capacitors, mixed values

(count by weight) 40p16167 80 {W. Resistors, mixed values 40ss'16168 5 pieces Assorted Ferrite rods 40p16169 2 pieces Tuning gangs MW/LW 40p16170 50 Single strand wire assorteo

wire 40p16171 10 Reed switches 40p16173 3 Micro switches 40p16176 20 Assorted electrolytics Trans type 40p16177 I pack Assorted hardware nuts/bolts, etc. 40p16179 20 Assorted tag strips and panels 40p16180 15 Assorted control knobs 40p16184 IS Assorted Fuses IOOmA-5 amp. 40p16188 60 }W. resistors, mixed values 40p16187 30 metres stranded wire assorted colours 40p

PRICE BARGAIN!£4 wore% (Min. Value) of

Project Books, Technical, SemiconductorEquirs.-Books of Assorted Titles.OUR CLEARANCE PRICE -Order No. 580 £2t per bundle

16173 IS Rotary Potentiometers. Assorted valuesand types 40p

16186 25 Pre-sets Assorted Values and typesSALE PRICE 40p

MULTI -TURN PRE-SETSS40 3 ,< 100 K LIN ONLY 50p

AUDIO PLUG ANDSOCKET PAKS

Order No.SI 5 x 3.5mm. Plastic Jack Plugs 40pS2 5 x 2.5mm. Plastic Jack Plugs 40p53 4 x Std. Plastic Jack Plugs 50p54 2 x Stereo Jack Plugs 30pSS 5 x 5 Pin 180° Din. Plugs 50pS6 8 x 2 Pin Loudspeaker Plugs 50pS7 6 x Phono Plugs Plastic 50p58 S x 3-5mm. Chassis Sockets (Switched) 2.5p59 S x 2 Smm. Chassis Sockets (Switched) 7.5p510 4 x Metal Std. Chassis Switched Jack Sockets 50p511 2 x Stereo Jack Sockets with instruction

leaflet for H/Phone connection 50pS12 5 x 5 Pin 180° Din. Chassis Sockets 40p513 8 x 2 Pin. Din. Chassis Sockets 50pS14 6 x Single Phono Sockets 40p

TypeCD4000CD4001CD4002CD4006CD4007CD4008CD4009CD4010CD4011CD4012CD4013CD4015CD4016CD4017CD4010CD4019CD4020C D4021

Price10.1410.1610 16/0 11010.17/0 SO10 SO/0-50[Ole10.17/0.4210 $0/0 4210 SO10 85£0 45[0.65£0 65

CMOS ICsTypeCD4022CD4023C 04024C D402SCD4026CD4027C 04028C 04029CD4030C134031C04035C 04037C 04040CD4041C 04042004043C 04044CD4045

PriceCO SO10 IS10.6410.1611.85/0.4010 001011510 4611.8011.4010 7810 7810.68[0.6610 71110.7811 15

TypeC 04046CD4047CD4049C 04050CD4054CD4055CD4056CD4069CD4070C 04071CD4072CD4081CD4082CD4510C0451 IC04516004516C 04520

Price10 11510 7110 4610 46104511 6011 IS10 3210 12CO 2010 1010 1010 1011 10/1 2511 1011 10

11 10

SUPER SOUNDSAVING !C60 METROS° U ND

LOW NOISECASSETTES

Order No. S53A 10 for £2.50*

BIB GROOVE -CLEANModel 60 Chrome Finish PlasticOrder No. 829 LI 40*

%i

ttlC60 's

AUDIO LEADSOrder No.117 A.C. Mains Connecting Lead for Cassette

Recorders and Radios:Telefunken Type 45p

118 Spin Din Headphone Plug to Stereo Socket 7119119 2 x 2 pin Plug to Inline Stereo Socket for

Headphones 60p123 20ft. of Coiled Guitar Lead (1.15124 3 Pin to 3 Pin Din Plug 50p125 Audio Lead 5 Pin Plug to 5 Pin Din Plug 50p126 Audio Lead S Pin Din Plug to Tinned open

Ends 50p127 Audio Lead 5 Pin Din Plug to 4 Phono Plugs 90p129 Audio Lead 5 Pin Plug to 5 Pin Din Plug -

Mirror Image 70p130 5 Meter Lead 2 Pin Din Plug to 2 Pin Din

Inline Socket 45p132 10 Meter Lead 2 Pin Din Plug 65p

AUDIO MODULE SALEType DescriptionAL30A IOW RMS Power AMPAL60 7.5W RMS Power AMPAL80 35W RMS Power AMPAL250 125W RMS Power AMPSPM80 35V Power SupplyPSI2 20-30V Power Supply for

AL30A II PA I2 Stereo Pre -Amp for AL30A tioPAI00 Stereo Pre -Amp for AL60/AL80 LIS450 Stereo F.M. TunerMPA30 Magnetic -Ceramic Pre -AmpStereo 30 Complete Audio Chassis

7W+7W RMS

Normal Pri e Sale Price03.65 [2.95C4.35. i3-55t6.95 LS 95

05.9 [14 45t3.7 0.10'

0O

-75.4585

11.15*15 95°

112 45°[18 65°[2 SS*

16.25° [14 95*

HOT OFFERANTEX SOLDERING IRONS

ORDER No. 1931 X25 25 WATT LOW LEAKAGEUsua11y...LI-40'NOW ONLY L295

PLUS FREE Heatehunt1948 Model C 15 watt General Purpose

Ustiallyi.3.48'SALE PRICE [295PLUS FREE Heatshunt

Order No. 1939 ST3 Soldering Iron Stand suitable for either Iron 0.20

HEAVY GAUGEBLACK PLASTIC BOX

WITH ALUMINIUM LIDand FIXING SCREWS Size 6k" x 3f" x 2"

Order No. 516 ONLY 75p

LOOK & LISTEN !GE 100 NINE CHANNEL

MONO -GRAPHIC EQUALIZER MODULEThe GEI00 has nine I octave adjustments using integrated circuitactive filters. Boost and Cut limits are * 12db. Max. Voltage hand-ling 2 V RMS, T.H D., 0-05%, input impedance 100 K. Outputimpedance less than 10 K. Frequency response 20 Hz -20 KHz (3DB).

The nine gain controls are centred at 50. 100. 200. 400, 800. 1,600,3,200, 6,400 and 12.800 Hz. The suggested gain controls are 10 KLIN sliders. (Not supplied with the module). See Paks 531 and16192.

ONLY L19.50SG30 POWER SUPPLY BOARD FOR GEI0015-0-15 Volt.

ONLY E4.50SEND S.A.E. FOR TECHNICAL DATA ON ANY OFTHE AUDIO MODULES

POSTAGE AND PACKINGAdd 25p for postage and packing unless otherwise shown

Add extra for airmail. Min. order CI

FOR EEL/ABLE f10/NTSEveryday Electronics, March 1978

ANT -EX IRONS!

NEW Siren Alarm ModuleAmerican Police Screamer powered from any 12 voltsupply into 4 or 8 ohm speaker. Ideal for car burglaralarm, Freezer breakdown, and other securitypurposes.

Order No. SIS ONLY 13.50

AVDEL BONDCyanocrylate adhesive Bonds -plastic, rubber.

Transistors, Components in Seconds.Order No. 143 SSp per 2gm. phial

vJ

ORDERINGPLEASE WORD YOUR ORDERS EXACTLY '10AS PRINTED, NOT FORGETTING TO IN-

CLUDErti

OUR PART NUMBER.VAT

ADD 12}% TO PRICES MARKED*. ADD 8%TO OTHERS EXCEPTING THOSE MARKED?. !A

THESE ARE ZERO RATEDPostage & Packing add 25p. Add extra for air mail

Dept. P.W.3, P.O. Box 6, Ware, Herts.COMPONENTS SHOP: 18 BALDOCK

STREET, WARE, HERTS.50H

EDITORF. E. BENNETT

ASSISTANT EDITORB. W. TERRELL B.Sc.

PRODUCTION EDITORD. BARRINGTON

TECHNICAL SUB -EDITORT. J. JOHNSON G8MGS

ART EDITORR. PALMER

TECHNICAL ILLUSTRATORSP. A. LOATESD. J. GOODING

EDITORIAL OFFICESKings Reach Tower,Stamford Street,London SE1 9LSPhone: 01-261 6873

ADVERTISEMENT MANAGERD. W. B. TILLEARDPhone: 01-261 6676

REPRESENTATIVEP. J. MEWPhone: 01-261 6676

CLASSIFIED MANAGERC. R. BROWNPhone: 01-261 5762

MAKE-UP AND COPYDEPARTMENT

Phone 01-261 6035

ADVERTISEMENT OFFICESKings Reach Tower,Stamford Street,London SE1 9LS

Projects... Theory...

and Popular Features ...

The electronics enthusiast seldomsticks just to constructing. A naturalextension of one's hobby leads quiteeasily to the checking and repairingof sundry domestic electronic equip-ment like radios and audio systems.

Many an enthusiast has been pro-pelled along this path by requests, ifnot demands, from friends or neigh-bours. It's a fact, once you're knownto be interested in electronics, it won'tbe long before you will be approachedto help out in a variety of ways, fromfixing a plug on a smoothing iron tohaving a look at the "telly".

Having warned you, there are twochoices. Be self - depreciating, andgently but firmly close the front door.Or, let your ego be flattered by thefirst approach and decide to make aname for yourself as the friendlyneighbourhood repairman.

We don't encourage you to adoptthe latter course without carefulthought however.

Forgetting all about reputations tobe made (or lost) in your neighbour-hood, there remains the possibility ofconfining your servicing expertise to asmaller circle, within the home.

To help equip you for a start in thisdirection we offer this month theAudiotest. The purpose of this equip-ment will be evident from its title.

A never -failing source of interestand amusement is the electronic soundgenerator.

Things have moved far since thosedays when radio receivers werenotorious for the unwanted grunts,howls and shrieks they emitted as thetuning and reaction controls werejuggled with desperately in attemptsto pick up the wanted station. A wholenew art has been created, based on theemployment of electronic sound pro-ducing and processing techniques.

No self-respecting TV cops androbbers drama dares to be presentedwithout some electronic sound back-ing. And we all know how indispen-sable electronic effects are to sciencefiction films like Star Wars.

Those with only modest means canget into the act without much diffi-culty. For many intriguing sounds canbe produced with a quite simpleoscillating circuit. Apart from anamusement, this month's Weird SoundEffects Generator will be a usefulinstrument for the creative taperecording enthusiast, for with its aidnot only effects but electronic musiccan be produced.

Our April Issue will be published on Friday, March 17. See page 341 for details.

Readers' EnquiriesWe cannot undertake to answer readers' letters requesting modifications,

designs or information on commercial equipment or subjects not publishedby us. All letters requiring a personal reply should be accompanied by astamped self-addressed envelope.

Telephone enquiries should be limited to those requiring only a brief reply.We cannot undertake to engage in discussions on the telephone, technicalo otherwise.

Component SuppliesReaders should note that we do not supply electronic components for

building the projects featured In EVERYDAY ELECTRONICS, but these

requirements can be met by our advertisers.

310Everyday Electronics, March 1978

ELECTRONICSVOL. 7 NO. 7 MARCH 1978

CONSTRUCTIONAL PROJECTSTHE AUDIOTEST A workshop aid for testing radio receivers and audio gear. Also includes a m.w. I.w. tuner

by D. W. Easterling 312

C -R SUBSTITUTION BOX Gives switched selection of resistors (100-10M0) and capacitors (.3.3pfy-P1 .0 GOO.poci)320

CATCH -A -LIGHT A reaction game with a difference by R.A. Penfold 332

WEIRD SOUND EFFECTS GENERATOR Create your own "Star Wars" with rhis simple project anda tape recorder by D. Edwards 438

GENERAL FEATURESEDITORIAL 310

TEACH -IN 78 Part 6. Conductance, matching, gain by George Hylton 324FOR YOUR ENTERTAINMENT Pioneers in audio by Adrian Hope 331

EVERYDAY NEWS What's happening in the world of electronics 338SQUARE ONE Choosing a case 340SHOP TALK New products and component buying for constructional projects by Brian Terrell 342

EE CROSSWORD No. 1 by D. P. Newton 343

AIRPORT RADAR A look at ASMI the latest high speed radar by G.A.G. Brooke 344READERS' LETTERS Your news and views 347

BRIGHT IDEAS Readers' hints and tips 351

JACK PLUG AND FAMILY by Doug Baker 351

PROFESSOR ERNEST EVERSURE The Extraordinary Experiments of. by Anthony J. Bassett 352

DOWN TO EARTH Measuring low resistance by George Hylton 354

4,FREE GIFT (Mounted on front cover)

^ TOOL RACK

Back issues of EVERYDAY ELECTRONICS (June 1977 onwards-October, November, December 1977 and January 1978 NOT available)are available worldwide at a cost of 60p per copy inclusive of postageand packing. Orders and remittance should be sent to: Post SalesDepartment, IPC Magazines Ltd., Lavington House, 25 LavingtonStreet, London SE1 OPF.

Binders for Volumes 1 to 7 (state which) are available from the aboveaddresS for £2.85 inclusive of postage and packing.is IPC Magazines Limited 1978. Copyright in all drawings, photographs and articlespublished in EVERYDAY ELECTRONICS is fully protected, and reproductions orimitations in whole or in part are expressly forbidden.

All .easonable precautions are taken to ensure that the advice and data given toreaders are reliable. We cannot however guarantee it, and we cannot accept legal re-sponsibility for it. Prices quoted are those current as we go to press.

337

Everyday Electronics, March 1978 3 1 1

1")

BBCBBC 1 '

RAD LON

AUDIO TESTa

MAINS

By D. W. Easterling

THE AUDICITESTA useful piece of audio test gear for the workshop incorporating a radio tuner

THE main purpose of the unit tobe described is to provide a

loudspeaker and loudspeakeramplifier stage for testing radioreceivers, record players, taperecorders, pre -amplifiers andsimilar equipment having an out-put power not exceeding 10 watts.A simple radio tuner is incor-porated as a speech and musicsource, and to provide entertain-ment in the workshop.

Since one side of the input andoutput connections are connectedto earth, the unit is generally un-suitable for testing equipment hav-

inca transformerless mains -drivenpower supply, such as that typicallyused with domestic television sets.

FACILITIESThe facilities provided by the

unit are best explained by refer-ence to the front panel layout asshown in the photograph above. Asingle input jack socket is mountedbelow the tuning control. Loadresistors across the input may beselected by the function switch tosimulate various loudspeaker im-

pedances ranging from 3 to 16ohms.

A high input impedance is pro-vided by a further position of theswitch, and two more positionsactivate the radio tuner on eitherthe medium or long waveband.

The output jack socket is at lowimpedance and is suitable for driv-ing small loudspeakers and head-phones as well as high impedancedevices such as amplifiers and taperecorders. Provision is made tomute the built-in loudspeaker inde-pendently of the signal at the out-put socket.


COMPONENTS

ResistorsR1 4.7ki2R2 1kf2R3 6800R4 100kflR5 MIR6 47012R7 1Okf2R8 4.7kfiR9 10k12R10 3.3010WR11 8.21210WR12 4.71210WR13 10kflR14 3900R15 11012R16 1.8kflR17 4712All 1W carbon ± 5%

PotentiometersVR1 2.2Mf2 carbon log.VR2 2.2M11 carbon lin.

CapacitorsC1 22/2F 10V elect.C2 350pF Jackson type 01C3 0.1µF polyesterC4 0.022/2F polyesterC5 10,uF 25V elect.C6 0.1µF polyesterC7 22/2F 10V elect.C8 0.01/2F polyesterC9 1/2F polyesterC10 2200p F plastic or ceramicC11 180pF ceramicC12 100pF ceramicC13 0.1µF polyesterC14 470/2F 25V elect.C15 4700/2F 25V elect.C16 47pF polystyrene

See

ShoTalk

page 342

ESTIMATED COSTOF COMPONENTS

£12'00excluding

case

SemiconductorsD1, 2, 3, 4 1N914 or similar silicon type (4 off)D5 TIL209 light emitting diodeD6, 7 1N4001 or similar 1 A silicon rectifier (2 off)D8 0A91 germaniumTR1 BC108 silicon npnIC1 ZN414 radio receiver i.c.IC2 LM380 2 watt audio amplifier i.c. 14 pin d.i.l.

SwitchesS1 2 -pole 6 -way rotaryS2 single pole on/off toggleS3 d.p.s.t. mains toggle

MiscellaneousT1 mains primary/12-0-12V 100mA secondarySK1 standard jack socket with one break contactSK2 standard jack socketLS1 8 ohm elliptical (5 x 3in.) moving coil speakerPL1 standard jack plugStripboard: 0.1 inch matrix 34 strips x 39 holes; plain matrix board:0.1 inch matrix 35 x 24 holes; rubber grommets (4 off); mains cableapproximately 1 metre; screened cable; connecting wire; aluminiumfor front panel; case materials; control knob (4 off); speaker grille tosuit LS1; ferrite beads (3 off); co -ax. cable; material for probe; 14 pind.i.l. socket.

STARTHERE FOR

CONSTRUCTION

PROTOTYPE CASEThe cabinet assembly can be

seen in the photographs, and em-ploys an aluminium front panelfitted to a simple wooden framewhich becomes the sides, top andbase. Pegboard was used for thebark panel. A metal front panel isconvenient for mounting the con-trols and jack sockets, and pro-vides partial screening. Full screen-ing could not be used because ofthe built-in ferrite -rod aerial, there-fore a metal box is ruled out.Plastic types are suitable if avail-able in this size.

FRONT PANELThe first step is to mark out and

drill the front panel. The loud-speaker averture is best traced outusing the loudspeaker chassis as aguide, and then redrawing it toallow a 6mm margin for the mount-ing flange. The panel size is400 x 150mm.

The method of cutting out theaperture depends on the toolsavailable. Usually the simplest wayis to drill a series of holes just in-side the cut-out mark, and linkthem with cuts made by a smallcold chisel. The edge may then becleaned to the line with a half -round file. A metalised plastic grillwas used to cover the aperture,and this is secured to the frontpanel by four pillars slotting intopush -fit holes. Again, the com-ponent was used as a template toarrive at the correct hole location.

In the prototype the frame wasmade from 80 x 13mm planedtimber with the joints mitred,glued and pinned, but the frontpanel is fixed with six small woodscrews and glued before the jointsbecome firm. Once the glue is dry,pin -holes, grain and cracks can befilled, and the whole assemblyrubbed down ready for painting.


AUDIO TEST

-AFEr#FE--'F'E-

;;.

oot

The completed Audiotest showing front panel control layout

The front panel was coveredwith Contact self-adhesive vinylwhich forms an ideal backgroundfor dry -transfer lettering. Forpermanency, the lettering shouldbe coated with a clear varnish; nailvarnish was found to be ideal.

MAIN COMPONENT BOARDMost of the components are

mounted on a piece of 0.1 inchmatrix stripboard size 34 strips x

HOW IT WORKS

RADIOTUNER

DUMMYSPEAKERLOADS

39 holes. The layout of the compo-nents on the topside of the boardis shown in Fig. 3 together with theunderside of the board showing thebreaks to be made along thecopper tracks. begin by makingthe breaks and then positioningand soldering in place the Vero -pins, link wires, resistors andcapacitors. In the prototype,sockets were used for mountingthe i.c. and the transistors, but thisis not essential although the i.c.

BUILT INLOUDSPEAKER

The Audio Test consists of two completely separate units. The first is asimple radio tuner, and the second an audio amplifier. An i.c., the ZN414, isused as the radio together with coils for both medium and long wave bands.This section provides an audio source for testing amplifiers, as well asproviding music for the workshop.

The amplifier is slightly unusual in that it has fixed resistors which areselected by a switch to replace the loudspeaker in the external equipment.Using this arrangement tests on an amplifier can be made, taking theoutput direct from the loudspeaker.

Other facilities exist that make the Audio Test most valuable in thetesting of radios and amplifiers.

socket is recommended. Whensoldering the transistors anddiodes, be sure to use a heatshunton the legs being soldered so asto eliminate possible damage fromthe hot soldering iron. The connec-tion between the board and othercomponents are made at the boardend to Veropins thus enabling theflying leads to be attached to theboard after it has been fitted inthe case.

FERRITE AERIAL WINDINGSThe ferrite rod aerial used in the

prototype unit used a rod withdimensions 100 x 10mm. Themedium wave winding, L2 is about20mm long and consists of a singlelayer of 30 turns of 42 s.w.g cottoncovered wire. The longwave coil,Ll is made with the same wire buthas 200 turns pile wound overabout 10mm. The aerial is securedto the main component board bynylon thread, rubber grommetsbeing slipped over the core to givesupport, see Fig. 3. Connection ofthe coils to the equipment boardare made to Veropins.

POWER SUPPLY BOARDThe power supply is constructed

on a small piece of plain matrixboard size 0.1 inch pitch 35 x 24holes. The layout of the com-ponents on this board is shown inFig. 4 together with interconnec-tion details on the underside. Thetransformer is bolted to the boardusing 4BA nuts, bolts and shake -proof washers, and when thisassembly is complete is fitted tothe case frame using three woodscrews with rubber grommets be-tween board and frame, Fig. 2.

The transformer used in theprototype had short stiff leadsemerging from the underside form-ing the secondary winding. This isnot essential as other types withflying leads can be employed andthe leads fed to the appropriateconnection points.

WIRING UP AND CHECKINGCare should be taken when con-

necting the mains cable to S3 andthe transformer to ensure thatthere is no possibility of it shortingto the low voltage part of thecircuit. A cleat fitted inside thecabinet will prevent strain on theelectrical connections.


3 T Tc

THE AUDIOTEST

ICI TR1 ANCOE

VIEWED FROM UNDERSIDE

240V AC MAINS

D5

FLAT

ATHODE A

B

0

FGH

JK

MN

C

S

r

U

P

XV

AABBCC00

EEFFG6HH

HHGG

FFEE00CCBBAA

X

V

S

P

U

KJ

H

F

N

CBA

C2

00 0 0000 0

D2

R2

CR3

2

DI

0 0 0 0 0 aOO 0 0 0 0 0 0 0 0FERRITE BEAD

SK2

FERRITEBEADS

LINKWIRE

LINKWIRE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2 22 23 u 25 26 27 2829 30 31 32 33 34

VR1

SKI

0 0000

R6 0

0 0 0 0 0 0 0 0 0 0 0000000000000000000000 00 00 00 00 00 00 00 0 ii: 00 0C lit 0C ei 0C 0O

irz0

0 c 1 U i, 000O 0O 00 00 00 00 0 0 00 00 00 0O 0O 00 0O 00 00 0O 0O 0000 0000000 0000 000000 0000000 00

Fig. 2. Front Panel assembly, viewed from rear. Fig. 3. Main component board layout, underside detail showing breaks incopper strips and interwiring to controls.

Everyday Electronics. March 1978 315

TST

11:.1

10014,

0

FE RAME'f ACK

=RM

02.14,P"

CIRCUITDESCRIPTION

TR1

12,

OEM

41,s#4.'

Rneo

2

33

0

0510

1C2

R1.12,2 70/..2

CIO C11 62200 'WY'

=ND

0C31,

'CAW" 0,

ti'AD 7,)'o lir:

t3aao-

1,;11,17

4,

Fig.1. Complete circuit diagram, including power supply, for the Audiotest.

The complete circuit diagram isshown in Fig. 1.

Integrated circuit IC1 is thewell known Ferranti ZN414 t.r.f.receiver i.c. and is tuned by thevariable capacitor C2 and theferrite -rod aerial winding Ll or L2,depending on the wavebandselected by switch Sla.

Resistor R5 across the tuned cir-cuit widens the bandwidth slightly,thus improving the sound quality.In difficult reception areas whensharper selectivity is required, itmay be omitted.

Network RI, R3, DI, D2 and de-coupling capacitor Cl reduce thesupply to ICI to the required 1.2volts, using the forward -voltagethreshold of the diodes to provideregulation.

The output from IC1 is fed tothe base of TR1, a voltage ampli-fier, r.f. being removed by theinclusion of C6 and the inductanceof the ferrite beads. This amplifierstage is connected in the common -emitter mode with decoupling net-works R7, C5 and R6, C7 establish-ing the correct d.c. working con-ditions.

Output from the collector ispassed via R9, filter C8, and ascreened lead, to the input jack

socket. SKI. This arrangementautomatically disconnects the radiotuner whenever the jack -plug is in-serted.

DUMMY LEADSSwitch Sib applies the appro-

priate load resistance across theinput when the amplifier circuit isbeing used to simulate a loud-speaker. This avoids having to usea specially wound transformer andfacilitates control of the signallevel to the built-in loudspeaker.The main purpose of R9 is toattenuate the radio signal when-ever the tuner is not selected.

The input via SK1 is fed viad.c. blocking capacitor C10 to thevolume -control and tone -controlnetwork VR1, VR2, C10. The tonecontrol acts as a simple top -cutfilter. The signal is then fed to theinput of the power amplifier inte-grated circuit IC2, which has afixed gain of 50.

The network C11, R13, C12 pro-vides further r.f. and noise filter-ing, and diodes D3 and D4 protectIC2 against input overload.Attempts to over -drive the ampli-fier will produce distortion butshould not damage the i.c. whichhas thermal protection.

The output of IC2 is coupled tothe output jack, SK2, by C14, andvia S2 to the built-in loudspeaker.

S

(1)

Network C13, R14 and a ferritebead on the output line discourageparasitic oscillation which mayoccur when long external outputleads are used. The output voltageand power delivered by IC2 willdepend on the external load. Al-though the circuit will operateover a wide range of load resist-ance 4 ohms should be considereda minimum. Care should be taken,however, to ensure that no poten-tial exceeding ±5 volts is fed fromthe load back to the amplifier. Ifnecessary, an external non -electro-lytic capacitor of appropriate work-ing voltage and capacitance shouldbe used.

POWER SUPPLYThe power supply circuit is a

conventional double -diode full -wave rectifier driven by a stepdown mains transformer having acentre -tapped secondary winding.Smoothing is by electrolyticcapacitor C15. The 4700 microfaradquoted is rather high and holds theunit operational for a second or soafter it has been switched off. Itcould be reduced to 1000 micro-farads with a slight increase in humlevel. The light -emitting diode D5and current limiting resistor R16are included to indicate that poweris being applied to the unit.


THE AUDIOTEST-POWER SUPPLY

DSANODE

PIN DON MAINBOARD

53

OOOO

tiJ D7

0* -1111:1 40 D6

0 00O

C15

OO0

OVO 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 000

PIN E CNMAINBOARD

R16

® VEROPINS

Fig.4. Power supply board component layout and underside wiring details.

THE AUDIOTEST PROBE

PL1

C16 Op,'

1-11PROBE TIP

R17 47k12

Dd0A91

PUr

/1//////./././././././././4a

Fig.5. Circuit diagram and construc-tional details for the probe.


Close-up of the circuit board showing the mounting of the ferrite aerial.

With the front panel drilled toaccept all the panel -mounted com-ponents these and the powersupply board should be secured inplace and interwiring carried outas in Fig. 2 and Fig. 4, with leadsthat are to connect to the board, ofsufficient length to reach the latterwhen fitted in the case.

In the prototype, the board washeld in position by means of p.c.b.slots that were taken from an oldcomputer panel, but an alternativewould be to use suitable hardwoodmouldings obtainable from manytimber merchants and d.i.y. stores.

It is a good idea to check thepower supply and l.e.d. indicatorbefore making connections to thecircuit board. The smoothed outputvoltage should be in the order of18 volts, and the l.e.d. should be atsatisfactory brilliance.

If all is well, the main componentboard should be fitted in place andwired up according to Fig. 3.

The audio amplifier circuit canbe checked by connecting, via SK1,a suitable test oscillator, or thephones output socket of a transis-tor radio, for example. The in-builtradio should operate at the appro-priate switch position. Unwantedoscillation may be cured by revers-ing the connections to the tuningcoils, Ll and L2.

SIGNAL TRACING WITHTHE UNIT

The unit is an ideal signal tracerespecially when used with theprobe shown in Fig. 5. The probe

enables the r.f. and i.f. stages of aradio tuner to be checked (long,medium and short waves-notf.m.) as well as being suitable forlow level audio stages. The smallcoupling capacitor restricts the lowfrequencies and makes the soundappear rather "toppy" but has theadvantage of reducing mains humwhich could be overwhelmingunder certain test conditions. Thefinal assessment of a.f. signalsshould be made with the test pointconnected to the audio -test by ascreened lead and not via theprobe.

PROBE CONSTRUCTIONThe probe consists of a small

coupling capacitor, diode detectorand filter resistor directly wired tothe end of a length of screenedaerial feeder. The whole assemblybeing subsequently slipped into alength of plastic tubing with a wiretip brought out through the endcap. The tube should be longenough to accommodate about twoinches of screened cable in orderto give it a place where it can beheld without introducing straypick-up. A separate earth returnwire is also required. Constructiondetails will be found in Fig. 5.

Signal tracing is simply a matterof working progressively along thesignal path of the system undertest from aerial, gramophone pick-up or recorder head to loud-speaker, with the object of locating

the point where the normal signalis no longer detected and hencethe faulty stage. A similar methodcan be used to trace distortion.

VOLTAGE CHECKSBefore attempting to signal trace

it is wise to make voltage checksto ensure that power supplies arenormal and metallic surfaces areeither fully isolated or at earthpotential. It is also a good idea atthis time to visually inspect thesystem for signs of overheating orother damage. Finally, beforeusing the probe, the earth returnlead should be connected to thecommon or earthy line of the sys-tem under test-NEVER TO THEMAINS LIVE OR NEUTRALLINES.

SUBSTITUTIONOnce the faulty part of a sound

system has been detected a furthercheck can be made by substitutingthe Audiotest. The unit tuned to asuitable radio programme can beused as a signal source in place ofthe original tuner, record deck orrecorder. The amplifier and loud-speaker can be similarly sub-stituted.

Suppose that a faulty loudspeakeris suspected. replace it with theAudiotest and switch in the appro-priate load resistor which shouldbe a similar value to the impe-dance specified for the system loud-speaker. If distortion is stillpresent try a higher load resistor,and if this is beneficial suspect theloudspeaker coupling capacitor.

Tablet: Approximate signal levelsfor various sources.

Source Output

Probe at r.f. stageProbe at i.f. stage -1

Magnetic pick-upRecorder tape head)Crystal microphoneCeramic pick-upCrystal pick-upRadio tunerPreamp tone control8 ohm loudspeakerat 0.5 watt

1mV

5mV

50mV100m V

500mV

2V


Table 2: Audiotest input sensitivity and output voltage levels using theloudspeaker as the measuring device.

Sound level(Audiotest L/S)

Input sensitivity Signal source(VR1 at maximum) output voltage

Just discernableSlightly louder

ComfortableLoud

lmV5mV50mV

reduce VR1 as necessary

less than 13mV50mV500mV

2V

Sometimes it is difficult toidentify the connections to a DINplug or socket. Standards are speci-fied but not always adopted by theequipment manufacturer. In anycase there may be a fault con-

dition or some confusion over theswitching arrangements. The con-nections can be quickly tracedusing the signal tracing andsignal source facilities of theAudiotest.

Signal tracing can sometimes beapplied to diagnose the cause ofa low stage gain. A disconnectedor unserviceable by-pass capacitormay introduce unwanted negativefeedback. Decoupling circuitsshould be checked to ensure thatno signal is detected at their junc-tions.

VOLUMEAt all times the volume control

of the Audiotest should be set toprovide adequate signal level with-out overloading either the test setor the equipment under test.Tables 1 and 2 give some idea ofthe signal levels to be expected. TI

\SOLO 0131\EN NOV- DEC -JAN

flew year ResolutionHAVE YOU KEPT YOURS ?...

if not -fill in thecoupon below

without delay!To

Please

NAME

(Name of Newsagent) ADDRESSreserve/deliver the APRIL issue of EVERY-

DAY ELECTRONICS (40p) ON SALE MARCH 17th,

and continue every month until further notice.


JI T

OBSIIIIIIION BOXINTRODUCTION

WHEN one is starting to equipa workshop with test equip-

ment one of the first items to bepurchased is a multimeter. Thenext likely item could either by ana.f. generator or power supply.

If all this expensive equipmentis bought it is so easy to forget theusefullness of really simple equip-ment. One such item is the resist-ance/capacitance substitution boxto be described here.

A substitution box is basically aselection of different standardvalue resistors and capacitorswhich can be connected into anexternal circuit and be used toreplace that component of itsentire function.

Typical examples are;1. Finding the value of a damaged

component in a circuit.2. Finding the correct bias re-

sistor for a transistor.3. By using both the resistors and

capacitors, the required periodin a timing circuit may befound.

There are other uses of coursebut the three mentioned are themost popular. It can of course bevery valuable to the professionalcircuit designer.

By P. C. O'Neil

STARTHERE FOR

CONSTRUCTION

RATINGSIn such a design as this the

voltage ratings and tolerances ofthe components become a slightproblem. On one hand hightolerance and high voltage com-ponents could be selected, thusincreasing the finished size andmore important increasing the cost.On the other hand one can selectstandard components thus produc-ing an inexpensive design and cut-ting down on the size.

It is the last method which isadopted in this design. Standardcapacitors with a reasonablevoltage rating have been used, andresistors w i th th e maximumtolerance required in most circuits.After all it is no good using 1 per-cent resistors when experimentingwith a circuit and then using stan-dard 10 percent types in the final


Tid SUBSTITUTION BOX

ResistorsR1 10f2R2 470R3 100f2R4 220f2R5 560f2R6 1kf2

R7 2.2kflR8 3.3kflR9 5.6kflR10 8.2kilR11 1042R12 15kfl

All resistors are IW carbon ±5%

Capacitors

R13 2242R14 39kflR15 56k12R16 100kf2R17 220f2kR18 560kfl

R19 82042R20 1MflR21 3.3MilR22 10Mf2

C1 3.3pF silver mica C9 22nF polyester C17 33/4F 40V elect,C2 10pF silver mica C10 47nF polyester C18 47/4F 40V elect.C3 100pF silver mica C11 100nF polyester C19 100/4F 40V elect.C4 470pF polystyrene C12 1/4F 35V tait. C20 220/4F 40V elect.C5 1nF polystyrene C13 2.2pF 35V tant. C21 470/4F 25V elect.C6 2.2nF polystyrene C14 4.7F 35V tant. C22 100C/IF 16V elect.C7 4.7nF polystyrene C15 10/4F 63V elect.C8 10nF polyester C16 22/4F 63V elect.

MiscellaneousS1, 2, 3, 4 1 -pole 12 -way rotary wafer switch (4 off)S5, 6 2 -pole 2 -way minature slide switch (2 off)Aluminimum case, 105mm x 105mm x4Omm or similar; four crocodileclips (3 off black, 1 off red); white card for front panel; four large roundknobs; two grommets; connecting wire; solder.

GUIDANCE.

: O,Leh ESTIMATED COSTOF COMPONENTS

£3.75excluding

case

See

ShoTalk

Pay( 342

TO CROCODILECLIPS

TO CROCODILECLIPS

Fig. 2. Component layout and wiring for the C -R Substitution Box. Note that the components have notbeen drawn to size to make wiring easier to follow.


iokn

13-2k 11

5.6140

3-3 kR

2.2k /LO

HI

10M11

3.3 Mn

IMO

820103./

550k11

OFF

10011

2200

\ 5600

1k11

OFF AAA

/151(1122kR

39k0

56k0

\ moo

220kR

OFF

100nF

47nF

22nF

*-11-3.1pF

/10pF

100pF

lOnF 470p1

11.7nF/

1000pF

470pF

220pF

1nF

2.2nF

OFF

1pF

HI

2.2pF

4.7pF

47pF / 22pF

33pF

Fig. 3 Front panel layout shown full size.

design. The results will not be thesame!

Follow the diagram in Fig. 2and wire up the resistors first thenthe capacitors next. Be sure toidentify the correct positions of

each, mistakes at this stage couldbe confusing when the unit is used.The remaining wires can then besoldered, leaving a generous lengthfor connecting to the crocodileclips.

CIRCUITDESCRIPTION

The complete circuit is shown inFig. 1. It may be considered as twoseparate sections; resistors andcapacitors. Each section consistsof two 1 -pole 12 -way switches, eachwith one spare position which isused as the OFF position.

Each rotary switch may beselected by a single pole slideswitch, which selects either HIGHor LOW values as required. Theseranges are set out in Table 1.

The output from either switchis applied via crocodile clips to theexternal circuit. It is important tonote that only one value com-ponent is selected at any one time,unlike some boxes which are called"decade" boxes.

When using the capacitor sectionthe polarity of the electrolyticcapacitors should be observed.With this particular unit both theresistors and the capacitors can beselected at the same time.

Once wiring has been completedthe front panel may be traced orredrawn on stiff white card usingthe layout in Fig. 3. The generalarrangement can be seen from thephotographs.

C R SUBSTITUTION BOX

The author's completed unit being used on a circuit board. On resistanceranges the decimal point has been represented by the multiplier, i.e. 8k262equals 8.21<12 and 3M3i2 equals 3.3MS2.


RI R2 R3 R5100 470 10011

24

200 5600R6 R7 R8 R9 )210 R11

ikn.OFF

2.2k0. 3.3k0 5-6k0 8.2k0 10kfl

o 51

0

R12 R13I5k0 22k0

0

S2

0

55

R14 R15 R16 R1739kfl 56k0 100kC1 220kfl

0

818 I R19ssokia 20k0

820Mn1l

R213.3Mf.

Fig. 1. Circuit diagram of the C -R Substitution Box

I Ohl

R22OMfl

Back of front panel showing wiring to range switches. Due tothe size of some of the components it may be necessary to tiltthem at an angle to fit into the area of the case.

High 15ki2 to 101\10

ICAPA:ITC850

_Lill! 11111c,3.3pF 10pF

Ci

100pF

MEM

470p) -C5 1061nF

0S3

OFF

O

C12

1FF

C13 C14 C15

2.21F 4.71F 10pF

O7TCS C10 C11

221-1F 47nF 100nF

56

2;26p17FC18 C19 C20 C21 C22

47pF 100pF 2201jF 470p 1000pF

TTTTTTTTTTT

Table. 1. R/C Ranges

Resistors Capacitors

Low 100 to 10k .I2 3.3pF to 100nF

tuF to 1000<uF

Ratings; Resistance -1W 5%

Capacitance -35 to 250V

OPERATIONThe major uses have already

been mentioned no doubt otheruses could be found in time. Twoimportant points should be noted.First the power rating, half a watt,of the resistors should not beexceeded. Secondly the voltagerating of the capacitors must beobserved. In fact these two pointsshould not cause any worry, sincemost of the circuits will probablybe low voltage anyway.

Using the substitution box is veryeasy, just connect the clips intothe circuit at the required pointand set the switches to the re-quired value, switch on the circuitand you're away!


1.11C111-11178BY

GEORGEHYLTON

AMPLIFIERS

Part

CONDUCTANCE - GAIN

AT THE end of last month the amplifier was intro-duced. So far in talking about amplifiers only

input impedance has been discussed. This as you havefound is most important when using amplifiers, somuch so that the matching of one stage to another isvery important.

These two qualities are of course very useful, butmean nothing if the gain is not known.

This then is our subject for this month, to measurethe gain of a practical amplifier which can be builtusing various modules.

MATCHINGWhen people talk of matching a signal source to

an amplifier they usually mean making the inputimpedance of the amplifier high enough, so that suffi-cient signal voltage is actually delivered to the ampli-fier by the source. Practical transistor amplifiers ofthe usual kind in which the emitter is earthed haveinput impedances from a few kilohms to a fewhundred kilohms.

An alternative transistor connection in which theemitter is not earthed directly can have an input

impedance of several megohms, but no voltage gain.A field effect transistor, is one example.

Signal sources with very low internal impedancescan be something of a problem, not because they arehard to match but because they usually deliver verysmall voltages. Transistors themselves generate smallvoltages in the form of electrical noise and these tendto interfere with small signal voltages. A typicalexample would be a microphone with an internalimpedance of 30 ohms.

This might deliver only a few tens of microvolts.Fortunately the transformer comes to the rescuehere. Remember that the voltage on the secondarycan be different from the voltage on the primary. Thetwo voltages are in fact in proportion to the numberof turns. If the secondary has ten times the turns ofthe primary then the secondary voltage is ten timesas great.

So 10/N from the microphone can in principle beincreased to 10.3IN at the secondary Fig. 6.1. Thereis no need to stop at tenfold increase. In principle,any turns ratio can be used and therefore any voltageincrease obtained.

Looking back from the amplifier into the secondary,the impedance seen is not 30f1 of the microphone butsomething much greater. It is in fact 3011 times theturns ratio times the turns ratio again. So if a turns


AMPUFIERI INPUT IMPEDANCE

Fig. 6.1. Using a transfo mer, the output of a microphone,represented here by the generator symbol and internalresistance R., may be increased to a level dependent on theturns ratio of the transformer.

ratio of 10 is used to increase the signal voltage tentimes the impedance is increased to 30 X 10 x 10=3000f2. If the turns ratio were 100 the impedancewould be increased 100 X 100=10,000 times.

It is this increased impedance which must be smallcompared to R.. So there is a limit to the amount ofstep-up of voltage which can be used.

Similar considerations apply to the output end ofan amplifier. There is a limit to the current and volt-age which any particular amplifier can deliver. Thisdetermines the lowest load resistance which cansafely be connected to the output. We shall look intothis in more detail when we consider power ampli-fiers, but it is worth a quick look now.

VOLTAGE AMPLIFIERSA common arrangement in voltage amplifiers Fig.

6.2a is to connect a d.c. load resistance R1 throughwhich the collector current passes. This currentincreases or decreases in sympathy with the signalsbeing amplified. These variations in current producea varying voltage drop in RI. In a.c. amplifiers suchas audio amplifiers these voltage variations are theoutput signal. They are often separated from thesteady d.c. voltage at the collector by a couplingcapacitor C which allows a.c. signals to pass butblocks d.c.

The capacitor transfers the output voltage to somekind of d.c. load impedance, R2.

If R2 is zero, there can be no voltage across it. Ifit is infinite, the voltage at the upper end must be thesame as the voltage at the collector when there is noR2 there. But what about real values of R2, in

Ic

RI

TOREMAINDEROF CIRCUIT

AC OUTPUT

R2 R2

between zero and infinity? To tackle this problem youneed the right tools. In this case the tools are twoideas.

The first idea is about mixtures of d.c. and a.c. Anamplifier, as we mentioned before, works by usingthe power of its battery to make enlarged copies ofits input signal. The battery supplies d.c. The inputsignal is usually a.c. It is inevitable that d.c. and a.c.get mixed up in the process of amplification. But youare at the moment not interested in the d.c., but onlyin the a.c.

IDEASThe idea you need to tackle this problem is that

when mixed a.c. and d.c. flow in circuits which containonly resistance, inductance and capacitance the a.c.and d.c. have no effect on one another.

So you can forget about the d.c. and think only ofthe a.c.

The second tool is the knowledge that in a.c., whichis periodically reversing its direction, the forwardflowing bits contain the same average energy as thereverse flowing bits. What is true of the forwardflowing bits, called positive half cycles, is equally trueof the backward flowing negative half cycles, exceptthat the direction of flow and polarity of voltage isreversed.

For the purpose of thinking about a.c. circuits youcan concentrate on one half cycle, when the voltages,are positive, and think about the circuit in terms ofd.c. polarities instead of reversing polarities. In effectyou take a short exposure snapshot of the a.c., freez-ing its motion.

This may sound rather difficult, but you will see itis really quite easy when we apply these ideas to ourcircuit. We shall consider the transistor as a source ofa.c. which flows out of the collector, through thecircuit and back to the emitter. The directions ofcurrent and polarities of voltage are then as markedin the diagram

The current being a.c. can in effect pass through Cso one path is via C and R2 and so back to the emitter.The other is through RI. If it goes through RI it muststill somehow find its way back to the emitter.

There is, in fact, an easy path via a component notshown on the diagram, the battery. Of course, thecurrent has to flow backwards through the battery,but remember we are talking about a.c., not d.c. Thebattery offers such a low impedance to the a.c. signals

R2

Fig. 6.2. The output from an amplifier is developed across resistor R1 as in (a). Because the battery has a low impedance to a.c.the two resistors can effectively be connected together as in (b). Now as C is effectively a short to a.c. the circuit can be simplifiedas shown in (c). We now have the situation where R1 and R2 are in parallel at a.c. and the conductances add together.


that for practical purposes it is a short circuit, asshown by the dotted line. This connects the top of R1to the bottom of R2.

So as far as a.c. signals concerned we can redrawthe circuit as in Fig. 6.2b, but even this can besimplified. If C has a very low a.c. impedance it toocan be effectively a short circuit to a.c. So we end upwith Fig. 6.2c.

This last circuit is a great help to understanding.The transistor can be regarded as a device whichpumps out a current. It is clear that this current mustdivide between RI and R2. If R2 is small, most of thecurrent goes through it, but the voltage across it islow. If R2 is large, most of the current goes throughR1 and the voltage is something close to what it wouldbe if R2 were not there at all.

If we are stuck with particular values of R1 andR2, and we need to know the output voltage we haveto know something about combinations of resistancesconnected together at both ends, like these. Theseare called parallel connections, for the obvious reasonthat the current divides and follows parallel paths,one through RI and the other through R2.

Earlier in the series it became clear from measure-ments in your RESISTOR CHAIN that resistances in series

PARALLEL RESISTANCEIt turns out that the conductance of a resistance is

just 1 divided by the resistance itself. The usualsymbol for conductance is G. We can not use Cbecause it is already used for capacitance. So inmathematical notation:

G=l+RIf we have two resistances, RI and R2,

G=1±.R1+1+R2In fact you can have as many resistances in parallel

as you like and add up all their conductances. Toturn the total value of conductance back into its cor-responding resistance you divide 1 by the con-ductance. That is:

R=1÷GTo take a simple example; what is the effective

resistance of 10, 100 and 1000 ohms, all in parallel?The corresponding conductances are 0.1, 0.01, and0.001 which add up to 0.111. If this is now dividedinto 1 it comes to 9, as near as makes no difference.So the net resistance is 9 ohms. Practical resistancevalues are often less easy to turn into conductances.If you have a calculator it is simple. If not you canuse a parallel resistance chart.

0.75(2

Fig. 6.3a. Two 20 resistors are in parallel, their total resistance is 1 ohm.Different values as in (b) can be thought of four resistors of the same valuein parallel (c) giving the result as 0.750.

just add together. This can not be true of resistancesin parallel because by allowing more paths for thecurrent to follow it can flow more easily. So the effectof connecting a resistance in parallel with anothermust be to reduce the resistance of the circuit as awhole.

CONDUCTANCEFortunately, though resistances in parallel do not

add together, something else does. You can think ofa resistance as something which conducts a current.A low resistance conducts current more readily thana high resistance.

If two resistances are connected in parallel thenthe conducting ability of the second is added to theconducting ability of the first.

The standard name for this conducting ability isconductance. So if you know the conductances of RIand R2 you can add them to obtain the overall con-ductance of the combination. That is you can get asingle quantity which takes both R1 and R2 intoaccount. If this can be turned back into an equivalentresistance you are home and dry.

You can replace RI and R2 by a single resistance RFig. 6.2d, and Ohm's Law will then tell you whatvoltage is dropped across it by the current from thetransistor. This must be the output voltage, since itappears at the right place in the circuit.

33n

311

3 X won

7I1

2 X 100n

Fig. 6.4. Using an extension of the idea that odd values canbe broken down into resistors of equal value. Here 3352and 470 resistors are connected in parallel. Each can beconsidered as so many 1000 resistors. Using the methodas found in Fig. 6.3c the answer is simply 100 = 5.

EQUAL RESISTORSFortunately it is often possible to find the equiva-

lent of two resistances in parallel by a simple trick.The basis of it is the fact that if equal resistancesare connected in parallel their equivalent resistanceis very easy to find. Take a look at Fig. 6.3a.

Here a 2V cell is connected to two resistances eachof 212. The voltage across each resistance is 2V, sothe current in each resistance is 2V+211=1A. Thetotal is therefore 2A. If you apply 2V to a singleresistance and 2A flows then Ohm's Law, which saysthat; Resistance= Voltage/Current, shows that theresistance must be la

Evidently, since the voltage and current is the samein each case, your two 212 resistors in parallel areequivalent to one HI resistor.

In the same way you can show that three equalresistances in parailel are the same as a singleresistance of one-third the value of one of theindividual resistances. Similarly four in parallel areone quarter, five one -fifth, and so on. So ten 10011 inparallel are the same as one 10012 divided by ten, i.e.1012.

To use this to work out resistances in parallel, youmentally turn the lower resistances into parallel com-binations of the higher ones. What is the equivalent


of ill and 311 resistors in parallel? Fig. 6.3b. The111 resistor can be thought of as three 311 resistors inparallel. The original 311 resistor thus makes up thefourth. So the circuit is equivalent to four 311 inparallel, which is 311±4 or 0.7511.

This trick cannot always be used, because some-- times the resistance comes in odd values which donot lend themselves to simple arithmetic. But it worksin most cases, especially if an approximate answer isacceptable.

For example, what is the equivalent of a 3311resistor and a 47.0 resistor in parallel? Fig. 6.4. Thesevalues look very awkward. But 4711 is not far from5011, and 5011 is the equivalent of two parallel 10011resistors. Also, 3311 is very nearly the equivalent ofthree parallel 10011. So altogether the circuit is theequivalent of five parallel 10011 which is of course thesame as one 2011 resistor.

PRACTICAL AMPLIFIERSLet us get back to amplifiers. Consider the simple

practical amplifier circuit of Fig. 6.5. This is for a.c.voltages and uses one transistor, two capacitors, andtwo resistors. The capacitors are to block d.c. while

INPUT

R2tkil

BI W I

minRI 200k11

Ar116.r4C1700nF OUTPUT

TR1BC108

C2 lOpf

Fig. 6.5. A simple and practical amplifier circuit. This maybe built using either of the NPN modules and the additionalcomponents shown.

allowing a.c. to pass. The transistor is turned on by asmall current through the base, which flows via R1,often called the base bias resistor. It might seem moresensible to connect the top end of R1 to batterypositive rather than to the collector.

However, the connection shown here has an advan-tage. The more base current flows, the more collectorcurrent flows. But the collector current flows throughR2 and as it does so it produces a voltage drop.Because of this the collector voltage is less than 9V.So the voltage available for driving base currentthrough R1 is less than 9V.

If for some reason the collector current increasesthe collector voltage falls, and this further reducesthe voltage available to provide base bias current.The collector current therefore tends to stay put atsomething near its original value. The arrangement isin fact a form of negative feedback and tends tostabilise the collector current.

This is useful in counteracting the effects of tem-perature changes. More important, it helps thedesigner to get more consistent performance fromtransistors with different current amplifications.

At present, it is not possible to control the currentamplification closely when transistors are manu-

factured. This is why your BC108 can have a currentamplification of about 100 to 800. Some manufac-turers restrict the "spread" of current amplification tosome lesser range such as 120 to 450, and some dividetheir BC108 into three groups identified by letters A,B, and C. A usually signifies a current amplification of125 to 260; B, 240 to 500; and C, 450 to 900.

The designer must take this variation into account.The bias system used here is a crude but useful wayof reducing its effects.

A useful "rule of thumb" is to make RI greaterthan R2 by the current amplification. For an amplifi-cation factor of 200, if R2 is 11(11 then R1 should be200ki2. This has the effect of making the collectorvoltage set itself to about half the battery voltage. SoR2 drops about half the battery voltage. It followsthat the collector current must be half the batteryvoltage divided by R2. With our values this means acollector current of 4.5mA.

CALCULATIONSIn engineeering shorthand the battery voltage is

written as; V,, the collector current as 1, and thecurrent amplification as hFE. So in shorthand form therule of thumb can be written;

R2= hFEXR11o= Vcr.± 2R2

CALCULATING RINow build up the circuit of Fig. 6.5 using either

of the two NPN modules. At this stage it does notmatter which. Make temporary connections to RI.Short lengths of connecting wire with crocodile clipsattached at each end will do.

For voltage amplification a collector current ofabout half a milliamo is suitable. The required valuefor R2 is then given by: 4.5V+0. 5= 91(11. The nearestprefered value is 10k11.

If the value of hFE is 100 to 800, then R1 must bein the range 1M1-1 to 8M11. A reasonable compromiseis 3.3M11, but with the aid of your VOLTAGE INDICATORand RESISTANCE SUBSTITUTION BOX you can find a valuewhich fixes VcE as near as 4.5V as possible.

At the same time this enables you to deduce thehFE value which is roughly R1±R2, if VcE isV±2. This is unlikely to give quite the same valueas you measured in Part Four, but should be reason-ably close.

Now repeat the experiment with the other NPNmodule. You will probably find that entirely differentvalues are required for each module. In fact we foundthat we had to use a value for RI of Mil for oneof the modules, and 3.3M11 for the other, in order toget VcE near to 4.5V.

These values for RI imply that the hFE in each caseis 100 and 300 respectively.

If you can not get close to 4.5V because oneresistance in your RESISTANCE BOX is too high and theother too low, then use the lower resistance. Oncethe values for both modules have been found solderactual resistors in place on top of the modules.

A single stage amplifier like this will give a voltagegain of about 200. Quite a high gain but not enoughfor our purposes. What we are going to do is experi-ment with using loudspeakers and earphones inreverse, as microphones.


LOUDSPEAKER DESIGNA loudspeaker such as your 8051 one is an electro-

magnetic device Fig. 6.6. A coil of very fine wire isattached to the back of the cone or diaphragm. Thiscoil sits in the field of a powerful magnet. When acurrent is passed through the coil it sets up a mag-netic field in the usual way. This field either assistsor opposes the field of the permanent magnet,depending on which way the current is flowing.

Fig. 6.6. Internal constructionaldetails of a typical loudspeaker.Essentially a powerful magnetand a coil is used to reproduceelectrical currents as soundwaves.

The result is that the cone is pushed outwards forone direction of current and pulled inwards for theother. If the current is audio frequency a.c. the conevibrates in sympathy with the current, and the result-ing pressure on the air in contact with the cone isthe beginning of a sound wave.

The device can also be worked backwards. That is,when a sound wave makes the cone vibrate, a voltageis induced in the coil.

If the loudspeaker is used as a microphone some-thing else must be used to listen with. A crystal ear-phone could be used here.

If you connect the loudspeaker to the input and anearphone to the output of your single stage amplifieryou should be able to hear something if a loud noiseis made near the microphone e.g. by tapping it.

C1

R1 3.3Mfl

L S 1'800

R210kfl

R4town

81 9V I

TL1

CRYSTALEARPHONE

Fig. 6.7. In this circuit an extra stage is added to give moregain. This can be constructed using the two NPN modules.The resistors in the previous experiment being omitted.

Evidently more amplification is needed. This canbe obtained by adding another stage of amplificationFig. 6.7. Since C2 blocks d.c. from the first stage itis not necessary to use a second capacitor for theother position. As a matter of fact it is not necessaryto use an output capacitor for the last stage either.A crystal earphone does not allow d.c. to flow through.It is a capacitor itself.

So connecting it between the collector of TR2 andearth does not upset the d.c. conditions. A loud-speaker or a magnetic earphone would pass d.c. androb the transistor of collector voltage. If you use amagnetic earphone it must either be connected be-tween collector and battery positive or an outputcapacitor must be used. But only a high impedanceearphone will be of any use.

The usual 8 ohm types supplied with the tranistorradios are no good. Large old fashioned ones fromheadphones are generally satisfactory.

Make the two stage circuit of Fig. 6.7 and experi-ment with it. You will probably notice a number ofthings. First, the amplification is much greater.Secondly, you hear a buzz, especially if you touch thewires of Cl. Thirdly, the quality of the sound is poor.Fourthly, after a sudden loud noise the amplifier goessilent for a while.

MAINS HUMThe buzz is "mains hum". You might think that the

mains has no place in a battery operated amplifier.However, minute mains frequency currents flow allover the house. When you touch the input terminalof the amplifier, mains hum is induced into your bodyand flows to earth.

On its way via your finger to earth it passes throughthe input of the amplifier, setting up a small voltagewhich thereby gets amplified to produce the hum.

It sounds more like a real humming noise in ampli-fiers whose speakers can reproduce very low fre-quencies. What you hear from your amplifier is mostly"harmonics", that is multiples of the mains frequencywhich get manufactured accidentally in various ways.

The distortion arises partly from the fact thatneither your microphone nor your earphone is hi-fi.Each reproduce some frequencies better than others.Speech and music, which are mixtures of frequencies,do not sound natural. Undue emphasis of particularfrequencies is called colouration of the sound.

WAVEFORMSThe jangly quality of loud sounds is also due to

distortion of quite a different kind. The amplificationis now so great that the a.c. from TR1 passing into thebase of TR2 is greater than the d.c. base bias currentin TR2. Fig. 6.8.

Fig. 6.8. Waveform obtained from the collector ofTR1. Here the amplification is so great that it islarger than the base bias on TR2.


During one half cycle, say the positive half cycle,the a.c. and d.c. combine to make TR2 pass morecollector current. But during negative half cycles apoint is reached at which the a.c. just cancels the d.c.Fig. 6.9.

Fig. 6.9. We now have a situation where the a.c.collector current of TR2 is so large that any furtherincrease is just cancelled out by the d.c. collectorcurrent.

At this instant the base current is zero so thecollector current is zero. Any further negative swingof the a.c. can have no further effect. You cannotreduce the collector current to less than zero. Theresult is that the tips of the negative half cycles donot get reproduced at all. They just get flattened.This kind of distortion, called peak clipping is anexample of overloading. The amplifier is being fedsignals too big for it to handle.

On tests carried out with the amplifier, an oscillo-scope was connected to the output. The resultantwaveform which appeared on the screen was not asine wave, or even clipped sine wave as shown inFig. 6.9 but a symmetrically clipped sine wave. Fig.6.10.

Vout

T1ME-o-

Fig. 6.10. The idea of overloading can be extendedfurther when both positive and negative peaks areclipped resulting in the waveform shown here.

Why is this? Well, the overloading can only explainthe cutting -off of collector current by the tips ofnegative half cycles of base signal current. Positivetips merely make the collector current increase. Thesolution to the problem lies in following this throughto its logical conclusion. The current increases.What happens to the collector voltage when itincreases?

If the collector current increases, the voltage dropin the 10kfl collector load resistance increases. Moreof the battery voltage is used up in the 10k.Q resistor.The collector voltage falls. Now, the circuit is set upso that with no signals the collector sits at half thesupply voltage, that is, 4.5V. The voltages can only

fall to zero. Any further attempt to increase thecollector current can have no effect on the collectorvoltage. Once ail the battery voltage is used up inthe resistor the collector can only remain at OV.

We are in a familiar situation, peak clipping occurs.Only this time it is on the other peaks, so, we end upwith double peak clipping.

OVERLOADINGOverloading is also the explanation of the silent

periods which follow sudden large input signals.There is evidently a time constant at work somewhereand one component likely to be associated with it isthe 10/iF capacitor. If this somehow gets charged upby a strong signal so that its negative plate makesTR2 base so negative that TR2 is cut off, the amplifierstops working. When the 10,4F discharges sufficiently,TR2 comes on again and the amplifier works onceagain.

The fact is that 10rIF is too large a capacitance, ifyou change it to 100nF (0.1/4F) you will find that thesilen: intervals disappear.

MEASUREMENT OF GAINJust how much voltage gain does the two stage

amplifier provide? You can make an estimate byexploiting positive feedback. An amplifier like thiswith two stages each of which has its emitter earthedis non -inverting. Feedback from output to input ispositive. If the amplifier has a voltage gain of 1000,then feeding back 1/1000 of the output will justmake it oscillate.

10

R1 1MI)

R2tkl

2S AGEAMPL FIER

FIG 5.19 Cll 100r1F

TL I

CRYSTALEARPHONE

Fig.Fig. 6.11. Using the amplifier of Fig. 6.7 and the RESISTORCHAIN, the gain can be measured by utilising ratios.

If you feed back the output of your amplifierthrough a voltage divider, and set the division ratioso that the circuit just oscillates then the ratio givesthe gain.

A double divider Fig. 6.11 enables the ratio to bevaried in a known way. The overall division is theproduct of the two divisions, that is the first one timesthe second, the second gives a ratio of about 1000.

The RESISTOR CHAIN gives ratios from 1 to 10. So,overall, ratios of 1000 to 10,000 are obtainable. Ouramplifier oscillated (producing a rasping noise) withthe RESISTOR CHAIN tapped at points 2 and above, butnot on point I. This indicates that the gain is over5003 (tap 2 gives a division ratio of 5) but under


10,000 (tap 1 is 10). This was with a crystal earphone.If you use a magnetic earphone it may greatlyreduce the effective load on TR2 and so reduce thegain.

You can connect it between the top of the 101(11 andbattery positive to avoid this. The amplifier givesquite a respectable gain, but should it not be higher?We said that a single stage amplifier should have again around 200. On this basis two similar stagesshould give 200 X 200=40,000, which is a lot morethan the measured gain.

UNKNOWN IMPEDANCESWhat has gone wrong? A careful inspection of the

circuit shows us that there are several examples init of impedances which are in parallel in just the rightway to reduce the gain. First the RESISTOR CHAIN,which has an impedance of 10kfl, is in parallel withthe 10kil load of TR2 Fig. 6.11. This reduces the effec-tive load to 101d1/2=5kn and halves the outputvoltage.

The part of the RESISTOR CHAIN between the tap andearth is "shunted" by the 'Mil+ Hal divider. Theinput impedance of TR1 (i.e. between input and earth)is in parallel with the lkfl resistor, reducing its effec-tive resistance and therefore increasing the divisionratio to something over 1000. The input impedanceof TR2 is in parallel with the 10k0 load of TR1.

Since we do not know the input impedances of thetransistors we cannot estimate their effect on gain.One thing we do know about (the values of theRESISTOR CHAIN resistances and the 1Mil and lkflresistors) should tell you, without bothering to do anysums, that the effect of the 1Mil -1- lkfl on the RESISTORCHAIN ratio is negligible, because very little currentwill be taken from the RESISTOR CHAIN by the 1Mflresistor. If our original guesstimate of 40,000 was

right, then the effect of the RESISTOR CHAIN across the10kfl load brings down the gain to 20,000 and theother factors must reduce it to less than half this.

Later, when we explore the characteristics of tran-sistors in more detail, you will be in a better positionto estimate the effects of their input impedances. Letus leave it for now, and do some more experiments.First, substitute your 100kf1 POT for the two voltagedividers, RI and R2 and see how the frequency ofoscillation varies as the knob is turned.

Disconnect the POT and connect the amplifier inputterminal to the positive of the battery.

Do you hear anything? Can you think why thecircuit is behaving as it does?

Next month we shall continue with amplifiers and discusssome of the more important parameters to be considered whendesigning amplifiers.

QUESTIONS6.1. A transistor collector load resistor drops 3 volts.

The voltage gain of the transistor is about;a. 30b. 120c. 300

6.2. A transistor has an hFE of 100, and an I. of 100,HA.Its a.c. input resistance is approximately;a. 25kb. 10kuc. 2501).

6.3. Noise is thermally generated in;a. inductorsb. resistorsc. capacitors

6.4. A gramophone pickup delivers 10mV to a pre-amplifier which then drives a power amplifier. If thepower amplifier needs 300mV to produce full output,the gain required in the pre -amplifier is;a. 10b. 30c. 300

6.5. A TV film crew is filming a speaker at a conferencefrom the back of the hall. The sound recordist com-bats the noise of the audience by;a. using a airectional microphoneb. increasing the amplifier gainc. inserting a sharply tuned filter

ANSWERS (To part five)1. positive (a)2. a voltage or e.m.f. (b)3. L and C in series (b)4. all the field of one passes through the other (c)5. very low (a)6.1 .3311 (c)7. 220 (a)

8. 480 (c)


Pioneers

IN THE field of audio electronics thework of two pioneers has quite

shamefully been underestimated, mis-understood and misinterpreted.

It was in 1933 that Major EdwinArmstrong of Columbia Universityinvented and patented the principlesof modern f.m. radio. He demon-strated multi -channel multiplexing justa year later. But it wasn't until theearly Sixties, some five years afterArmstrong's suicide, that f.m. andstereo multiplexing was authorised inthe U.S.A.

Likewise, EMI engineer Alan Blum-lein never really saw the fruits of his128 patents in action. Whereas Arm-strong died of frustration, in 1954,Blumlein was killed by an accident in1942. The Halifax bomber in which hewas testing the H2S radar system thathelped win the War crashed in theWyeValley and all on board were killed.

Armstrong's life and work is publi-cised by the Armstrong MemorialResearch Foundation at ColumbiaUniversity in New York City. But nosuch "official" body has ever soughtrecognition for Blumlein in the U.K.

Blue PlaqueThe situation has, however, changed

recently for the better, due mainlyto the enthusiasm of Blumlein'sofficial biographer, F. P. Thompson,who encouraged the Greater LondonCouncil to commemorate Blumlein'slast London home at No. 37 TheRidings, Ealing, with a traditional blueplaque. The official unveiling wasfollowed by a ceremony sponsored byBlumlein's original employers, EMI.

The ceremony took place almost tothe day 35 years after his death, andten years after the publication in theJune 1967 issue of Electronics andPower of an article by engineer, B. J.Benzimra. This article meticulously

By ADRIAN HOPE

researched and tabulated all 128 ofBlumlein's patents, and probably trig-gered all the current interest.

Stereo QuadOf these, British Patent No. 394 325,

which was filed in 1931, is perhaps themost significant , for it laid the founda-tion to all modern stereo sound.Every stereo record in the shopstoday has its left and right channelscut in the single groove as suggestedby Blumlein.

In January 1934 the invention wasput into practice at EMI's Abbey Roadstudios when Sir Thomas Beechamtook time out of a mono recordingsession to let the young Blumleinmake a test version of some sectionsin stereo. Sadly, these historic re-cordings have never been issued orMade public, although some are nowin the hands of the British Institute ofRecorded Sound in London, who maywell play some at public demonstra-tions.

It's fascinating to see how evennow, despite the flurry of interestthat came with the unveiling of theBlumlein plaque in the centenary yearof recorded sound, the man's workstill continues to be underestimatedand misinterpreted. In April theTelegraph Magazine was censured bythe Press Council for belittling Blum-lein by suggesting that his work wasso secret that no one had ever heard ofhim and that there was this comedy inthe idea of erecting a blue plaque tosuch a man.

Then, in his speech at the officiaunveiling of the plaque, Sir AlarHodgkin perpetuated an erroneousmyth which has passed into audiofolklore. Referring to the recent quad-raphonic broadcasts made by the BBC,Sir Alan spoke of Blumlein's originalstereo work and suggested that"Not only did his patent specification

refer to the invention of stereo: it alsoincluded an outline of quadraphonicand ambisonic techniques."

In fact, although Blumlein paved theway to such modern techniques asthe NRDC's Ambisonics surroundsound system, by describing sum anddifference techniques of signal hand-ling, Blumlein did not specificallyinvent and describe "quad". Butwith the text of the speech released byEMI and quoted verbatim by news-papers round the world, Blumlein isnow assured a place in history as theinventor of quad.

It's ironical, because it's probablyone of the few aspects of modernaudio that Blumlein didn't invent.

Television GamesThe price of television tennis,

football, shooting and race games isnow dropping like a stone, just asthe price of calculators plummettedorce the basic chips had been de-veloped and put on the market formanufacturers to incorporate in theirown brand name machines.

In a previous column we warnedabout the risk of buying too early andthus spending too much on a t.v.game, and personally I have no doubtthat the price of games will drop fur-ther as more and more new chips andgames are developed. Already pro-grammable games are on sale, whichmakes the basic style seem very tame.

Another development soon will bedevices which connect with an audiotape recorder, (cassette or reel-to-reel) and enable you to take part inpre -taped sports, such as racing andtarget shooting, complete with asoundtrack of advice, sound effectsand music.

But, sorry to be a wet blanket,here's another word of warning.Do beware of tube burn. I havetalked with several t.v. engineers andwithout doubt there is a risk of dama-ging the cathode ray tube of a t.v.set if a game puts a fixed field patternon the screen for hours on end ath gh brilliance and contrast. And, ofcourse, this is just what might happendjring the school holidays.

Because the beam current of acolour set is fairly low, there is lessrisk likely with a colour t.v. than witha black -and -white set. And there isleast risk of all with a colour gamethat puts a single colour field patternon a colour t.v. screen, because thisway the beam current is reducedeven further.

But if you use a black -and -whitegame with a black -and -white set,don't play on the same static fieldpattern for too long. Change forinstance from ping-pong to footballand back again every quarter hour.And reduce brilliance and contrast,using the set controls.


INTRODUCTION

MODERN electronics readily lendsitself to the production of

simple but amusing games, manyof which could not be accom-plished using more traditionalmaterials. The electronic gamedescribed below is for two playersand is a competitive test of theirreaction speeds.

GAME BASICSA set of six lights forms the basis

of the game. These are arrangedin the pattern shown in Fig. 1, andhave been numbered in the dia-gram for the convenience of thisexplanation.

The game begins by one playerthrowing a switch, which causeslights 3 and 4 to blink on and offat a rate of several times persecond. As one light turns on, the

other switches off. After a fewseconds this process ceases withone light remaining on and theother turned off.

Each player has two push buttonswitches, one player's switchesoperating lights 1 and 2, and theother player's switches operating

01 =off

O 3

0, 60

LEDS

Fig. 1. Layout of the light emitting diodeson the front panel.

Do you think youhave fast reactions?

Test yours againstopponents withthis easy to buildreaction game.

By R.A. Penfold

lights 5 and 6. If, for instance,light 4 remains on at the endof the blinking process, the twoplayers must try to switch onwhichever of their lights is nearestto light 4 by depressing the appro-priate push buttons.

If the player operating the upperset of lights manages to get lamp2 on first, he is the winner of thatround. There can be no argumentas to who is the winner, as oncea player turns a light on, the lightat the other end of the row isblocked and cannot be switched onuntil the winning light is extin-guished.

There is obviously no effectiveway of cheating at this game, sinceto do so it would be necessary toanticipate both the end of theblinking period and the randommatter of whether light 3 or 4 isthe one which will remain on atthe end. The game is thus purely atest of the skill and reaction speedsof the two players.


MULTIVIBRATORSThe winking light part of the cir-

cuit is based on two forms of multi -vibrator circuit, the astable andbistable types. The basic circuitdiagrams of these two types ofmultivibrator are shown in Figs. 2aand 2b respectively.

ASTABLEAn astable circuit operates in

the following manner. When thesupply is initially connected bothtransistors will receive base cur-rents and will begin to turn on.Due to inevitable circuit im-balances, one transistor will beginto turn on faster than the other.For the sake of this explanationwe will assume that TRa turns onfaster than TRb. As TRa turns on,a negative signal will be fed via C.to the base of TRb. This will holdTRb in the off state while basecurrent via R,, turns TRa hard on.

When TRa is turned fully on, nofurther signal will be developed atits collector and transmitted toTRb base via Ca. The charge acrossC. will quickly die away and asTRb now starts to receive base cur-rent via R., it begins to turn on.As it does so, a negative signal isfed via CI, to the base of TRa, thusturning TRa off. This sends a posi-tive pulse to the base of TRb byway of C., quickly turning TRbhard on in the process. As a resultof this regenerative action TRanow becomes turned off and TRbbecomes turned on.

The charge across C1, thenquickly dies away and this processcontinues with both transistors con-tinuously changing state.

HOW IT WORKS

INSTABLE/

ASTABLE

PLAYER A PLAYER B

A simple timing circuit consisting of R and Cprovides a variable timing period to the combinedbistable/astaole. This part of the circuit producesrandom flashing of two I.e.d.s. At the end of eachperiod this circuit will stop and only one I.e.d. willremain illuminated. Whateve I.e.d. remains on,either 1 or 2, the players press their switches cor-responding to that I.e.d. In the case shown this is3 or 5.

As one switch is pressed, say 5, the associatedI.e.d. illuminates, at tha same time the bistable"latches" on in one state thus preventing the otherI.e.d. from turning on. Due to the "cross coupling"between the two bistables, the other bistable isprevented from turning on.

Thus the circuit indicates wh ch player has "caughtthe light" first.

BISTA3LEA bistable operates in a similar

manner, except that as it is d.c.coupled it latches with one tran-sistor turned off and the otherturned on. Referring to Fig. 2b,when the supply is initially con-nected both transistors will begin toturn on, and again one will turn onfaster than the other at the outset.If, for example, TRc begins to turn

Fig. 2a. Basic astable multivibrator circuit.

Fig. 2b. Basic bistable multivibrator circuit.

on faster than TRd, TRd will bestarved of base current as the col-lector potential of TRc quicklyfalls. TRc on the other hand, willbe able to draw plenty of base cur-rent from the collector of TRd andin consequence it will quickly turnhard on. In doing so it completelystarves TRc of base current, andthe circuit will rest with TRcturned on and TRd turned off.

The completed Catch -A -Light reaction tester.


Rear view showing the circuit board mounted on the front panel.

STARTHERE FOR

CONSTRUCTION

Begin construction by cutting a16 strip width of 0.1 inch matrixVeroboard from a 3.75 inch lengthof board, using a hacksaw. Thenreferring to Fig. 4, make the 14cuts in the copper strips, and solderin all the components, leaving thetransistors until last. The compo-nents are fairly closely spaced andcare must be taken to avoid shortcircuiting adjacent copper stripswith blobs of excess solder.

With the exceptions of the pushbutton switches and battery, theremaining components aremounted on the front panel of thecase. The author used an inexpen-sive plastic case having a steelfront panel, but any case of asimilar size (130 X 100 X 50mm)should be suitable.

Two small cases are used tohouse the push button switches,one box for each player's switches.A couple of commercially madeplastic boxes are used on theprototype, but if suitable readymade boxes cannot be obtained,suitable containers can be made

from wood or aluminum. The con-tainers do not even have to have afull six sides, and can be very basic.

WIRINGThe final wiring of the unit is

also shown in Fig. 4. The compo-nent panel is positioned behind D2,D5 and D6, with its component sidefacing the front panel. Thick insu-lated single strand wire (about 16to 18 s.w.g.) is used to connect thepanel to VR1 and Si, and providedthese leads are kept as short aspossible they will provide a firmmounting for the panel. The restof the circuit can then be wired upaccording to Fig. 4.

To avoid mistakes, use adifferent colour connecting wirefor each switch when wiring in S3and S5, and also when connectingS2 and S4. These are connected tothe main unit by leads about 20cmlong. It is advisable to fit the frontpanel holes for these leads withgrommets if a metal panel is used.

The battery fits beneath D1 andD3, and some foam rubber can beused to hold it in place more firmlyif it is found to be at all loose.

Components

ResistorsRiR2R3R4R5R6R7R8R9

lOkf24.7K/2.7kf268kf268kf268k1268kf22.7kfl10f2

:,.ism

All resistors are carbon W ± 5%

PotentiometerVR1 100kf2 Lin.

R10R11R12R13R14R15R16R17

27kf)2.7k122.7kfl27kfl27k122.7kf22.7kf227k12

CapacitorsC1 100pF 10V elect.C2 2.2,4F 10V elect.C3 2.2µF 10V elect.

SemiconductorsTR1, 2, 3, BC109 silicon npn (3 off)TR4, 5, 6, 7 BC108 silicon npn (4 off)D1 to 6 TIL209 red light emitting diode (6 off)

See

page 342

MiscellaneousS1 d.p.d.t. standard toggle switchS2 to 5 push to make, release to break push switches (4 off)B1 PP3 9V batteryStripboard 0.1 inch matrix 37 holes by 16 strips; battery clip; case typeM3 or similar size, 130mm x 100mm x 50mm; two small boxes to suitS2 to S5; small round knob; grommets as required; connecting wire;mounting clips for I.e.d.s (if not supplied); solder.


3736

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

2019

18

17

16

15

14

13

12

11

10

9

B

7

6

5

4

3

2

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c ,

0t t

0 .....r1 .. 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0ABCDEFGH I J KLMNOP

oo000

0 0 0 0 0 0 0 0 00

0 0 0 0000000

0 cox COM 00o 1

00 48 0

00 00

0om00000.000.0000000

0

0

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0 00 00 0 0 0 0 0 0 00 0 0 0

a

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k k!.10.ia

02

0033

c49 0

a ft k

06

....,)a k

01

06

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k c) a

D1 -D6

TR1-TR7

S3 SS

Fig. 4. Interwiring details, component layout and underside of the stripboard showing breaks in the copper tracks.Wires from the reaction switches and front panel mounted components should be connected to their correspondingletters on the circuit board.


rTIMER

VR1100ku

R9 131:1

Sib

R1

1014 +

R2

7kfl

TR18C109

COMBINED ASTABLE/BISTABLE

Iti_209

R3 Re

2 7k12 2 7kt1

C2 2 2uF c3 2 2uF

R4 68k1) R76876

BC109

TR2

RS 6876

RE 6 Bia:

BC109

TR3

T1L209 (T1L209

S2

c-

B1STABLE ONE

R11 R12

2 7k0 2 7kI1

BISTABLE Two

111_209 115.209

TR4 TR513BC106 BC10

a"

Fig. 3. The complete circuit diagram for the Catch -A -Light.

The complete circuit diagram ofthe reaction game appears inFig. 3.

Transistor TR1 is used as a timeswitch, and when the supply is con-nected Cl charges via RI and VRI.Normally there is virtually the fullsupply voltage at TR1 collector,but when about 0.6 volts is de-veloped across Cl, TRI will turnon and its collector voltage will fallto almost zero.

BIASFor the period that TR1 collector

is high, TR2 and TR3 will operateas an astable multivibrator withbase bias current being providedvia R2, R5, and R6. Resistors R4and R7 have no significant effect onthe circuit yet.

As TR2 and TR3 repetitivelyturn on and off, their l.e.d. loads,DI and D2, will flash on and off.When TR1 turns on, the base biasvia R2 is cut off and the circuitwill no longer operate in theastable mode. It will, instead,operate as a bistable multivibratorwith R4 and R7 providing the cross

CIRCUIT DESCRIPTION

coupling. These cause the circuit tolatch in whichever state it was inwhen the bias was removed.

For instance, if TR2 is on andTR3 is off when the bias is re-moved, bias current flowingthrough R7 will hold TR2 in theon state while TR3 will be unableto receive any base current fromTR2 collector and so remains off.

Thus, when the supply is turnedon, DI and D2 will flash on and offfor a few seconds until TR1 turnson, whereupon the oscillationceases with one of the 1.e.d.s re-maining on.

TIMINGThe timing period can be varied

from about 2 to 12 seconds usingVR1. It is largely a matter ofchance whether it is D1 or D2which remains on at the end of thetiming period, but with VR1 at anygiven setting the circuit is likelyto be imbalanced in favour of oneor other of these. Altering thetiming period at the end of each

R15 R16

2 7k0 2 7k0

IRS T R7BC108 9:108

D61'11_209

SS

round using VR1 helps to iron outthis imbalance, as well as makingit more difficult to estimate andanticipate the end of the period.

PRECEDENCE CIRCUITTwo identical bistable circuits

form the remainder of the unit.These ensure that only one win-ning light can come on at one time,and one bistable is used for eachrow of lights.

In practice, D3 and D4 aremounted at opposite ends of thedisplay (positions 1 and 5 in Fig. 1),and one player has S2 while theother has 53. If S2 is pressed, basecurrent will flow from the positivesupply to TR5 via D3, R11, andRIO. Transistor TR5 will turn onand D4 will light. If S3 is now de-pressed, as TR5 collector is atalmost the same potential as thenegative supply rail, TR4 can re-ceive no significant base currentvia R13 and D3 will not light. Thecircuit is extremely fast in opera-tion, and the moment one pushbutton is depressed the other isblocked.


,,....

ei-

r

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. 'TY% . . . . .

-77-77:1"

Close-up of the rear of the front panel.

Top view of the circuit board showing component layout

IN USETo start the unit it is merely

necessary to switch S1 into the onposition. At the end of the roundS1 is put back in the off position,and Cl will then almost instan-taneously discharge through Slband current limiting resistor, R9.After the setting of VR1 has beenaltered, the unit is then ready tobe turned on again and anotherround played.

A simple method of playing agame using the unit is to have thefirst player to score 10 points asthe winner. One point is scored foreach round that is won, and a pointis deducted in the event that aplayer either pushes the wrongbutton or pushes both at once.

An element of luck can be intro-duced into the game by havingadditional points scored by playersbetting on which light they thinkwill remain on at the end of eachround.

The precedence switch part ofthe circuit could be used as a quizmonitor, and the other part of thecircuit a sort of electronic headsor tails, and could possibly haveuses in other games. 1=1

YOUR FREEr

Presented Free with this issue is a very usefulitem to have in the workshop. How many times afteryou have finished working on your latest project, putyour tools down and walked away, only to discoverthat when you return the tools have grown legs andvanished. A tool rack solves this problem, by havingthe tools mounted in an easy to see quick to findposition.

As received the EVERYDAY ELECTRONICS Tool Rackis flat, before using therefore it must be folded tothe correct shape.

With the Tool Rack on a flat surface and the letter-ing facing you, bend up at an approximate angle of45 degrees, the five locating strips. Next bend up thebottom half to meet the strips, at the same timepushing the lugs on the strips into the slots providedand lock them in position by pushing the lugs back.

Do this operation with all five strips, and your toolrack is then ready for use. The photographs shouldmake this clear.

It can be mounted in any convenient positionusing small screws. A half -inch number 8 wood screwwould be ideal. Once the tool rack is mounted thetools can be put in, and is then ready for use.


Everyday NewsEXPLORE THE WORLD OF SCIENCE

To those familiar with the layout of the Science Museumin London, there have been a few changes just recently. Thelatest being the conversion of the ground floor ElectricPower Gallery into an Exploration exhibition illustratingsome of the ways in which scientific and technical skills arebeing used to explore the world and beyond.

The exhibits include the actual Apollo 10 spacecraft thatcompleted a moon orbit in 1969, a full size reconstruction ofthe Apollo 11 moonbase (with additional "Keep off theMOON" signs so necessary to stop children of all ages from'stepping boldly'), a lifesize model of a manned submersibleand an EMI brain scanner in simulated working operation.

Electronics play such a vital part in the modern worldit is good to see this exhibition, which is on for at leastthree years, depicting so dearly the use to which it can beput, from remote sensing where we can look at familiarobjects at wavelengths outside the range of our own eyesand truly see the world in a new light, to earth -based radiotelescopes and orbiting observatories.

Each section has its own story to tell, and audio visualaids are everywhere including several video-taped displays.The museum staff are to be congratulated on this excitingand well presented exhibition which is proving so popular.

Britain's largest computercompany, ICI., is now gainingmore than half its revenuefrom exports. Total turnoveris now running at over £400million a year, a 45 per centincrease over the previousyear and pre-tax profit is up31 per cent. All this despiterestriction on output throughlabour disputes.

Full scale model of Apollo Moon Lander, on display in the ExplorationGallery. (Science Museum Photo.)

Transatlantic Radio MessagesThe 240Hz "song" of the

early Marconi rotary sparkgap transmitter was heardonce again on the air wavesduring a week of commem-oration of the first two-waytransatlantic radio messagesachieved by Marconi and hisassistants in 1903.

HOME COMPUTING

The popularity and someidea of the tremendousgrowth of home computingwas demonstrated at a recentseminar in London.

The seminar, entitled"Home Computing", wasorganised by Lynx Elec-tronics Ltd and was a com-plete sell out. Such was thepopularity that they had toexpand the accommodationto take 550 people whoturned up on the day.

The final line-up ofspecialist guests who agreedto speak set a standard thatwould be very hard to better.be it hobbyist or profes-sional. In order of appear.ance the guests were: GaryTysoe of Mostek; Jim Ayres

of harrow College of Tech-nology; Phil Pittman ofZilog; Phil Cooke of SussexUniversity and Tony Rundleof Compeda.

The seminar was given tocommemorate the launchingof the Nascom 1 Microcom-puter Kit. This kit is builtaround the Mostek 2.80 CPUand includes a full keyboard,1K EPROM which provideseight basic operator com-mands including single stepand a 2K RAM.

The kit has interfaces forconnecting to a domestic TVas a monitor, a domesticcassette recorder for datastorage and a teletype. Theprice of the kit is £197.50(excluding VAT).

Amateur stations on theoriginal sites at Poldhu,Cornwall, and Cape Cod,Massachusetts, gave thous-ands of amateurs throughoutthe world the opportunity of"working" the historic sites.The control operator at CapeCod, Robert J. Doherty, re -

Manufacturers of advancedautomatic test equipmentand a leader in the field,Membrain Ltd, has beenacquired by SchlumbergerMeasurement and Control(UK) Ltd for an undisclosedcash sum. The company willcontinue to operate as aseparate business and withits existing management.

ceived special permission toreproduce by audio meansthe famous Marconi soundon his c.w. transmissions.

The amateur stations alsooperated on single sidebandR/T, radio teletype and slow -scan t.v., all unknown tech-niques 75 years ago.

Signing OffOver 50 million QSL cards

I exchanged by radio amateursto confirm contact/ havebeen handled during the 38years Arthur Milne, G2MI,and his wife have beenrunning the QSL bureau forthe Radio Society of GreatBritain (RSGB) which cur-rently has 20,000 members.

Arthur Milne has now re-tired and the new managerof the QSL Bureau is E. G.Allen, G3DRN, 30 BodnantGardens, London SW20 OUD.

Marine CallRadiotelephone calls

through the Post Office v.h.f.maritime radio service haveincreased more than tenfoldin ten years. Annual callsare now 200,000 a year com-pared with 17,000 ten yearsago.

In the same period thenumber of small coastalcraft fitted with R/T hasrisen from 1,500 to 8,000,much of the increase beingattributed to safety -consciousleisure yachtsmen and boat -owners.


. . . from the World of Electronics

ANALYSIS

MICROS AND JOBS

MICROELECTRONICS in general and microprocessorsparticular are putting employment at risk. Today's

electronics revolution, so technologically sweet, can besocially very sour indeed.

This was one of the messages emerging from the Impact ofElectronics conference organised recently by our contempor-ary, "Electronics Weekly".

The consumer benefits are plain to see. Jack Akermal.managing director of Mullard, reminds us that the domest cradio set of 1936 cost about E13 when a Morris Minor carcost about £100. Today the equivalent radio still costs £13while the cost of a Mini is about E2,000. Electronics has neverbeen better value.

The Social cost, however, is highlighted by Kenneth Co --field, chairman and managing director of STC. Moderntechniques like I.s.i. and p.c.m. have reduced equipmentswitching size in telephone exchanges by a factor of 15.

This means smaller buildings, less work for the building in-dustry as well as those manufacturing exchange equipment.The present generation of electronic exchanges has reduceddirect operatives in manufacture by a factor of 2.6 and thenext generation will reduce labour by a further factor of 10:1.

The vice-president of Siemens, W. Germany, Dr. AlfredPrommer, reinforces the argument by pointing out thatmodern technology has reduced the cost of producing a singecomponent in an integrated circuit by a thousandfold with acorresponding decrease in manpower required for its produc-tion.

And Alex d'Agapayeff, chairman of Computer Analysts ardProgrammers, referring to the broader impact in commerceand industry, suggests that it is inevitable that 2i millionpeople will endure substantial changes in their jobs and thatI million jobs may actually be lost through the applicationof microelectronics.

Have we, then created a technological monster whichthreatens to destroy our society? Not at all, provided we allrecognise the challenge and abandon complacency.

Electronics has and can enrich our lives. But clearly weshall have to accept and adapt to change in work habits ardattitudes if we are to continue to enjoy the riches yet to come.

Brian G. Peck.

The 13th Appleton Lecturedelivered in the IEE lecturetheatre had the title 'Solid-state devices and the age ofindolence'.

Olympics TAMEDThe new television centre

in Moscow, now under con-struction for world coverageof the 1980 Olympic Games,will be equipped with Mar-coni Instruments TelevisionAutomatic Monitoring Equip-ment (TAME).

TAME will monitor all in-coming signals from thecompetition areas and alltwenty outgoing programmechannels. Picture quality iscontinuously checked on 24parameters.

AWARDThe 1977 Royal TelevisionSociety Southern CentreAward has been won by Mr.P. A. Crozier -Cole for hiscontribution to the develop-ment of the IndependentBroadcasting A uthorit y'sRegional Operations Centres.For four years he has beenproject manager leading theteam of engineers whosework is recognised by thisaward.

The RTS Southern CentreAward is given annually forthe most significant contribu-tion to television during theyear in the Southern Centrearea. It is not confined to theengineering field.

COMPETITIONThe "Young Engineer for

Britain 1978" competitionhas now been launched andis organised by PETT (Pro-ject -Engineers and Technolo-gists for Tomorrow).

Regional competitions arescheduled for August, fol-lowed by the national final.This year's prizes includeConcorde flights to the USAand visits to American en-gineering projects and aspacecraft centre.

One of last year's winnerswas 14 -year -old electronichobbyist Matthew Tonkswith an ingenious integratedcircuit test set.

Details of this year's com-petition can be obtainedfrom the Secretary, PETT,Department of Industry,Room 114, Abell House, JohnIslip Street, London SWIP4LN.

The US -based company,Teradyne. has sold $25 mil-lion worth of laser -trimmingequipment to thick filmmicrocircuit manufacturersin the past six years.

The recently appointedAssistant Managing Director,Post Office Telecommunica-tions, John Harper, buildsradio equipment as a sparetime occupation. In his newJob he has special responsi-bilities for business planningand system efficiency.

FarnboroughAir Show

The giant Farnborough AirShow, a shop window foravionics, is already fullybooked by exhibitors. Theexhibition areas have beenre -designed for the 1978show and will be larger andhigher, allowing completeaircraft and space vehiclesto be displayed under cover.

A quarter of a million visi-tors are expected to visitFarnborough International'78 which runs from Septem-ber 3-10. The general publicwill be admitted only on thethree public days September8-10 when a full flying pro-gramme will also be staged.

Deer Wins First PrizeThe ninth Scotch Wildlife Sound Recording Contestattracted over three hundred entrants with top prize andSound Recordist of the Year title (plus equipment worth£500) going to Keith Biggadyke, seen below, for his chancerecording of "Muntjac Deer and Nightingale".

He and other winners were recently presented with theirprizes by Lord Alec -Douglas Home in the plush surround-ings of the Hilton Hotel, London.

The annual competition is open to amateur tape record-ists whose task it is to capture on tape cassette or spool,the sounds of British Wildlife. Entry forms for this year'scontest, with first prize valued at £1,000 are available fromScotch tape retail outlets.


SQUARE

ONETHE HOUSING SITUATION

Thanks to miniature componentsand low power requirements, manyelectronic projects can be accom-modated in quite small cases orhousings.

Constructors have always beenenterprising when it comes toproviding a suitable case for someproject. Tobacco and mustard tins,even baking tins, have all been pressedinto service over the years! Morerecently, the large variety of cast-offplastic receptacles for all kinds ofproducts have been seized upon byconstructors and put to good use.

CUSTOM BUILTWhile not decrying these inspired

efforts at improvisation, we recom-mend the use of "custom-made" caseswherever possible. This implies,generally, commercial products, yetdoes not exclude home-made cases inmetal or even wood.

No matter how simple the project,it is possible to finish it off in style.Metal and plastic containers designedexpressively to house electronic equip-ment are available in considerablevariety-and even in a choice ofcolours. So every home -built unit canhave a professional look.

Our constructional articles showand specify all the mechanical details,including recommended cabinets orcases, where applicable.

We say "where applicable" becausecertain projects intended to be usedin association with other equipmentor apparatus can usually be installedwithin that equipment without anyspecial housing for the unit itself. Forexample, the Electronic Touch Switch(January '78).

Where - projects are designed forincorporation within other equipment,

... FOR BEGINNERS

WE ARE HERE TO HELP YOU -NO MATTER HOW NON -TECHNICALYOU MAY BE, JUST READ ON !

obviously we cannot provide exactdetails regarding the installation, theinterwiring and the drilling of holesfor controls and so on. The builderhas to think this out for himself.

The greater majority of projectsare, however, entirely enclosed withintheir own box or case. This is generallydesirable. It is mandatory for portableor hand-held units-and is necessaryalso for safety reasons in many in-stances where the electronic unit ispermanently installed in position.Electronic projects for motor cars,for example.

PLASTIC OR METALPlastic cases are a boon to the

amateur constructor since they canbe obtained in a variety of styles, sizesand even colours. They enhance theappearance of the finished project.Plastic cases are generally toughenough for the environmental hazardswe normally have to take intoaccount.

For exceptionally hazardous situa-tions a metal housing might be better.

Burglar Alarms and power con-trollers are examples where thegreater security afforded by a sheetaluminium or a die-cast box might bebeneficial.

Great ruggedness, rigidity, andresistance to fire are not the onlyadvantages offered by metal housings.There is the important considerationof electro-magnetic screening.

Some electronic circuits must beenclosed within a metal container inorder to prevent either the radiationor the pick-up of electro-magneticsignals. Such extraneous signals mayaffect the proper operation of elec-tronic equipment or cause inter-ference on radio, TV, or audio equip-ment.

HOME-MADE CASESWith the wide selection of plastic

and metal cases to choose from, theincentive to build one's own case haslargely vanished. But for those wholike working with metal or wood, theopportunities are still there.

The construction of metal casesrequires certain skills and tools-andwill appeal to a specialist minority.

Wood is a more commonly handledmaterial. In fact wood was one timeextensively used for home-made casesand cabinets. Modern timber materialswith or without some plastic claddingare readily available in variety, andoffer a solution for anyone proficientin woodworking.

Examples of projects using woodencases are not very common nowadays,but as it happens one does appearthis month, see the Audiotest.

CHOOSE WITH CAREEVERYDAY ELECTRONICS projects are

presented in full detail, right throughto the housing stage. Suitable casesare specified in the articles, but theindividual can use his own initiativeand employ something different ofcourse if he wishes.

However, if a metal case is specifiedthere will be a good reason for this,and a plastic (or wooden) case shouldnot be used as a substitute withoutdue regard to the possible consequen-ces.

Conversely, should a metal box becontemplated in place of a specifiedplastic (or wooden) case, rememberthat proper electrical isolation of allcircuit components and wiring fromthe metal case is vital.

If in doubt, always stick to the kindof case specified in the articles.


NEXTMONTH

Have you ever been frightened at the state of theload on your car roof rack? If so set your mind atrest with our simple design.

MAINS DELAY SWITCHThis timing circuit can be used with TV's, radios, taperecorders etc. With the option of a remote control the uses areendless.

POCKET TIMERA portable unit for indicating elapsed time from a fewseconds to several hours. Ideal for use in games, thekitchen and as a parking meter reminder.

A selectionf proven

desieverybody!varied application.

Something ns ofg for

dritiffmcs APRILISSUE ON SALE

FRIDAY, MARCH 17


By Brian TerrellNew products and component

buying for constructional projects.

Printed Circuit BoardsIn last months issue of EVERYDAY

ELECTRONICS you will have noticed thattwo of our constructional projects,Chaser Light Display and Stereo' MonoLead Tester use printed circuit boardsas the basis of construction as wasthe case with the Automatic Phase Boxin the December issue.

Although details of fabricatinghome-made p.c.b.s were included ineach of these issues it is felt thatcertain constructors may not wish toget involved in this and may prefer toobtain the p.c.b.s ready etched anddrilled. The use of p.c.b.s in con-structional projects is known to be inthe minority as opposed to stripboardand other general purpose boards, butit is surprising to find no firms ad-vertising such a service to readers. Soadvertisers (or would -be -advertisers)how about it? We feel sure it will bewell received by our readers.

New From VeroA new addition to the Verobox

range of small plastic enclosures hasrecently become available. Known asthe Flip -Top Box, it has a unique flip -over hinged cover to protect panelmounted items.

The cover, hinge and base aremoulded in one piece of polypropyleneand has a textured matt exterior. Analuminium alloy panel is supplied withthe enclosure which clips in andrequires no fixing bolts or screws. Theinside of the enclosure containspillars, integrally moulded to facilitatehorizontally mounted componentsboards. Six self -tapping screws aresupplied for this purpose.

The boxes are available in two sizes,130 x 75mm (pocket size) and 196 x127mm and there is a colour choice inboth sizes-black or yellow.

The cases will find many uses andare ideal for keyboard units, field testequipment etc. Across the counterprices are V -92 and E2.50 excludingVAT and are available from the usualVero stockists.

Ultrasonic TransducersAs constructors will no doubt have

found, there are very few suppliers tothe amateur market of ultrasonictransducers. We are pleased to informreaders that Ace Mailtronix, TootalStreet, Wakefield, West Yorkshire cansupply a pair of ultrasonic transducersat a cost of Z4.00 including VAT andpost and packing. This is a very goodprice and we believe it to be the lowest.

The transducers known as type 7505are a suitable replacement for theRL400P as used in the UltrasonicRemote Control System featured in theNovember and December 1977 issuesof EVERYDAY ELECTRONICS (all issuessold). Ace Mailtronix tell us that theycan supply a set of electronic com-ponents to build this project and cansupply a priced component list for thisproject on the receipt of a stampedself-addressed envelope. Also theycan supply a photostat copy of thearticle at a cost of 25p per part.

The two sizes of Flip -Top Box from Vero.

The company extends this service toall other constructional projectsfeatured in EVERYDAY ELECTRONICS fromthe October 1977 issue onwards, seetheir advertisement in this issue. Forcomponents they are unable to supply,they will advise constructors of com-petitive sources of supply. An in-valuable service to the constructor-especially the beginner!

Logic I.C. StickersUsers and designers of circuits

embodying rri_ integrated circuits willbe pleased to hear of an ingenious aid,called STICKIES. These are self-adhesive printed labels --same size asi.c.s-and show pin -outs for 61 of themost popular 74 series of integratedcircuits, both 14 and 16 pin.

The labels are peeled from a mastersheet and then positioned on theupper body of the i.c. thereby givinginstant recognition of pin function, seephotograph. They can be used forconstructing and de -bugging proto-types, and useful in fault location.Another application is evident whendesigning printed circuit and othercomponent board layouts.

These STICKIES are available insets of 450 and come complete withcomprehensive instructions. The costis E2.80 for one sheet containing 450labels with discounts on quantities,and can be obtained from ConceptElectronics, (Dept. EE.), 8 BayhamRoad, Sevenoaks, Kent, Tel: Seven -oaks 514110.

Constructional Projects ThisMonth

No buying problems should beexperienced when getting together thecomponents for the C -R SubstitutionBox. The types of capacitors specifiedare those that appeared in the author'smodel, but these are not critical andcan be of any composition that may beavailable. Attention should be paid tothe working voltages however, sincethese will limit its application. The

A STICKIE in position on an integrated circuit.


voltage ratings should, if necessary,be raised to be compatible with thevoltage levels in the equipment likely tobe used with the unit.

A d.p.d.t. slide switch is specifiedalthough only a s.p.d.t. version iscalled for as the latter type is un-available.

All the components for the WeirdSound Effects Generator are availablefrom a number of sources and shouldpresent no problems. The silicon pnp

transistors specified may be unfamiliarto some readers and were chosensince they are complements to theBC108. However, any silicon pnp typemay be substituted if more readilyavailable. A suitable replacement isthe 2N3702, but remember that this hasa different pin configuration to theB C178.

The "heavyweight" pro.ect thismonth is the Audiotest and a fewcomponents may prove difficult to

locate. These are the 10 watt resistors,the 350pF variable capacitor, and theterrite beads. All of these componentsare listed in the Maplin catalogue.Also, the 10 watt resistors are stockedby Marshall's. Both of these suppliers'addresses can be found in the ad-vertisement pages of EVERYDAY ELECT-RONICS.

All components required to buildthe Catch -A -Light are available from anumber of sources and should presentno buying problems.

CROSSWORD HP/ BY D.P. NEWTON

1

8

6 75

9

10 11 12 13

14

15 16 17 18

2219 23 20 M

24 25 26 27

28 29 30 31

32 33

34

35

ACROSS1 A bet on a battery?6 Stand open for an old capacitor.8 Pure and tidy.9 Frequency modulation.

10 Entrance terminal.12 Guiding lights.15 Send across.17 Musical devices for selection.19 Useful for fishy work.20 Grounded.22 A small item of computer informa-

tion.24 Mistakes27 A device of usefully low conductivity.28 Pert. to the stars.

7 Take arms in part to give the equiva-lent D.C.

11 A sense from a state. (Anag.13 A wire along which signals are sent.14 The fixed plates of a capacitor also

found in some motors.16 With tar about we don't get science.

( Anag.)18 The above and beyond state when

nothing extra is to be gained.21 Electronic wandering.22 To offer money against a chance of

gain.23 Not a gate.25 Not quite'a blank screen.26 To choose in a tuned circuit.28 The dielectric for some capacitors.29 A cornucopia of a loudspeaker.31 Against.

Solution to Crossword on page 351

30 There's a stoat about for a drink.( Anag. )

32 A small piece of a circuit.33 Reflect to give a t.v. ghost.34 A light switch with watery connec-

tions.35 Diminution of a signal.

DOWN1 Impedance reciprocates to

entry.2 A semiconductor found on

shoulders.3 If this is missing it can cause

trouble.4 Delivery terminal.5 Roof beam.

allow

some

some

"This computer my wife constructed tends to bea bit garrulous"


7p,

4

PAN11""

I No F.

EVERYONE appreciates the needfor airport radars that keep

Air Traffic Controllers (A.T.C.) "inthe picture" as to where all arriv-ing and departing aircraft are atany one moment. But most peoplewould be surprised to know that,at major airports anyway, exactknowledge of aircraft on theground (on runways and taxiways)is likely to become of even greaterimportance.

BLIND LANDINGWhat really makes this ground

knowledge so vital are the increas-ing pressures on A.T.C. staff. Thesecondense to more aircraft leadingto tighter schedules (there can beeighty movements an hour), largerairports with 4km runways makingvisual checks difficult or impos-sible, and the introduction of blindlandings in fog.

,1111111=0"-'

46

101111111110

IRPORTDAR

BR

40

Blind landings whatever thecloud height and with runway visi-bility at only 50m will soon be used3y British Airways at Heathrowand any other airport fitted withthe necessary equipment. Gatwickand eight other British airportscan handle landings with a ceilingof 30 feet and runway visibilityof 400m.

It is the Civil Aviation Author-ity's aim to bring all major air-ports up to Gatwick standard assoon as possible. So, where pre-viously fog meant a rest for A.T.C.staff it now means even greaterpressure and necessity to knowwhere everyone is on the ground.

AIRFIELD SURFACEMOVEMENT INDICATION

How in fact do they do it at all?'Nell at Heathrow, Paris (Orly)and Rome (Fiumicino) and shortly

I I

1

at Milan and Turin they rely onthe ASMI (Airfield Surface Move-ment Indication) radar. To anyonefamiliar with ordinary radar theproblems would seem immense.

The surface of an airport, notunlike a small town, with its con-trol buildings, hangars, stores, carparks and so on would normallyprovide a radar picture that wasan indiscriminate jumble of echoesat best. ASMI is not, however, anormal radar; it was purposedesigned, in collaboration with theCivil Air Authority, for this oneairport job.

Radar does not pass throughsolids like radio, so that a targetin front of another will throw a"shadow" on the display in theform of a blank space. So the firstrequirement was to give the radar

Heading photo via Flight International.


a high viewpoint, obviously the topof the control tower.

This has the advantage that ithas the same view as the Con-troller a few feet below, but italso has a disadvantage in that theweight of the aerial (and its turn-ing mechanism) restricts aerialsize, with an important result aswe shall see. At Heathrow andother places the control tower issuitably central, but at Rome it isnot, needing a second ASMI tocover the outlying area, its radardata being transmitted back bymicrowave link.

PERFORMANCEThe next question was how to

give ASMI an unheard of radarperformance in all parametersexcept range, which needs in mostcases to be not more than 4,500m.Arriving at the eventual specifica-tion was an interesting exercise injuggling the various characteristicsinvolved so that they "meshed"suitably for the ASMI require-ment.

Radar design is no different tothat of ships, cars or in fact anycomplex system in that it is a com-promise. In this case tremendousresolution was vital, but range, aswe have seen,designers started with two othervariables fixed, or at least limited,one of them unusual to the pointof it being unique.

AERIAL ROTATIONThis was the necessity for a very

high aerial rotation rate. Aerialsof ordinary radars revolve at from24 to 60 r.p.m., which means thatthe synchronised trace in the dis-play crosses the target, renewingecho brightness, every one to twoseconds. This is good enough forslow moving targets (or even fasttargets) at long range, but if youconsider an aircraft only 800maway taking off at 120 knots acrossthe front of the observer, it willtravel 60 yards between each pas-sage of the trace and thus appearto be leaping forward in discon-nected jerks.

With the unique rotation rate of750 r.p.m. however, targets are"repainted" 12 times a second andthe aircraft will only advance afew yards between renewals; inpractice it is seen to proceedsmoothly but in a slight blur.


An aircraft on a taxiway at Fiumicino(Rome) airport as seen by the DaccaAirfield Surface Movement Indication(ASMI) radar. The aerial is on the controltower, centre circle, hence the "shadow"thrown by the aircraft's echo.

Another advantage of the highrotation rate and only slightlysecondary in importance is thatthe resulting picture is virtuallysteady and flicker -free, mucheasier on the eyes of Control Offi-cers who may have to watch it forlong periods, than the usual brighttrace circling round a dark display.

The other limited variable wassize of aerial. Mounted on top ofa tower, the weight of the aerialand its turning mechanism limitedsize to 6ft and it will soon be seenhow this influenced other factors.It is actually encased in a radometo reduce wind resistance and sothe power and size of turningmotor required.

RESOLUTIONWith a steady picture providing

smooth motion we are alreadysome way towards that "unheardof radar performance." Providingvery high resolution takes us therest of the way.

Ir. broad terms resolution meansthe ability to see the shape of atarget and is therefore divisibleinto definition (accurate delinea-tion of the extremities of thetarget) and discrimination (abilityto separate adjacent echoes). Todo these two things well you haveto measure the target in range;front to back, and in azimuth;from side to side.

The keys to these measurementsare pulse length and aerial beam -width (pulse repetition frequencyalso comes into it, but only to theextent that at the high aerialrotation rate used here, a highp.r.f. is necessary in order torecord the target at all). Theshorter the pulse length the betterfor purposes of range discrimina-tion; short pulse represents ashorter distance over the groundso that two close objects are moreeasily resolved than with a longpulse. ASMI pulse length is 0.03microseconds, resulting in a rangediscrimination of 13.8m as ob-

In-dicator (P.P.I.) at 1.800m.

BEAMWIDTHGood bearing discrimination

comes from narrow beamwidth(obviously a broad beam is not so

A view of theHeathrow controltower. Top rightis ASMI, encasedin a "radome".

accurate). Beamwidthdepends on the size ofthe aerial; the longerthe aerial the more con-centrated the beam andthe narrower the beam -width.

Using the standardradar X -band frequency(about 9.4MHz) anaerial of about 25ft (asemployed in major har-bour surveillance sys-tems) would be neces-sary to provide thebeamwidth required byASMI, i.e. 0.4 degrees.We have seen that alarge aerial is ruled outfor practical reasonsand so recourse has tobe had to very high fre-quency in order to per-mit the use of a 6ftaerial. The frequencychosen is Q - band(35MHz).

A radar picture of Heat-ircw airport slowilg controltower and runways. A Jumbo jet is waiting to rake off.

INCANDESCENT MAPThe end result is startling, more

like an incandescent map than aradar picture; grass is reproducedas a bright, speckled expanse, flatconcrete as jet black, so the run-ways and taxiways of airports aredistinctly drawn. Resolution issuch that dead hares, six inch

countersunk runway lights andpuddles of paint only half an inchthick show up.

With ordinary radars an aircraftshows up as a blob, but ASMI"draws" its shape, faithful yenough for seasoned operators tobe able to recognise different

One of four Decca ASMI Bright Displays in the visual control room at Heathrow. Anyparticular section of the radar coverage can be displayed, enlarged as required.

types: and of course aJumbo stands out amile. Due to an effectalready mentioned, air-craft just off the groundthrow a "shadow" onthe grass and the exactmoment of take -off ortouch -down can be toldby the behaviour of thisshadow. Though theycan see directly when-ever the visibility isgood, many A.T.C. offi-cers have come to relyon ASMI all the time.

BRIGHT DISPLAYA problem common to

all radars but particu-larly applicable to thosein the Visual ControlRoom of an airport con-trol tower, is that theradar screen is difficultto see in bright light.This has been over-

come in recent years by the provi-sion of Bright Display, which em-ploys a television technique.

The Visual Control Room atHeathrow has just been com-pletely redesigned, the opportunitybeing taken to fit four ASMI BrightDisplays; one for the western andone for the eastern facing controlconsoles, one for the Ground Con-troller and one for the LightingController. The aerial itself is in aradome conspicuously mountedabove and a little to one side, atthe top of the Control Tower).

The radar in the form of azi-muth, video and trigger signals isfed to a P.P.I. display employinga small (11cm) but high resolutioncathode ray tube. The P.P.I. pic-ture is optically coupled to a vidi-con camera channel employing astorage vidicon pick-up tube. Thecamera channel will operate onvarious scanning standards with2:1 interlace, and may be inter-nally or externally driven.

The camera channel output canbe fed to any desired number oft.v. displays either directly or viaa t.v. distribution system or a t.v.link. The P.P.I. and camera chan-nel are specially compensated toensure a substantially uniformvideo signal over the total t.v. fieldarea.

The P.P.I. is provided with fourrange scales and an off -centringfacility, both of which can beremotely controlled if necessary,for example at the t.v. viewing


position. Thus, by off -centring inthe desired direction and switchingto the shortest range (to give thelargest scale) the whole displaycan be filled with a fairly smallsection of the original total cover-age. A useful facility when, forinstance, there has been an "inci-dent", such as a crashed aircraft.

OTHER TASKSThis leads to the fact that there

are, indeed, several subsidiaryuses for ASMI itself, in additionto the general surveillance role. Itcan be used, in conjunction withthe runway lighting, to assist pilotsto leave the runway at the desig-nated point, more quickly andwithout fear of collision with otheraircraft, thus speeding up opera-tions generally. It can ensure thataircraft and vehicles (both beingfitted with v.h.f. communication)do not encroach on the instrumentlanding system restricted area, astheir presence can cause distortionto the glide path and localiserbeams.

As we have seen, fire engines,ambulances and other emergencyvehicles can be directed to the

scene of an accident, which mightbe just ot.tside the airport. This isimportant in foggy weather; therehave been instances of precioustime being lost by such vehiclesbeing unable to find the spot.

Lastly, and this was probablynot in the manufacturer's mindwhen an ASMI was first installedat Heathrow ten years ago, is itsundoubted value to security. Ifhares show up, a gentleman witha bazooka will be only too appar-ent!

What of the famous hare? Bitsdrop off aircraft, believe it or not,usually inspection plates and so on,and when the ASMI detects one ofthese, a Land -Rover is sent out torecover the object before itobstructs an aircraft. On oneoccasion at Heathrow somethingwas indicated on the runway anda Land -Rover despatched, only tofind that it was a dead hare! Thosewith radar experience find suchsensitivity hard to believe, but itis true.

A lot of people are alreadytaking a second look at this un-usual radar that does a specific jobextremely well. I=1

04. tsiippesa-

The ASMI "radome" being lowered intoplace at Orly airport, France.

Wiper ModsHaving recently obtained my first car,

I found it annoying to switch the wind-screen wipers on and off during finedrizzle.

With that in mind, I decided to build anelectronic slave to do the job for me. Thesensor for this device comprised of apiece of stripboard wired in the form of"interlocking fingers", and was held inplace on the radiator grille with the helpof two elastic bands.

However, this did not prove satisfactory,as a droplet of water between two adjacenttracks on the stripboard, took an ap-preciable time to disperse.

This is where your colleague's idea,mentioned in Shop Ta/k, which was topaint a pattern of "interlocking fingers"on the windscreen using electricallyconductive paint gave me the idea ofwriting to you.

Although I have not tried the paint, I

feel sure it would work. For this reasonI've enclosed the circuit diagram for myoriginal idea, which although very simpleI hope will be of interest to your readers.

It is at this point I feel I should point outthe need for two models, i.e one forpositive and one for negative earthvehicles. Should a negative earth designbe fitted to a positive earth car, in heavyrain, a virtual short circuit across thebattery would be inevitable. A model ofthe correct polarity could not let morethan a current of 1.2mA (R1 -10kf) drainto earth.

J. M. Hayes,Keig hley,

Yorks.

A

TERMINALS A t II TO SECONNECTED ACROSSWIPER SWITCH

ov

Fig.1. Circuit diagram for a negative earthvehicle wiper delay. The terminals A d, Bare connected across the car wiperswitch. For positive earth vehicles TR1should be any medium power npn switch-ing transistor.

ThyristorIn the February issue of EVERYDAY

ELECTRONICS a design for a WindscreenWiper was published.

So far I have been unable to obtain thespecified thyristor. I would be grateful ifyou could list a supplier or an alternative.

G. L. Clifford,Warwickshire.

A suitable alternative for the thyristor isthe C126M available from Maplin Elec-tronics, PO Box 3, Rayleigh, Essex. Thelatest price we have is £1.42. In fact anythyristor with a rating of 10A at 50V wouldsuffice.

TriacI am in the process of building the

Enlarger Timer featured in the October '77issue of EVERYDAY ELECTRONICS. Onecomponent I cannot seem to obtain isthe 2N5756 triac.

I wonder If you could possibly adviseme where to obtain this part.

B. AppletonBucks.

You can obtain the triac from ArrowElectronics, Leader House, Coptfold Road,Brentwood, Essex. The cost is £2.04including VAT and post/packing.


SatifiIEN112

EINLETE3By. D. Ewards

INTRODUCTION

THIS unit came about as aresult of experiments in elec-

tronic music and musique con-crete. While the unit's musicalvalidity may be open to some ques-tion, tastefully used it can be putto good effect. For those who in-dulge in making tape plays, theWeird Sound Effects Generator canprovide a whole host of futuristicsounds to rival even those heardin the film Star Wars. The circuit,in fact, was designed with thecreative tape recording enthusiastin mind and the prototype, in ex-tensive use, has proved its worth.

STARTHERE FOR

CONSTRUCTION

The circuit is built on a piece of0.15 inch matrix perforated boardsize 15 X 9 holes. The layout of thecomponents on the topside of theboard and the interwiring on theunderside is shown in Fig. 1.

HOW IT WORKS

EFFECTOSCiLLATOR 2

ti

OSCILLATOR 3

eN,

It is advisable to leave the com-ponent leads untrimmed as thesecan form the wiring for the under-side connections. Leave the transis-tors until last. Unless you are anexperienced constructor, able tocarry out speedy soldering, a heat -shunt is recommended when solder-ing the transistors in place.

CASEThe next stage of construction

concerns the case. In the proto-type, a plastic hinged case measur-ing 90 X 90 X 30mm that was de-signed as a presentation case hasbeen effectively utilised by theauthor. Make the cut-outs in anysuitable case to suit the DINsocket and the three potentio-meters, and then fix these in posi-tion. The layout of these com-ponents within the case is shown inFig. 1, but this is not critical andmay be changed to suit individualrequirements. Attach a battery clipas shown and then wire up betweenboard and case mounted com-ponents as detailed in Fig. 1.

One important point to be awareof when selecting a suitable case,is the overall depth of VR1/S1body. The type used in the proto-type was a "midget" type whichjust fitted in the case. If a largerpotentiometer/switch assembly isobtained the height of the casewill need to be increased.

...,....,,...,. 7 >

Three oscillators running at different frequencies have their outputsconnected to form a composite output signal. The output signal of eachoscillator when combined with the other two is both affected by and affectsthe other two outputs. No oscillator however is dominant and a strangemixture of electronic sounds appears at the unit output socket.


Resistors COMPONENTS"....0 - -14R1 39ki1R2 1.5k0R3 391(0R4 1.5kflR5 39kilR6 1.5kilR7 1110

All 1 watt carbon ± 10%

PotentiometersVR1/S1 47041 lin. carbon with ganged on/off switchVR2 470k11 lin. carbonVR3 470ki2 lin. carbon

CapacitorsC1 0.047pF plastic or ceramicC2 0.1µF plastic or ceramicC3 0.01µF plastic or ceramicC4 0.1µF plastic or ceramic

SemiconductorsTR1 BC178 silicon pnpTR2 BC108 silicon npnTR3 BC178 silicon pnpTR4 BC108 silicon npnTR5 BC178 silicon pnpTR6 BC108 silicon npn

page 342

MiscellaneousB1 9V type PP3SK1 5 pin 180° DIN socket0.15 inch plain matrix board size 15 x 9 holes; battery connector to suitB1; plastic case approximate size 90 x 90 x 30 mm; control kiobs(3 off); connecting wire.

Components and circuit boardmounted inside the case.

TR1-TR6VIEWED FROMUNDERSIDE

e

Fig.1. Component layout for the circuit boardand interwiring to the controls and undersideof board.

SK1 VIEWEDLOOKING ATPINS

ESTIMATED COSTOF COMPONENTS

£3.00excluding

case

e VR3 VR1VR2

0 f-11-111. - 0-4-0 0

o'er. iiis 01 0,./ B1

0 0 0 0 0 0 0 0 00000 eri

2 1


L..

TPtBC.118

C'o niajuFRI39k 11

4704

TR2BC 108

R2/.5kI2

TR3sc178

C 2 0/pF

<70kti

BC 108

R4

1.54.17

TR5sc 178

C 3 0 01),F

IIEPrEC 'I

SC108

470;4

SI

C4Lar

SKI

9Vmoo

Fig.2. Circuit diagram of the Weird Sound Effects Generator

CIRCUIT DESCRIPTIONThe complete circuit diagram of

the Weird Sound Effects Genera-tor is shown in Fig. 2 and can beseen to consist of three distinct,similar sections, each being anoscillator of different frequencyfrom the complementary astablemultivibrator family.

The operation of each of theseoscillators is identical, so only thecircuit action of that comprisingTR1 and TR2 and associated com-ponents will be described.

When power is applied to thecircuit, capacitor CI initially un-charged begins to charge up via R7,R2, RI and VR1 with TR1 and TR2off (non -conducting). The capacitorcontinues to charge up until thevoltage on the base of TR1 becomesnegative with respect to its emitter.This causes TR1 to turn on (con-duct) and at the same time thepotential on TR2 collector, coupledto TR1 base rises and causes TR2to be biased on (conducting).

Under these conditions the capa-citor is presented with a dischargepath through TR1 and continues to

Although a DIN socket was usedin the prototype, there is no reasonwhy any other socket cannot besubstituted to be compatible withthe tape -recorder or other equip-ment that will be used with theWeird Sound Effects Generator.Finally connect the battery, fixthree control knobs and the unit isready for testing.

TESTINGConnect the unit via a suitably

terminated lead to an amplifieror tape recorder set to record and

discharge until the voltage on thebase of TR1 becomes positive withrespect to its emitter and turns off.The base bias of TR2 is thus re-moved and so TR2 turns off. Thecapacitor then starts to charge upagain and the above cycle of eventsrepeats, and continues for as longas power is applied to the circuit.

During each cycle, the voltagelevel on TR2 collector rises andfalls and this is coupled to the out-put socket via d.c. blocking capaci-tor C4 from a split collector loadR2, R7.

The frequency of oscillation,being the inverse of the chargingtime of Cl, is therefore governedchiefly by the setting of VR1 withR1 (since their values are muchgreater than the combination R2,R7 which completes the chargingroute).

Since all the oscillators share acommon load resistor R7, the out-put is a complex signal that is sub-stantially changed by the opera-tion of a single effect potentio-meter.

switch on the unit. Rotating eachcontrol knob in turn should alterthe sound heard. This conditionwill indicate that the unit is func-tioning satisfactorily and is readyfor use.

IN USEThe Weird SoundEffects Genera-tor consists of three separate

oscillators but there is no reasonwhy the number of oscillators can-not he increased to provide aneven greater range of sounds.

Different values of tuning capaci-tors would, of course, be requiredfor each of the additional oscilla-tors.

Alternatively by using only twooscillators coupled together andgiving the timing capacitors valuesof a few microfarads, the unit canbe used as a simple rhythm genera-tor. In this mode, of course, theremaining oscillator could be usedas a metronome. There is no needto rigorously adhere to the valuesof the timing capacitors suggested,try any values from 0.001 micro-farads to 10 microfarads (withelectrolytics, the positive plateshould be connected to the collec-tor of the npn transistor at eachoscillator). The higher the value,the lower the frequency of theoscillator.

Also, switches ganged with VR2and VR3 to switch these oscillatorsin or out of circuit as desired couldbe incorporated for extending thenumber of effects.

The author has used the proto-type generator to create electroniccompositions in conjunction witha mixer, tape recorder, organ, fuzzbox and waa-waa unit to produce"out of this world" sounds andeffects.

The output level and the wiringof the output socket makes thegenerator compatible with mostautomatic recording level tape re-corders (eg. Philips, as used bythe author) although it shouldwork equally well with othermachines.

However, in the case of an ampli-fier or similar high input levelequipment, a small booster ampli-fier should be added and the out-put for this could be wired to pin3 of the DIN socket SK1. Aseparate lead should then be madeup to allow this output to be fed tothe high input level equipment, ie.DIN plug with connections at pin3 and earth.

1=1


Readers' Bright Ideas; any idea that is publishedwill be awarded payment according to its merit. Theideas have not been proved by us.

RESISTOR CLIPSIt is sometimes found when measuring resistors

with a multimeter that body resistance across theprobes cause misleading readings. I thereforedesigned the following.

A small piece of plastic is drilled with two sets ofholes one above the other, four cable buckles are thenbent as shown and fixed over the holes. It is then asimple matter to connect the probes to one set, andthe resistors to the other set. The buckles can easilybe bent over to hold the resistor in place when takingmeasurements.

C. Ashcroft, Dorset.

0

0

METER CASEI have just discovered an economical case for the

Exposure Meter featured in the August issue ofEVERYDAY ELECTRONICS.

An empty plastic sticking plaster reel is used. Thel.d.r. is mounted in the centre of the tube and heldin by Araldite. The meter and switch are mounted inthe base of the reel so that the detachable base canbe used as a cover. Finally the case may be paintedto hide any printing on the surface.

S. D. H. Saunders, Surrey.

TIME DELAYHaving read the Flasher Bleeper article in the

September issue Of EVERYDAY ELECTRONICS, I thoughtthat a sensible improvement would be to incorporatea time delay. In normal circumstances the unit wouldnot operate, but if the car indicators were left onlonger than normal, the time delay would come intooperation.

PROBES

RESISTORS

0 HW R

R. L. Johnson. Leicester.

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-REED C R U 0

R AffENuA-floN

JACKPM&FAMIIILY... BY DOUG BAKER

YES, I KNOW YOU'RE HUNGRY AND WANT YOURDINNER, DARLING. WE ALL DO. BUT THE ULTRASONICREMOTE CONTROL SYSTEM ONLY SWITCHES OFFELECTRICAL APPLIANCES - I'M AFRAID IT WOULDN'T._

t


The Extraordinal,

Experi-ments

of

ProfessErnes

Evemureby phony John Bassett

BOB has been assisting MissLilian Whiteley, in some

"Extraordinary Experiments inDowsing" in the presence of theProf. and Dr. Angus R. Paterson,who is highly sceptical of the pro-cedures of dowsing. Lily has beenusing a Radiesthetic Preamplifierto assist her in the identificationof concealed substances, and Bobhas queried the function of thispre -amplifier.

DISTURBING INFLUENCEThe Prof's experimental Robot

soldered a small copper plate toeach end of a length of copper wireof just over 1 metre, and acting onthe Prof's instruction, has arrangedthis wire in place of the Radies-thetic Preamplifier with its copperinput plate and output plate.

This is to test Bob's surmise thatthe influence which Lily picks upby dowsing will travel along aplain piece of wire without theassistance of the Radiesthetic Pre-amplifier.

Following the same procedure asbefore, Bob secretly placed asample of one of the five sub-stances onto the copper plate atone end of the wire, concealedfrom Lily by means of an opaquescreen. The wire led past thescreen to the plate at the other

end, over which Lily now swungher pendulum.

"It's no good, Bob", sheannounced, "I'm not getting anyresponse at this end".

Bob was astounded. "I felt surethat it would work!" he said inpuzzled disappointment.

"I thought that, if this influencewould travel along a conductingwire with the assistance of the pre-amplifier, it should equally do sowithout one!"

"Actually I think you're right",Lily told him, "But there is afactor which you have not allowedfor. It is the disturbing influenceof the nearby samples in their con-tainers. If you move the containersaway to a distance of severalmetres I'll try again."

Bob placed the five sample con-tainers on a small tray and re-moved them from the vicinity. Afew moments later Lily was ableto correctly identify the samplewhich was concealed from her bythe opaque screen, but connectedby a wire to the copper plate overwhich she swung her small perspexpendulum.

NEW ARRIVALSHowever, this had been wit-

nessed by an increased audienceas the Prof. had meanwhile

admitted two more visitors to thelaboratory; Tom and Maurice, whoenthusiastically devoted most oftheir spare time to the running ofevents for their favourite charityorganization.

Lily explained to the smallassembly: "The influence of thesample on the plate at one end ofthe wire travels along the conduc-tor to the plate at the other endand by dowsing at this end I amable to identify the sample at theother end. But this process is sub-ject to a number of disturbinginfluences, such as the presence ofa nearby sample of the same sub-stance in a container. One of thefunctions of the Radiesthetic Pre-amplifier is to enable me to over-come disturbing influences such asthese-and it helps me to obtainmore accurate results."

DOWSING COMPETITIONTom and Maurice were very

interested. "We are running yetanother Charity Fair", saidMaurice, "Bob I wonder, MissWhiteley, whether you might verykindly give a dowsing demonstra-tion as one of the events there."

"That's a really good idea,Maurice", Tom chipped in enthu-siastically, "Maybe you could also


help us to run a Dowsing Compe-tition so that members of the Pub-lic could try their hand. We couldeasily fix up one or two tables withsmall opaque screens, and a wireleading to two copper plates justlike you have here!"

"Maybe!" Angus thought outaloud, "Maybe the Prof. couldconstruct a Dowsing Robot."

"Angus!" warned Lily, "For thisCharity Fair, we're going to dressyou up in a Robot suit made ofsheets of cardboard covered inshiny aluminium foil, with flashinglights, electronic bleepers and anelectronic Cyberman voice pro-jector". She turned to Tom andMaurice, "I'd be delighted to helpat the Charity Fair, boys, maybewe could arrange details later asAngus and I must leave now.Thanks, Prof., and Bob for mend-ing my preamplifier."

Fig. 1. Arrangement of equip-ment for the "dowsing" experi-ment without the inclusion of apreamplifier.

WORKBENCH

SAMPLE

COPPER.PLATE

might be able to advise on how toconnect them up electrically tooperate various amusing gadgetsor competitions".

Angus placed a coin in the slot,and, to his surprise the box beganto rock and move, with a mys-terious grinding noise. A flapopened and out slunk a greenplastic hand which slowlyapproached the coin, grabbed itand suddenly slammed back intothe box with a bang! "I think I'llbe leaving now!" remarked Anguswith mock horror, then, "Cherrio,everyone, Au Revoir". Angus andLily left the Laboratory.

DRACULA'S BANKThe Prof. now picked up the

small black plastic box. "So this isDracula's Bank! !" he remarked,examining the writing on one side,

OPAQUESCREEN

0'

COPPERPLATE

"It has been a most interestingvisit", said Angus, then suddenly,"Hey! Whatever is that?" His eyeshad fallen on a brightly colouredsmall box which Maurice hadplaced on one of the laboratoryworkbenches.

It was garishly labelled "TheThing-from the Crypt!" Mauriceopened the packet and brought outa small box with a slot in the top."This is what I have come to con-sult the Prof. about," he toldAngus, "We are thinking of usingseveral of these as coin collectormechanisms at the Charity Fairand I have the idea that the Prof.

"What a good idea of yours to con-vert it for the benefit of Charity.

From my initial observations itwould seem that the mechanismin this box could be used for twochief functions when activated bythe donation of a coin. One wouldbe to initiate or enable an actionor the start of a small game, dis-play or competition sideshow.

The other function would be toact as a timing mechanism depen-dent upon the time taken fromfirst placing the coin in the slot,until the coin is snatched away by"The Thing". Of course timingaccuracy may not be very good.

but is probably adequate foramusement purposes".

"Prof., we tried to look insidethe box in order to see whether wecould work anything out for our-selves, but the box is sealed withblobs of molten plastic which wecould not cut away as the bladein our craft knife has beenbroken."

"Yes, Maurice, I can see them".The Prof. had removed thebatteries and also unscrewedthe bottom of the box. After tip-ping out a small heap of coins hecould see that further access tothe mechanism was sealed with afew small blobs of molten plastic.

"I will be able to remove thoseseals quite quickly, Prof." heoffered. "Okay, Tom, and you canuse some of my tools if you like,as some of the plastic which youmust cut away is rather awkwardto remove. Meanwhile I think Bobhas something he wishes to askme".

MORE INFO'The Prof. had noticed that whilst

he was discussing the mechanicalmoney -box with Tom and Maurice,Bob had become increasinglyagitated, and now looked aboutready to burst! "Prof!" he said, "Imust know more about the Radi-esthetic Preamplifier! What circuitdoes it use? When I mended it Icould see that it did not look verycomplicated, but I am really keento know more about the circuit!'

To be continued

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GEORGE HYLTONbrings it

ownMeasuring Low Resistance

AREADER, sent us a circuit for usingan ordinary 1mA meter to measure

low resistances. He has doubts aboutwhether it's right.

First, the problem. The circuit(Fig. 1) is first set up without the un-known resistance R.. The variable(potentiometer) R. is adjusted so thatthe meter (whose resistance is Rm)reads some convenient value, suchas exactly full scale.

When R. is connected (by closingthe switch), some of the current isdiverted from the meter. The resis-tance Rs can then (according to ourreader's source) be calculated by aformula which includes the two meterreadings and Rm but not V or R..

When R is connected the resis-tance of the circuit as a whole mustfall. So the current must increase.Doesn't this change of current affectthe measurement?

Ohm's LawWhen confronted with this sort of

problem there are two ways of attack.One is to do a lot of algebra and geta thorough understanding.

The other method is to put someplausible circuit values on the diagramand see what happens using nothingmore complicated than Ohm's Law.I've done this (bracketed values). If Vis 1V, then to set the meter to 1mA(R. out) it's necessary to have a totalresistance of 1000 ohms (1V/1mA =1k0). The meter is 10012 so Rs mustsupply 9000.

When R. is connected across themeter there are two 1000 in parallel,making 500. With R. the total is now9500 and the current is 1V/95011 =1.052mA. Half this goes through themeter, which then reads 0-526mA.The question is, do these figures,inserted into the formula, provide

tearoth

the correct value for R.? The formulais:

R=(meter resist.) x (2nd meter reading)

(1st meter reading) - (2nd reading)

Without doing an exact calculationyou can see that this gives the wronganswer. The right answer, we know,is 1000. But the formula says it is 1000times 0-526/0-474. Since 0.526/0-474is greater than 1, the answer is greaterthan 10011 (actually about 1110), sothe system "reads high".

To produce the right answer, thecurrents at the top and bottom of thefraction must be equal. One can'tavoid the suspicion that the formula"wants" them both to be 0-5mA Toobtain this value the total currentmust not change when R. is con-nected. This is impossible, but youcan get as close to it as you likeby making V and R. large.

In this case, R. "swamps" theeffect. If V = 10V, then for 1mA, thetotal resistance must be 10k, of whichR. supplies 99000 and Rm 1000.Connecting R. reduces the totalresistance to 9900 + 50 = 99500,which is low by only 0.5 per cent.The error is roughly twice this, orabout 1 per cent, which is accept-able for most practical purposes.

Linear ScaleThe circuit has one distinct ad-

vantage. It does not rely on the accur-acy of the meter. So long as the meteris linear, the system works. Even if themeter calibration is "out" by quite asubstantial amount it still works.

But there is also a disadvantage.If R. is small compared with Rm, thenon connecting R the reading fallsto a very small value. This makes ithard to get an accurate reading.

For example, if Rx is 1/100 of Rmthen the reading falls from full-scaleto less than 1 per cent of full scale.This is less than one small scale

Fig. 1 vov)

Fig. 2V

175190001

division on most meters, and cannotbe read accurately. In fact the systemis most accurate when Rx is aboutequal to Rm.

Many of the cheaper multimetershave only a few current ranges, forthe higher currents, together with agood selection of voltage ranges. Inthis case, a different approach to theproblem of measuring low resistanceis preferable.

Current/VoltageIf a known current is passed through

a resistance (Fig.2) then the resultingvoltage drop tells you what the re-sistance is. If, for instance, you pass100mA through 10, the voltage dropis 100mV (0-1V).

So, a multimeter can be used firstto set the current then to measurethe voltage drop. A meter whichreads 100mA full scale is likely to havea low resistance, perhaps about 10.If R, is around 1000 the meter re-sistance has little effect on the cur-rent. If V is 9V, R. must be 900 for100mA, enough for most purposes.

When the meter is switched to avoltage range and connected acrossRx to measure the voltage drop, itwill not seriously effect the accuracyso long as it takes only a small amountof the current. Most multimeters take1mA or less for full-scale deflection.So if 90mA is flowing through Rs theaccuracy of measurement won't beseriously impaired by the 1mA bledoff by the meter.

To measure R., then, you use a cur-rent range to set a suitable currentand a voltage range to measure thevoltage drop. For a current of 100mAand voltage range of 100mV (0.1 V) aresistance (Rx) of 10 gives full-scaledeflection (f.s.d.).

Smaller resistances give proportion-ally less deflection: 5052gives a readingof 50mV (0.05V) for example.

This scheme has the disadvantagethat any inaccuracy in meter cali-bration affects the result.

55 1 Everyday Electronics, March 1978

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35(i Everyday Electronics, March 1978

TVNE/NVINI MOW Drmokomoassroar

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Give your friends a warm welcomeHere's the Chroma-Chime-a perfect exampleof British scientific achievement brought rightto your own front door. Now-you can beamong the first enthusiasts in the world tobuild your own electronic musical doorchime-a door chime with no moving parts.There are 24 of the world's favourite and bestknown tunes pre-programmed onto themicrocomputer chip so that all you have to dois to set the Chroma Chime's built-in selectorswitches to a code to index the "tune of thecay" from the repertoire:

GreensleevesGod Save the QueenRule Britannia'Land of Hope and GloryOh Come All Ye FaithfulOranges and LemonsWestminster Chimes'Sailors Hornpipe

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Bold IMPfrie Ow-Avec

Mime for

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'These tunes play longer if the push button is kept pressed

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Not only visitors to the front door will be amazed, if ycUlike you can connect an additional push button for a backdoor which plays a different tune!

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NO LICENCE EXAMS NEEDEDTo operate this miniature, solid-stateTransmitter -Receiver Kit. Only £9-75plus 25p P&P.'Brain -Freeze' em with a MINI -STROBE Kit, pocket -sized 'lightningflashes'. vari-speed, for discos and',allies A mere £4.10 plus 20p P&P.Experiment with a psychedelicDREAM LAB, or pick up faintspeech/sounds with the BIG EARsound -catcher; ready-made multi-function modules. £5.00 each plus20p P&P.LOTS MORE! Send 20p for lists.Prices include VAT. (Mail orderU.K. only).

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Service SheetsB ELL'S TELEVISION SERVICE for servicecheats of Radio, TV etc. 78p plus 11A8.Colour TV Service Manuals on request.SAE with enquiries to RTC 100 King'sRoad, Harrogate, N. Yorkshire. Tel: 042361581165,

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AUDIO MIXERA superb sterec audio mixer. Itcan be equipped with up to 16

input modules of your choice andits performance matches that of thevery best tape -recorders and hi-fi

equipment. It meets the requirementsof professional recording studios,FM rad o stations, concert hallsand theatres. Full construction

details in our catalogue. Acomponent schedule is available

on request

I it i! .

f

10 CHANNELSTEREO GRAPHIC EQUALISERA new design with no difficult coils

to wind, but a specification that putsit in the top-flight hi-fi :lass. All this for

less than £70 including. fully punched andprimed metalwork and woodwork. Send

fc r our component schedule now.Ft II construction details price 40p.

0 ;: 11 a

'7

- `1,-.

PEDAL UNITA completely self-contained pedalunit. 13 -note, 2 -Octave range. 4 organ

stops. It can be added to anyorgan. A really unusual extra isthe bass guitar stop which usesfour envelope shapers to givea real bass guitar sound. Amust for the solo guitarist.Full construction detailsin our catalogue-postthe coupon below now!

EOARDS AND PCB'sPages 59 to 61 of our

catalogue show you ourrange of Veroboaris andS-Decs or if you prefer to

make you' own pcb's a rangeof e-ching systems including

the nc vel etch -in -a -bagsystem pl JS prirted circuit

transfers fo that pro'essionalis, to your boards.

Who says the MaplinCatalogue's worth having?

in our 'musts' for readers -to -collect list"-P.E."contains ... just a7out everything the DIY

electronics enthusiast requires"-'.W."probably the most comp-ehensive catalogue we hay,

ever come across"-E.E."has been carefully prepared and is very well

presented..-R.E.C."make the job of ordering components an easy, accurate

and enjoyable pastime"-P.W."Only one word describes the publicatIor-superbl-

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THE 'DRUMSETTE' RHYTHMGENERATOR

Organists, pianists, guitaristsan automatic drum set to accompanyyou! Nine highly realistic instrumentsplay fifteen different rhythms. Fifteenrhythm -select touch switches and atouch plate for stop;start without

rhythm change gives absolute easeof operation. Build it yourself for

under £65 including smart teak -effectcabinet. See it and hear it in our shop!

Send for full construction detailsnow: MES 49 price 25p

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look! here's how · complete with insulated probes. leads, battery, circuit diagram and...

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