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J U L Y1 9 7 8 V o l u m e3 1 N o . 1 1 PublishedMonthly (2nd of precedingMonthl First Published | 947 lncorporating The Radio Amateur Editorial and Advertising Offices 57 MAIDA VALE LONDON W9 ISN Telephone 01-2866141 Annual Subscription: f7.0O, Overseas t 8 . O O( U . S . Aa. n d C a n a d a$ 1 5 . 0 O )i n c l u d ing postage.Remittances shouldbe made payableto "Data Publications Ltd". Overseas readers,please pay by chgque or InternationalMoney Order. Technical Queiles, We regret that ws aro unable to answer quoriesother than thos€ arising from articles appearing .advise in this magazine nor can we on modifications to €quipment described. queries regret that We such cannot be answerod over tho telephone; they be must submitted in writing and accompanied by a stamped addreesed lor reply. onvelop6 Correspondenceshould be addressedto tho Editor, Advertising Manager, Subscription Manager or the Publishgrsas sppropriEto. Opinions expressod by contributors are not necesltarily those of tho Editor or proprietors. Offset. P u b l i s h e di n G r e a tB r i t a i nb y t h e P r o p r i e t o rasn d P u b l i s h e r s ,D a t a P u b l i c a t i o n sL t d . 5 7 M a i d a V a l e , L o n d o nW g ' l S N The Radio & Electronics Constructor is orinteb b y S w a l e P r e s sL t d . JTILY 1978 652 NEWS AND COMMENT 656 3-WAY TOUCH BUTTONS - Suggested Circuit by G. A. French 658 ULTRASOUND DISPLAY SYSTEM by Michael Lorant 661 SINGLETUNED CIRCUIT F.M. RECEIVER bv P. R. Arthur 662 DECIMALRANDCVALUES by R. D. Smith 668 RECENT PUBLICATIONS 670 C M O S D I G I T A L F R E O U E N C YM E T E R Part 1 (2 parts) by R. A. Penfold 672 T H E R M O M I G R A T I O N b v M i c h a e lL o r a n t 681 SHORT WAVE NEWS b y F r a n kA . B a l d w i n 682 Talegrams Databux,London '' Data Publications Ltd.. 1977. Contents may only be reproducedafter ':btaining prior permissionfrom the Editor. Short a b s t r a c t s o r r e f e r e n c e sa r e a l l o w a b l e provided acknowledgement of source is given. Pto d u ctio n.-Web CORDLESSHEADPHONE RECEIVER by A. P. Roberts For DX Listeners WATCH THE WEATHER bv T. F. Weatherley 684 CLOCK AUTO-DIMMER bY R. A. Penfold 68G BOOK REVIEW 689 SCAN AND FLYBACK RECTIFICATION - In Your WorkshoP 690 RADIO TOPICS by Recorder, 696 iii E L E C T R O N I C SD A T A N o . 3 5 (For The Beginner- Simple Measuring Bridges) T H E A U G U S TI S S t . ' E W I L L B E P U B L I S H E DO N 3rd JULY CORDLESS HEADPHONE RECEIVER By A. P. Roberts * A unique design mounting the electnonicsonto the headphones * Caseonly 2t*2x1in. app. * No trailing wires Although initially designed and built for its A.M. RECEIVER novelty value, this receiver does have advantages over sets which feed a pair of headphonesor an Obviouslythe output of the setwill not be of true earphonein the conventionalway. this little set Hi-Fi quality as the receiveris an a.m.model.AIso, differs from conventionin that the electronicsare the output is only monophonic, with the two -wired built into the headphones, or perhapsit would be headphonesbeing in series.Nevertheless, more accurateto sav that thev are built onto the results are goodwith regard to quality, and the set headphones. A case-whichm'e-asures only ?1.5 by comparesvery favourablywith most otherportable 49 by 24.5mm. housesa completelyself-contained receiversin this respect,regardless ofwhetherthey fully tuneable medium wave receiver,including a are of the loudspeaker or elrphone output varieti. -course ferrite aerial and battery. As can be seenfrom fhe It would of be pbssible to^ feed moit accompanyingphotograilhs,the set is mounted on monophonic-receiversinto-a pair of inexpensive one ,earpieceof a pair of inexpensive8o stereo stereoheadphonesand obtain very similar iesults, neaopnones. but there would then be the disadvantageof a trail652 RADIO AND ELECTRONICS CONSTRUCTOR Fig. l. f he circuit of tha headphone receiver. The headphones dre two 8Q units in series and give adegudte volume after the a.f. provided amplification by TRI 2 zN4t4 L6d-outs ing lead between the headphonesand the receiver. Ordinary simple earphone receivers, with which this set is roughly comparable in complexity, also suffer from the same disadvantage. One might expect this radio to be rather difficult to build due to the small size into which the electronics must be condensed. However, this is not really the case, and construction of the project is quite simple and straightforward. All the components are standard readily available items, and power is obtained from a single HP7 1.5 volt cell. The set is therefore very economical to run, esneciallv when one considersthat the cell will have a iong liie even if the receiver is used extensively. TRr BCrO9C BC rO9C Lcod-outs taken by the output stage.Someconstructorsmay be worriedby the fact that a standingcurrent from the output stage flows through the headphones. This current is not sufficiently high to adversely affect the performanceof the headphones, evenin the long term. The transducersusedin inexpensive stereoheadphones are almost invariablesimilar to miniature 8o loudspeakersof the type that are found in small radib sets,cassetteriiorders and similar items. T H EC I R C U I T As can be seen from Fig. 1, the circuit of the receiveris extremelysimple, and is basedon the popular2N414 integratedcircuit.This devicecontains most of the componentsfor a sensitive medium wave t.r.f. receiver,including the r.f. amplifier, detectorand a.g.c.circuitry. A ferrite aerial and tuning capacitor are required,and theseare Ll and VCl respectively.Cl nrovidesan earth return oath for one side of the tuned circuit, and Rl is ihe bias resistorfor the 2N414.R2 is its.loadresistor,and it is acrossthis component that the audio output signal is develoned.C2 is an r.f. filter canacitor. The'2N414 providesan audio output level of about30 millivolts r.m.s.from a sourceimpedance of a few hundredohms.This is not really sufficient to adequatelydrive 8(l headphones, and someadditional audio amplification must be employed. The amplification is provided by TRl, which is wired as a high gain common emitter amplifier. TRi is biasedby R3, and the headphones form the collector Ioad. C3 provides interstage coupling betweenthe 2N414 and the transistor,whilst C4 rolls off the high frequency response.This is necessary as the frequencyresponseof TRl would otherwiseextendwell into the r.f. spectrum,which would almost certainly result in instability (especiallywhen one considersthe very compact componentlayout which must be used). S1 is tire on-offswitci"r, and no supplydecoupling componentsare required. The current consumption of the receiveris about 8mA. most of which is JULY 1978 Resistors Rl 100ko ! watt 51h R2 2.2ko pre-setpotentiometer,0.1 watt skeleton,horizontal R3 47kcrI watt 5% Capacitors Ct O.Otpntype C280 (Mullard) C2 0.l1tF type C280 (Mullard) C3 0.1pFtype C280 (Mullard) C4 3,300pFceramicplate VCl 20-250pFtrimmer (seetext) Inductor L1 ferrite aerial (seetext) Semiconductors 1cl 2N414 TR1 BClOgC Switch S1 s.p.s.t.sub-miniaturetoggle Miscellaneous Verobox type 75-1469-L Stereoheadphones, 8cr per headphone Veroboard,0.1in.matrix Trimmer converter(seetext) Control knob 1.5 volt cell type HP? (Ever Ready) Ferrite rod, 2] x fin. (seetext) 32 s.w.g.enamelledcopperwire (for Ll) Insulatingtape, soldertags,wire, etc. 653 are cut to about 50mm. in length, and then the enamel insulation is scraped off the ends of these Ieads so that they can be tinned with solder. If difficulty is experienced in obtaining |in. ferrite rod, it is in order to use the more readily available rod having a diameter of Smm. This is cut down from a longer piece in the same manner as the iin. rod, and the winding has the same number of turns. There is iust sufficient room in the lavout for 8mm. rod. Bondsof insulotiontope Fig. 2. Details of thc ferrite rod aerial winding FERRITE AERIAL Commercially produced ferrite aerials all seem to be far too large for the present applicaticn, and so it is necessary for the aeriaf to be homeconstructed. Details of this component are provide d' fihneF i g . 2 . aerialis wound on a 2l by lin. ferriterod. It w i l l p r o b a b l y n o t b e p o s s i b l e - t oo b t a i n a r o d o f t h e required Iength, and so it will be necessaryto cut a piece from q longer rod. Ferrite is an extremely hard and brittle substance which cannot be easilv cut through with, say, a hacksaw. It should b-e possible to cut a deep groove around the circumferenceof the rod at the point where it is to be cut, after which the rod can be easilv broken at this point by tapping it gently against ihe edge of the bench. This does not always produce a very neat break, but any rough protruding edgescan be filed off, and any general roughnesswill not affect per[ormance. The winding consists of 80 turns of 32 s.w.g. enamelled copper wire wound around the middle third of the rod in a single layer. The turns should be spaced as closely together as possible. If it is found that the length of the winding is slightly different from the fin. shown in Fig. 2]so thaiit is not fully central on the rod, this is not of importance. Bands of insulating tape are used to hold the lead-out wires in position and to thus prevent the winding from springing apart. The lead-out wires CONSTRUCTION Most of the components are assembled on a 0.1in. matrix Veroboard panel which has 14 holes by 7 copper strips. The component layout of this panel is shown in Fig. 3. Start by cutting out a panel of the required size using a small hacksaw, and then make the single break in the copper strips. No mounting holes are drilled in the board. Then solder the components into position. Note that R2 is a skeleton potentiometer having 0.2in. spacing between track tags, and 0.4in. spacing between track and slider tags. The general arrangement of the receiver can be seen from the accompanying photographs. A Verobox type 75-1469-L is used as a housing, and as the receiver has been designed to fit into this case it is probably best not to attempt to use an alternative. The ferrite aerial is glued in position at the top of the case using a good gap filling adhesivesuch as an el)oxv type. The HP? cell fits into the space opposite this at the bottom of the case.A 250pF mica compression trimmer is used as the tuning capacitor, and this is fitted with a "trimmer converter" which replaces the adjusting screw and has a ]in. shaft for a knob. The trimmer and the trimmer converter are available from Home Radio (Oomponenls) Ltd. The tuning capacitor is mounted on the extreme right hand side of the case, and its two tags must be bent forwards slightly so that they are not obstructed by the ferrite aerial. The tuning capacitor is mounted by the bush and nut which are situated at the rear of this component. It does not matter which way round the connections are made to the tuning capacitor and to the ferrite aerlal. RADIO AND ELECTRONICS CONSTRUCTOR S1 is mounted towards the top of the left hand side panel of the case, and it is important that this be a sub-miniature toggleswitch as there is very little space to accommodate it. An entrance hole for the headohone lead is made in the side of the case -i u s t b e l o w S 1 . When all this has been completed the component panel can be wired up to the rest of the unit. The banel fits into the spice in the centre of the case. The headphone lead is crtt down to only about 5in. or so in length, and then it is soldered into the circuit. Only two of the three wires in the headphone Bottcry pos Bottcfy :; /: frl- [n' t tot to |tela| t YI _ H. vD - I to a oto neg headohone lead may initiallv make temporary connectiirns from the 6oard to its jack plug. The wire mav then be cut and finally fitted after tests have beeh comuleted.) The baitery leads are terminated in 68A solder tass and theie are held against the appropriate baiterv terminals with the aid of a rubber band' Take 6are to ensure that the cell is connected with correct polarity. The cell terminals will be marked with their respective polarities. 'fhe lid of the case must be drilled with a ftin. diameter hole through which the trimmer converter shaft can nass. Thd case lid can then be screwed into nositidn and the trimmer converter shaft fitted with'a small control knob. However, this cannot finally be done until R2 has been adjusted to the correct settlng. When com*pleted, ' wthe h i c hreceiver is -glued to the the headphone lead headnhone from .*"rner. It will need to be mounted fairly low down so that it does not obstruct the headband assembly, ancl it is nreferable for it to be mounted on a flat surface. Ho*eve., if an epoxy adhesive, or other tvn e h a v i n q a s o o d e a p - f i l l i n g - a b i l i t y ,i s e m p l o y e d 'should be p*ossible'toobtain a good bond to a ii if necessarY. surface, curved ffiffi? o la o\ ?*, oo ADJUSTMENT R2 shouldbe adjustedsothat its slideris at apB n E F G proximately the ientre of its track - before the is" initially switched on and tested. If i;;"i;;; ,u.uftr .""- to be Jatisfactory,adjustingthe slider ,,1 ttz slichtlv in an anticlockwiiediriction will pr,,ll;rlllvIr,,ducea srnallimprovementin sensitivitr,,rn<lseiectivity.l)o not adjustR2 too far in the , r i r t i c l o c k w i s ed i r e c t i o n , t h o u g h , a s t h i s w i l l lr,'rrlt irt the circuit becoming unstable.Also, Fis.3.Mostorthacomponentsarawiredup-o.7 f"ri,l i:,,[,,]';til:11]f,,"t*tr:".3$iffit"?'rnltt;; thetaYout emptovins asmattveroboard,Panet' ii;fll,tlrj:i.,.';lixr*lxwznirl:tut'u"tt : I p I cableare actuaily connected;the commonleadbeinnin"o*d. It is possibleto determinewhich leadis the iommon one by inspectingthe wiring at the iack plug. The commoniead iJthe one which conhecis'to[he main part of the barrel of the plug,and who ii is usuallv coloui codedblack. (Constru6tors preferto check out the receiverbeforecutting the Jt LY 1978 If there is any sign of instability when first testingthe receivLr(iuch as a whistling sound as the set is tuned acrossa transmission),ther R2 slider should be adjusted slightly in a clockwise directionto eliminatethis instability.Do not adju.st it any.furtherin the clockwisedirectionthan is abthis wotlld causea needless solut-elynecessary,^as degradationof performance.R2 should be set up rvith a new HP7 cell connected. ri55 NEWS AND A N A L O G U E M A G N E T I C T A P E R E C O R D E RL O G S VITAL DATA IN SEA WAVE ENERGY RESEARCH An EMI'lechnology high-performance magnetic tape recorder is playing an important role in the I)epartment of Energy's continuing R&D programme for winning power from the waves around Britain's coast. One of their portable S87000 analogue instrumentation recorders is bei n g r r s e dw i t h e q u i p m e n t p r o v i d e d b v T h e N a t i o n a l M a r i t i m e I n s t i [ r r t i r( N M i ) a n d t h e B r i t i s h H o v e r craft Corporation for experiments being undertaken in.the.Sole^nt by Wavepower Ltd., a io-pu.ry comprising Sir Christopher Cockerell (inventbr of the Hovercraft) and E.W.H. Gifford and Partners ( < r o n s u l t i negn g i n e e r s ) . The Wavepower project* is concerned with evaluating the effectivenessof the 'Cockerell Contorrring Rafts' (seediagram). These have a seriesof hvdraulic motors/pumps situated between each raft in order to convert the energy of the raft motion into high pressurein a fluid. Initially, trials of g_thscale ralt strings were conducted by fhe British Hovercraft Corporation in test tanks oir the Isle of Wight, and- now {th scale trials are being held at seain the Solent. The recorder is being used to monitor a number o.f.critical parameters relating to the design, efficiencv and survival capabilily of the Coikerell O o n t o u r i n gR a f t s ( C C R ) . B a s e d o n a n a n a l y s i s o f the data obtained, a full size prototype riill be d e s i g n e df o r f u r t h e r e v a l u a t i o n . For the fith scale experiments the SE7000 is housed, together with all the other measuring and real-time data analysis equipment, in a 20-to; ISO container secured in the hold of a moored 60ft seagoing barge. This barge is linked to the CCR by means of an electrical'umbilical' cable. The tape transport is being used to record pulse code modulated (pcm) signalsderived from 64data channels. With such a large number of channels, conventional analogre tape recording would have fg.n - prohibitiv-ely expelsive; pcm techniques (developedby Microconsultants Ltd in association ,r'',i"li'^ii".'9!ll3il''lt ii5io"". The Cockerell Contouring Rafts are hydraulicl electrical systems whereby the waveinduced movdments of the articulated sections are converted into a hydraulic pressure, which is then itself converted into electrical energy using an altemator. with the NMI) were therefore used. since these enable up to 21 data channels to be recorded on only one track of the 14-track recorder. The measurements being monitored relate to environmental conditions such as wind and currenr speeds and directions, tide and wave heights and directions (using a device developed bv th.-eBritish Ship Research Association), baiomeiric pressure etc. Other parameters being recorded include the mooring fgr.ceson the CCR pontoons, loadings on ttre raft hinges, hydraulic pressures, electlical power output and so on. Analysis of the recorded data is finally carried out at one of the NMI's land bases, using Honeywell mainframe computers. * I-n all, four - each based on a R&D projects different mode of wave-to-mechanical energy.transfer- are being financed by the s2j-million DofE wave inersv resdarch programme. The other three - 6eine undertaken by both private industry and Government bodies - concern the-use of Salter Ducks, Oscillating Water Columns and the Russell Rectifier. M U L T I C O R E S O L D E R SA T T H E BRITISH INDUSTRIAL EXH|B|T|ON, HONG KONG Picture shows H.R.H, The Duke of Kant discussing Mufticore Solders Limited Solder Creams with Gordon Arbib the company's Managing Director. Also shown are four axecutives from Multicore's Hong Kong Distributors Rory Electric Co, Ltd It wasa questionof History repeatingitself when H.R.H. The Duke of Kent met Gordon Arbib, ManagingDirector of Multicore SoldersLimited, at the British Industrial Exhibition in Hong Kong. His Royal Highness stopped at the Multicore Solders' Stand to see demonstrationsof their Oxide-FreeSolder Cream and specialsoldersfor the solderingof aluminium. Twenty-eightyears ago QueenMary visited a Multicore Soldersexhibition stand at the Radio Showin Londonwhereshemet Mr. Arbib's father, the late Richard Arbib. The Hong Kong exhibition attracted exhibits from 120 British Companies. RADIO AND ELECTRONICS CONSTRUCTOR COMMENT ADDIITGSOUND TO CANCET NOISE Oneof the problemsof workingin industrial environments is that of noise, and the usual approach towards obtainine a reduction in this particular field of audio pollution has been the use of soundproofing materials,baffling and the like. Plus, of course, attempts to reduce the level of noise generated at the sourceitself. All such devicesare of a passive nature, and it is of considerableinterest to learn. from the National Research Development Corporation, of stepsto reducenoiseby an active method. A further noise is added to the original noise and achievespartial cancellationof it. To date, the technique has only been developedto the state where noise at low frequencies can be dealt with. The work carried out has beenon the low frequency noise from gas turbines, which have presented what has always up to now seemed to be an insoluble problem. However. recent research at ChelseaCollegeinto a systemdevised by Dr. M. A. Swinbankssuggests that the answer mav lie in absorbing soundby generatingadditional sound to interfere with and cancel the offending frequencies. Work on the Swinbankssystemis currently being financed by the National Research Development Corporation and. at the present state of development,the method appears to be particularly appropriate for absorbingsoundin air ducts and fluid pipelines. It is commonly acceptedthat a loudspeakercan be used to cancel soundwavesin one direction at the expenseof doublingthe soundlevels in another. but the fact that a to abloudspeakercan be employed -forming a sorb iound energy (thus "sound sucker") is lesswell known. Assuminsthat the noisefrom a fan, for insta-nce,is travelling alo4g a duct and that it is required to silencethis noise before it reaches the area servicedby the duct, the following method is adopted. Two microphonesare placed in the duct and their outputs are combined to detect only the sound arriving from the noise source.The microphonesignal is electronically processedand is thpn fed to two Ioudspeakersmounted on the wall of the duct. These operate to produce a "plane wave" which travels down the duct towards the zoneto be silenced but not towards the original noisesource.The plane wave has identical amplitude but oooosite ohase to the unwanted sound and the end result is silence in the duct downstream from the l o u d s p e a k e r sw i t h a n u n a l t e r e d sound field upstream. Under such circumstancesit can be shownthat the sound energyis first "trapped" between the loudspeakers,and is then progressivelyabsorbedby the loudspeaker nearer the mlcrophones. The experimental system has sicnificantlv cut random noiseover th-eranee of gO to 200H2 in a duct measuring approximatelv 300mm. square.WittL i fan havirig a broad noise peak at 160H2,the attenuation over the rangewas t4dB. The precise attenuation and frequency iange of operationis largely governed bv the need to ensure accurate repr6duction of the initial sound fidld; for example, 20dB attenuation correspondsto 907oaccuracyin the operation of the loudspeakers. IUEW SCIEilTIFIG CALCUIATOR The student rnarket is particularly well cateredfor with the introductionof Commodore'snew scientific calculator. which represents a breakthrough in calculator features/price: Model SR4912is an 8+2 digit tED calculator, and undoubtedlv regresents fine valuein students'scientificmodels.At a price of s12.50,it features 50 key functions including log and trig, mean and standard deviation, polar rectangular conversionand many more power keys. the^ Followingupon the aboveinterestingnewsitem we ex-press hopethat souridpollution from radios-etc.will not be a featureof ouldoor life this-summer. The position has certainly improved sincethe first advent of transist6r portables,and we can all set a good example. PRIIUTEDCIRCUIT BOARDS AVAIIABLE Messrs. Ramar Constructor Services of Masons Road, Stratford-on-Avon,Warwickshire CV37 9NF, inform us that they can provide printed circuit boards for the 'Duette' Stereo Amplifier describedin our January issue and for the 2 Metre Converterdescribedin our Februarv issue. 'Duette' are The p.c.b.'s for the frriced at 82.45the pair including'VAT and postage.The boards being in fibreglass,roller tinned and fullv drilled. The p.c.b. foi the 2 Metre Converteris produced as a double sided 6oard, one side being plain for scieeningon fibrqglass, roller tinned and drilled. Tf,e-price is €2.38incl[ding VAT and postage. JULY1978 tce page & bo, 3-WAYTOUCH BUTTONS By G . A . French 'f he current availabilitY of CMOS digital i.c.'smakes it possible to rnake up a nu.m.berol slmple foci; citcuiti whit'h would be relaiiuetv difficult to- bring into o r a c t i c a lo p e r a t i o nw i t h t h e e a r l l e r i . t . i . d . u i . . t . A l s o , - C M O Si ' c ' ' s . h a v e the advantages of not requlrrng a iezutatedpoier suPPIY.and of havini rvtt.tm'etticalinput and output .*it.iling from higtr to low voltages .na ui.. versa, excePtionallY low c u r r e n t c o n s u m p t i o na n d v t r t u a l l y infinit. gate input resistance' On the debii side is'the fact that many CMOS gates have a low outPut EapabilitY, with the result *ri."t ihut .*t.tital discrete transistors are required if a CMOS device is to controi a load having a current requirement in excess of a few rnilliamps. ;ihe circuit to be described in this Circuit' article -*ttt'.'suggested highlights hith these advantages and the disadvantage. LATCHING CIRCUIT !'ic. I shows a theoretical lat"hT"s circuit incorporating three 2-inrrut-NAND gates. As readers *itt'U. aware, the qutPut of .a 2(postttve) inout NANI) gateis high when either both its inPuts or onlY of its inputs is low (negattve)' one 'fh. NAND gate output goes ,lo.w o" n l v w h e n b o t h i t s i n P u t s a r e n t g n ' i'n the circuit the outPut of gate A connectsto one input o-Jsgle g apo to one input of gate C' Slmllarly' the outPul of gate B connectsto one and t-'ne inPut of i"""iirgate"A gate ( sale C, whilst the output of lonnects to one inPut ol gate A ano of gate B' one ' " r h input ;' cir.uTr has three stable staiei: either the outp-ut of gate A is low and the outputs ol the other two nui.i ui" hieh, or the output of gate bD6 dition on its own, as may the outPut of sate C. Whatever gate output ls l o w , t h e r e m a i n i n gt w o g a t eo u t p u t s a r e h i g h r e i n l o r c i n g ,a s i t w e r e , t h e of the first gate. low output 'circuit of Fig. I has little The Dractical use because, once the suoolv has been switched on, we .uhhoi chanse it from whichever of the three staTesit chooses to select' if we attemPt to forceanYinPut low trr hieh we will' find that we are short-circuiting a gate output to-an ,rppositepolarity from that which it ttiia., *itn a consequent risk of damase to the gate and, certainlY, the flow of excessive cunent. ;fhe situation alters dramatically if we introduce three current the iirr.ritineresistors in series with 'l'hes.e outptrti, as we do- in l'ig...2. vDo r e s i s t o r sh a v e v a l u e s w h l c h l l m l t current to a safelY the sate ' v a l uoutput e when it is desired to low c h a n s et h e s t a t e o f t h e c i r c u i t ' A l s o inclJded in lhe circuit is a llying lead frorn the negative VSS suPPlY .uit inr the gates.Let us say that the ( ) u t D u to f e a t e C i s l o w a n d t h a t u e * ^ n t t o a l t e r t h e s t a t eo t t h e c i r c u i t so that it is gate B output.which is l o w . T o d o t h i s w e a P P I Yt h e t l Y t n g l e a d t o P o n t Y , t h e r e b Yt a k i n g t h t s circuit trointlow. Although the outr-rut of eate B is high, the current which fiows from it is still at a sate Ievel because of the series current limitine resistor. The negative connection'to point Y causesone input oi eate C io go low, whereuPo-nits outiut eoeshi"eh.The output of gate A, ;hi;h is already high' remains u n a l t e r e d .I n c o n s e q u e n c eb. o t h l n vss puts of gate B go high and its outPut coes low. When the negative connection is taken from point X. gate B output is low and remains low' Fig. 1. Theoretical circuit in' with the other two outPuts high' corporating three CMOS Note that the outPut of gate B is not NAND gates. The circuit has three stable states fttrt'L'd \ow bY the negative connecRADIO AND ELECTRONICS CONSTRUCTOR B on its own is low, or the outPut of gate C on its own is low' The oPe-raliun of the circuit can be readilY visualised. If, for instance. the outout of gate 'gateB is low, so also is one inA and one inPut of gate ilut of C. crr.rsittethe outputs of gate A and gate C to be high. These high.outputs connect to the lnputs ol gale b. iirmlv latching its output in the low cnndition. Siirilar reasoning .will show that gate A output may allernativelv be latched in the low con- I t tion to point Y. The sequenceof operations is that the negative connection causesboth its inputs to be high by way of the other iwo gates, whereupon its output goes low following standard NAND gate principles.In practice,the negative connectionto point Y may be of a momentary nature only. If we next want to make the low output that of gate A, we momentarily apply the negative connection to point X. Subsequentlyapplying thi negative connection to point Z will then make the output of gate C go low. Should we apply the negativeconnectionto. say,-pointK when the output of gate A is already low nothins happens;both ends of the current limiting resistor associa"ied are at the same potential and the circuit state is unaltered. The circuit of Fig. 2 can be made to operate with t.t.l. gates but the choibe of current lim-iting resistor value is rather critical if we are to ensure that sate output current and that ratings are no1 exceed-ed gateinput current requirementsare 5atisfied. Also, the presence of resistors between one gate outPut and the input of anothergatemeans that the circuit is not operatingto proper t.t.l, standards. None of itreie difficulties is present with CMOS gates.The fact that virtually t Vgp Fig, 2, Addlng output cunent limiting resistors anahles the circuit to bc changed from one srate to another TRt BC2t4L Outputs - Touchbuttons BC2r4L Lcod-outl R1-R9 l/4wott too/o one of the Fig. 3. A ptracticalcircuit in which three outputs can be provided by momentarily touching threa touch buttons JULY 1978 659 vpg 9v+ zerocurrent flows into a CMOS gate input means that the resistorscan have any value above that which limits gale output currents to a safe value. TOUCH BUTTON OPERATION One obviousapplication for a 3wav CMOS latch is as a touch-button switch. in which one of three outputs can be selectedby applying a finger to the appropriatetouch button-.A working S-way touch button circuit is illustrated in Fig. 3, where it will be seenthat the CMOS i.c. emplovedis a quad 2-input NAND eati tvoe CD40i 1. Onlv three of the Eatesin the i.c. are useil.The fourth has its inputs at pins 12 and 13 taken to the negative rail, and no connectonis made to its output at pin 11. The three remaining gates are connectedin the circuit of Fig. 2, the three current limiting resistors beinsRl, R2 and R3. If the circuit is traled throughit will be seenthat the output of each gate connects first to its current limitins resistor and then to one input of eachof the other two gates. In Fig. 2 the right-hand ends of the curient limiting resistorswere taken negativeby meansof a direct In Fig.3 the corresponconnection. dine circuit points are taken negitiue by way of the touch buttons; bridsing the two contacts of anv touch-button with a finger tiP provides the negativb coupli4g. Althoueh the resistancescross the touch button will be of a relatively hieh valueit will still be much lower thin the valuesof Rl, R2 and R3, therebytaking the appropriategate inputssufficientlylow to changethe circuit state.R4. R5 and R6 are included in the touch button circuits to limit anv input currents which may flow b6cauieof static voltages to L level which can be reliably handled by the protection diodes inside the i.c. The gate outputs also connect, via R7.R8 and R9. to the basesof vss the three p.n.p. transistors.TRl' TR2 and TR3. When any gateoutput goes low, the corresPonding transistor passescollector current. and can supply outPut loads uP to some 30mA or so. When touch button 1 is touched, oins 9 and 2 of the i.c. are taken low. The output of the remaining eate.at pin 4,-alsogoeslow, causing TRL to turn on. If touch button 2 is bridged,pins 8 and 5 go low, as also then doesthe remaininggateoutput at pin 3. TR2 is therebymade conduitive and output 2 is turned on. Bridcine touch button 3 takes the negaliuJcoupling to pins I and 6 of thd i.c.. wittr the result that pin 10 goeslow and TR3 conducts. It is a little difficult to devise a simole meansof biasingthe circuit such that a particulai outPut is alwavsthe first one to be turned on which maY when -poweris applied, be a desirable feature in some applications.In practice,however, it would seem that this facility is automatically provided by the nature of the circuit itself, since there must be a difference in the current gains of the three gates. With the p,rototypecircuit it was found that ihe same output alwaYs came on first wheneverthe suPPlY was applied. Such a performance cannot be guaranteedwith all CD4011i.c.'s,of course,and the oarticular output which comeson hrst will varvbetweenone i.c' and the next. The outputs to be controlled depend upon the requirementsof the bonstructor.If a r6lay is to be used in an output circuit, it will reguire the usual reverse connecteddlocte across its coil, as is illustrated in Fig. 4. "Thethree touch buttons will need to be home constructed,each consistinsof two small piecesof metal a shinv surface which gives havine "contacts to the skin of the sood Iineer. The heads of two nickelola"tedbolts of around 4BA or 2BA mounted close to each other on a oiece of insulating material can brovide a simple a-ndeasilYmade louch button.- The touch buttons themselvesshould be sPaced'aPart such that there is little risk of two buttons being inadvertentlytouched at the same gime' This will not cause any damage to the i.c. but mav prevent correct output selecwhich the tiori. If the equipmentin 'emPloYed touch button circuit is has a chassis which is at earth ootential. this mav be made common with the touch button negative supply. On no accountmust the circuit be so used with equipment having a live chassisconnectedto the mains supply, aSoccurswith manY televiI sion receivers, Mail Order Protection Scheme If a mail order trader fails. readersare advisedto The publishersof this magazinehave given to the lodgea claim with the AdvertisementManagerof this Director General of Fair Trading an undertaking to magazinewithin 3 months of the appearanceof the refund monev sent bv readers in responseto mail advertisement. order adverti6ementsfilacedin this malazine by mail order traders who fail to supplv goods or refund For the purpose of this schememail order advermoney and who have becomeihe-sulject of liquidatising is definedas: tion or bankruptcv procbedings.These refunds are made voluntarilv and are subiect to proof that pav"Direct responseadvertisements,display or ment was made to the advertiser fof goodsord-er6d postal bargainswhere cash has to be sent in through an advertisement in this magazine. The advanceof goodsbeing delivered." arrangementdoesnot apply to any failure to supply Classifiedand cataloguemail order advertisingare goodsadvertisedin a catalogueor direct mail solicitaexcluded. tion. RADIO AND ELECTRONICS CONSTRUCTOR ULTRASOUND DISPLAY SYSTEM By Michael Lorant X-ray-like TV displays employ colour changes to indicate varying d e n si ti e s i n human tissue. John E. Jacobs, executive director of the Northwestern Universitv Institute's Biomedical EngineeringCentre in the United Stateshas, with a team of graduatestudents,developedthe first ultrasound imaging systemcapable of producing Xray-like coloured imageson a television screenof the inside of a human finger, muscle or any other livins tissue. Clearly discernible blood veisels in the fingers of a hand appear as pulsatingblue, greenor orange lines. The array of colours shifts as the hand Working with the first ultrasound colour imaging system, John E. Jocobs, director of executive University Northwestam lnstitute's Tdchnological Engiieering Biomedical Centra, watchds the image on tha colour TV screen oiven bY the hand of the braduat6 student on the left. The student's hand is immersed in a tank of water contaihing tha 3.58MHz ultnsonic r*eiving (ln tha transducar. American N.T.S.C. colour TV system, 3.58MHz is the chrominance subcarrier frequency,) 7::/..,.. '174ir,,,,,:,: /7' ,z I changesposition in a tank of water containingthe transducer device. The colours do not representreal colours in the internal structure ofthe}and. Instead,they represent varying rates at which ultrasound waves pass through-the hand when it is dipped in a tank of water in the path of the waves. A normal unimpededsound showsup on the TV screenas red. Sound wavesarriving slightly later, and therebv at different phases.appearsuccessively as purfle, blue, gre'enand oiange. All hues JULY 1978 recognisableby the human eye on a standard colour scale appear as the sound wave phase patterns alter. Ultrasound wave interference patterns are set up by the varying arrival times of the waveson a two-inchround quartzplate operatingat 3.58MH2. The plate is part of an imaging tube at one end of the tank which translates the ultrasound waves emanating from the other end into the different colours seen on the TV tube. There are three important advantagesof the Tbnt svstein which will allow it to be used as a medical diagnostic tool. First, it can produce bright simultaneousimagesof pulsating blood vesselsin muscle and other tissues not normallv visible bv conventionalX-rays. Second,the colour televisioir svstem is 20 to 40 times more sensitiveto tissue changesthan existing black-and-white displays of ultrasound images. Finally, by using scattering and reflecting techniques, similar to those employed in SONAR, the systemcould be used to diagnosediseasedconditions in human tissue. I 661 SINGLE TTJNED CIRCI-]IT F.M. RECE,IVER ri .l 'i i I By P. R. Arthur Local Station v.h.f. receiverusing the synchronous detector principle. Simplev.h.f. broadcastreceiverscan make an alternative to the more usual a.m. interestirie -broiect. They provide good quality receiver of B.B.C.and relativelvinteiference-fieeleception commeri:ialstationsin areas of reasonablygood of a simple simal strength.The main disadvantage v.-h.f.set islhat it is not really suitable for use in mediocreor poor receptionareas.A simple v.h.l. detectorcan be very sensitive,but tuning and adiustmentbecomeeicessivelycritical on very weak iimals. and thereis a markedlossof audioquality. T[e orototvpe is used approximately 25 miles awav'fromihe B.B.C. Wrotham transmitter, and excellentresults are obtained. The receiver uses four transistors including a Jugfet,and it providesan output which is suitable crystal earearheadpnones or a crystal impedanceheadphones hish lmpedance for"hish tbr reasonablysenreas.onably sensen; piece. drive any drive any reasonably piece.-In it wtll will even will evendnve faci tt faci ln tact oiece.-In 3itive pair of low or medium impedance The set has a telescopicaerial and is headohdnes. poweredfrom an internal PP3 battery. It is thus iompletelv self-contained. This is-not intendedto be a miniature pocket tvne receiver,and the unit is built into a case *tricn measuresapproximately 185 by 109 by 60mm.This, coupledwith the useof varicapdiode tuning, enables good freedom from hand capacitanceeffects to be obtained' 662 THE CIRCUIT The circuit of the receiverappearsin Fig. 1. It a synhas four stages:a groundedgateinput stage-,. chronousdelector,a common emitter audio preamplifier and a commonemitter output stage. Whilst the groundedgateJugfetin the TRl position providesa srnall amount of gain, its primary funcfion is to isolate the aerial from the detector, becausethe detecTR2. Suchisolationis necessarv -the result ttrat- lhp tor is oscillating in use, with receiver could otherwise radiate a signal which would interfere with other sets. Rl is the source bias resistorfor TR1 and no input tuned circuit is emnloved as the aerial couples directly to the .ource.Ll is a low value r.f. chokewhich forms the drain load fbr TR1, and the output from this stage is coupledto the detectorvia C1. detectorand appearsin a TR2 is the sSmchronous srounded base Colpitts circuit which is adjusted, bv meansof VR2, tb the point whereit is just gentlv oscillatine.In this statathe oscillatorwill tend to lock onto iny similar or very close frequency, which in this caseis an f.m. transmission' As the innut sisnal deviates around its central frequencvthe oscillatorfollowsit, resultingin fluctuationsin the amplitudeof the oscillationsand,.in turn, the current cbnsumptionof the oscillator circuit.'The oscillatorcurrent flows through R4, the RADIO AND ELECTRONICS CONSTRUCTOR CrO0.rpF typeC280(Mullard) (Mullard) iype tvpe C280 C280(Mullard) Cl1 0.015pF 0.015r2F Resistors (All fixed valuesi watt 5%) R1 lkn R2 1ko R3 120ko R4 2.2ka R5 18kn RG 10krr R7 220a R8 1.2Mo R9 4.7ko R10 220CI R11 2.7kct R12 560kn R13 56 r) R14 lk n VRl 1Okolinear, with insulatedspindle (see text) VR2 sko log, with switch51 Semiconductors TRl BF244B TR2 BF194 TR3 BC1O9 TR4 BC1O9 Dl BZY88C7V5 D2 BA1O2 Capacitors Cl 1.8pF ceramicor silveredmica C2 226uFelectrolvtic,10V. Wkg' C3 220uFelectrolytic,l0V. Wkg. C4 0.011Fcliscceratrtic C5 4.7pF ceramic C6 0.001rI"disc ceramic C7 0.41iF-electrolvtic,1t)V.Wkg. (seetext) C8 100rF electrolytit'.10V. $'ke. C 9 0 . 0 l l F d i s ct ' t ' r r i r t t i i ' MisceLlaneous Plastic case,185 by 109 by 60mm. (seetext) 9 volt battery type PP3 (Ever Ready) Battery connector Telescopicaerial (seetext) Veroboird.0.1in.matrix 18 s.w.g.aluminium sheet 2 control knobs Enamelledwire, resistor,for L1, L2 (seetext) Grommet, nuts bolts etc. 1001F electrolytic,10V. Wkg. C12 100aF 10! Wkg. C13 100pF 1001lFelectrolytic,10V. TCI 25pF trimmer, type C801 (Jackson) Switch S1 s.p.s.t.,part of VR2 Sochet SK1 3.5mm.jack socket *ou c [:"3 dgs B F2 4 4 8 L€qd-o!ts tlLl H B FI 9 4 Leod-outs Fig. l. The circuit of the singte tuned circuit f.m. receiver. dioda D2 JULY 1978 )'P4 e /---\b /o o\ \-v. BC l09 t-?od-ou ts Tuning is carried out by VRI and varicap 663 voltageacrosswhich varies in sympathy with the then or.iii"uioi curtent, and thesevoliage chan-ges detector' theof output audio the constitute CapacitorC4 providesr.f. filtering and decoupling' The detector is tuned over the t.m' band Dy diode D2 which, in company with the u"ri."o-tti.n*er TC1 and' L2, completes the os..ri.. cillator tuned circuit. R2 and D1 form a zener ihunt t"gtt"tor circuit which providesa.slrbilized uottae.-?orthe tuning potentiometer,VR1. The uoit"Eetapped off bv VRt slider is applied to D2 throu?h R3, and the circuit can be set up for a tunine ra*nseof about 88 to 100MHz. "CZcdupletthe audio output from the detectorto TR3. R? and C9 provide additi.onalr'f' -utta the baseof tni. is necess'arysince both audio fiii.;i";. ;1;;; ftt; responseswhich eitend into the v'h'f' Insfability could easilv -result if a ;;;?tit;. .i*iii.unt amount o-f oscillator sigiral were to be alTowedto enter the audio stages.The filter comnonentsalso provide the necesiaryde-emphasisto in.- *ai" simal. This de-emphasisis. merely a .iit"ti-utii"uni of treble attentuation which como."."t.. for the treble boost (pre-emphqlis) 'I'he ipplied to the audio signalat the transmitter' rman provtde piripose9f t{re pre-emphasisis to ratlo. proved slgnal-to-nolse 'Both ilie audio stagesare conventionalhigh gain .o--on emitter circ-uits.R'13,the emitter resistor i;; tR4, is unbypassedand providesq.measureof nueutiuif".dbalk in the output stage.This reduces ;ir?'l r ;ih;;wise a marginallv.excessivegain, and also sliehtlv improvesthe audto quallty. -. Stpptv d"conplittgin the varioussectionsof the provloeooy C2, C3,C8, RtQ pn-4Cl2' 51 crrcurtrs -ofi switch and is ginged i. tt* ot -the with VR2' The receiver-it -qPtuiient consumption of the PPU whereupon only' 6mA oroximatelv batterv has a reasonablylong life. COMPONENTS The telescopicaerialemployedwith the receiver has a cioted length of 1?6min. qn-dan extended for operation at t""*tft of t.t meties, and is hin-ge-da-ifiaii"i aneles. This is available from Maplin nieittonic Silpplies. It has a bracket on the lowest ...tio" *tti.his not requiredh-ere;this bracketcan ry aritting out the two rivets which il;;;";td secureit to the aerial. Other telescopicaerials of ."me tengttrwith the hineg't facility and "l"t'itit! a 4BA tapped hole in the centre of the basesnoulo ''also be suitable' as 0.47wF electrolytic 10V' -b";i; bf"it""'tp."ined practice it will be found verv diffiwil.. .uit- io oUtain an electrolytic capacito: of thts a lo* working voltage' It will be rii""" having a much o-ttli to use a capa"citor. ;;l;; i""t".ricii uott"g",evenas-bishas 100volts' tilh;;;;;filg for T.Ri. is available speci?ied f fi;"^BFt4+ii-Juppliets, including .EIectroi;;;";;ial value. The 185 by 109 by 60mm' plasttc caseIn is assembledis retailed by ;ili;il ihe "eceiu"er giian Finally, the C801 trimmer Reed. J' fvi"ttt.. for TCl cln be obtained from ;;;";it";;pecified Home Radio. COMPONENT PANEL Most of the componentsare-assembledon a o.iii,. *jit.tt Veroboardhaving32 holes!v 1-4gtrips' V"."Ui"ta is not often usedln r'f' and v'h'f' conprojects as thera.pacitance between r;u;;i";i strips can causeproblems' This was not found to 6" ttt" case her-e,and an eqlly prototvpe was- ur.ilid.i"l t-Dec! Ddtailsof the panel J,i.""".inrirn the other rtt"*""i" Fig. 2, which also illustrates "t. *tfi:ft L1 and L2 are home-constructed comoo"u"lt. h consistsof about 40 trrr-nsof 0'18mm' ei;;;;;-("i-eo goPpe1 ..*.g.) ename-ll-e4 :vi:g ;;;;bi;-;outtd - o.t anlrdinarv- 270k o,r watt 10% ;;;iffi:- The iesi.tor has a bodv diameter of u.ou"d 0.1in.The endsof the-winding,alter scraPti"ning, are solderedto- the resistor leadi"n ""a ttt" resistorbodv, so that the win;;?;i;;t;i;;"ib ;i#;;;;;;.i'i"!rfi.1. wiresof the rhe lead-out then act as ftte lead-out wires of the choxe' -ualue resistor of the resistor is not important' proTtt" it it ioot'n or more' since,it is merely "^ia:.a'iir"i being used as a former' L2-is a tuned winding and has to be wound with 'wiie 0'9mm (ot ?9 r.r*"lUt.-piecision. It employs qld is selfcopper rlile.t- ;"^';iit.a is woundon .itiri"iti"il it ffi exactlv4 tur.ns"and coil a temDorarv *tn. diamdter coil former' The holes in the and passthroug!. ;;;;;-b;;t'dow" 'I'he turns are apart' 0.3in. are which Veroboard RADIO AND ELECTRONICSCONSTRUCTOR \o ooo o : : : :::'*;";,' o o [o o o o o o o ooo :i:j I('._":: l: | : t"ltt T']'d;T;l'tl ffiTT-l[J oooooo o I o o o o o o o o " o o . I \ o\ o o .@- a C1 - -il": m-H, o co I' (tlt'.'dI o o 0 o r.^otkilql. 'h o ?o l'^3GSl.rP t I o o o"f, o o I o t9 A ooaooooo 2l 23 o o 25 o ;t.l o 27 29 3l aaoooooooaooa 6 a o E F G aaaaa H J K M ooooooooooooooooo00 N Fig. 2. The wiring of tha receiver. Nearly alt the pafts are assembled on the Veroboard 'l'he coil ends are snacedout over this lensth. sbrapedclean of enamel ind tinned before being ;;id;tedlotheappropriatestripsof theVeroboardl The Veroboaidpahel is initially cut to sizeby the copperstrips over which they pass.In fact, the lead-out wires of all componentsin the r.l. circuitry should be no lonjer than is reasonably necessary. mounted'and solderedin b;;rd'6;p;"J"t. "r" and C5 are mountedon the -Their iliti""- N;te that C4 lead-out wires ;;;;;; ;id; ;a ah"- p"n.t. ;fi;td be tepi a" rttoif aspossible.Thoseof C4 are ;itti ri"ruitrg to i.euetrt short-circuits to ;;;;d . Ttt.photographsfelntoshowthegenerallavout inside the case,qld.lt is strongly recommended that this is not ra.dicallyalteredin any way. In this applicationthe lid of the casebecomesthe rear phnel, with the controls,output socketand aerial appearingat the upper face' VR1 can be mounted LAyour r':.:tlTfcAsE :*'lni'ly8 r,ffi'"iJ fi.xTei\'ltrTgr tli;-;"pd.;;arips are tn." -ad., after which the 1 I l{ tt I t,. ts" panel JULY 1978 Detail illustrating the Veroboard assembly more closely. The boarr! was not holted to the case when t h i s s h c t l _ 1 , : . f. ,. ) r o 4 direct to this upper face,but greater freedom fiom via this tag. The aerial passesthrough a suitable hand capacitanceeffectswill be givenif it is a type hole in the top of tlre case,and a groirmet can be having a long plastic spindle and if its body is fitted hcre to prr-rduce a near finislr. positionedlower down, fitted to a metal bracket. The comnonentnanelis securedto the bottom of This bracket is made up, as shown in Fig. 3 (a), the case b! meani of two lin. long counte;sunk from 18 s.w.g.aluminium sheetand it is bolted to tiBA screws.Spacing rvashers,or"spacing ^ensuienuts the baseof the caseby two countersunk4BA bolts suitably positioired,ire employed to that with nuts. It shouldbe positioned.as shownin the the copperside is clearof the centrebottom mounphotographof the interior, such that there is just ting pillar. The board is oriented such that the room for the PP3 battery betweenits upright sec- holesin row No. 1 of Fig. 2 are nearerthe aerial.It tion and the adiacentcorner nillar of the "case.It is not finallv mounted until all connectionsfrom it will be necessaryto alter the fength of the upright have been made to the other components.As can sectionof the biacket if the spiridle lencth bf [ne be seenfrom Fie. 2. C2 and C3 are mountedon the pol;entiometerdiffers from thdt of the c6mponent tagsof VR2, Dl-and RZ on the tagsof VR1 and C13 usgdby the author,and this point shouldbe check- on one of the taes of SK1. edrbeforemaking up the bricket. As alreadv mentioned.the batterv fits between ilhe mounting bracket for TCl is also made the braclietirr VFil and the adiaceirtside mounfrcim 18 s.w.g.aluminium sheet,and this is shown, ting pillar of the case.A piece of foam rubber or in Fig. 3 (b). The trimmer is securedto it by two plasticwill hold it in positionwhen the rear of the short-6BAbolts and care should be taken to ensure caiieis screwedon. that the adjusting spindle and bush do not touch thb insideedgeof the centralhole.The capacitoris mDuntedwith its moving vane tag pointing au'av - A D J U S T M E N T S from the bend in the bracket. The bracket is glued Initially, TCI should be adjusted for almost torthe inside surfaceof the front panel with a good maximum capacitance(i.e. its vanes should be quality adhesivesuchas an epoxytype. Those-who nearlv fullv inmeshed). With an earphone or ptefer to do so may alternativelydrill two holesin headphoneiplugged into SK1, the set ca'nthen be .-d o:i ...;,"\ '' ' r,lvanred.It will probablynot the panel and the bracket to take countr:i'sru:1.: I RA r , ' r ' .l ' , ' i ' I r ' l , : v m c a n so f V R 1 u n bolt-swith nuts. but this will mean :r. hgadswill be visible from the front ol tire sei. til ';iiz is weii advanc'rri, ii VR2 is advancedtoo The aerialis bolted.in the positionshownin the far it will not be possibleto producea properaudio photographs,to the bdttom ofthe caseby meansof output from the set as the oscillating detectorwill a 4BA countersunk screw. A solder tas is fitted not lock on to the carrier ofthe receivedsignal,and over this screw between the bottom oflhe aerial all that will be heard is the beat note produced by alrd the case,and connectionto the aerial is urade the carrier of the tr l:rission heterodyningwith 66rj .' i-, iii,UC'l'RONICS CONSTRUCTOR 5h6"aio ttl2" ttti' t\z'n ] l I rtd I tttt ! lvst 'lA -l __l - L! I -ilil n::;, Altcr bcnding- tl_____] Fig. 3. (a) The mounting bracketforVRl. lf this component has d non-stdndard spindle length. bracket dimensions may need to be modified as described in the text r) The mounting bracket for VCI I i$ i& ., Two of the r'f' bYpass capacitors JULY 19?S are sotdered mto circuit on the copper side of the board 667 where oscillation ceasesthe audio outPJI level will #;;1h;;^ld ;;;"C;;"ilI' qqqlilv ii'a -iswiil be a little for op""W"tofvRz needed adjustment results. iimum -'il;fu;ii;-"it r.-. broadcasttransmittersuse "p"I" ti, ot'" tta .o thete Iescopi c aer i aI n" ii'"""it?ii "Ti " ratherthanleft in a .t;;ff;;.I;;i.a to the.posi"[+r'a"gtees rotated be it shoull il;ti;;i;;itT;nl in il;^;hi[h iit". ttt" strongestsignal, bearing as critical less Vn2f,ecomes in?..ti"i"f "ri^tia'iiiit rrgii"t".itttgit i"-"t?u*.. A strongsignalalsogives broader "'T;id and eastertunlng' ;;nse at the ldw frequencv end of the #;'il4;;;;1iea-uv-tct' rcl for inAdjustind rre-;;J ."p".it"tt.""extends tfie range in the low i? iG rangedodsnot extend Hffi;;tT;;;i;;. -A; *itii tci at pqli-mpm capacitance' i"i""ii,"iigtt its be compressedslightlv to increase ;;ii t inductance. '^'Ot;;;ilIy, when there is insufficient covemge i" ttt"-itiei'tfi6qnettcyditection, \2 g"\be stretch- ;d ;1 ;ii;i';itl;;;a;" In its i{rductance' to reducg adjust TC1. for decreased ini. .itu^1ioti, ao "o[ would reduce the overall tuncaia;iffi;;;'i.ittit Another view of tha receiver' The telescopic aerial is stightly more than ona metre in langth when it is fullY extended the oscillatinsdetector.VR2 should then be lgrned i;ff;ffi;;;'i':iii withoytoscillation i' possible "' wh6n oscillationhas beenlost i"i.ine. ti is obvious than backgroundnoise, are other u.-no-timuls, point ;;"4;.;J. ii Vnz is adjusted too closeto the lng range. "'fffr"fr o". considers the simplicityof the circuit' Is very very p-rototypetq the prototype +ho orrdin nrrqlitv of the quality orrtnut output of the audio o Edadphones headphones impedance-16 t"* I*p.dance'16 to* il;iii;"4;ili il;d ii;"4;lii -^' Alihough il;dphones. Alihough hioh l;;"d"r,"" irnnerlance 4.0iib'; h;;dphones' - 4,oiib'; :;"ti;i' a with as nof sood are y,ill ^3 qo3d .il'i'?I."pT"nt t, t-.*-. ..itt.. a notlceaDly .3t . ticea-blyreduced q edrPiece.This gves crvstal earpiece. crystal ;;J;;;1;';;;--idaL ."Uj;"ii".iv, TltiP which' ?equency resPonse results in a'lower signal-to-noiseratio' t DECIMAL R AND C VALI]ES By R. D. Smith "round number" Combining two El2 resistors or capacitors to obtain decimal values. We occasionally require -resistanceand ."o"iit""." valuesritrictrire in the deiimal series ;;i0:t0: 50,+0,so,60,?0,80 a-n{90,andit canbe wi rina that theonlv com*t ;ii;i;il".tiatihe have hddo'tan P-urchasi ;; "n ;o#ffi;;^h;"". you will series''T^his, ptlr"tred" E12 itt.#il;i" r0,ri,15r ;#fi, il#;" iri.r.itt6*i"g-;;;;i' 1q' no provi47,56,68and82.There-is Ti,1i ,'g5,-gg, .i-,i" i6t tit" deciinal'valuesof 20 to 90' If. however,we ar" preparedto use tyo EJ2 rn serres resistorsin parallelor two E12 capacttors which combinations up e -"t i"i'Jitv ^pt*iJ.tv ffi ffi' it a d'ecimalvalue, or which J""r."i"i. of calculate at the decimal value with an error 0.5% or less. SERIES-C PARALLEL-R ' in vaG of two resistorsin parallel' as til;;"I is Fig. 1, RlxR2 R:rTEz where R1 and R2 are the values of the individual resistors. Fig. l. Two resistors connected in parallel Fig. 2. A serias combination of two capacitors TABLE Parallel-Ror Series-C Total Value 22 and220 20 33 and 330 30 47 and270 40 100and 100 50 100and 150 60 82 and 470 70 100and 390 80 180and 180 90 JULY 1978 The Table showshow two E12 valuesof parallel resistanceor seriescapacitancecan be combinedto give total values in the decimal series.Two of the entries are glaringly obvious; 100 and 100 must give 50, and 180 and 180 must give 90. Not quite so obvious,at first sight perhaps,is that 100 and 150 give exactly 60. It is a minor matter of mathematical moment that22 and220 give precisely 20, and that 33 and 330 give precisely30. The remaining valuesare not quite so exact.47 and270 gsve40.0to three significant figrres, the calculated value working out at 40.03. 82 and 470 work out to 69.8 instead of 70 precisely;the error is 2 out of 700 or two-sevenths of 1%.100 and 390 are the worst,thesecalculating out to 79.6. Compared with 80, this representsan error of 4 out of 800 or one-halfof l%.ln practice, not too much to lrorrv about. Lets try out the Table with a few examples.If we want a 4.0ko resistancewe mav use a 4.7kO resistorin oarallel with a 2?ko resistor.If we want a 30pF cipacitance we can connect in series a 33pF capacitorand a 330pF capacitor.Another example: an 800d. resistanceis conveniently given by the parallel combination of a 1ko. resistorand a 3.9k o- resistor. You may find it convenientto cut out or copy out the tdfle and put it up on the wall of voui workroom. It could-save vou ouite a little h-eadscratchinein the future. pdrticularlv with the more awkwardiombineddeciiral values-of20, 30,40,?0 and 80. I 669 . v J' o P - A M P S - T H E I R P R I N C I P L E SA N - g A P P L I C A T I O N S B diirn"iitice. 96 pig"s, 215 x 135mm. (8{ x af,in.) Publishedbv NewnesTechnicalBooks.Price f2.25. integra-tedcircuit. operational ln terms of time, it is not so long ago when one of the first This was in 1965, and the 7O2 was amplifiers, the 702, was introduceO ti ttri electronic scene. 13 vears is a very extended iJif"*"J, inlgOSTOO, Uv tf'" immensely successful 7O9' But concerned. and the presentis development circuit "".i" p"iioO ,o f.|. as the exubeiant tiefO of integrated devices' each havmonolithic of range dav user of i.c. operational amplifierscan choose from a wide characteristics. ing its own individual and very informative style' Mr. Dance's book deals with current op-amps in an uncomplicated complete with component The approach is non-mathematical, and ihere are numerous circuits values. of the 741' which is inexThe first part of the book covers operational amplifier basics in terms principles'.Other wilh and readily demonstrates operational amplifier pensive,is easy to The book then compensation' frequency is "rp"iit"nt also as witil, dealt ift"n integrated circuit op-ampt .t" These are pre-amplifiers' audio lownoise and turns to devices witfr t.e.[.-inputs,audio power circuits good index' a glossary and by a followed work with operational The book will be of excellent value to anyone who is commencing in particular to the non-academic technician and the home constructor' amplifiers,and will "pp""i H t G H P E R F O R M A N C EL O U D S P E A K E R SB. y M a r t i nc o l l o m s 2. 4 6 pug"r, ZIS ^ f gSrr. iatr t s{in.) Publishedby PentechPressLimited' Pricef8.95. perimprovem.ents.in has resultedin considerable The developmentof high fidelityloudspeakers all other virtually with as well as these with deals tnit'book OEcaOe, formanceover the fast lt hardlyneedsto be statedthat "nO and their enclosures. aspectsof modernhigh qualityloudspeakers is the componentwhich is most dependent in the highfidelitycnaindi repioductibnthe loudspeaker functioning loudspeaker's on subiectiveevaluation,aitnougftit is possiblenowadaysfor much of a to be iudged by objectiveanalysisand measurement' radiators,practical After a,$eneratreview,ttre book dealswith theoreticalaspectsof diaphragm drive radiators' direct moving-coil analysis, system frequency low. diaphragms,acousticloading, chapteris Each performance' loudspeaker crossoversystems,enclosurisand the assessmentof originatingin the U'S'A" in work takes book the and references, of listing extensive an by followed J a p a na n d A u s t r a l i aa s w e l l a s i n t h e U ' K ' to studentsof The book will appealio-r *iO" rangeof readers,from do-it-yourselfenthusiasts professionalloudspeaker to valud it of make will wtricn .l"ptX ti wriiten is anO electronics, There is a minimum of designers,studio engineersand technicalwriters on hi-fi subiects' in practical personal involvement mathematics,and the work clearly reflects the author's manufacture. and design loudspeaker , . S c .1 3 6 M O D E L R A D I O C O N T R O LT, h i r dE d i t i o nB. y P a u lN e w e l l B frg"r, 21O x 14Omm. (8+ x s{in.) Publishedby Radio Control PublishingCo. Ltd. Pricef2.95. Radiocontrolof models,includingin particularmodelaircraft,hasitsownspecialfascination'The reierencein the U.K.and appears,enlargedand re-visedinits oresentbook has becomea standar-d fh; book is availablefrom model shops or may be obtained direct from Eadro Il*-iftiiJ"liii;;. Berks,SLS 0NF' Modelter magazine,High Street,Sunningdale, The book siartswhhi Uti"t historicalsectionthen proceedsto its maintheme,proportionalcontrol systems.Theseare consideredin greatdetail,and includedare chapterson digitaltransmitters, Jigit.l-i"""i"ers, digital decodersand-digital servo amplifiers.The text is accompaniedby clear of modelsand equipment'Furtherto be found Jiifrr.r, and theriis alsoa wealthof ph-otographs are circuitsand printed board layoutsfor an advancedi.c. digital system. This is definitely a book for the radio control buff. AND ELECTRONICSCONSTRUCTOR CMOS DlGtTt FREAUENCY N Part | 2 partd BvR.A.Pen Range: lOHz to 42MHz For the ultimate in accuracyand convenience in use, a digital frequencymeter easily betters the alternative methods of frequency measurement. The unit which is described here is capable of measurementsfrom low audio frequenciei up to a typical maximum of about 5OiMHz (42MHz minimum) with a minimum resolutionof 10H2. The accuracyof the unit is largelydependentupon the adjustment of the clock oscillator, and should be within 0.1%.The unit is quite sensitiveand at middle frequenciesrequiresan innut level of onlv about 20mV r.m.s. However.the'sensitivitvdoei fall away somewhattowards the upper and-lower Iirnits of the operating range. Digital frequencymeters tend to be rather complicated instruments,and although this particular unit has beendesignedto be as simple as possible without sacrificingaccuracyor sensitivity,it is still quite an involvedprojectwhich is only suitablefor the more experiencedconstructor.The circuitrv is largely based on eleven CMOS i.c.'s, but tliree otheri.c.'sand ten transistorsare alsoemployedin the unit. An r.f. sisnal seneratoris reoirir6d for checkingand adjust-ingtfie meter after if has been completed. BASIC PRINCIPLE The.manner in which a digital frequencymeter operatesis very simple. In fact, in this respectit is probably the most simple form of frequencymeter. The block diagram of Fig. 1 shows the basic arrangement. The input frequency is displayed on an electronic counter which showsthe answerin ordinarv denary form. There are severaltypes of digital diiplay which can be used in this application, and the type actually used here are seven-segmentl.e.d. displays. The functioning of these displays has been described a number of times in previous issuesof this journal and soneed not be considered further here.In the presentunit a four digit display is employed. An electronic switch operating as an enabling gate appearsat the input of the counter, and this only allows the input signal to pass the counter when a suitable pulse is applied to a secondinput of the gate. A pr-ecisionpulse generatorfeedst'his secondinput, and it is the length of the pulse that this geneiates which deteniines the'range of measurementprovided by the unit. If, for example, the length of the pulse is-l secondthen a four digit counter will operate as a 0 to 9.999kH2frequency meter. A little considerationwill show that this is the case.If the input signal were lkHz then 1,000 input signalpulseswould be fed to the counterduring the 1 secondperiod. The counter will then display 1,000H2or 1.000kH2accordingto the position of its decimal point. It follows thaf the miximum frequencywhich can be measuredis 9.999kH2,as any frequency higher than this will cause the counter to overflow. Higher rangescan be provided bv using shorter gateenablep-ulses. A gatetime of 10b milliseconds, or 0.1 second,would increasethe range to a max- Electronic switch Fig. l. The basic form of a digital frequency meter, The enabling gate is held closed for a pr*determined period and the counter then displays the number of inPut' signal cycles which have passed through 672 Electronic countcr Input signol RADIO AND ELECTRONICSCONSTRUCTOR IL qETER ll old Crystalcontrolledgating pulse Printed circuit modular construction imum of 99.99kH2as therewouldbe onlv one-tenth of the previoustime for the maximum iount to be reached,-andthereforete! times the input frequency would be neededto ach_ieve this. Shbrteningthe gatetime to 10 millisecondswould producea range of up to 999.9kH2, further shoftenine it to "1 millisecondwould increasethe rangeto g:gggMHz, and so on. In. the present unit the frequency ranges have figure-s of __99.g'ilkH;, 99"9.9kH2, TgIlIlyJn 9.999MH2 and 99.99MH2. As was mentioned earlier.this last range iloperable in practiceup ro a typical figure of SOVgz. ACTUAL ARRANGEMENT A practical digital frequency meter tends to be v.erymuch more complicate-d -than the diagram of t wgul{.suggest.This will beco*e i--Eaiateiy liq.. appare.ntwith referenceto the block diagram of th'e unit which is shown in Fig. 2. Dividc by O pfa6colar ond intartoca Clock cnoblc Rcrt ! I ^ $ q F Fig' 2' ln practica the frequencY meter is moro compticated than is evident from Fig. | . However, the use of integrated circuits simptifies construction to a considerabte extent JULY I9?8 673 ] .. "f, {t',Y" ,:ifuk ',lll . i;' i I The display and counter sectionis basedon four 4026 CMOS i.c.'s. each of which drives a DL704 commoncathodel.e.d.display.The 4026 contains all the necessarydecodingand driver circuitry to enableit to directly drive an l.e.d.display.It also has a divided-by-ten output which is used to drive the input of thi subsequent4026. Thus the first 4026 ieeds the units display, the next only increasesthe displayed count by one for every ten input pulses and so diaplays the tens on its l.e.d. ieadout, the next countei displays the hundreds, and the last one the thousands.Note that in practice the l.e.d. displays are not mounted in the fashion shown, Uui witn the units display at the right and the thousandsdisplay at the left, so that the output is shown in conventionalmanner. I iL GATE PULSE Not only is it necessaryfor the gatepulse to haie a hieh decreeof accuracv. it must also have good LC and RC circuits do not have-the repe"atabil"ity. One problem with the 4026i.c. is that it will only operate up to frequenciesof about 5MHz, which necessarylong-term stability, and it is doubtful meansthat frequenciesabovethis figure cannot be whetherthey would evenhave adequateshort-term stability. The gatepulse is, therefore,derived from handled as the first 4026 in the chain would fail to operate.This can be overcomeby usinga prescaler, a crvstal oscillator and divider chain. H-ere,a singletransistor lMHz crystal oscillator which is merely a divide-by-ten circuit which is feedsa seriesof six divide-by-ten circuits via stage added ahead of the counter circuitrv. To satisfv The latter is neededto ensure -sutficient normal amateur requirementsit is ne6essaryfor air a birffer amplifier. input signalto drive the first instrument such as this to operate up to at least that there is 30MHz, and this can be accomplishedby usingan divider stage reliably. Each of the divider stages SN7490ANt.t.l. decadecounter.This will operate usesa 4017CMOS i.c. Another 4017 i.c. is used to convert the conat frequencies up to at least 42MHz, and will tvpicallv function at frequenciesof about 50MHz tinuous output signalfrom the dividers into a single gate pulse. The 4017 is not just a straightforward oi'so. Thus, the input frequencyis divided down to a level which can be satisfactorilyprocessedby the divide-bv-tencircuit. and anart from the ordinarv carry oui output it hd'sten other outputs.Thesear-e CMOS counter circuitry. numbered "0" to "9", and they each go high, for An interface circuit between the prescaler and one complete input cycle, in sequence. ':, the counter circuitrv is neededbecausethe t.t.l. reset push-button switch is the Thus, when prescaleroperatesfiom a 5 volt supply (which is operated,the four countersbre resetto zero andiso the normal supply voltagefor t.t.l. devices)and the is'the seventh401? i.c. The "0" output of this i.c. CMOS counter circuitry needs to operate from a goes high until the next positive edge input 'is higher voltage than this in order to drive the dis- received from the divider chain. The "0" outpllt plays with adequate current. In this case the then returns to the low state and the "1" outpgt CMOS circuitry is fed froin a 12 volt supply. The goeshigh for one completeinput cycle,and precisepurposeof the interface circuit is to raisethe peak- ly one input cycle. It is this ou@ut which is usedto to-peakoutput voltagefrom the prescalerto a level control the input to the counter circuitry. Thereiis which will properly drive the CMOS counter. no need to use an external gate,as the 4026i.c. has There are reaily three sectionswhich comprise an internal clock enable gate. Since this has to be the input circuit. First, there is a buffer stagewhich taken low to enable the counter to operate,an inis designedto provide the unit with a high input im- verter is incorporated after the "1" output of the 6?6 JULY19'8 PRESCALER I pedancesothat it placesas little loadingas possible bn the equipment irom which the input signalis heing drawn.'Second,there is an amplifier section *fri"h provides the'instrument with a reasonably hieh sehsitivitv. Third, there is a Schmitt trigger ciicuit which frovides an output signalhaving fabt rise and fall times, even when the input signalis a type which doesnot. such as a low frequencysine wave.The Schmitt trigger is an essentialpart of the unit, as without it the prescalerinput will not be cleanly switchedfrom onelogic stateto the other dn slow wave forms. and this would result in 'a malfunction with-probably more output than input pulsesbeing produced. NN." J\ :i ,N) N* {irllowinsinput capacitanceand which would gteatlv recluclthb sensitivityof the unit at suchfrequenc"^"& les. couplesthe output from TRl sgq1geto the baseof the commonetititter amplifier, T&2' This is io the emitter fbllower,-TR3, with ii;.;l'.;;pled resistorcontrolling-bothstages' bias a fr'a *titts ls Ai frr.f .t'igttt,an emitter follower at the output of itt" amnllfier mav seem superfluous, since it ;;;"i,1;.';; voltagdgain. The ioad resistorvalues it t"Tn"i on the low side.-However, il;;';t.;';;"-r'.t-ir" remembered that this amp-lifier has to i"".ii"" ui-it"qu"""ies up to-about50MHz,-q!d so it'h;; l"-.;plov high sireedtransistors(TR2 to 'l'R6 have an fT of 500MHz) which ,requrrecomhigh operatingcurrents'Also, the input "ototiu.tu -rithe subsequentstages-tends to lall imoedanie o"f tiigti frequencies.arldso the use of i'"tl.iJ"i'"tit" f"lio*ei buff6r stagehelps to maintain ;;;;itt.; hieh frequencYresponse. the ""Ti{i'""a' foim a fairly. conventional tn6 -circuit, and this .drives the S"tt-itt-Ttigger ;;;;;;1". uu ii"u"toof another emitter follower, TR6' produce the best possible senfrg it the set"llGteJ low radio frequencies *il"ltu.If "tia particularlv ""aio no! potentiometer is i-hi. ;i;;'";i .iiii.ut, but in'order to obtain good sensitivity at ;;;;hfuhliequencies it must be set up Quite acC6' is a speed-up.capa-citor which "ui'^t"ti. at high frequeniies ahd provides an Rii b;;J. irirbroved high frequency response' 'Ihe SN7490AN decade counter actually contains a divide-by-five and a divide-by-hro counter.It must be connectedto first divide by five and then bv two as the divide-bv-five counterwill operateup to at least 2MHz,-whereas the divide-by-two INPUTCIRCUIT counterwill only operateup to about-16MHzor so. The circuit diagram of the input and prescaler It must be noted that an orilinary SN7490Ndecade stagesis shownin Fig. 3rT.h.inpu!.[qffelamplifier 'fhis- 'stage counter is not suitable for use in this circuit as it is The Jugfet source follower, TRl. will only operatesatisfactorilyup tg about 16MHz. lMsr as providesa'highinput im.pedanc.e-of.abo'r,t tne SNz,ig0AN can be obiaiired from Maplin Cl givesd.c.blockiu.tt u. a low input'capacilance. Electronic Supplies,who can also supply the tranine at the indut, anC Rl' Di and D2 provide sistors tvpe 2TX313 and the voltage regulators With high levelinputs the two ov?rloadprote'ction. snecified'for IC1 and IC14. The remaining at TRl-gateto about 1'3 volts signal the clip diodu. semiconductordevices.apart from the l.e.d. diswilh Rl rrrovidingcurrentlimiting' oeak-to-n^eak, plays, are available frbm Maplin Electronic f u..u lirse input signallevel would be neededto Supplies and from other suppliers. C2.bypasses ariy of ihe input components. dama"ge TR7 is a common emitter amplifier which Ht aitrietr frequencies,thereby preventinga top betweenthe t't.l' and CMOS p.oliicies'itttitfacing the given Rl and by be cut filteriffect which would 1978 4017. It is only necessary to connect the inverted control oulse tL the clrrcli enable input of the firsl c o u n t e r s i n c e ,i f t h e i n p u t s i g n a l i s b l o c k e d a t t h e first counter it c,buic,uslvcannot reach any subsequent counter. The three other 4026 i.c.'s therefore have their clock enable inputs permanently conn e c t e d t o t h e n e g a t i v es u p p l y r a i l . The 401? i.c. aiso has a clock enable input, and its "2" output is connected to this i.nput' During c i r c u i t n p e r a t i o na s s o f a r d e s c r i b c dt h e " 2 " . o u t p u t i s l o w , a l l o w i n gt h e 4 0 l 7 t o o p e r a t en o r t t l a l l y ,b u t after the "1" output has gone higlr fol one output c v c l et h e " 2 " o u t p u t g o e sh i g h . I t l h e r e b y t a k e s t h e a " O t Zc l o c k e n a h l l i n i i u l h i g h a s w e l l , a n d t h e c i r c u i t l a t c h e si n t h i s s i a t e r v i t h t h e " 2 " o u t p u t h i g h and the input clock signal received from the divider chain blocked. The length of the output pulse of the 4017 i'c' depends upon the position o-f the range switch' If this switch conneit. a lHz clock signal to the device, then obviou,sly an output pulse.of one secona (i.e. the length-of one inp-ut cycle) will be produced. 10H2, i00Hz and -1kllz inPut frequenbies oroduce output pulses of 100 milliseconds, -10 rnilliseconds and i millisecond respectively' Therefore, allowing for the fact that the- input signal is diulded by ten by the prescaler, the unit f"our ratt"ge.: 0' to 99.99kH?,- g-!o iri.'iit."f"tt"*ing 999.9kH2, 0 to'-9.999MH2 and 0 to 99.99MH2 .{sain, it must be carefully noted that the upper to be much lessthan limit on the last range is lik-ely -explained,it is detergg.SgN4H,since, as*already mined by the capability of the prescaler and the first counter stage t.c. 3 H 'r'-l o $i x-./?s! c)E oo G1" Io 16 €..1" xl kE J "E$i a-@ -9 o- z l i I t circuits. This transistor is poweredfrom the main 12 volt supply line so that it providesvirtually 12 volts peak-to-peak for the following CMOS counter. R15 Iimits TR? base current to a safe value and C10 is a speed-upcapacitor. The prescalerand ihput stagesare poweredfrom a 5 volf line which is ddrived fiom th6 main 12 volt supply by means of the monolithic voltage CONSTRUCTOR RADIOANDELECTRONICS regulator,IC1. R17 lowersthe voltageat the input of the rezulator and thus reducesits dissipation. C3, C7, C8 and C9 are the usualsupplydecoupling capacitors.It should be mentionedthat somedata tables show the sourceand drain lead-outsfor the 2N5245 transposedfrom the layout given in the lead-out inset. The 2N5245functionssatisfactorily with the lead-out connectionsgiven in Fig. 3. 677 4llzo, a *Ep_*@"' .,m ry F{ "m ffi"oF w rt@-ft,"-fii' ,il$,W*;$6$, :@Y, nt {'f{H$il$+'+** a$@'wf -+\s); [l *ffi$" Q[WCI\s Fig,4. Component and copper sides of thc printed board on which are assembled the input, Schmitt trigger and prescaler sfrges. The board is reproduced full size CONSTR UCTI ON D I S P L A YA N D D I V I D E R S all the componentsare mounted on four Nearlv -circuit orinted boardi. One board containsthe inbut and nrescaler circuitrv. whilst the others are ior the mains power supply, the counter/divider chain circuits and the Le.d. displays. Details of the input circuit and prescalerprinted circuit board are shown in Fig. 4. This showsboth the componentlayout of the board and the copper backine oattern. The diaeram is reproducedactual sizesoih'at it can be easily copied.This board, like the other three which will be described later, is fairly complex,but it is not difficult to produceif a p.c.b-. etch reeist pen having a fine point is available. The two mounting holes are drilled for ,68A clearance. 67E The circuit diagram of the oscillator, divider chain, counter and display sectionsof the unit is shown in Fig. 5. TR8 is the crystal oscillator, and this usesthe well-known Pierce configuration.Cl1 enablesthe oscillator frequency to be trimmed to within less than 1Hz of the nominal crvstal frequency.TR9 is a buffer stage,and this is a straightforward common emitter amolifier. The six 4017 integrated circuits, IC3 to IC9 inclusive, form the lMHz divider chain. The input signalis applied to the clock input (pin 14) of each device,and the divided output is obtainedfrom pin 12. The clock enable (pin 13) and reset (pin 15) terminals of each i.c. are-simplytaken to the negative supply rail, and the ten unused outputs are igCONSTRUCTOR RADIOAND ELECTRONICS Il o( ol $r -= F. E TI ()Oul<A 0 N9 o..d c ci ct ct JULY 19?8 6?e .,,,/K;Kffi nored, w_ithno connectionsmade to their ,pins. Sl(a)(b) is the range switch, and this connects the output of the appropriate4017 divider to the 4017 pulse generator,IC9. The manner in which IC9 ploduces the control pulse has already been discussed,and go will nof be considerediurther here.The commonemitter amplifier, TR10, functions as an inverter stage. R24 limits its base current to a safe value and C15 is a speed-up capacitor. It is essentialthat Sl(aXb) is a break-beforemake type. If a make-before-breakswitch were employed,two of the divider i.c. outputswould be briefly short-circujted together when changing from one rangeto the next. Sincethe outputs coulii well be at o[posite logic states when this occurs there would be a high risk of damageto the i.c.'s concerned.Break-before-makerotar*vswitchesare cur_rentlylisted by Maplin Electronic Supplies, and the type employedshould be a 3-pole-4-way swltch, wrth connectionsmade to onlv two of the poles. The 4026 decoder-driveri.c.'s can drive the seven-segment l.e.d.displaysdirectly, and there is no need to use output cuirent limitine resistors. With the 12 volt supply which is usedhe.-re, an output current of about 1OmAper segmentis obtained. This is more than adequatefor high brightnessdis680 plays such as the DL704 devices which are specified.The Ietters"A" to "G" aroundeach4026 apply to the appropriate display segments. Each 4026i.c. has a display enableoutput, but only that of IC12 is used her6.This outprit drives the applicabledecimalpoint via S1(b),which is, of course,part of the range switch. Incidentally, the DL704 displays have a right-hand decimal point. 32 is the reset switch, and when this push-button is operatedthe resetterminalsof IC9 lo IC13 are connectedto the positive supply rail. This resets them all to zero,and they areh'eti in this stateuntil the switch is released. The unit then makes a measurementof the input frequencyin the manner describedearler. R26,R27 and R28 are includedto ensurethat at no time is an input of any i.c. left floating.This could otherwisebccur with the modular f6rm of constructionwhich is employedhere if it happened that externalconnectionitoiheprinted boaid were not completed.A floating i.c. input is highly undesirable as CMOS i.c.'s can be damagedif high static voltagesappear at an input. Oneadvantageof CMOS i.c.'sis that they havea high degreeof lmmunity to noiseand ther6forerequire very little in the way of supply decoupling.In this case only one decouplingcomponent is used, this being C16. RADIO AND ELDCTRONICS CONSTRUCTOR NEXT MONTH In next month's concludingarticle we shall carry on to the construction of the printed board oir which is assembledthe circuitrv of Fis. 5. For conveniencea full ComponentsList accompanies this article, although it will be appreciated that the functions of somecomponentswill not be fully apparent until they are de-altwith in the constructionaldetailsto be givenin next month's issue. Nevertheless,some notes on the componentshere will be of value. The availability of the semiconductors has already been deali with. The mains transformer employedin the unit is the type "MlN TR 15V" retailed by Maplin ElectronicSupplies.The lMHz crystal arid holder are available'fiom severaloutlets. The display filter is an optical filter suitablefor red displaysand that emplovedbv the aurhorwas the Electrovaluetype PNF2i. Th6 author's frequency meter was assembledin a Veropak casetype 491470L,which has dimensionsof it Uv l.l bv'g.ein. This has a p.v.c. clad steel shell anf, an a-nodised aluminium front panel, and should be available from stockistsof Vero casesor from Retail Department, Vero Electronics Ltd., Industrial E6tate, Chandlers Ford, Hants., SO5 3ZR. Other metal casesof the samedimensionsor larger mav alsobe used. The DL704 displays can be"obtairiedfrom Ambit International. Dealing with smaller matters, switch S3 should be suitable for switching mains voltages. The horizontal skeletonpotentiometer,R9, shouldbe a type having 0.2 in. spacingbetweentrack tags,and 0.4 in, spacingbetweentrack and slider tags.The 18 s.w.g. aluminium sheet is used for making a chassis,a bracket and a heat sink, and details of these will be given next month. (To be concluded) THERMOMIGRATION New doping procosscould herald a fantastic advance in semiconductor manufacture. by Michael Lorant Dr. Thomas R. Anthonv and Dr. Harvev E. Cline, staff research scientists of General Ele-ctric Research and Development Centre. Schenectadv. N.Y., have developed an entirely new techniqu-e, called "Thermomigration", whicli reducesthe time required to fabricate a semiconductor device by as Badiant Heat Sowce / This drawing demonstrates General Electric,s 'Thermomigration" process. Droplets of dopant in the silicon wafar migrate frcm the cold side of tha slice towards tha hot side, taking one-thousandth of the time needed lo( the intoduction of dopants with prcvious techniqueg much as a thousand.fold.In addition the novel technique,which relies on a temperature gradient to drive a liquid dopant through a silicon wafer, reduces fabrication temperatures and increases processingyields. The patented innovation is also expected to result in another important by-product:^a significant savingof energyin the production of semiconductor components. In the manufacture of a semiconductordevicea crystal of silicon is first gxownat the highestpossible purity and is then slicedinto thin wafers.Next, preciseamountsof an impurity, called a "dopant", are introduced into the wafer to change its electrical properties and turn it into a semiconductor device with n and p regions. In the new processone side of the silicon wafer is heated whilst the opposite side is cooled. The temperature difference forces the dopant, in the form of a droplet of liquid, to migrate through the wafer from the cooler side to the hotter side. The thermomigration technique can be accomplished in minutes. By contrast, the best previouscommercialmethod for equivalentdoping of wafersrequiresnearly a weekof processingtime. Also, the new process can be performed at a temperature several hundred degreesFahrenheit below that requiredup to now. With established techniques, excessivewafer breakageis often a problem becauseshallow dopant penetration, of around two to three thousandthsof an inch, necessitates the use of correspondinglythin wafers.With the new approachit is possible,by manipulating wafer alignment and temperature,to control the size,Shapeand concentration of doped regionsin the wafer. The end result of this new technolog3'can be the creation of whole new classesof semiconductor deviceswhich never existed before. I 6Er By Frank A. Baldwin We 0ommence this month by listing some stationson the LF' bands for the interest of the Dxer, much that liriio',';:;lrcing on the higher frequenctes. O - CHINA RadioPekirt:;r-'rr446O at22lO,YL in Chirtesein I progratnme,scheduledht'rt: the DomesticS;i,r'.'icc from 2000 to 2300 with this transmission. RadioPekingon 48OOat 2110,YL in Chineeein sameprograllrrrleas above. the ---Harbin,'Hillongjiang, on 484O at 2113,-YLin from 0825 to 1430 nnd sch6huli'is The Ctiines". *from 2040 to 06i)5. Gnchow, Gansu,on 4865 at 2215,OM and YL alternatein Chinese,scheduledhere from 0950 to 2120to 0100 and from 0320 to 0600. 1600, at1728,signing'off Xinjiang, on 49?O Urumchi,'renditiori of the 'Internationale'.In adwith ch,rral dition to local pro{ratnmes,this transmitter relays the Peking Doriestic Service in Kazakh,--the scheduleii'frnm 2300 to 0030 and from 0130 to 1730. North America in parallel on 11666, 154O6 and 1?815 and to Asia on 16670. .- E. GERMANY Berlin on 21540 at 1241, OM with annuii"."-""ts and identification at the end of the bnslish proffrailme to South East Asia. Announced i r , i i a r a l i eo l n 15115 and 17880. O - SOUTH AFRICA in iofr^..u.burg on 21636 at 1356,progr-amme blnslish about lrocodile conservation,identificacheck at 1400 followed by a local li"f;-".a-ti-e iirj*t.u.t. Announced in parallel on 110OO and t5220. O - COLOMBIA' Emisora Nuevo Mundo, Bogota, on 4766 at 041?. Latin American music, OM with song in Snanish.This stationhasra 24-hourscheduleand a p6*ut of lkW, sometimesidentifying as Radio Caracol. --Ona^" del Meta, Villavicencio,on 4885 at0245, 'OM with iri+ntifit':rliunin Spanish and into a O - NORWAY ?i*t";; zlzgo at 1405,oM and YL with a talk about Norwegian affairs in English. Identification at 1411. O - EGYPT L767o at 1419,local-typemusic, YL c"fi; u"J iitvt in AraUicin the DomesticSCrviceGeneral scheduledfrom 1300 to 1830 on this Ft"nt"--., ;ith-piostutntn"s intended fo-r the Arab ;ffi;I ffi;ia: East ind" Central Africa and Southern !)urope. O - PORTUGAL LirU". on f Zggf at 1608,YL with newsof Port,reu"r" ;ffuirs in-ttre English programmedirected , t i . ' ' : , ! ' ,l " 1 r i i r t n f e l l ' t ! t s i c :T h e pfogt",,. ir''t-n" Mlaale East, sche?uledfrom 1600 to 1630 'ictrJdul , ,,; and the priwel is (nr.rt ii{,,ri i :}(Jir1r:iL}i;t.t{) Sundays). 'gz+O ut 2030,OM with identification t irU"" 1kw. "ri bv the news in English for Europe., f"ll";;d scheduledfiom 2030to 2100(Englishprogramme) a VEN iiZUEL,r' and in parallel on 6O25. at 0232, RadioFrontera,SanAntonio,on 476O pops,6M announcer' YL with songsin local-stvle 'J'heichedrrle is from1000to 0300andthe Spanish. pirweris 1kW. A R O U : ' . r ,1! ' : i i ' ' i , i i t- ISRAEL -English, on 215O0 at1220,pops'commercials "t"t,it"tern and into identification,announceme+lls in and Europe Western to Attno,rnced uiiz30. Fiu-n.Tr682 .- TURKEY on 9515 at 2!32, OM with a local Attt ".^ nu*..*lln-ihe-English programmeto Europe and i.t;;h-A;;'1"i, *f,.a,il;d F;; zrgoto 2255and in - parallelon 7170. f;;kiu; FolisRadvosu(TurkishPoliceRadio), a"lu.^."on ogaO uf tstg, local-typemusic,YL RADIO AND ELECTRONICS CONSTRUCTOR with songs,OM in Turkish. Scheduledfrom 0600to 1100,1200 to 1600and from 1TB0to 1900. O BRAZIL Radio Relogio,Rio de Janeiro,on 4gOE at 02b8i: OM in Portuguesewith announcements, time piffi O GREECE off at 0302 without National Anthem. Athens on 1?88O.at 1582,.typical local_type A Voz do Oeste,CuiaUa,on'+Zi{-it001b, OldJ r-nryrc, OM and YL alternate,i,itt{innouncements with identification in Portuzuese.local-stvlemusicr ln ureek. Schedule is from 1000 to bgOOand the power fil O PAKISTAN _-Karachi on 62Bb at 1943,OM with a talk in Urdu.in the World Serviceto ftre U.X.-r"a nuroo". schedyledhere aqd.in_paraliefo" 47i8 ;;;06; trom -1915to 211bin Urdu and from 2t I; t"-zt6 in Sylheti. . W. GERMANY ., Cgloqle.on 6lOO at 0130,OM with a newscastin tne-!:nghshprogrammeintendedfor the East Coast ot North ASner-ic_a, scheduled from 0130 to 0200 ancttransmitted from the relay station at Cyclops, Malta, on this channer. O AUSTRALIA Melbourne on llgo0 at tblg, yL with the 'Australian Editorial Opinion' in RroqTa.mme Flnglrshtransmissionto Asia and the-pacific. ihe Time 9.h""l1.5prp. and a newscastin EGIish after sta_ uon lclenttfrcation at 1600. Melbourneon llZOb at 0g46,yL with listeners leco.r-qLeguestsin the English brog."--" to the f;acific Isfands. _ Melbourne on SBZO at 14b0, OM with the ppslish programmeto Papua/Ne* Gulnea,ider,_ trllcatronand world news at 1b00. . 'Wellington NEW ZEALAND on 1182O at 0900, OM with time $eql., station identification and the news in English. O PHILIPPINES (fgr Company) ,l.Fq,BC ^East Broad^c_asting Manila on 11766 at.0918,OM with a religiols programme to Australia, New Zealand and -New Guinea. ,Radio.Veritas,I{a.nilq, orr llgbb at 141b,yL wrth a talk on travel in t-heFar East in the English 5#ffi;i-", t'\ scheduled from 1400to 15-00 on"this I . CHINA je.king on gg4b at 118b,yL in Viet_ -^1{i" namese,.Uhlnese music in a programme for Viet_ n4.pr,scheduledhere from tt50 t"oiiiio, fr"_ 1330.to 1430. "li" gg2o at 1480,OM and yL in .,,,fi.a$,ioPeking on propramme Domestic Minority il-1..1^"tifh. Fr!up-s,schecluletiom 1400to 14bb. ,fadjo , Peking. on 9880 ^i- :/LZs, Chinese 9rFnestratpu-slg in a programmeintended for In_ oonesra,scheduledfrom 1400to 1480. . Radio.Pekingon 9B9O at ta+0, Vi;; Tagalogto the Ph.ilipJines-, schedut"a frbm' f igOio^ i soo. *,K,qd,toPekrng on_1165O at 1429, OM irr the programmefor South East Asia, scheduled 5,.ngllsh. tlorn 1400to 1500. ' Radio Peking on lt69b at 1433,OM with a plgsgmme for Cambodia,scheduled'fr;m 1400t; 1500bn this channel. JUI,Y1978 1.skw. Radio Pioneira, Teresina, on a measuredb016 at 0242, announcementsin Portugtrese.OM with ballad. Schedule is from 0800 to 033b and the power is 1kW. Radio Ribamar, Maranhao, on 4786 at 0126, folk musicand sonss,identificationbv OM at 0180. The scheduleof this one is from 1100 to 0400 and the power is 5kW. Radio B_orborema, Campina Grande,on 6O26 at 02_10,OM in Portuguesb, several mentions of "Campina Grande". Scheduleis from 0830to 0b00 and the power is 1kW; sometimegwandersdown to 5023 and/or identifiesas A Princesado Sul. - Emiso-ra Rural, Santarem, on 4788 at 0238, local-style music, love song by OM in Portuguese - quite a torried affair I gathered!The scheduleis from 0830 to 0400 and the power is 5kW. ! t', . WORLD RADIO TV HANDBOOK i, There must be many readers of these columnS; who would like to know more about the world of short wave listening and the short wave station$" mentionedhere month by month but do not knor+i" where to obtain such information. It is to this tvpe of readerthat the followingreview is presentei.'., The 1978 issue (32nd Edition) oi the World' Radio TV Handbookcomprises512 paeespacked. with facts, figures and iirformation- aboui shoit' wave stations, frequencies,schedules,interval. signals, addresses,callsims, powers and per:l sonalities.Quite apart from c6mpletedetailS of' Long and Medium wave stations and TV transmitters, there is also a wealth of information,' about such matters as DX Drosrammesof the world, maps completewith tim'e-c6nversion charts, broadcastsin English, frequency lists, standard frequencyand time-signalstations,solar activity Ibr 1978,co-operatorsand monitors and co-operating' operating DC clubs, together with much othei, information of assistanceto the modern-davshort' wave enthusiast. Also included betweenthe coversof the WRTIf is a separatesectionentitled Listen to the World,. the contents of which include the followinA chapters - Eight New Short Wave Receivers(d review of receivers currently being offered to SWL's); Antennas for Broadcasting Reception (several practical aerial designs suitable foi thq stated purposes);DXing in Paraguay (listening resultsobtainedby Tony Jones,a well-knownDxel residentin that country); FrequencyCountersfo{ the Dxer (learn all about measuringfrequencies); Build Your Own Log-PeriodicAntenna (}ieh-eain switchableor rotata6leaerial svstemsfor thd SWIr! and Unofficial Radio (all aboirt radio pirates and 'i suchlike). The 32nd Edition of the World RadioTV Handj bookis thoroughlyrecommended to all who operate over the short wave spectrum and it is available direct from The Modern Book Company, 19-21 PraedStreet,LondonW2 lNP at e8.16inciusiveof postage. 1i 688 WATCH THEVVEATHER By T. F. Weotherley Red sky at night . . . seaweed,rain before seven, all are traditional ways of telling the weather but today thingsare different. Today there are weather satellitescontinually sendingcloud cover pictures of the earth from f,0oo mi6s up. These irictures can be receivedhere on earth bv anvoneequipped with a groundstation to receiveihem and it neidn't cost a fortune. A sophisticatedsatellite receiving station can be built for less than e50. The interestedreader should first write to the Home Office Radio Regulatory Dept for permission to receive satellite signals from satellites operatingin the 135-138Mhz band and a letter grantingauthority will allow; "authority to receivesignalsemitted by artificial earth satellites engaged in scientificspaceresearchfor the purpose of making observationson the technical NOAA 5 visible light deprassion off Greenland characteristics of such sisnals or otherwise carrying on techiical investigationsinto radio technique." Obviously a receiver to receive this band is required. The author's route was to feed the output from a converter into a VHF receiver.A suitable converterfor the satelliteband can be purchasedor built. The 2M converter in the RSGts handbook can be readily adjustedto cover 136-138Mhz. At the present time (1977) there are two American satellitesin orbit NOAA4 and NOAA b. T-h_e_p_rlpe gqtellr_teis NOAA 5 transmitting on 137.5Mhz with NOAA 4 as back up on 137.62Mhz. The signal from the satellite is tiansmitted as a wideband FM signal with a 2.4 Mhz AM subcarrier, to the ear this sounds like 'lub-dub'. The satellite passesoverhead at about 10.30 am local time each dav. NOAA 5 Same depression over UK RADIO AND ELECTRONICS CONSTRUCTOR The Display unit Actrve fi lter Timc- bose i drivcrs Block diagram ' Receiving of Sateilite set up - The block diagram (Fig. 1) showsthe sectionsof the displayunit.-Onreceivethe unit generates a 2.4 Mhz referencetone which is recorde-don one track of a ste_reorecording while the satellite signal is recordedon the other. In the displav mo-dethe received-s.ignal and the referencetone dre replayed into the display unit. The receivedsignalis cieaired up with an active filter and is used to brighten the trace on a CathodeRay tube. The referenle signal is phaselockedand usedto generatethe horizo'ntal time baseG I 5 Hz\ . The verticaltime baseis seven minutesand is resetwith a switch. With sucha slowpicturebuild un a conventional CRT will not displaj,a picturebut'an ex radar tube with an orangephosphgrgivesa reasonablepicture in a darkenedroom. The picture is best reiorded JULY T978 NOAA 5 Composite of two photos cloud cover over Westarn Europe and UK at night photographedin lnfrc Red Light photographically and the photographs show some of the results to date. The interested experimenter will find the "Weather Satellite Handbook" published by 73 Magazine gives much useful constructional informatron. I bdi) I t l CLOCK AUTO-DIMMEN By R. A. Penfold A neat little unit which dims digital clock display brightness under dark conditions. This device has been designedprimarily as an add-on unit for the "Single I.C.-Disital-Clock" -issue. which was described in last month's Some digital clock i.c.'s have an auto-dim facility built in, so that very few external discrete compbnents are required to add this feature. However,such is not the casewith the AY-5-1224A devicewhich is employedin the "Single I.C. Digital Clock", as this i.c. was chosento cater for a goodand very simple basic clock design. The auto-dimmercircuit to be describedis quite simple and can easily be added to the clock with a minimum of modification to its desim. The experienced constructor could probablf -clocks adapt the circuit to operatewith similar incorporating the AY-5-1224A,but this has not beencheckedbv the author and should not be attempted by anyone who is not fullv competent. The purposeof the'auto-dimmer is to reducethe brightness of the display under verv dark con-display ditions. Under such conditions the can become a little difficult to read due to glare, and the facility is especiallyuseful if the clock is to be used in a bedroom. Pos. (lC.pin 2) Output to clock N.9. ( l C . p i n5 ) Fig. l. How the auto-dimmer unit functions. lt takes up the positive and negative outputs of the clock powar supply and then varies the positive supply voltage fed to tha clock itself 686 THE CIRCUIT Fig. 1 shows the manner in which the autodimmer is added to the clock. It is connected between the positive output of the clock power supply and thi positive iriput to the clock circuit. The dimmer has two output states:one where the output voltage is virtudlly equal to the input voltage, and one where the output voltage is. at about half this level.The former is the normal state of the circuit. and the latter occurswhen the clock is in almost total darkness.It will be apparentthat the circuit doesnot just reducethe voltagesupplied to the clock display but it also reducesthe operating voltage for the entire clock. In practice this does not a-dverselyaffect clock operalion providbd the dimmed outbut voltage is irot made excessively low. (It *ay be noted that in Fig. 1 the positiv-e supply rail is showh above the negative rail, whereasin the clock circuit in last month's issue the positive rail was the lower one.) The complete circuit of the auto-dimmer appearsin Fig. 2. Here, a photoconductivecell type ORP12 is coupled to the input of the Schmitt trigger consistingof TR1, TR2-and their associated components.The output of the Schmitt trigger couplesto the emitter follower TR3. The circuit is arrangedsuch that TRl is turned off in the dark condition, with its base-emitter voltagebeing below the 0.6 volt level required for a silicon transistor to pass collector current. TR2 is then made conductivi by way of the basecurrent it receivesvia R3 and R4. In a more conventional Schmitt trigger R7 would have a value which is much higher than that of R6, so that the oulpgt voltage at TR2 collector would be only slightly oositive of the nesative rail. But in the present ipplication the ouiput voltageneedsto fallio only abbut half that on-the positive rail, and this requirement is achievedwi-ththe valuesof R6 and R7 which are specified. The resistor values around TR2 are, also, such that this transistor does not turn hard on when it becomesconductive, as is CONSTRUCTOR RADIOANDELECTRONICS Pos. in BFY5I Leod-outs Fig. 2. The auto-dimmer circuit. TRI and TR2 form a Schmitt trigger which is actuated by the voltage on TRI base. TR3 is an emitter follower feeding the ctock circuit usual with the more common. versions of the Schmitt trigser. If .the voltage at TRI base is caused to so p o s r t r v ea, l e v e l w i l l b e r e a c h e da t w h i c h T R I c o r i _ mences to pass collector current. This will cause the base current available to TR2 via RB to be reduced, whelgqpoq a .lower voltage is dropped Across R6. This further increasis the 6aseernitter voltage of TRl and a resenerative a c t i o n t a k e s p l a c e w h i c h c o n c l u " d e sw i t h 'fRl turned fully on and TR2 cut off. The change of state in the transistors takes place, of course, in a minute fraction of a second and is triggeredwhen the appropriate voltage is applied to TRl base. With TR2 cut off the output voltage of the auto-dim circuit is only slightly lower thanlhat on the positive rail. The circuit will revert to its previous state if the base of TRl is taken negative, but the voltage at which the changeoveroccurs will be lower thanthat at which TR1 was turned on. This hvsteresiseffect is a characteristic of Schmitt triggeioperation and is useful here as it ensuresthat the trigger does not continually change states for very small changesin input potential. The output current available from TR3 when TR2 is turned off is that flowing through R7 multiplied by the current gain of the transistor. Despite the relatively high current drawn by the clock circuit the voltage drop acrossTR3 under this condition is only about 0.7 volt, which is not significant. The base of TR1 is fed from a potential divider circuit of which one arm consistsof the photoconductive cell PCCl and current limiting reiistor R1, rvhilstthe other arm consistsof the pre-set variable resistor,R2. Under light conditions PCCl exhibits a low resistance, this increasing to a very high value, in €xcess of several megohms, under very dark conditions. In consequence,light condition-s cause the base of TRl to be taken sufficiently positive for this transistor to be turned on and TR2 to be cut off, whereupon the auto-dimmer output is J U L Y1 9 7 8 n * I Resisfors | (All fixed valuesj wart 5%) I R l 2 . 2 ka I R2 50k o or 47k a pre-setpotentiometer,0.2S watt horizontal I I R3 3.9o I R4 22ko I R5 12ko I RG 390 o (seetext) R7 r-160 0 I I Semiconductors I TRl 'fR2 BC109C | 'fR3 BC1OeC BFY51 | CeIl I Photocondu,ctiue i PCC1ORP12 I Miscellaneous I Printedboard materials I Wire, solder,etc. at its maximum level. If PCCI is in a dark situa-. tion its resistanceis high, and TRl basegoessuf-' ficiently negative for il to be turned off and the. Schmitt triggerto changeover to its alternatestate. The darknesslevel at which the Schmitt trigger operatesis chosenby adjustingR2. CONSTRUCTION The auto-dimmer is constructed of a small printed circuit board, and the copperpattern and componentlayout of this are illustrdted, actual The auto-dimmer assemhled components their printed board on 687 L size,in Fig. 3. Note that R2 is a standardand not a mrniature pre-set potentiometer.It is advisableto mark up the three holesit requiresfrom the tagsof the actual component,as these mav varv sliehtlv from the hole positioning shown iri the diadani. I ne two mountrng holes are 3 to 3.5mm. in diameter.Insulated leads about 1b0mm.long are connectedto the board at the points where if will eventuallyconnectto PCCI and the clock circuit. The photoconductivecell is,grountedon the rear panel of the clock case, between the zero reset switch and the adjacentend cheek.It is mounted by first drilling two holesof about 3mm. diameter throug.lJ. which its leads,-coveredby -insleeving,can pass. The photgcell.is the_nglued positi"oirby meansof.a good_qualityadhesivesuch is an epoxy type. Make absolutely certain that its lead-ou^ts dir not short-circuitto the case. The printed circuit board is mounted on the inside of the right-hand end cheek of the case,using two small short woodscrews.It is necessaryto fii several washers qver these screws betwein the board and the case,as otherwisethere is a strong risk of the board cracking when the mountin[ screwsare tightened. CLOCKMODIFICATIONS After the board has been fitted in position it can be connectedto the photoconductivecell lead-outs. AII the insulated leads from the board are shortened as necessarywhen they connectto the cell and to the main printed board of the clock. process is to modify the clock, -The next -becomes whereupon it necessarvt6 consult the printed-circuit board diagram which was published ag Fig. 5 in the article describing the clbck. First identify th9 coppertrack which calries the negative supply rail. In the view of the copper side of the board this is the track which runs dl-onethe bottom and up the right of the board, conne-ctingto the emitters of TRI to TR4. Drill an additional hole through this track at anv convenient point and solder the negative input lead from ihe autodimmer board to the track. At the qpper centre edge of the diagram of the copper side of the main clock board islhe positive supply track, which connectsto the -break positiveleadout of Cl and to C3. Make a small in this track between the connections to these two capacitors. The break can be made quite easily with a sharpmodellingknife. Drill additionalholes h0 Mounting holc6 N /7\J G-------./ (-_-----1 F _: {/\ ----o -\' tS | o NcA. in Flg, 3. Details of tha printed board, which is raproduced fult size 6E8 RADIO AND ELECTRONICSCONSTRUCTO ...: "'i'\ $ throughthe track on either side of this break.The hole nearer C1 positive lead-out provides the positive input to the auto-dimmer ind the hole nearerC3 takes the auto-dimmerpositiveoutput. This completesthe modificatioirsto the main printed board of the clock. The board can now be remountedin the case. R2 may next be set up, althoueh in most instanc.es .it will probably be foundlatisfactory to simply leave it with its slider at about the centre positionof the track. The reasonis that the most usualrequirementof the auto-dimmerwill be that it shouldoperatewhen the clock is in virtually total darkness,and this will be achievedat almost anv settingof R?, It- is, however,possibleto adjust R2 so that the display will dim rinder semi-darkcon- cell beingbrightly illuminatedduring the dav bv sunlightor an electriclamp. This is be6ausethe cefi will then exhibit an extremelylow resistance, giv-the ing a slight risk of excessivecurrent flow in potentiometertrack. The risk disappearsif the potentiometeradjustmentis suchthai it alwaysin_ sertsabout 5k o or moreof resistanceinto cirtuits. 5k o is, of course,just one-tenthof the total track resrstance. . If desired,it is possibleto vary the dimmed display brrghtness to someextentbv alteringthe value in 9f R6. Raising its value will causean i"ncrease brightn_ess, and reduci-ngit will have the opposite effect.Its value shouldnot, however,be increased to more than 470o or reducedto lessthan 800 o. TR3 will run rather warm but thereis no needto dltlons. fit it with a heatsink,even if the clock circuit has R2 slider should not be turned fully clockwise, been modified to providc an increaseddisplay whereuponminimum resistance current (as was describedat the end of the articli is inseitedinto circuit, if thereis a possibilityof the photoconductive on the clock). I BOOKREVIEW T E S T E 0 U I P M E N T F o R T H E R A D I O A M A T E U R ,s e c o n dE d i t i o n . , . E n g .M , . l . E . EG . , 2 B U p .1 5 1 p a g e s , 2 4 Sx 1 g 5 m m . P f H . L . G i b s o nC (9f, x 7f,in.)Published by Radiosocietyof GreatBritain.pricef3.7s. The amateur radio transmitting enthusiast require.sbasic test equipment not only for servicing but also to ensure that he is satisfying his licence conditions. Furtheriest equipment is desirable and much of this can be home-constructed with a corresponding saving of cost and increase in experience.The book under review gives detailed information oir the biriloing of tesr equipment and the manner in which it may be used. The present second edition has a completely revised text and has n e w d e s i g n s , i n c l u d i n gd i g i t a l i n s t r u m e n t s a n d m i c r o w a v e t e s t g e a r . .many The test equipment covered includes everything that can conceivably b! required for normar amateur work, and takes.in the measurement of voltage, current, frequency, r.f. power, noise, aerial standing waves and many other quantities. The book also has the aitractiue well-drawn diagrams which are associated with R.S.G.B. publications. The price at the head of this review ij tnat a p p l i c a b l ei n t h e U . K . , a n d t h e v o l u m e m a y b e o b t a i n e d d i r e c t f r o m R a d i o S o c i e t yo f G r e a t B r i t a i n , 3 5 D o u g h t y S t r e e t , L o n d o n ,W C l N 2 A E , a t t h i s p r i c e p l u s ( a t t h e t i m e o f w r i t i n g ) 6 7 p p o s t a g ea n d packing. JULY 1978 n@H y0uril u|0r $mr U { I Cheerfully, Dick plugged an aerial lead into the 14 inch black and white television receiver, after which he inserted its PIug into one , r f t h e m a i n s s o c k e t sr a n g e d a l o n g the rear r-,fhis bench. He switched the set on, whereuPon the sound sisnal frcm one of the local u.h.f' channels became audible from its soeaker. Expectantlv, he waited for the cathode rav tube to warm uP and the consequentaPPearanceof a uicture. There was no Nothing' nhappened. ot even a blank unoicture: nrodulated raster. He leaned forward, located the brightness contrr!,I,and aijusted. it e x n e r i t n e n t a l l vT. h e s c r e e nr e m a l n e d ^ b l a n k a t a i l s e t t i n g so f t h e c o n trol. Returning it to a central po-sition, Dick frowned as he mentallY totted up the faults which could cause the loss of raster and picture: a faultv tube, a fauitv tube heater . u r r p l v , a f a i l u r e i n t h e e . h . t 'v o l t a g e . u i l b l v t o t h e t u b e f i n a l a n o d e ,i n c , , i i e r l to n e r a t i o no f t h e l i n e o u t p u t , line oscillator or line driver stages, or an unsetviceablesupply to any of the remaining tube electrodes. "Rlimev." he rnuttered glumlY, g to be a stinker"' " t h i s o n e t s g o i n-off the receiver. At He switclied once the screen of the tube lit up, to fnde awaY into its Previous darknessafter a few short moments. B R I G H T N E SC SONTROL I n c r e d u l o u s l y ,D i c k g a P e da t t h e receiver. Witlr i trembl-ing hand he turned it on again, with the result that the sound-'signalbecame audible once rnore. The face of the Pict u r e t u b e r e p e a t e di t s ,p e r f o r m a n c e irnd retnatneclcompletely olanK' Dick waited for a minute then turn690 Scan and Flyback Rectification +iu'iil[-$ $ililil$ 'lhe screenbecame iled the set off. luminated for a brief period before it returned to its previous darkness. "Hey, Smithy!" The Serviceman, preoccupied with a dismembered music centre on his own bench, gave no indication that he had heard his assistant. "Hev. SmilhY/" lrritibli', Smithy Put down his test nrods. "For goodness' sake, what is it r1()w i "I've eot a set here that's haunted. it only comes on when I switch it off!" With a sigh,Smithy got up from his stool and walked over to Dick's bench. "Show me." l)ick dernonstratedthe behaviour of the receiver. Smithv watched. unimpressed.when the screencame momentarilyto Iife as Dick switched the set off. "There's nothing verY comn l i c a t e dt h e r e , " h e c o m m e n t e d".l ihould look for a snag in the briehtnesscontrolcircuit' In fact, I strriirtdcheck the brightnesscontrol o o t e n t i o m e t e ri t s e l f . P e r h a P s ihere'san open-circuitbetweenone end ol the tiack and its tag.You can do a simple d.c. ohms test there." With which words, SmithY stumnedback to his music centre' Deflated, Dick took the television .et plue' from its mains socket, ..-nued the back of the receiver and. after maniPulation of the orintedboard,foundhimselfableto ieach the tags of the brightness p o t e n t i o m e t e i .H e s w i t c h e d h i s iestmeter to an ohms range and applied its test cliPs to two of the o b i e n t i o m e t et ra g s .( F i g ' 1 ( a ) ' ) Brightncss control (o) (b) Fig. | (a). Following Smithy's bidding' Dick checked the continuity oi the brightness control potentiometer track in his faulty monochrome TV raceiver lb). When he checked between the slider and the track tag. when the control was turned fultv cloclrwise he found an open-circuit between thd tnck end and the tag RADIO AND EI,ECTRONICSCONSTRUCTOR Frcm video output stoge ill lll lll ill ttl To tlywhccl sync stoge l2V bottcry - Fig. 2. Line output stagd of a monochrome mains/battery television receiver. (This is a stightty simplified version of the circuit emptoyed in the Thorn lsbo-lsgl Seriis or television receiverc.) He turned the control in an anticlockwise direction. The meter needle fell to a near-zero reading as the potentiometer slider reached the end of its track. He transferred one of the clips to the other end of the track, then adjusted the controt so lhal its slider approached and reached the fully ilbckwise end of its track. This time the meter continually indicateda high resistance reading. It was obvious that Smithy's d.iagnosiswas completely c o r r e c t .( F i g . 1 ( b ) . ) . "Hey, Smithy! You must be havtng me on!" Absent-mindedly, Smithy looked rouno. "I bee vour pardon." "I_ said'you must be having me on. You must have had a look at this set yourself, because the fault ]v?s.an open-circuit to the .bnghtness control track. exactly like vou said." _"Was it?" said Smithy, pleased. "Desp.ite. my declining year., I haven t lost my grip then. No, I haven't examined the particular set vou've got there but I-do know the model fairly well. I made a guessat the snag, working from that knowledge,and it looks as thoush my guess was pretty inspired." " JULY 1978 "But I don't get it," wailed Dick. "How on earth can the symptoms of the screen lightine up at switch-off lead you io d-ir6ctly to the brightness control pot?" Resignedlv, Sniittrv rose and walked over to the filing cabinet in which the service sheetswere kept. Afler some moments he removed a manual, carried it over to Dick and laid it alongside the television receiver.-He opened out the pages bearrng the receiver circuit and indicated the line output and picture t u b e s e c t i o n .( F i e . 2 . ) "I need the reieiver circuit to be ahle to answer your question," he remarked. "Now. if vou look at that circuit you'll see there's a winding o n t h e l i n e o u t -pulses put transformei which feeds scan via a rectifier diode to a lpF electrolytic and the brightness control pot." "Here, hang on a minute, what's all this about scan pulses? And what's a winding doing on the line output transformer providing these sc-anpulses, anyway? I thouglt that all a l-ineoutpui transformer"is supposed to do is to feed the line scann i n g c o i l s a n d g e n e r a t ee . h . t . f o r t h e final anode of ihe picture tube." "Even in dn ordinarv monochrome receiver where the h.[. is derived direct from the mains. the line output transfbrmer does more than just that. It so happens that the set you have here is one of the popular current models which can run either from the mains or from a 12 volt battery. If you connect a battery to ii the- batterv voltagepassesto a stabilizingciicuit which providesa regulateiloutput of around 11.5volts.And when you connect the mains to it, the mains is passed to a step-down transformer and a rectifier and smoothing circuit, the output of which 4lso goesto the stabilizlngcircuit. So, whether it's mains or battery operatedthe set still has to work with a supply of only 11.5 volts." BOOSTED H.T. SUPPLY "Humph," gruntedDick. "Well, !g-t].gg back to the ordinary type of TV where the power supply ii obtained direct from the mains.What extra job does the line output transformer do in these sets?" __"It providesboostedh.t.," replied Smithy promptly. "The simplest example here is given with the earlier sets having line output valvesrather than line outout trhnsistorsbecausethe circuitrv is a little easierto understand.H6re'spart 691 B@stcd HT+ oscillator and do similar iobs where f a i r l v h i g h v o l t a g ew a s n e e d e d . " "Is there a boosted positive supply in this transistor circuit?" Srnithy pointed to the circuit in t h e s e r v i c em a n u a l . ( F i e . 3 ( b ) . ) "'Ihere it is," he stated, "right at the top of the line output transformer primary winding, just as with the valve version. Since the supply voltage in this set is only altout 11.5 volts, the boosted voltage is correspondingly low. In practice it's approximately 25 volts, and it's rrsed for supplving the i.f. stages and part of the video amplifier. But it does not supply the video output stage,which requires a much higher v o lt a g e ". +25V boostcd voltogc (200v opprox.) Boost Rs€rvoir coPoc[or Port ol line oUtput tronstormer tii lil eortor rinc output lil lilt'on"o""' H r r r n g r l r ,r"e p e a t e dI ) i c k . He thought for a rnomenl. " \ V h ; r l v o l l a g e s .h e a s k e d , , . d o e s . t h e l r r r er ) u l l ) u tl r a n s f o r m e rp - sr u, rpvpi ldye other than the boosted vol tage?" Linc output volvc 'tr Fig. 3(d. A look at one of the earlier valve line output stages readily demonstrates the formation of a boosted h.t. vottage. With the h.t. voltage shown, the boosted h.t. voltdge would be of the order of 5OO volts (U. Virtually the same circuit is given semiconductor version ofa typical valve line output stage." Smithy pulled Dick's note-pad towards him, took out a pen and sketched out a circuit on its too p a g e .( F i g . 3 ( a ) . ) "We haven't got time to go through the whole line output cycle," he went on, "but, as we're only interested in the boosted h.t. supply we need only consider part of it anyway. And I need only show transformer vou the line outout -which primary winding, produces the boosted h.t. Now, during the latter half of the scan section of the line cycle, when the spot is being deflected horizontally across the screen of the picture tube, the line output valve is on and is drawing a continually increasing anode current from the positive h.t. rail through the booster diode, and through the line output winding below the cathode of the booster diode. Since the lower end of this winding is negative of the booster diode cathode, simple transformer action causesthe top end ofthe winding, above the booster diode cathode, to be positive. The result is that the boosted h.t. reservoir capacitor charges up, causing a voltage which is considerably positive of the h.t. positive rail to be available on the terminal of the capacitor which connects to the top end of the winding." "And that's the boosted h.t. voltage?" 692 the more modern "lt is," confirmed Smithy. "The capacitor losesa bit ol' its charge d u r i n g t h e l ' i r s tp a r t o f t h e s c a n s e c tion of the cycle but it still retains enough to provide a voltage which is considerably positive of the h.t. p o s i t i v er a i l . W i t h s i m p l e l i n e o u t put transformer circuits you - p o s ican tive alwavs find the boosted voltage, because it's sitting at the top end of the winding whose bottom end goes to the line output valve. Or. nowadavs.to the line outpu1 transistor. In-the older sets the boosted h.t. voltage was normally used to supply the frame timebase "As vou've already mentioned," said Srnithy, "it supplies the extra high tension voltage for the final anode of the oicture tube. In this particular circuit the e.h.t. is provided by its own separate winding. Since the current drawn by t h e f i n a l a n o d e i s q u i t e s r n a l l ,b e i n g rrllorra l tenth of a milliamp or so, t h e w i n d i r r ci s c o n n e c t e ds o t h a t i t fceds positive-going flyback pulses to the e.h.t. rectifier. The conseq r r c n t e . h . t . v o l t a g e ,w i t h t h i s s e t , i s then allout i 1kV." (Fie.4.) "F-lvback pulses I can understand," complained l)ick, frowning. ",lust now, though, you were talking about scan pulses." "Ah ves," said Smithy, picking r u ph i s p e n a g a i n . " W e l l , l e t ' s n e x t take a look at the voltasewhich is induced in that e.h.t. windine. It w i l l l r a v e n w a v e s h a p e. o m e t h i n g like this, with spaced-out high voltage pulses appearing at flyback ancl long cornparatively flat sections between these pulses. The average voltage of the waveform will be just slightlv above the flat sections, w h i c h r e p r e s e n tl h e s c e n n i n g p a r l of the line output waveform during ill c H L I,OOOpF winding Fig.4. The e.h.t. section of the line output stage. ln most receivers the e.h.t. resentoir capacitor is given by the capacitance between the inside and outside graphite coatings of the picture tube. ln this receiver a I,OOOpF capacitor is added in parallel RADIO AND ELECTRONICS CONSTRUCTOR Averogc LolJle: Scon Flybock (o) Averoge vol toge Fig. 5. Acgording to the way connactiong are mada to it e windlng on a llrto oupt tnnsform& mdy pmduce flybac* puls* which tn positiva-golng wlth rcspect to crrdsstc, ds in bL or wDn pulsas which .ra p6tltlv*. going. as in (b) . which the spot travels across the picture tube screen." Smithy drew the waveform with its average voltage carefully. (Fig. 5(a).) SCAN PULSES "Now that waveform." said Dick musingly, "is the one that's passed to the e.h.t. rectifier." "That's right," ageed Smithy, "and, as we've already said, it's got positive-goingflyback pulses." An agonised expression creased Dick's face. "I think I'm beginning to understand what this scan Dulse business is about," he stated ilowly. "Am I right in saying that it is all right to pass spaced-out positive-going flyback pulses to the e.h.t. rectifier because the e.h.t. current is so small?" "You are. The e.h.t. reservoir capacitor is charged up by each pulse, and the smhll e.h.t. iurrent only discharges it a little bit before the next pulse comes along." "Right," said Dick briskly. "Now what happens if we want the line output transformer to supply a circuit that requires a high current? could we do that by turning the waveform uoside-down so that the flyback pulies go negative?" (Fig. 5 ( b ) .) JULY 1978 "We could." "Blimev." remarked Dick delightedly. "I think I've discoveredsomethinghere! If we turn the waveform upside-downso that the scan sectionsare positive-going and then apply fftrs to the rectifier, we'll also get a positive rectified voltageoutput. The positiveinput to the rectifier will be presentnearly all the time too, won't it? That is to say, not spaced out like the flyback pulsesare. This meansthat the rectified output will be much better regulatedand will be able to supply quite a high current with only a small value of reservoir capacitor." "You're exactlv rieht." "Am I? Gosh!'-' "To use the usual terms." continuedSmithy, "you cangeta much higherrectified output current from scanrectificationthan vou can from flyback rectification. But there's a snag. "When isn't there?" "Can you," askedSmithy gently, "see it?" "Let me think about it." said Dick. "Now, when we have flyback rectification the peak rectified positive voltagewill be the voltage by which the flyback peaks go positive of the waveform average voltage.I think that's right, isn't it?" "Apart from the very small forward voltagedropped in the rectifier, which can be ignored here, you're completelyright," "Good," said Dick, encouraged. "Then, when we use scan rectification, the peak rectified voltageis the maximum amount by which the scan part of the waveform goes positiveof the averagevoltage.For a givenline output transformerwinding this will be very much smaller than the peak flyback rectification voltage.Now, what doesthat bring us to?" Suddenly his face lit up. "I've got it!" he said excitedly. "I've got it! If you want a certain rectified voltagewith scanrectification vou need a lot more turns on the line output transformerwinding than you do to get the samevoltage with flvback rectification." "That," remarked Smithy approvingly, "is it precisely.The last thine the TV set-makerwants to do is to'put an unnecessarynumber of turns on the line output transformer and so,if he wantsto obtaina supply voltagefor a low current circuit, he uses flyback rectification. It is only when the supply voltagehas to feed a high current circuit that he usesscanrectification.The polarity of the waveform is governed, of course,by the end of the winding which is connectedto chassis.In this particular TV circuit we've already seenthe flyback rectification for the e.h.t. supply. Can you see another case of flvback rectification?" THE MODERN BOOKCO. Largest selection A.merican radio of English and & technical books in the country 19-21 PRAEDSTREET L O N D O NW 2 1 N P Tel: Ol-723 418512926 5,OOO O D D SA N D E N D S Mechanical Electrical CATFREE W H I S T O ND E P TR . E . C . N E W M I L L S .S T O C K P O R T cABlNErs BEC (BOOK END CI{ASSIS) T h e D U E ' l . T ES l e r e o A m p l i f i e r u s e s o u r G B I case and can be supplied punched or unpunched. The P.C. Boards for this oroiect are also available:For details of this and our stanclard range of cases send 1Sp (refundable) to: H. M. ELECTRONICS 275a Fulwood Road Broomhill Sheffield S10 3BD Ril[0tlfltulsffi For the convenienceof lrish Radio & Electronics Constructor Dsta Books F a n e i$ i g n s I rnnsfers Aiso a postal setvice 693 the line output transformer circuit sives scan- rectification and a ielatively high voltage, of the order of 95 voits, ippearsicross the lPF reservoir ca[dcitor following the rectifier. This voltage feeds the collector load of the video output transistor. It also feeds the brishtness control pot, which is the one vou found to be faulty. It will be almost certain that the track was open-circuit at the high brightness end, where it connects to chassis." "That's right," agreed Dick. "It was the end where the knob was turned fully clockwise. Which reminds me all over again: how on earth did you know that the fault was there?" "Recause this particular set has a rather unusual on-off switching circuit which we ha.ren't talked about vet." reolied Smithv. "The on-off iwitch has three poles, two of which switch the mains supply whilst the third switches battery negative to chassis when the set operates from a 12 volt battery. When this third pole 'ofl" position it connects soes to the ihe slider of the briehtness control pot to chassis, whereupon the tube Dasses a fairlv heavY current between cathode and fihal anode, and larselv discharges the e.h't. reservoii cioacitance. The result is lhat vou don't get that little sPot anpeiring on ihe centre of the *.ieen aiier vou switch off whilst the tube cathode is still hot. Also, vou don't have a lot of e.h.t. volts i r a n g i n ga r o u n d . " "0f cour*e. of course. I see it nowl" said Dick. "When the set was on, the open-circuit to the pot track rneant that the tube cathode was alwavs highly positive with respect Fig. 6. Ftyback rectificdtion is used to feed the tube first anode and the focus Potentiometer FOCUS CIRCUIT f)ick peered anxiously at the circuit. "Could it," he asked eventually, "be after the winding which supplies the focus pot circuit?" (Fie. 6.) "You've nlt lt llrst go. l)ear me, vorr're in verv good form today." "l rnust admit." stated Dick modestlv. "that t have the occasionai Ilash of genius." "Right,then..." "l)own at.loe's Caff, for instance, t h e l ' L r t r ku p o n m e a s b e i n g e x c e p tionallv brainv." "Ok-av. so lbt's . . ." "Naturallv. I have to draw a cloak over mv brilliance." "This flvbick rectification . . ." "Ottrerwise people might look uoon ' " W i lme l as heing big-headed." v o u f l a r n i n g . w e l lb e l t u p l " "Ulimv, Smithy. what's up with 'vou I "I want to set on with this discussion," retorted $mithy heatedly. "Not listen to you and Your mouth." "Fair enough," replied Dick equabl5'."Well, if you remember, we weie talking about the flYback rectification bit which, as I just oointed out, is used in the focus conirol circuit." Smithy slowly simmered down. "And," he grated, "you were risht in pointing out that flYback rectification is used there. Now, let me collect my thoughts. Ah yesl The associatedwinding on the line output t ransformer aPPlies oosiiive-eoineflvback pulses to the iectifier' ,nd the follbwing l0lF reservoir capacitor, and the resultant positivi voltage then supplies the focus oot. the slider of which connects to the focus electrode of the tube. The rectified voltage, which is around 300 volts, also goes to the first anode, incidentally. The first anode and the focus electrode both draw very small currents and so flyback rectification is quite in order here." "There's another rectifier," remarked Dick, "and that's at the bottom of the same winding which passespositive-goingflyback pulses i,r the'focus pot circuit rectifier' 't'here's a chassis tap into the *ind i n c i n b e t w e e n ,t o o . " ( F i g . 7 . ) ''And that," remarked Smithy, "is the scan rectification circuit I rcferred to right at the beginning. Frorn the way that the comPlete winding is drawn in the diagram v,rrr worrld expect the wavefortn at its hottom end to be of opPosite polaritv to that at the top end. And s o , i n l i r n c t i c e ,i t i s . T h i s l a s t p a r t o f llllllllr Scction on-off of switch Fig. 7. The higher current 95 volt supply for the video output stage is provided by scan rectification. The cross indicates the position of the open-circuit in the faulty brightness control potentiometer ELECTRONICS CONSTRUCTOR If H BY THE B M E T H O! D Tlrrse colrses. which have been sold for ovcr 2 3 year.; havt: been proved rnany times l ) e to the fastest Inethod of learning Morse. Y()il slarl riqlrt away by learninA the SOUndS ()f llrt various letters n1'nbers etc.. aS yo' will in fat:t Lrse thern Nol a series of dols an(l r l i r s l r r r sw l r i c h l a l e r V o r r w i l l h a v e l o t r a n s l a t e iilli) l{,llpr:t arrrl worcis lJsrnr; sr:ienlifically prr:parerl 3 speecl rccords V()| dillorudlically learrr to recoqnise the lrrrio RIiYTHM wilhoul lranslatinq You r : i r r rI l r r ' l p i l l l s a s e a s y n s l c a r n i n q a t u n e in 4 weeks .lua6r)teed 1B WPM Tlli' (l()rJrt)li,tI C()llrse c()nSiSts of three rllorrls Fig. 8. Yet some further outputs from the line ouvut trdnsformer. The tap in the lowar winding provides line blanking pulsas for the picture tube grid together with nagative-going flyback pulses to tha flywhdel sync stage to the tube grid regardlessof the positionof the pot slider.Switching off shortedthe pot slider to chassis and took the tube cathode sufficiently negative for the tube to passcurrentand light up the screen. It's obviousnow, when you look at it." POWERHOUSE "Exactlv." chimed in Smithv. "Taking ihe tube cathode highly positiveof the grid causedthe tube to cut off. becausethis is the same as taking the grid highly negativeof the cathode. As vou can see from the main circuit, ihe grid is held at potentialby way of the 10kO chassis and 33ko resistorswhich coupleit to the chassis.So there vou are. Dick. You'vegot a TV setin front of you which hasa positivesupplyrail of only 11.5 volts feedingthe line output stage. And the line output stage,with its transformer,acts like a veritable power house.It knocks out a boosted supply voltage of around 25 volts for the i.f. stages and some of the video circuitry, it producesan e.h.t. voltage of about 1lkV for the final anodeofthe tube. it generatessome 300 volts for the tube first anode and the focus potentiometerand, finally, it gives 95 volts for the video output transistorand the brightnesscontrol circuit. That's not bad going for just one stage." "I'll say it isn't," agreedDick. "Stap me, Smithy, I've just noticed anotherdiode! "There's a tap in that last windingwe dealt with and it connectsvia a 4,700pFcapacitor anda 100koresistorto this diode.It also goes by way of a 0.1pF capacitorto anotherpart of the circuit. Blimey, there's no end to the JULY 1978 external circuits which are tacked on to this line output transformer!" ( F i g .8 . ) "That last diode," chuckled Smithy, as he glanceddown at the service manual, "is in the line blanking circuit, and it clampsthe waveform with positive-goingscan oulses so that the most positive parts are held at chassispotential. The result is that the grid of the tube is at chassispotential during the scan period and then goes negativeduring the flyback period, caisins the tube to be cut offduring that period. And the waveform passedvia the 0.lpF capacitorgoes back to the line flywheel sync diodes.The important parts of the waveform in this case are the n e g a t i v e - g o i n gf l y b a c k p u l s e s . These make the flywheel sync diodes conductive, but I don't intend to start diesing into tftof part of the circuit no-ri.tsspeciallyarithe only thing that started off this particular gen session was a faulty brightneis control pot!" "Well," said Dick, "You've certainlv openedmY eyesso far as the line 6utbut stageof this set is concerned.It's a v-erybusy little stage, isn't it?" It certainlv is," rePliedSmithY' "Which reminds me that it's time that I became busY too, and got back to mv work instead of sPending my time nattering awaY like this." "Still, it's nice to have a little break from work everY now and again." ""I Smithy' supposeso,"conceded "provided it doesn't haPPen too often." "With us." crinned Dick, "it onlY I happensoncel month!" rs wtll as irtstrrrclion l)ooks. F o r C o m p l e t eC o u r s es e n d f 5 . O 0 p l u s p a r t postage5Op {overseassurtacemail C1 extra). Norv available Shrouded Morso Kovs f2.7O inc. UK Postage THE MORSE CENTRE Box 8, 45 Green Lane, Purley, Surrcy. I c r r r : l o sI e5 0 O r l r l a r q es a e f o r e x p l a n a l o r y booklet Name.................. Address............. GAREX 2-motre RECEIVER NR56: fully tunable 1 4 4 - 1 4 6 M H z , a l s o I 1 x t a l p o s i t i o n sf o r m o n i t o r i n gs p e c i f i cc h a n n e l sC . o m p a c t ,s e n sitive, idoal for fixed or mobile listening. B u i l t - i nL . S . ,1 2 v D C o p e r a t i o nf.6 4 . O O i n c . V A T . C r y s t a l s i, f r e q u i r e d :f 2 . 5 0 e a c h A l l p o p u l a r2 m . c h a n n e l si n s t o c k .C r e d i tt e r m s a v a i l a b l e ,s . a . e . d e t a i l s . M a r i n e B a n d R x ( 1 5 6 - 1 6 2 M H z s) i m i l a rt o N R 5 6 . . . . . . . f 5 9 . 4 O ( x t a l sf 2 . 7 9 1 R e l a y s6 v c o i l ,2 5 4 c o n t a c t sS , p m a k aB O p , 2P make90p Neons min. wire eno. O b p / fO ; f 4 l l o o S l i d e S w i t c h e s m i n D P D T1 8 p e a ; 5 + : 1 4 p 2 p o l e , 3 p o s i t i o n 2 2 pe a c h ; 5 + : 1 8 p Resistor Kits E'l2 series,22 O to tMO 5 7 v a l u e s .5 ' %c a r b o nl i t n r . { W o r } W . f2.95 S t a r t e rp a c k , 5 e a c h v a l u e ( 2 8 5 ) M i x e d p a c k ,5 e a c h * W + ] W ( 5 7 O l f 5 ' 4 O 'lO f6.40 each (570) S t a n d a r dp a c k , f13.25 G i a n tD a c k .2 5 e a c h ( 1 , 4 2 5 ) f . C ' s ( n e w ) 7 4 1 O 2 5 o C D 4 O O 1 A E2 6 P S N 7 6 6 6 O 7 5 p N E 5 5 5 5 5 P 7 2 3 ( T O 5 )7 5 P 7 O g ( T O 5 ) . 7 4 1 ( D l L - 8 ) O Pa m p s 3 O P ; 2Op: BNC Cable mtg socket 50n r 8P; 5 + : 1 5 p P 1 2 5 9 U H F P l u g & R e d u c e6 5+: 6Op; SO239 UHF Socket Panelmtd. 5 5 p ; 5 + : 4 5 p N i c a d T e c h a r g e a b l e sp h y sically equiv. to zinc-carbon types: AAA { U1 6 )f , l . 6 4 ; A A ( U 7 )f 1 ' 1 5 ; c ( U1 1 ) f 3 ' 1 5 ; D(U2) f4.94; PP3 f5.20 AnY 5+: less 1 O % ,A n y 1 O + l e s s 2 O % . W a s t o c k a m a t a u r V . H . F . c q u i p m o n tl n d mobiloaerial.,s.a.o.datrlls. Distriburo6 for J. H. Asrocirtc! Ltd. (switches PRICES INCLUDE end lampr) UK POST. PACKING & VAT Solc Addros8; Mail order only GAREX ELECTRONICS 7 NORVIC ROAD, MARSWORTH. TRING, HERTS HP23 4Lg Cheddington {STD 0296) 008084 Radio Topics By Recorder A R A L D I T EP A C K S I should imagine that few readers will reouire an introduction to that excellent adhesive, Araldite. In the Recorder household, for instance, this enoxv resin has been used fbr manv'jolis ranging from the mounting of radio components on a chassisto the successfulblockins of holes in. of all things, an enamel bucket. In industry, Araldite is widelv employed for the potting of comnonents and small electronic assemblies.And it is made up by cornbining the requisite amounts of resinand hardener. Araldite has been available in two-part sachets containing 500 sramsof the material for industrial p o t t i n g a y r p l i c a t i o n s ,a n d t h e t n a n r r f a c t r t r e rn o w a n n o u n c e s t h e introduction of smaller 200 grarn 'lhese s:rchets. should be of special interest to userswho need to employ l r o t t . i n gr e s i n s o n l y o c c a s i o n a l l y ,o r w h o s ea l l l l l i c a t i o n sd o n o t r n e r i t t h e rrse of an automatic metering rnachine t() obtain the correct mixt r r'l'he r eo f r e s i n a n d h a r d e n e r . 200 grarn pack is in two Araldite from a 2OO gram sachet being poured to form a small potted assembly. The resin and hardener are in separate halves of the sachet, being brought together just before use by removing a clip p:rrts, one containing the resin and the other the hardener, with the two sections kept apart by means of a clip. When the clip is removed the two comDonent materials come into contact with each other and can be t h o r o u g h l vr n i x e d b y m a n i p u l a t i o n . 'l'he bag is then pierced at one end lnd t,hemixture squeezedout to be rrsedas required. In the photograph it. is being poured into a can cont a i n i n g e l e c t r o n i c c o m p o n e n t s .A s cllr lle seen,there is no requirement firr the operator to come into llhvsical contact with the mixture at all. 'f he performance of epoxy resins cart depart from specification if the trvo components ate not mixed in the correct ratio, and the two-part sachet is a good way of ensuring lh;rt optimum performance is achieved. The resin and hardener differ in colour, whereupon the operator has a visual means of checking that they are thoroughly rnixed beforeuse. 'l'he rnixture consistsof CW 1404 (lR resin and HY 956 hardener. 'l'his has a low shrinkage on curing, low exotherrnic temperature rise end a low coefficient of thermal exp i r r r s i o nA. s w i t h o l h e r e p o x y r e s i n s , thc electrical properties are particrrlarlvgood. 'fhe hardener was chosen to give the mixture rapid curing properties at moderate temperatures, and it is therefore particularly suitable for 'lhe s.mall castings. mixture will cure fully rrt 25'C in 24 to 36 hours, but will cure in as little as 3 hours at {i0'(-'.Once mixed, the system has a life at 25"C of about 2 to 3 hours. Further details on the 200 eram sachetsmay be obtained from Ciba('leigv Plastics and Additives Com1ranv, [)lastics Division, Duxford, O a r n b r i d g e ,C B 2 4 Q A . Central Electricitv Generating Board, are precisely defined in terms of time. It may be that the time element becomes PartlY hidden because virtually all the measurements are expressedas the number of cvcles which exist within of 1 second. The the fixed period -terminology from cycles change of per iecond to Hertz may also be responsible for shifting the time qo n c e p t , r n e l a v e r l o w e r i n o u r thinking. We can control the timing of electronic circuits by mechanical rneans. as with a quartz crystal, or hv purelv electronic means. In the latter casewe take advantage of the llrollerties ot capacitanceand incluctance, of capacitance and resistance or inductance and resistance. When we combine canacitance and inductance we have. of corrrse, a tuned circuit. (-arrncitanceand resistance can be directlv related to time becausewe can make un bscillators which are rlependent ulron the time taken for ru capacitor to charge or discharge, via a resistor. to a specific voltage. Notable examples here are the twotransistor sllmmetricaI multivibrator and circuits incorp o r a t i n gt h e 5 5 5 t i m e r i . c . I t i s a l o t more difficult to give an example of timing control bv inductance and resistance, so far as generating an rrlternating voltage is concerned, Irt'causeit is irnpossible in practice to ohtain inductance on its own without self-capacitance stealing o n t o t h e s c e n e .A b l o c k i n e o s c i l l a t o r ittcrrrporating an ir-on-cored Ir:ln\{i)rller is a rough approximation to frequency control by means of inductance and the inevitable r e s i s t a n c ei n t h e i n d u c t a n c e , a n d t h e i n d u c t a n c eo f a T V l i n e o u t p u t l r ; r n s f o r m e r( a n d t h e l i n e s c a n n i n g t''ils) cert;rinlvcontrols the timin[ ol a line output stage during the s c a n n i n gs e c t i o no f t h e l i n e o u t p u t cvcle. l'erhzrlls the most fascinating of time controlling circuits are those emlrloving Capacitance and resistance.because it is so easy to select the tirning by choice of c a p a c i t a n c ev a l u e . I f y o u h a v e n ' t g o t a n o s c i l l o s c o p ea v a i l a b l e a n d vou want to seethe waveform given irr'. sav. a 555 oscillator,you can . i r r r p l v s l o r vi l d o r v n a b i t b y u s i n g a l:rrge value electrolytic for the c r r r r a c i t a n c e .S e v e r a l h u n d r e d DOING TIME rnicrofhrads in coniunction with r e s i s t a n c e si n t h e o r d e r o f l 0 0 k O Although we do not always pav can give an overall cycle lasting much attention to the fact. as soon s o n r el 0 o r 2 0 s e c o n d s . , I u sct o n n e c t r r s w e c h a n g e ,i n a n v e l e c t r i c a l o r r r v o l t r n e t e r a c r o s st h e o u t p u t o f a c l e c tr o n i c w o r k . f r o r n d i r e c t v o l t a g e iir.-r slorved dorvn in this manner :rrrd current to alternating voltafe rrrrd r,'oucall see its waveshape. and current we are introducing-'a t',,tnllletelynew dimension. All I l i r i r r ' . e v e r v r l ( ) wa n d a g a i n j u s t a l t e r n a t in g q u a n t i t i e s ,w h e t h e r t h e v 1 , ,r e r n e r r r b et hr a t a l l t h e a l t e r n a t i n g l r e t h e u . h . t . t e l e v i s i o ns r g x a l v o ' u i-olt:rges:rttd cr.trrentswe deal with p i c k u p o n t h e r o o f - t o pa e r i a l . i h e ;rre all accurately determined in auclio orrtput from a hi-fi amplifier, r r l r ; r ti . p e r h a l t st h e m o s t e p h e m e r a l ,'r the mains supply which is all: tinreitself. rlirnenri,rn,,f-them h r r , r r g h t1 , r r r s b y c o u r t e s y o f t h e I RADIO AND ELECTRONICS CONSTRUCTOR