July 1986 ekrto* Rs. 7.50 High-power AF Amplifier \ Scope converter \ Volume - 4 Number- 7 July 1 98B Publisher: C.R. Chandrana Editor: Surendra Iyer Editorial Assistance: Ashok Dongre General Manager: J. Dhas Advertising: B.M. Mehta Production: C.N. Mithagari Address: ELEKTOR ELECTRONICS PVT LTD 52. C. Proctor Road. Bombay • 400007. India Overseas editions Glentop Publishers Limited Standfast House. Bath Place High Street Barnet London EN5 1 ED Editor: Len Seymour Elektuur B.V. Peter Treckpoelstraat 2-4 6191 VK Seek — the Netherlands Editor: PEL Kersemakers Elektor sari Route Nationale; Le Seau; B.P. 53 59270 Bailleul — France Editors: D R S Meyer; G C P Raedersdorf Elektor Verlag GmbH Susterfeld-StraSe 25 100 Aachen,— West Germany Editor: E J A Krempelsauer Elektor EPE Karaiskaki 14 16673 Voula — Athens — Greece Editor: E Xanthoulis Elektor JCE Via Rosellini 12 20124 Milano - Italy Editor: D Fumagalli Ferreira Et Bento Lda. R.D. Estefdnia, 32-1° 1000 Lisboa — Portugal Editor: Jorge Goncalves Ingelek S.A. Av. Alfonso XIII, 141 Madrid 16 — Spain Editor: A M Ferrer International co-ordinating £t technical manager: K S M Wa I raven The Circuits are 'or domestic use only the submission ot designs, of articles to Elektor India implies permission to the publishers to alter and translate the text and design, and to use the contents in other Elektor publications and activities The publishers cannot guarantee to return any material submitted to them All drawings, photographs, printed circuit boards and articles published in Elektor India are copyright anc may not be reproduced or imitated m whole or part without prior written permission of the publishers Patent protection may exist in respect of circuits, devices, components etc described in this magazine The publishers do not accept responsibility for (ailing to identify such patent or other protection Distributors: Blaze Publishers & Distributors Pvt Ltd Printed At: Trupti Offset Bombay - 400 013 Copyright© 1986 Elektuur B.V. The Netherlands electronics technology CCD video memory systems . . . Loudspeaker efficiency A compact radar for helicopters . Monitoring highways electronically projects High-power AF amplifier Single-trace CRT converter Eight-way relay board TV interference suppression . . . , Supply failure indicator Portable mixer - 2 VHF/UHF-TV modulator information Readers services Corrections guide lines New products Switchboard Appointments Index of advertisers selex - 1 4 Digi-Course II (Chapter 8) Threshold voltage and the LED Capacitance decade box High current and magnetic fields 7.28 7.44 7.51 7.52 7.18 7.25 7.33 7.36 7.37 7.39 7.48 7.08 7.74 7.62 7.69 7.66 7.74 7.54 7.56 7.58 7.61 elektor indie July 1986 7.03 BE PROUD INDIAN SUPPORT INDIAN TECHNOLOGY PLA HAND HELD DMM TYPE: DM-14B1 'PLA’ introduces VERY ECONOMIC model with extra ordinary features not available in IMPORTED/SMUGGLED DMMs. in market. Low prices, THANKS to EXCISE REDUCTIONS and our streamlined DMM production. OUTSTANDING FEATURES (USER MUST CONFIRM THESE BEFORE PURCHASE OF ANY DMM ) • Battery Life: 1000 operating hrs. due to special LSI. • Ohms Protection: FOOLPROOF protection upto 450V DC/RMS by means of PTC and not merely by fuse as in IMPORTED/SMUGGLED DMMs. Burnt fuse replaced by ordinary wire may damage the DMM completely later on. • Servicing: NO PTH PCBs hence servicing easy. In PTH PCB component replacement very difficult. • PLUG-iN LCD/LSi/iCs: Use of easy to replace plug- in LCD/LSI/ICs. • Push Button SW: Push button sw (No rotaries) for easy and real single hand operation. • Warranty. 12 Months free servicing warranty, servicing/spares available for years. • Price: All these features available at rock bottom price. FOR DETAILS CONTACT : PLA ELECTRO APPLIANCES PVT. LTD. THAKOR ESTATE, KURLA KIROL ROAD, VIDYAVIHAR - BOMBAY - 400 086.' *5132667/8, 5133048. elektor india july 1986 7.07 INDUSTRIAL MICROPROCESSOR TRAINING AND DEVELOPMENT SYSTEM with built-in Ideal choice for Process Control and Data Aquisition System Development. Can be converted into a powerful Process Control Computer. Important Features: ■ Eight Channel ADC with 50 micro seconds conversion time. ■ Single Channel DAC with 0 to 12 V analog output. ■ 8085A CPU @ 3 MHz. ■ Fully decoded Memory and I/O. ■ Battery Back Up for RAM. ■ Powerful Monitor Firmware. ■ Fully buffered STD bus for expansion. ■ Fast, Intelligent on board EPROM programmer. ■ Built-in ASCII keyboard interface, Centronics compatible printer interface and Cassette interface. ■ Eight channel Interrupt Controller with programmable priorities. Options: ■ Real Time Clock/Calendar and 64K extra memory with battery back up available on STD bus through Bank Switching. ■ VDU card with Editor/Assembler/Disassembler on STD bus. ■ Floppy Drive Controller card with CP/M BIOS. ■ Full fledged FORTH language and Tiny BASIC in ROM. Introduced for the First Time in India by: Creative Data Systems 14, Hanuman Terrace, Tara Temple Lane, Lamington Road, Bombay 400 007 Tel: 36242 1,353029 Telex: 01 1-7561 4 SEVKIN Gram: ELMADEV1CE. An associate of Dynalog Micro-Systems, who gave you the most popular MICROFRIEND series of Microprocessor Training and Development Systems. elektor india july 1986 7.09 ADDMark AUTHORISED STOCKISTS ALLAHABAD ALLAHABAD ELECTRICAL & ELECTRONICS 533 Nehru Nagar. j«4ul' Meerapur. i a^abad 211 003 AHMEDABAD RADIO ELECTRONICS Opp Gulabbai Hospital Bldg Relief Road. A" -dabad 380 001 BANGALORE BANGALORE ELECTRICAL AND ELECTRONICS CO 122 1st Floor Sadar patrappa Road Bangalore 560 002 BARODA MANDAR ELECTRONICS Ganeshwadi Behind Khanderao Market Baroda 390 001 ANOTHER WINNER BOMBAY PRECIOUS ELECTRONICS CORPORATION Chotam Building. 52. C Proctor Road. Grant Road (East) Bombay 400 007 ELECTROMARK 304. Lotus House 33 A New Marine Lines Bombay 400 020 CALCUTTA GENERAL RADIO COMPANY 6 Madan Street Calcutta 700 072 ELECTRONIC COMPONENTS & SYSTEMS 32 Ezra Street ®i> t No 418 4th Floor, Calcutta 700 001 DELHI INDIAN TECHNOLOGICAL PRODUCTS Flat No 305 306 Bldg No 35 36 N.-hru Place. New Delhi 1 10 01 SPARK GAP CAPACITORS Spark Gap Capacitors to protect picture tubes. Box configuration. Encapsulated in flame retardant polypropylene and protected by a dust cover. Miniature size. Available in popular ratings. Competitively priced. DEWAN RADIOS 1681 28. Bhagimath Palace Chandhi Chowk Delhi 110 006 HYDERABAD SHILPA ELECTRONICS 107 Parklane Secunderabad 500 003 INDORE ATLAS SALES AGENCIES 18 2 Siyagani Indore 452 001 JAIPUR EL TRONICS 87 A Dhuleshwar Garden Jaipur 302 001 KANPUR EVERON ASSOCIATES 18 1 79. 1st Floor. Opp Phool Bagh. The Mall Kanpur 208 001 CTR MANUFACTURING INDUSTRIES LTD NAGAR ROAD. POONA 41 1 014. CAPACITORS KOTA CHAMBAL ELECTRONICS 4 C 16 Housing Board colony KOTA CHAMBAL ELECTRONICS 4 C 16 Housing Board colony Talwadi. Kota 324 005 WAMS 027/85/D LUCKNOW ARTEK ENTERPRISES 14. Cantonment Road Opp Shilpi Cinema Lucknow 226 C91 TRIVANDRUM ACE INDIA T C No 1 978 Bank colony Road Kumarapuram Trivandrum 695 01 1 MADRAS TEXONIC INSTRUMENTS Post Box No 3746 9. Athipattan street Mount Road. Madras 600 002 POONA NAGESH ELECTRIC & ELECTRONICS CROP 476. Budhwar Peth Pune 411 002 PRECIOUS ELECTRONICS CORPORATION 9. Athipattan street Mount Road. Madras 600 00? TRIO RADIO & ELECTRONICS CORPORATION 468. Budhwar Peth Pune 411 002 STOCKISTS REQUIRED FOR UNREPRESENTED CITIES offers m €L€CTRONIC EDUCATIONAL & TESTING DEVICES 1 DL-400 Logic Trainer 1 DL-800 Digital Lab we stock: Solderiess Bread Board 1 DL-600 Analog Lab DT-01 Digital Trainer elektof Magazines Semiconductors ic IZUMIYA 1C INC. P.C.B. Drafting Aids KOIMTAKT Cleaners TELEDYNE RELAYS 2 Semiconductors Trimming Potentiometers Ribbon Cable 05! 227. Pnnnprtnr Panpls Connector Panels connector raneis Semiconductors SPECIAL EX STOCK OFFER E-Prom D-RAM Regulators SCR/Triacs TTL/CMOS 27 J9 27128 4116 41256 7805 - 24 4 amp - 40 amp 74 LSOO - 629 2732 27256 4164 42128 7905 - 24 400 V - 600 V 4001 - 40257 2764 Bulk importers and users in India may Kindly forward their specific enquiries Denise Electronics Pte. Ltd. 101 Kitchener Road *02-04, Singapore Electrical, Electronics and Hardware Centre, Singapore 0820. Telex : DEVICE RS 33250 FOR ENQUIRIES CALL: 298 6455 (4 lines) elektor india july 1 986 7.1 3 HIGH-POWER AF AMPLIFIER- 1 Here is an amplifier that meets the demand for good quality sound reproduction at very high sound pressure levels. Capable of delivering either 2x500 W in a stereo arrangement, or 1000 W in a bridge configuration, this design may be called powerful in the true sense of the word. Technical characteristics Input sensitivity: Input impedance: 3dB bandwidth: Distortion: Damping factor: Features:* 775 mVrms for maximum output power power amplifier: 22 kQ. preamplifier: 47 kQ.* 8 Hz.. . 100 kHz. <0.1 % at 1000 W in 8 Q, or 2 x 500 W in 4 Q, or 2 x 250 W in 8 Q; measured within 10 Hz 30 kHz band. <0.01 % at 600 W in 8 Q. or 2 x 300 W in 4 Q, or 2 x 200 W in 8 Q; measured within 10 Hz . .30 kHz band. >100. mono/stereo selection on preamplifier with symmetrical or asymmetrical input and volume control. Transformer current limit at power-on; DC level monitoring at amplifier output; delayed loudspeaker connection; thermal control of fan relay. * to be discussed in part two. The considerable power reserve of the amplifier described in this article will be of definite interest for appli- cations in discotheques as well as in large PA (public address) systems, where a sufficiently high SPL (sound pressure level) for the low and lower middle audio frequency ranges is normally only attainable through a combination of amplifiers and a number of stacked, high-efficiency bass bins. Apart from presenting an amplifier with outstanding features, both as to performance and reliability, this article is also interesting from a theoretical point of view, since pro- cessing small audio signals to ten odd amperes of output current re- 7 .1 8 elektor indla |uly 1 986 quires quite a lot of attention to overall efficiency and problems per- taining to stability, as well as to opti- mum transmission of dissipated heat. Genera/ considerations An amplifier with an output capa- bility of the order of 1000 watts poses problems as to the heat dissipation of the power output stage. In order to shed light on these problems, their theoretical aspects will be briefly discussed below. In theory, the output stage has a maximum efficiency of 78.5%; that is, with maximum drive level applied and disregarding the transistors' drain-source saturation voltage of about 2.5 V. At 1000 W output power, therefore, the DC input, Pm, to the fi- nal stage amounts to Pm =1000(100/78.5) =1274 watts. The maximum dissipation, however, does not occur at full drive, since the overall efficiency drops with lower drive levels to the output stage, but, theoretically, at a drive level of 64%, and amounts to Pdtss = 0. 4P out =0.4 x 500=200 W per channel. Since this is a stereo design, we can expect 400 W in the worst case con- perature and the output DC level are complementary, differential ampli- Fig. 1. Basic cir- dition. The losses due to the quiesc- under constant surveillance in order her stages (T 9 -T 10 ; T4-T5), each with cuit arrangement ent current add a further few watts to to timely detect amplifier malfunc- its associated current source. Alter- of the amplifier the total dissipation. Given a quiesc- tions are/or gross distortion occur- nating voltages at the inputs ‘see’ the input section. If ent current of 100 mA per transistor, ring in overdriving conditions. All of differential amplifiers as connected correctly dimen- ie 400 mA per channel, the ad- these protections aim at preventing in parallel. The advantages offered stoned, it offers ditional power demand Pqc is costly and disastrous bangs in the by this setup may be summarized as excellent per- calculated from loudspeakers) and blown mains follows: first, the complementary formance. Pqc =0.4x75 Vx 2=60 W ner fuses when the amplifier is switched transistor types and the equal cur- channel on. rents, supplied by Ii and I 2 , cause Fig. 2. The This article discusses theory and the base currents of T 9 and T 10 to driver circuit of Again, this figure should be construction of the main high-power counterbalance with respect to the the high-power doubled, since there are two ident- amplifier board, two of which are re- input; secondly, the four transistors amplifier is ical channels. Note that the factor quired for a 2 x 500W (4Q, stereo operate at virtually constant basically a sym- two in the above calculation setup) or a single 1000 W unit (8 S, collector-emitter voltage, which metrical and represents the symmetrical ±75 V bridge connection). Next month’s makes for constancy of capacitive complementary supply. In conclusion, it is seen that, issue of Elektor Electronics will deal feedback characteristics and, conse- cascade con- theoretically, each power transistor with the power supply for the input quently, a further reduction of poss- figuration. The dissipates some 33 W in a worst case and driver sections, a stereo/bridge ible non-linear operation. Further- application of condition. Obviously, this calls for a preamplifier, details for setting up more, the constant voltage ensures VMOSFET suitable heat-sink with very low ther- and testing, the protective circuitry, pure current amplifier operation of drivers ensures mal resistance, supported by a and constructional hints. the differential configuration so as to ultra-linear oper- powerful fan which is switched on obviate the need for the internal tran- ation over a automatically when the heat-sink Rryqjr' sistor ca P acltances t0 be charged wide range of temperature exceeds a safe value. ^ ^ dCdL^//Li// and discharged at audio speed; this audio signal In order to achieve maximum ef- C/&S/CfD works out to be a great asset as to the levels. ficiency and a large signal handling ^ quality of amplification at low collec- capability, both at instantaneous and The functional division of the pres- tor currents, and, therefore, the low- continuous operation near the peak ent amplifier board into input stage, noise and high cut-off frequency output power level, the amplifier in- driver stage, and power stage is a properties of the design. In short, the put and driver sections have been ar- logical consequence of the specific input section achieves a remarkably ranged to operate from a higher task assigned to each circuit section, low TIM (transient intermodulation) supply voltage than the output stage; All functions have been thoroughly distortion figure. Driver transistors T 3 this ensures full drive reserve in all analysed and the resulting basic sec- and Ts must provide clean voltage conditions and thus avoids driver tion designs will be discussed amplification; however, contrary to ‘pulling’ at peak output currents. below. | the basic arrangement shown in Of necessity, several protective The input section has been devised Fig. 1, these transistors have, in fact, measures have been incorporated in for optimum characteristics as been cascaded and connected to the present amplifier design, since regards low noise level, stability, and driver MOSFETs — see Fig. 2. its huge power reserve is capable to frequency response. Figure 1 shows The typical advantage of the driver destroy even the most rugged of the basic concept of this section cascade setup is a further improve- high-power loudspeakers in the which exhibits outstanding qualities ment upon the already highly linear absence of suitable circuitry to delay by virtue of its symmetrical arrange- Id=lfUd) curve, relevant to these both the speaker connection and the ment At the left is the audio input, at complementary MOSFET devices, presence of the full mains voltage at the right the input for a portion of the Moreover, the extensive frequency the power transformer primary amplifier output signal (feedback). range of this driver design fully winding. Also, the heat-sink tern- Basically, the circuit consists of two matches that of the input stage as de- elektor india july 1986 7.19 Fig. 3. The basic circuits of Fig. I and 2 can easily be spotted in this circuit diagram of the high- power amplifier. Note that this is but one of two identical units! scribed. The power output section is basi- cally a conventional push-pull design with complementary N- and P-channel power MOSFETs of the horizontal type, selected for good transient response and linearity at all possible drive levels. Circuit details A careful examination of the circuit diagram shown in Fig. 3 reveals the practical realizations of the sections discussed above. Note that the part numbers have been retained for this purpose. There is a fair number of zener diodes in the circuit; D1...D4, together with ICi and IC 2 , provide the stable +80 V supply voltage for the input and driver section. D 7 . . .D 12 ensure the presence of the correct supply voltage for the com- plementary, low-noise transistors Types BC550C-BC560C. Ti and T6 supply a constant collector current to each differential amplifier, set to about 0.45 mA per transistor, this current constitutes the right com- promise between minimum noise level and maximum cut-off fre- quency of the input stage. T 2 and T? have been connected as diodes to reduce the voltage excursion at the collectors of T 9 and T4, as well as to correct any thermal runaway effects in T 3 and Ta. The quiescent current 7 20 elektor india July 1 986 86031-4 90 V 5 of the driver stage has been ar- ranged at a fixed 25 mA, which flows through T3, Tn, Tia, and Pi. The lat- ter is used to set the quiescent cur- rent of 400 mA for the power output stage. Unfortunately, power MOSFETs of the type used in the present ampli- fier tend to oscillate quite easily, es- pecially when connected in parallel. In order to combat this tendency, each MOSFET is fitted with a low- value gate resistor. Owing to essen- tial differences in their internal struc- ture, the N-channel MOSFET Types 2SK135 and 2SK175 typically present lower gate-to-source and gate-to- drain capacitances than the com- plementary, P-channel Types 2SJ50 and 2SJ55. To avoid output stage un- balancing and resultant instability, a number of small ceramic capacitors, C18...C25, are fitted at suitable points around T13. . .Ti6. Diodes D13. . Dm limit the drain cur- rent of each MOSFET to 5 A in case of an output short circuit. This effec- tive protection causes no measurable distortion during normal operation. Each MOSFET source terminal is connected to the loudspeaker out- put rail by four parallel connected 1 watt type resisistors. These are used instead of a single 4 watt type, which is typically a wirewound type. This type of resistor cannot be used here since it would present a stray in- ductance in a highly critical location, causing amplifier instability and a strong tendency to oscillate. Power supplies The circuit diagram of Fig. 4 shows the +90 V supply for the input and driver sections of two amplifier boards, as well as the necessary supply voltages for the protective circuitry. This combined power supply will be reverted to in next month’s second article. Figures shows the +75 V, high- current power supply for the two amplifier boards as described in this article. It should be made quite clear at this stage that the final sound quality of the proposed amplifier depends direct and inevitably on the current sourcing capability of this power supply. Any attempt to skimp on this vital section will result in failure of the amplifier to produce good sound quality, especially in the low and lower middle frequency ranges where generally most of the music power is contained. The pro- posed supply ensures good ampli- fier response to continuous as well as short-duration signal peaks generated by musical instruments such as electric bass guitars, bass drums, or synthesizers. To meet the current demand of the amplifier boards, the proposed + 75V supply incorporates two identical, toroidal 750 VA mains transformers, a 25 A bridge rectifier, and 2 x 30,000 nF smoothing capacitors. It stands to reason that the construction of such a supply unit deserves the necessary care and attention, and this will also be reverted to in next month’s article. Fig. 4. The driver and pre- amplifier supply section provides a higher output voltage than the power stage supply to ensure sufficient drive at continuous oper- ation near peak amplifier output. The construction of this supply unit will be reverted to in part 2 of this article. Fig. 5. Vital to the correct oper- ation of the amplifier, this powerful mains supply unit is equipped with two toroidal transformers and a suitably dimen- sioned smoothing section, capable of catering for the amplifier's high current demand. elektor India julv 1 986 7.21 Fig. 7. Compo- nent mounting plan for the high- power amplifier. Parts list (relevant to a single amplifier board) Resistors: Rt=22 k R2 = 4k7 R3;R4 = 8k2 R5;Re = 1k8 R7;R8 = 33Q Rs Ri2 = 180Q R 13 . . R 22 = 220 Q R23;R24 = 10 9; 1 W R2s. ..Rs6=1 Q; 1 W R57= 470 Q,1% Rss = 22k;1% Pi = 250 Q preset (good quality!) Capacitors: Ci;C3;C4;C6;C9; Cio=r47p; 100 V; electrolytic C 2 = 100 n Cs=330 n C7;C8;Ci3=47 jj; 16 V; electrolytic Clt = 1 p; MKT Cl2 = 220 p 04 = 100 p; 16 V; electrolytic Ci5;Ci6 = 10O p; 100 V; electrolytic Ci8...C2i=330p C22...C25 = 33 p Semiconductors: Di;02= zener diode 33 V; 1.3 W D3;D4 = zener diode 39 V;1.3W Ds;D6;Di7;Di8- 1ISI4002 D7. . .012= zener diode 47 V; 0.4 W- Dt3;Dt4- zener diode 10 V: 0.4 W Di5;Dt6 = 1N4148 Ti...T5=BC560C T6. . .Tto= BC550C Tti = IRF9610/9612/ 9620/9622 (International Rectifier! Ti2 = IRF610/612/620/ 622 (International Rectifier) Ti3...Tt6 = 2SK135/ 2SK175* (Hitachi) Tt7...T20=2SJ50/ 2SJ55' (Hitachi) ICi = 7808 + finned heat-sink IC 2 = 7908 + finned heat-sink Miscellaneous: heat-sinks for Tu;Ti 2 (37.5 mm e.g. Fischer SK59) 2 PCB-mount fuse holders To get the most out of the amplifier, all supply wiring should be of 2.5 mm* cross-sectional area, preferably heat-resistant stranded wire. Do not fail to observe due precautions when working with this power supply; 150 volts is a dangerous level! Power resistor R 59 in the mains supply line prevents the mains and/or the domestic 13 A fuse(s) from blowing when the amplifier, or rather the power supply, is switched on. Without this current limiting device, the discharged capacitors and the absence of a magnetic field in the mains transformers would cause a very high, momentary mains current, enough to blow the fuses. The protective circuitry, which is dis- cussed next month, energizes Rei (i.e. short circuits R 59 ) after a short ‘power-on’ delay, which is long enough to allow the transformer magnetic field to be built up and the smooting capacitors to be given an initial charge. Construction and initial test Before commencing the construc- tion of the amplifier, it is advisable to be quite clear as to its intended ap- plications. If it is to be used as a bridge-connected 1000-watt mono type, the power supply should be configured as outlined above. MOSFETs Types 2SK175 and 2SJ55 are then preferred to Types 2SK135 and 2SJ50: the former are more rug- ged and better capable of withstand- ing high-voltage surges. If the amplifier is intended for use as a 2x250 watt stereo type with 8-ohm loudspeakers, the toroidal trans- formers may be rated at only 7 A each, and the total smoothing capacitance may be halved. Note that all amplifier configurations men- tioned so far require two items of all , parts as indicated in Fig. 3, including the ready-made PCB. Fitting the parts as per Fig. 7 should present few problems, but the eight TO-3 style power transistors and the heat- sink require some skill in mechanics; this will be explained later on. It is strongly advised to use first-class components of known make in all lo- cations. Never use cheap, baker’s dozen capacitors or resistors, and closely observe tolerance and maxi- mum rating of each and every part before soldering it into place. Also opt for safety where the high-voltage supply rails and the amplifier output are involved. The MOSFET power transistors are 7 22 elektor mdia july 1 986 7 fined onto the board last, along with a suitably drilled, 5 mm thick alu- minium angled bracket; see Fig. 6 for the relevant dimensions. Do not forget to fit the transistors with good quality mica washers; ceramic (AI 2 O 2 ) types are preferred, but more expensive and harder to get. Also remember to use a generous amount of heat conducting paste. Check for any short circuits between the transistors and the bracket once these have been bolted together. It is strongly suggested to take ample time for a thorough inspection of all parts when they are fitted on the amplifier board; verify the cor- rect polarization of all zener diodes and electrolytic capacitors; make sure that the NPN and PNP tran- sistors have been fitted in the correct PCB positions. Keep in mind that any mistake, however trifling it may ap- pear, may have costly consequences for the output stage and/or the power supply, not to mention the loudspeakers. . . If everything appears to be in perfect order, proceed with bolting the amplifier board to a large heat-sink with a thermal resistance of no more than 0.3 K/W. Now consider whether you want to test the board right away, or wait until next month’s issue is on your work-bench. It should be noted that testing at this stage of construc- tion involves a number of risks, ow- ing to the fact that the protective circuitry is not present as yet. Therefore, if you feel less sure about taking a risk, wait till next month and have the protective circuits correct any of your mistakes. When in doubt, opt for the safe way! For an initial test, it is assumed that the amplifier board has been bolted to a heat-sink, and the ±75 V supply has been constructed in an ex- perimental setup. Connect the + 75 V to the +90 V, and the —75 V to the —90 V terminals on the amplifier board. Replace the 6.3 A fuses with 1 8, 4 W resistors, and solder 5K6, 1 W resistors in parallel with zener diodes Da and D+ Now put the board aside for a moment and test the +75 V supply. Temporarily short out R59 and insert a 10 A anti-surge fuse in the mains line to the transformers. Make sure that the experimental setup is safe as regards the presence of the mains voltage at several points. Now switch on. Should the 10 A fuse blow, re- place the wire across R59 with a suitably rated switch. Verify that the switch is open and apply power again. Close the switch as fast as you can; the new fuse should not blow this time. Leave the power supply on for a few minutes and measure the output voltages; these should be of 3 car-type terminals for ±75 V and earth connections 2 fuses 6.3 A anti-surge soldering pins as required aluminium bracket* large heat-sink 0.3 K/W* 140x15 cm, e.g. Fischer SK39I PCB 86031 8 insulating washers TO-3 style* Parts for + 75 V power supply: (purchase in quantity as listedl Rs 9=100 Q;10 W Tr-vTrs toroid transformer: 55 V-15 A secondary or 2x28 V-15 A* le.g. IIP Type 9B656I B = B200C25000;BYW64 C26...C3i = 10,000 p; 100 V* * see text and/or relevant Figure. eleklor mdia july 1986 7.23 Fig. 6. Dimen- sional outlines of the support bracket which forms the ther- mal contact be- tween transistors and heat-sink. Also shown are transistor mount- ing details. the order of +75 V to +80 V, de- I pending on the exact secondary voltages of the transformers in use. Switch off and slowly discharge the smoothing capacitors with a 500 Q- 10 W resistor. If applicable, set the auxiliary switch to the off position again. Connect the supply to the relevant terminals on the amplifier PCB and tum Pi to its minimal resistance pos- ition (fully counter clockwise). It is not necessary as yet to have a load connected to the amplifier output; hook up an oscilloscope instead. Switch on as outlined above and carefully measure the voltage drop across the fuse replacements; this should be 0 V. Slowly tum Pi for a reading of 0,4 V across each ‘fuse’ to set a quiescent current of 100 mA per output transistor. Observe the measured value for a while and verify that the amplifier does not oscillate at slightly different quiesc- ent currents; neither should there be any tendency to thermal instability. Measure the DC level at the amplifier output; this should not exceed about + 50 mV. If everything appears to be in order, a suitably rated loud- speaker may be connected to verify distortion-free amplification. Do not test for maximum power in this test setup! Finally, replace the 1 8, 4 W resistors with the fuses again, remove the supply wiring, and unsolder the resistors across Da and D+ The test procedure for the other amplifier board is, naturally, entirely identical to that outlined. NOTE: The next part of this article will be featured in our October issue. 7.24 elektor pndia july 1 986 The single-trace type of oscilloscope is definitely one of the most widespread items of measuring equipment, and as such it is generally appreciated by those who do any kind of testing or repair on (home made) audio circuitry However, the single-trace scope has its limitations, which are the more keenly fett when trying to compare, say, an amplifier input to an output signal. Here is an add-on design to achieve double-trace operation from that old, simple scope of yours! SINGLE-TRACE CRT CONVERTER The obvious advantages of having a second, simultaneously visible, channel available on an oscilloscope are likely to be so well known to any electronics enthusiast as to obviate the need for any further discussion. However, a close examination of the typical operation principles of the two-channel and dual-trace type of oscilloscope is essential to a basic understanding of the present add-on unit. As will be generally known, the main circuits in a standard oscilloscope may be represented schematically as shown in Fig. 1. The input signal to the scope is amplified before it can deflect the cathode ray tube (CRT) electron beam in the vertical (Y) direction. Also the signal is used to modulate the sawtooth voltage, generated by the timebase section (horizontal or X deflection). The setup as shown allows the displaying on the CRT screen of a single trace (i.e. input signal) only. Basically, there are two methods of simultaneously displaying two or more curves on a single CRT screen. The dual-beam configuration is the rarer and also the more expensive of the two, since it involves a CRT with two independent sets of X and Y deflection systems and associated electronic circuits. However easy the latter may be built, it will be readily understood that providing a single- trace CRT with an additional elec- tron beam is definitely out of the question as a means for single-to- two-channel conversion of an existing oscilloscope. Contrary to Fig. 1. Functional sections in a single-channel oscilloscope. Fig. 2. The chop- per mode in- volves very hast trace switching between the two input channels. If the timing is cor- rect, the curves will appear as smooth and con- tinuous to the observer. the dual-beam type, the typical two- channel oscilloscope has only one CRT electron beam and. conse- quently, only one X and Y deflection system. The trigger and timebase sections are also single circuits; the difference with a single channel type lies in the presence of two at- tenuators and a fast switching chan- nel selector, which operates at a speed, high enough to make both channels appear simultaneously and correctly positioned on the CRT screen. Obviously, such a channel switching unit may be used as a separate add-on item in conjunction with any single-channel oscillo- scope to obtain the enhancement as outlined above. eleklor mdia juty 1 986 7.25 Fig. 3 Block schematic presentation of the two-channel scope add-on unit. Fig. 4. Circuit diagram of the two-channel add- on unit. Note that only two of the four bilateral switches, con- tained in ICj have been used,- the control inputs of the remaining two have been grounded to preclude inter- ference caused by the chopper oscillator Nt-Ni Chopping or alternating? Most commercially available two- channel oscilloscopes offer two modes of operation: chopping or alternating. Operation in the alter- nating mode is basically as follows; assuming that the electronic switch circuitry has selected channel 1, then a trigger pulse enables the scope to display the curve relevant to the signal as applied to the channel 1 input attenuator. On completion of the horizontal sweep of the luminous spot, it is arranged to return to the left of the CRT screen again, ready to be set off by the next trigger pulse. However, not only does the trigger pulse start a new horizontal sweep, it 7.26 elektor india July 1986 also causes the electronic switch to select the other input channel for display on the CRT screen. There- fore, both channels are alternately displayed, but the mode has one distinct disadvantage, which should not be left unmentioned. If, for in- stance, the scope is to display two complete cycles of a 1000 Hz sinus- oidal input voltage, the timebase is set to the 0.2 ms/div. range, given a screen graticule of ten by ten squares. In this setup, the travelling electron beam needs a minimum of 4 ms to display two times two com- plete cycles of the sine wave. The display frequency relevant to this measurement equals 1/0.004 = 250 Hz, which is high enough to en- sure a stable, flicker-free image on the CRT screen. However, a less favourable situation arises in the case of input signals in the lower than 100 Hz frequency range, since these are displayed at a frequency of 25 Hz or less, which typically causes the display to flicker to the degree of disturbing the visibility of the signal curves. Chopper operation, on the other hand, is typically devoid of the above disadvantage, since the channel selector is controlled with a rela- tively high-frequency signal (several kilohertz), independent of the trig- ger pulse and the input signal fre- quency. Assuming that the chopper frequency is 50 kHz, and the signal frequency 1000 Hz, the luminous CRT spot is arranged to alternately display tiny (chopped) sections of the curves on both channels; the principle is illustrated in Fig. 2, which shows that the displayed waveforms are, in fact, chopped into some 50 sections each. The switching rate of the CRT beam is so high as to make the gaps in the curves invisible to the human eye; the curves, therefore, appear as smooth and continuously present. If the chopper frequency is well in ex- cess of the signal frequency, as in the above example (50 to 1 ratio), this os- cilloscope display mode ensures stable, flicker-free visibility of the ap- plied signals on the CRT screen. In case the input signal frequency ex- ceeds that of the chopper section to the extent of resulting in a ratio of, say, 6 to 1, the situation that ensures is not necessarily dramatic as yet, since the curves on both channels are each displayed three times over. Problems are only anticipated in case the chopper and signal fre- quencies are either about equal or in some fixed relation to one another; the resulting effect on the CRT screen is comparable to that out- lined above in the section on the alternating mode. However, the sol- ution to the problem is relatively simple in this case, since the chop- per frequency may conveniently be made variable; in case of display in- stability, the chopper oscillator is slightly detuned. The circuit The block diagram of Fig. 3 aims at offering an insight into the basic op- eration of the present scope add-on unit. Two input amplifier sections, each with a vertical trace positioning preset, pass the signals to two elec- tronic switches, which are antiphase controlled by a central chopper os- cillator section. All of the above functional blocks can be seen in Fig. 4, the circuit diagram of the add-on unit. At the left are two identical, fast opamps Type CA3130, which amplify the input signals to both channels. Presets Pi and P 2 are the trace positioning con- trols; they elevate the AC signal to a certain DC level in order to obtain the correct vertical position of each trace on the CRT screen. Electronic switches ESi and ES 2 are contained in a Type 4066 CMOS quad bilateral switch IC. To prevent the input capacitance of the oscilloscope from delaying the steep edges of the chopper signal — this would make them visible on the screen — , IC 4 has been incorporated as a fast output buffer opamp. The chopper oscil- lator is a conventional design using Schmitt-trigger NAND gates; P 3 pro- vides the tuning control. The necessary phase difference be- tween the output control signals is realized by taking them from the in- put and the output of N 2 . The ex- pected frequency range of the proposed setup should be about 50 to 100 kHz. Gates N«-Ns and Ns-Ne prevent the switching moments of ESi and ESi from coinciding. Finally, IC 7 creates a virtual earth level in order to enable the circuit to work off a single 18 V supply. Construction, adjustment and use In order to preclude undesirable spurious radiation caused by the chopper oscillator from manifesting itself in domestic receiving equip- ment, the present add-on unit should be fitted in a suitably dimensioned metal enclosure. After connection of the completed board to the oscilloscope, Pi and P 2 are adjusted to obtain the correct trace position for each channel on the CRT. Now adjust P 3 to obtain a stable display of the chopper switch signal with the oscilloscope time- base set to 10 ps/div. Presets P 4 and Ps may now be adjusted for maxi- mum edge steepness of the chopper signal, i.e. it should, ideally, become invisible on the screen. This com- pletes the necessary adjustments. The use in practice of the present add-on unit is, of course, subject to the limitations brought about by the relative simplicity of the proposed circuit. Given the absence of input attenuator sections, the measured voltages should not exceed 12 V peak-to-peak (4.3 Vrms ). The use of opamps in the circuit inevitably limits the attainable bandwidth to several hundred kilohertz, but this need not be a drawback if the user mainly intends to measure audio signals. Should the chopper fre- quency become visible on the screen, then P 3 may be set to a slightly different position to make the signal edges invisible again. Finally, the present design does not incorporate a power supply; the user must either avail himself of an exist- ing mains supply, or construct a separate unit to this end, capable of delivering 18 V at about 50 mA. Also note that no ready-made PCB exists for this project; the true scale track layout, however, is given in Make your own PCBs, elsewhere in this issue, while the component mount- ing plan is given in Fig. 5. KD;TS Parts list Resistors: Ri;R 2 ;Ri,R 6 = 100 k Rj;R4 = 120 k Rt. . .Rio = 4k7 Rti;Ri 2 = 10 k Pt;P 2 = 100 k linear potentiometer P 3 . , .Ps=5 k preset Capacitors: Ci;Ct=220 p Cj = 150p CUiCs.'Cio^ 10O n Cs = 6 n 8 Cr;C«=100p C 9 - 100 p;25 V; electrolytic Cn;Ci 2 = 10 p;25 V; electrolytic Semiconductors: ICt;ICi;IC4 = CA3130 IC 3 = 4066 ICs;IC. = 4093 ICt = 741 Miscellaneous: Si = single-pole toggle switch 2 knobs for Pi and P 2 4 sockets for inputs and outputs metal enclosure PCB 86013 Inot available through Readers Servicesl suitable power supply; 18 V; 50 mA regulated elektor india july 1986 7.27 tt seems fairly certain that over the next few years more and more video systems will incorporate picture frame memories. With these, the picture quality of monitors and television receivers can be improved, white at the same time the way is opened for a host of new features. Video memories are used in satellite receivers; in medical scanners; in material testing by infra- red, supersonic, and X-ray techniques; in astronomy and photography; and, last but not least, security equipment. Such memories are, in the main, dynamic RAMs. CCD (charge-coupled device) memories are inherently slower than RAMs, but also cheaper and more compact. This makes them suitable for applications that are either serial In nature or that do not require the fast operating speeds ot RAMs. Now that digital signal processing is used in modern TV receivers, video memories can be incorporated to offer a number of new oper- ational aspects. • Improved picture qual- ity through more effec- tive noise suppression, greater freedom from flicker, and better colour separation. • Picture freeze facility, and the possibility ot conveying such pictures over telephone networks. 7.28 elektor india july 1 986 CCD VIDEO MEMORY SYSTEMS Fig. 1. Digit 2000 prototype board offering complete digital processing of video, audio, and teletext signals. ; Photo courtesy of ITT) Fig. 2. Block schematic of ITT's Digit 2000 digital colour television receiver. Fig. 3. Illustrating the prin- ciple of Valvo’s video signal processing. The clock generator is syn- chronized with the line time-base generator. 88027-2 3 analogue I digital I analogue 80027-3 • Superimposition of pic- tures on one another. • Zoom-in facility. • Teletext storage with instant access. It would also be possible to use the video memory in conjunction with a video cassette recorder and microcomputer to obtain an editing facility. Digital television techniques Since the early 1980s, a number of semiconductor manufacturers have introduced digital video signal processing devices. International Telephone and Telegraph Corpor- ation— ITT— was the first to put such a device into standard production (in 1983). This Digit 2000 offers complete picture, sound, and teletext processing and is already used in hundreds of thousands TV receivers. Valvo, iri conjunction with Philips and Siemens, have developed another system that is now being used in a number of TV receivers under development. The main difference be- tween the two approaches lies in the choice of scan- ning frequency. ITT links the clock frequency to that of the chrominance subcarriers, whereas in the Philips/Valvo/Siemens system the scanning fre- quency is synchronous with the line frequency. In the line-based concept the video memory is organized on the basis of picture build-up. This makes it possible for additional signal process- ing to be carried out by including adjoining pic- ture elements in suc- cessive rasters. In this technique, use is made of specially designed CCD memories in which the data is stored line by line. ITT prefers standard RAMs as video memories. Although these are more expensive than CCDs, fewer of them are required: the ITT system requires five 256 K DRAMs, whereas the Philips/Valvo/Siemens set- up needs seven 317 K CCDs. DRAM system ITT has had a TV receiver elektor india july 1 986 7.29 (Digivision) with a 12 Kbyte RAM in production for just over a year. This enables two video signal sources to be displayed simul- taneously on the same screen. The video signal to be faded in is taken from one of the SCART connec- tors via a single chip PAL decoder. The RGB signals at the output of the decoder are converted in a multiplex process by a single digitizer at a scan- ning rate of 1.5 MHz. A 4:1 data reduction results from 5 6 Covt'W v>o«e • 86027-6 the simple process of reading only every fourth line from the RAM that syn- chronizes the pictures Because of the small for- mat of the superimposed picture, it is sufficient to store just one raster. This requires only 4 Kbyte per colour, making a total of 12 Kbyte. Control of the memory as well as addressing the RAM is carried out by two gate arrays which replace no fewer than thirty standard ICs Fig. 4. Photograph showing the display of two different video signals onto the screen of an ITT Digivision* TV receiver. The secondary picture is identified by a coloured band at its lower edge. (Courtesy of ITT). Fig. 5. Block schematic of an ITT Digivision chassis containing a 12 Kbyte video RAM. Fig. 6. TTT's Type VMC2260 Video Memory Controller drives a video memory consisting of five 256 K DRAMs. Thanks to the doubling of the frame frequency, the picture is 1 virtually free of any flicker 7.30 elektor India july 1986 Fig. 7. Prototype CCD video memory board from Philips. Next to the seven Type SAA9001 CCD memories (left-hand side) are four control ICs which provide a number of features, such as still pic- ture, noise reduction, and recall picture. Fig. 8. Block schematic showing how the various special features are obtained in the Valvo system. Fig. 9. Block schematic of the Type SAA9001 CCD memory. Fig. 10. The SAA9001 is arranged in 294 lines of 1080 bits each 8 Digital , colour decoderl Video (Analogue)! Digital sweep" I I processor | H~ v 1 analogue 9 However, gate arrays are not suitable for a com- plete video memory with five 256 K DRAMs. For that purpose, ITT has devel- oped a special video memory controller, the Type VMC2260. Apart from doubling the frame fre- quency to 100 Hz, this device also provides the still picture, zoom, superim- position, and teletext memory facilities. CCD technique In the Philips/Valvo/ Siemens system, the analogue video signal is converted into 7-bit digital words synchronous with the line frequency. The clock frequency of 13.5 MHz results in a fixed scanning rate of the luminance signal (Y-signal) of 720 samples per line Because of their limited bandwidth, the chromi- nance signals (U and V signals) at the output of the decoder, however, are scanned at only 3.375 MHz, ie. 180 samples per line. All together there are, therefore, 720+2x180= 1080 samples per line, resulting in a frequency of the multiplexed signal (Y+U+V) of 20.25 MHz. The video memory, built up in accordance with the scanned frame struc- ture, is based on CCD Type SAA9001. In this device, 317 Kbit can be contained on a small crystal surface, arranged in 294 lines of 1080 bits each. The visible part of a nor- mal raster (two rasters con- stitute a complete picture or frame) in the 625-lines- per-frame system is com- posed of 288 fifty-two- microsecond lines. The SAA9001 is, therefore, able to store a complete raster with one bit per sample The relevant 1080-bit line of the SAA9001 receives 720 luminance bits and 2x180 chrominance bits from each of the 720 scanned pixels in a raster line. Since each scanned pixel results in seven bits, the memory consists of eloktor India July 1986 7.31 seven CCDs, which together store the infor- mation pertaining to 720x288=207 360 pixels. In contrast to other CCD memories, the SAA9001 uses serial-parallel-serial transfer ot information. In this method of operation, the first 1080-bit data line is input serially to the first row in the array at high speed. When this row is filled, the bits are trans- ferred in parallel at a slower rate, while the next data line is input. Suc- cessive lines are thus transferred through the array. After 294 data lines have been input, the first of them appears at the output from where it is transmitted serially at high speed. The line shifts in the memory are synchronous with the line frequency. Higher line or field fre- quencies are not yet planned in this concept. None the less, it offers these features: • cross-colour reduction; • noise reduction on noisy signals (particu- larly from video recorders); • store and recall picture during normal TV oper- ation; • picture freeze during normal TV operation; • storage for up to 252 teletext pages with instant access. Fig. 11. Multiplex structure of a CCD video memory containing seven Type SAA9001 devices. Fig. 12. Possible set-up of a video editing aid using a computer and a CCD video memory. Other features are poss- ible, but their incorpor- ation will depend largely on consumer demand. The SAA9001 is also an interesting memory device for other than television applications. Since it has only three control inputs. its use is straightforward and allows the construc- tion of digital video and audio memory units at relatively low cost. Its facility for accessing parts of a video picture via a computer should be of interest to microcomputer and television (slow scan TV) experts and amateurs alike. Interested readers are also referred to The Accordion image Sensor (EE India, March 1986) EK New wideband opamp National Semiconductor Corporation have recently introduced a wideband, FET-input operational amplifier that can provide 100 mA continuous output current. Designated the Type LH4101, the chip eliminates the need for a buffer to provide the ad- ditional current drive not available with other wide- band opamps. The Type LH4101 provides internal compensation for unity gain stability and all the internal gain set resistors for most popular gain settings; also of interest are its 45 MHz bandwidth and capability to drive 50 ohm loads directly. The new part, as com- pensated. is claimed to represent an optimum compromise between slew rate, bandwidth, settling time, and gain linearity, at the same time replacing compensation and bypass capacitors, and gain set resistors. Applications of the hybrid opamp include video distribution, summing amplifiers, fast sample and hold circuits and speed integrator circuits. The Type LH4101 is the first in a series of opamps from National Semiconductor that will be combining internal bypassing com- pensation and providing all external components normally found in high speed opamp configur- ations, and it is currently available in a 24-pin dual- in-line plastic (DIP) pack- age. National Semiconductor (UK) limited 301 Harpur Centre Horne Lane Bedford MK40 1TR Telephone: (0234) 47147 Telex: 826209 (3459:12) i 7.32 elefctor india july 1 986 EIGHT-WAY RELAY BOARD Whatever they say don’t believe that computer interfacing is within reach of the average owner of a personal micro equipped with a parallel output port. Always remember that the way from CPU a ecu to, say automatic control relays is a mighty long one, and stick to these beliefs until you have constructed this universal board. Table 1. Boolean algebra functions of the XOR and NOR type of logic gate. Despite its heavy accent on versa- tility and compatibility with any type of computer having a parallel output port, the present relay controller board comprises only very few com- ponents, as can be seen from the cir- cuit diagram shown in Fig. 1. No LSI chips, dedicated I/O controllers, or handshaking hardware; the pro- posed relay controller along with a few BASIC instructions puts you in control of any of eight DIL type (reed) relays, merely using four databits from the computer's par»'l- output port. A seif-strobing decoder IC3 is a Type 4099 CMOS 8-bit ad- dressable latch which can pass the logic level at the D (data) input to one of eight outputs selected by the com- bination of bits at the A 0 , Ai and A 2 inputs; latching of databit and ad- dress takes place when the enable (E) input is pulled low. In addition, the Type 4099 has a RESET input to clear the internal latch and pull all i chip outputs low. 4-input NOR 1 input output 1 2 3 4 0 0 0 1 0 0 0 1 1 1 0 l 0 1 , 1 1 1 1 1 1 1 1 0 0 0 0 _o 1 Exclusive OR input 1 2 output 0 1 1 1 0 1 1 1 0 0 0 0 elektor india july 1 Fig. 1. Circuit diagram of the universal relay controller board. Several types of DIL relay may be accommo- dated as ex- plained in the text. Fig. 2. Showing how different types of PCB- mount relays may be fitted onto the board. 2 Component side view i * underlined number refers to manufacturer's coil voltage designation system 86039-2 Siemens PCB mount relay V23127 B...2...* V23057 B . . .2. . . * Siemens DLR relay V23100 V4012.. . . • Clare Dry Reed- relay 1A012* OMRON G2V-2 DC12V Since the circuit is to be suitable for connection to any computer having a parallel output port, a means other than any kind of output strobe pulse had to be devised for clocking the latch, since many computer manu- facturers do not even seem to be bothered by, say, the rules laid down in the Centronics standard. The present circuit therefore needs no computer generated strobe pulse; it provides its own whenever data is written to the relevant four bits comprised in the output port data. Table 1 shows that the output of an exclusive-OR (XOR) gate does not go high until the logic levels at its inputs are complementary. ICi, a Type 4030, contains four XOR gates, each of which has one input driven direct by an address bit A 0 ...A 3 , while the other input receives the same level, but slightly delayed by a R-C network. Therefore, every logic change on any of the A 0 . . .A 3 lines causes the relevant capacitor to be either charged or discharged over the associated resistor, providing a short-duration complementary pulse combination at the XOR gate inputs, which fact causes the gate to pro- duce a high level pulse at its output. Quad input NOR gate Type 4002 7.34 elektor mdi8 july 1 986 receives the output levels of the four XOR gates, executes the the logic function as per Table 1, and supplies IC3 with an E pulse, which causes the databit on A 3 and the rel- evant channel (relay) number to be strobed into the device, which ac- tivates or deactivates the corre- sponding output Qe . . . 0 ?, each driving a transistor with a relay and associated indicator LED connected in the collector supply line. Figure 2 shows how a number of relays by different manufacturers may be fitted onto the board. For types not listed, you may use the spare holes, but check the internal configuration as well as the coil re- sistance and voltage before using any unlisted type of PCB-mount relay. Supply voltages The relay board requires two regulated supply rails; one of +5V for the CMOS ICs, and another, + 12 V, for the relay coils and driver transistors. The latter supply should be connected to point + + on the ready-made PCB The circuit as shown in Fig. 1 has been designed for the incorporation of relays with a 12 V DC coil voltage, but differently rated types may also be used, provided the LED series resistors are dimensioned according to Ris=(Vcoii — Vled)/1led . Since the circuit as shown incor- porates 12 V type relays and LEDs which draw 20 mA at 2 V, the given resistor value of 560 Q is accounted for by Ri-s=(12-2)/0.02= 500 2, Ri 8 having the next higher value in the E12 series. Construction It is suggested to start the construc- tion with fitting the IC sockets and soldering pins, followed by the re- maining passive components (Fig. 4). Note that Ri to R 16 and pro- tective diodes Di to Ds are fitted ver- tically to save board space. The LEDs may be mounted either at the soldering or the component side of the PCB, depending on the type of enclosure you have in mind for the project. Reset switch Si is connec- ted to a pair of soldering pins, using two short wires. Despite the tempting presence of soldering pins for the supply wires to other equipment, it must be strongly advised not to have the relay contacts switch or carry currents or voltages in excess of the manufac- turer’s specificatidns, since doing so may cause the PCB tracks to bum out after the relay and possibly the driver transistor have been de- stroyed internally. Practical use Users of the well-known Com- modore C64 computer may readily wire the present relay controller board to a parallel output port, whether this is a DIY or ready-made type. The program listing shown in Fig. 3 is intended as an initial test to verify the correct function of the relay board. Owners of other types of computer having a parallel output port may refer to Table 2 to find the relevant bit combination for each relay as well as the code to turn it on and off (effected with A3). Finally, the Reset switch may be pushed at any time while in the pro- Tabie 2. Relay A2 A1 A0 on” off' 1 0 0 0 X8 X0 2 0 e 1 X9 XI 3 0 1 0 XA X2 4 0 1 1 XB X3 5 1 0 0 xc X4 6 1 0 1 XD X5 7 1 1 0 XE X6 8 1 1 1 XF X7 In hexadecimal notation and assuming that Ae. . .A 3 are connected to port bits Do. . .03 in that order. cess of writing and debugging relay control subroutines, which, as any serious programmer will admit, is usually by way of trial and error as well as frequently occurring com- puter hangups. HS;CK 10 POKE 56579,15: REM P0 to P3 are outputs 20 POKE 56577,0: REM soft reset for relay board 25 FOR 1-0 TO 7: R(l) = 0: NEXT 30 INPUT "WHICH RELAY";R$ 35 IF VAL(R$) < 1 OR VALIRS) > 8 THEN 30 40 l = VAL(R$)-1 50 IF Rill = 1 THEN RID = 0: GOTO 60 55 Rill = t 60 POKE 56577,1 + 8'R(I) 70 GOTO 30 Table 2. Sum- mary of relay ad- dresses and output port data codes. Fig. 3. This short programme may be keyed in to test the relay board as an ex- tension to the C64 computer's parallel output port. Fig. 4. Track layout and com- ponent overlay for the relay con- troller board. Parts list Resistors: Ri...Rs = 560Q - R9 . . . Rt6; Ri8 . R 21 = 4k7 Ri? = 10 k Capacitors: Ci. .0 = 1 n C5= 10 p;16 V electrolytic C6= 100 n Semiconductors: Dt . . . Ds = 1N4148 D8 . Dt6 = LED Tt. . .T8= BC547B 10 = 4030 IC2 = 4002 IC3 = 4099 Miscellaneous: Si - push to make button Ret. . . Res = PCB mount DIL relay' 2 off 14-way IC sockets 1 off 16 way IC socket 34 off soldering pins PCB 86039 Suitable enclosure Sockets for relays, if required Sockets for computer and relay connections 'see text and/or relevant Figure. elektor India July 1986 7.35 In actual fact, it is not always the fault of (amateur)transmitters that they cause interference on TV sets. As a rule, it is the 'broad-band aerial amplifier' in- cluded in the TV set's aerial system which is at the root of the problem. Broad-band amplifiers have the dis- advantage of being rather indiscriminate. They pick up and amplify everything, including signals which are not meant for them at all. When powerful broad- cast, amateur or mobile transmitters are around, the voltage in the aerial ampli- fier rises to such an extent that the amplifier becomes completely 'jammed' and this makes a clear reception of TV signals very difficult. the broad band amplifier, is stripped at a certain point and connected to one end of a piece of coax. This coax, believe it or not, is the filter. It should be exactly % wave length of the signal that is to be eliminated. The other end of this piece of coax, which is known as ’4 X (quarter-lambda) stub, remains open. This is how it works: Radio waves reaching the open end of the % X stub are reflected. For the un- wanted signal, the stub is exactly %X long, so that the reflected waves have travelled a distance of 2 x 'A X = V4X by the time they get back to the beginning of the stub. Consequently, the reflected wave is in exact phase-opposition with TV Interference suppression Nearly everybody will agree that interference on TV can be extremely annoying. Interference can be caused, among other things, by local transmitters. Usually, however, this can be dealt with in a fairly simple and effective way. So what do you do? Well, after reading the above, it would seem an obvious conclusion that it is probably better to do without an aerial amplifier altogether. For that matter, very often one is included in the aerial system 'just to be on the safe side', without it being strictly necessary. It is a much better (and cheaper!) idea to simply use a good TV aerial which is a powerful 'amplifier' anyway (and will have a more accurate directional effect and an improved front-back ratio — both important factors). If, on the other hand, you cannot manage without an amplifier, it is advisable to use tuned aerial amplifiers (also known as channel amplifiers). These, being narrow-band, do not pick up unnecessary signals and so interference is no longer a problem. However, if you already have an aerial system which is fitted with a broad- band amplifier, it is rather frustrating to talk about the kind of aerial you should really have. Quite a few interference problems can be dealt with in an inexpensive way by simply inserting a band-stop filter in the broad band amplifier's input. This elim- inates the interfering signal (produced by an amateur transmitter, for example) before it reaches the broad band ampli- fier. The so-called V4 X-fi Iter is a good choice: it is easy to make — all you need is a piece of coax cable! The % X-fi Iter Figure 1 shows what the filter looks like. In passing, it should be noted that this filter can be used for all kinds of purposes - not only eliminating inter- ference in broad band amplifiers! As the drawing demonstrates, the (coax) aerial cable, leading from the aerial to 80006 ^ k_J Figure 1. The filter is a piece of coax, connected in the lead from the aerial to the broad band aerial amplifier. In practice, it is often best to connect the % A stub at the input of the amplifier. 80006 2c Figure 2. The filter works as follows: The voltage reflected in the stub (2b) is in exact anti-phase to the input voltage (2a), so that the resulting voltage (2c) is nil. 7 36 elektor ir>dia iuly 1 986 the input signal, so that the resulting voltage is nil. This is illustrated in fig- ure 2. Figure 2a shows the input voltage, figure 2b shows the reflected voltage and figure 2c gives the result. Everything always sounds marvellous in theory, but often turns out differently in practice. Here too, unfortunately this is the case. What happens is that the A X stub attenuates the reflected wave. MB m Figure 3. A spectrum-analyser photo of a coax V* X-filter for the 2-metre band. The attenuation is approximately 36 dB. “3 Km -10 — !1 -20 !A|_ 1 1 — — • -50 i - - Figure 4. The rejection filter intended for the 2-metre band can also be used for the 70-centimetre band, with marginally poorer results. 80006 s Figure 5. A spectrum-analyser picture over a much wider frequency range (100 MHz per division) shows that there are many more frequencies at which the input signal and the signal reflected by the filter are in anti phase. so that the resulting voltage is not completely nil, as shown so optimisti- cally in figure 2c. It doesn't have to be! A reduction by about 30 dB (32 times) is usually achieved with the aid of the filter and nine times out of ten that is enough. Furthermore, the filter not only blocks interference on the wave length which is four times as long as the A X stub, but it also works for wave lengths corresponding to %X, %X, 7 /»X etc. The input signal and the reflected wave are in anti phase at these fre- quencies as well! In practice As far as the exact length of the filter is concerned, simple theory is one thing, practice another. The speed at which radio waves travel along coax is not the same as that in air. For this reason, the wave length inside the cable is shorter than that outside: a radio wave may have a wave length of 3 ft. outside and as little as 2 ft. inside the coax cable. The reduction factor, in that case, is: % = 0.67. Let us consider a rejection filter for a 2-metre amateur transmitter. Amateur transmitters on the two-metre and 70-centimetre bands seem to be prime targets for complaints about inter- ference. On the two-metre band A X corresponds to A x 2 = 0.5 metres. In order to find out what the exact length of the AX stub should be, this figure must be multiplied by the reduction factor of the coax. Every manufacturer (and reliable retailer) will be able to supply this information. It is advisable to make the cable slightly longer than the calculated length, so that once the stub has been connected, it can be trimmed for maximum suppression of the interfering signal. This can be done by cutting off small bits at a time. When you have found the correct length, the 'AX stub can be rolled up. It looks neater, that way. One of the characteristics of this type of filter, as mentioned earlier, is that it will eliminate several frequencies. This can be an advantage: a filter for the 2-metre band can be used for signals on the 70-centimetre band as well. The spectrum-analyser photo's (figures 3 and 4) illustrate this. Figure 3 shows how the filter attenuates interference at the frequency for which it was originally intended: 144 MHz (the 2-metre band). Figure 4 illustrates the effect at 432 MHz (70-centimetre band). Since the damping of the coax cable is greater at higher frequencies, the attenuation achieved is less than that at 144 MHz. As the photo's- illustrate, the difference is approximately 6 dB. The spectrum-analyser photograph in fig- ure 5 gives an idea of the attenuation over the whole frequency range (hori- zontally 100 MHz per division). M supply failure indicator Many circuits, especially digital systems such as random access memories and digital clocks, must have a continuous power supply to ensure correct operation. If the supply to a RAM is interrupted then the stored information is lost, as is the time in the case of a digital clock. The supply failure indicator described here will sense the inter- ruption of the power supply and will light a LED when the supply is restored, thus informing the microprocessor user that the information stored in RAM is garbage and must be re-entered, and telling the digital clock owner that his clock must be reset to the correct time. S 12V When the supply is initially switched on the inverting input of IC1 is held at 0.6 V below positive supply by D1 . Pressing the reset button takes the non-inverting input of 1C1 to positive supply potential, so the output of 1C 1 swings high, holding the non-inverting input high even when the reset button is released. LED D2 is therefore not lit. When the supply is interrupted all voltages, of course, fall to zero. Upon restoration of the supply the inverting input of IC1 is immediately pulled up to its previous potential via D1 . However, Cl is uncharged and holds the non-inverting input low, so the output of 1C1 remains low and D2 lights. eleklor india |ulv 1986 7.37 *■*•’•*•*•*•*• *.*.*»•■ m m mm m mm M PORTABLE MIXER — 2 by A Schmeets This second part in the series continues with the construction of the first output module, which incorporates tone controls, an output level indication and a balanced as well as an unbalanced line output. As explained in the preceding article (Elektor India , June 1986), there are two output modules to the portable mixer: one for general usage, and one for more specific ap- plications, incorporating a monitor and effects amplifier, as well as a parametric equalizer section. The former is described below, whereas the latter will be discussed in next month’s issue. The circuit The circuit diagram of the first output module is shown in Fig. 1. Opamps Ai and Af are summing amplifiers for the left and right channel respect- ively. The active tone control section of each channel consists of a number of R-C networks in conjunction with an operational amplifier. Note that the tone control potentiometers are stereo types to ensure identical and simultaneous tone setting on both channels. The HPl and HPr signals, as well as the mono MONITOR line (P 4 ), go to the relevant inputs of the second output module, to be de- scribed next time. P6 is the balance control and Ps the master output slide potentiometer. Provision has been made to connect the LINE out- put signal to the PFL section by elektor mdia juty 1986 7.39 Fig. 1. Circuit means of R19, R20, and Si. unbalanced mixer output signal is photographs illustrating the first part diagram of out- The LED VU section for each output available at the U terminals. Two ad- of this series of articles. The only put module 1. channel consists of an opamp-diode ditional opamps, At and A«’, provide parts common to both PCBs are the which has a combination (As-Di) which rectifies balanced output signals, which are stereo potentiometers and the 13 -way 3-way tone con- the signal level at the wiper of the available across high-stability ( 1 %) PCB connector. The compactness of trol, balanced master fader. The variable DC level resistors R13-R14 (L) and R13-R14' (R). the unit necessitates vertical mount- and unbalanced is next applied to a special LED VU ing of some resistors and capacitors; outputs, and a driver, IC3. The division in five out- the terminals of D2...D6 and LED VU meter put signal levels is sufficiently ac- OODStrUCtiOfl D2’. . .De’ should be bent to suit the for each output curate for most purposes; OdB cor- protruding LED positions in the front channel. responds to about lVrms. Output module number 1 is fitted on panel, which is made to the outlines LINE output amplifiers A 3 and A 3 ' the PCB shown in Fig. 2. The sand- given in Fig. 3. Fitting the output also receive their input signals from wich construction of the completed sockets, the potentiometer spindles the wipers of the master fader. The module should be familiar from the and the PFL switch should present 7.40 elektor India july 1 986 2 Parts list 6.3mm cinch-type- socket (stereo) 13-way PCB-type connector to DIM 161 7 5-way XLR Cannon type socket PCB 86012-3A;3B* knobs for potentiometers as required front panel foil 86012-3F* D5;D5' = 3mm LED; amber D6;D6' = 3mm LED;red ICi;ICi';IC2;IC2* = TL072 IC3;IC3' = U267B* (AEG-Telefunken) see Table 1 IC4;IC4' = XR4195* IC5;IC5' = 741 Resistors: P5= 10k log stereo slide potentiometer; 58mm travel* P6= 10k linear potentiometer + C5;Cs' =22n C6;C6';C7;C/ = 4n7 C8;C8' = lOp Cio;Cio' = 470n Cii;Cn' = V;16V; electrolytic Ci2;Ci2';Ci3; Ci4;Ci9= lOOn Ci5;Ci6= lOOp Ci7;Cis=10m; 16V; electrolytic Ri;Ri';Ris; Ri5';Ri9*;R2o* = 22k R2;R2';R3;R3'; R4;R4';R7;R/ = 10k R5;R5';R6;R6'-3k3 R8;R8';Ri6;Rt6'; Ri7;Ri7' ^ 47k R9;R9' = 100k Rio;Rio';Rn;Rn' = 100Q Ri2;Ri2' = 2k2 Ri3;Ri3'; Ri4;Ri4' = 33k;1% Ri8=1M Pi;P2;P3 = 100k stereo linear potentiometer + P4 = 25k stereo ibg potentiometer * not mounted on PCB with 4mm spindle for PCB mounting see text Capacitors: Ci;Cr;C2;C2'; C9;C9' = 10m; ' 40V bipolar electrolytic C3;C3' = 47n C4.C4' = 5n6 Semiconductors: Di;Di';D7;D8 = 1N4148 D2;D2';D3;D3';D4, D4 =3mm LED;green Miscellaneous: Si = double miniature switch elekior India j i . 86 7.41 Fig. 2. Track layout and com- ponent mounting plans for the out- put module PCBs. few problems after a careful look at the relevant drawings and the photo- graph in this article. Finally, the on-board voltage regu- lator, IC4, may be replaced by regulators Types 79L15 and 78L1S as explained and illustrated in last month’s article. Modules and amplification Below are a number of useful hints to obtain the correct total amplification of the modules as described so far. Where necessary, some resistor and/or capacitor values may have to be adapted to suit the individual signal levels of equipment connec- ted to the portable mixer. MIC/LINE-module the amplifi- cation of output opamp A 3 depends on the ratio R9/((R7-Pi)+R6) so that any of these resistive elements may be given a different value to obtain the desired total amplification. Note, however, that Rs-Re and Ri2/Rio=Ri3/Rii. Alternatively, Rio and R11 may be changed, but these should keep identical values. The re- sistance is inversely proportional to the resulting total amplification. Stereo input module: the amplifi- cation of the MD preamplifier is ar- ranged at 35dB at 1kHz. R3 and R3' may be given different values; ampli- fication is inversely proportional to the value of these resistors. C3 and C3’, however, should also be changed in inverse proportion to R3 and R3' to ensure the correct cut-off frequency of the preamplifier; the lower the value of R3, the higher that of C3, and vice versa. The total amplification of this module depends on the resistor arrange- ment around A 2 , to the effect that the amplification, a, of this opamp equals a(A2)=l+Ri3/Ri2. The value of R 12 is inversely pro- portional to the resulting total ampli- fication of the module. Like C3, C 9 must also be dimensioned accord- ingly. It is even possible to turn A 2 into a variable amplifier, Fig. 4 shows the necessary circuit modification, 7.42 olaktor india juty 1 98f which may be useful to correct level differences between, for instance, 33 and 45 rpm records. Output module 1: the amplification of the summation opamps Ai and At’ has been arranged at unity (OdB); this value may be changed, if desired, to a maximum of lOdB by suitable dimensioning of Hi between values of 47k to 100k. The amplification of the output buffers A3 and A3’ is 6dB. Since this value is determined by the ratio Rs/Rs, Rs may be given a lower value and C 10 a higher value to ob- tain an increase in output amplifi- cation. A final word about the VU indication: a mixer output level of OdB cor- responds to about lVrms at the input of As. If A 3 is arranged, to have a higher amplification, the amplifi- cation of As should be reduced, and vice versa, of course, to ensure that the VU meter indicates the correct output level. The amplification a of opamp As is given by afAs)=l+Rw/Rn. Table 1 shows a number of alterna- tive LED drivers with different input level ranges and linear or logarithmic characteristics. Current consumption The typical current consumption at + 18V of all types of module in the portable mixer is summarized in Table 2. NOTE: The next part of this article will be featured in our October issue. 3 . 22 . 5 ., 14.5 f* 14 ^ »l lij M 14-1 ! ! I u? 4- 4- i- m -4- I i dimensions in mm Table 1 Input levels for VU meter ICs Type input threshold units characteristic U237B 0.2 0.4 0.6 0.8 1.0 IVrms] linear U247B 0.1 0.3 0.5 0.7 0.9 | Vrms) linear U257B 0.18 0.5 0.84 1.19 2.0 IVrms) logarithmic -15 -6 -1.5 + 1.5 + 6 IdBI U267B 0.1 0.32 0.71 1.0 1.14 IVrms] logarithmic -20 -10 -3 0 3 IdBI Current consumption of mixer modules ImA]. supply voltage IVI MIC/LINE STEREO OUTPUT 1 OUTPUT 2 + 18 20 30 60 80 -18 30 40 25 20 4 Fig. 3. Front panel foil and drilling template for the output module. Fig. 4. Showing how the circuit with As may be modified to operate as a variable ampli- fier stage. Table 1. Sum- mary of different IC types which may be used in the 1C 3 and IC 3 ' positions. Table 2. Refer to this table when in doubt about the expected total current con- sumption of the portable mixer. elektor mdia july 1 986 7.43 LOUDSPEAKER EFFICIENCY by D J Schulz The power handling capacity of a loudspeaker system is seen by many as, perhaps not the only, but certainty as the most important factor as to its quality, whereas, arguably, it is one of the least important ones. Loudspeakers convert only about 0.25 to 2.5 per cent of the electrical energy supplied to them into acoustic energy. The re- maining 97.5 odd per cent is converted into heat. The energy efficiency, or simply efficiency, rjo, of a loudspeaker is the ratio of the useful acoustic energy to the signal energy input. >/o=^0logw(Pi/Pc\ [dB] [1] where Pl is the total radiated acoustic power in watts, and Pe is the elec- tric power delivered to the speech coil. The efficiency may, of course, also be expressed as a percentage, when it is no =\mPi\Pi\ [%] [ 2 ] Nowadays, it is customary for producers to state the sensitivity of a drive unit in the relevant data sheet. The sensitivity is the inten- sity level in decibels at a distance of 1 metre from the unit (dB nr'), when the electrical signal input is 1 watt, referred to the inter- national standard refer- ence intensity. The intensity, /, of a plane or spherical "free” sound wave (no reflections) in the direction of propagation is I t=p>lW e c [Wo cm’] [3] 7.44 ttleMor itdia jufy 19P-6 where p is the effective sound pressure in pascals, e is the density of dry air at 20 °C (1205 kg cm 3 at an atmospheric pressure of 1.01325x10 5 Pa), and c is the velocity of propa- gation of a sound wave of small amplitude; its value is c=330.6+0.61(? [m s ’] [4] where 6 is the temperature in degrees centigrade. The standard reference intensity is 10 16 Wo cm 1 The intensity level in dB of a plane or spherical "free" sound wave in the direction of propagation is Z/=10logio (2.42x10 6 p 2 ) [dB] [5] It should be noted that the decibel is not a measure of loudness, since the sen- sitivity of the human ear to changes in intensity varies with frequency. The unit of equivalent loudness of a sound is the phon: this is a measure of the intensity level relative to a refer- ence tone of defined intensity and frequency. The internationally ac- cepted standard reference tone has a root-mean- square sound pressure of 2.04x10 Pa and a fre- quency of 1000 Hz: this is equivalent to an intensity of 10' 6 Wa cm 2 . One decibel represents an in- crease in intensity of 26 per cent, which is about the smallest change the human ear can detect. The standard intensity level of 112 dB at 1 m distance from the sound source (112 dB m* 1 ) is equivalent to a sound pressure of 20 Pa and an acoustic power of 1 watt (Wa). This is a very high level for the human ear (about the same as a jet engine at 6 metres distance), which, in an average living room, results in a mean intensity level of 104 dB The operating input power, Pw, is a useful character- istic indicated primarily on enclosures and loud- speaker system test sheets. It is the electrical power required to produce an intensity level of 90 dB/m (formerly 96 dB/m). If a loudspeaker system produces an intensity level of 112 dB rrr' when the electrical input power is 1 watt, its efficiency is 100 per cent. If follows from for- mula [1] that for each decibel the actual inten- sity level is lower than 112 dB the efficiency is re- duced to 0.7944 of its previous value. In other words, if the intensity level for an electrical input power of 1 waft is 102 dB nr’ (10 dB nr' below the standard of 112 dB nr’) the efficiency is only 10 per cent of that at the standard (0.7944 ">=0.10). The reference efficiency, rjo, of a drive unit may be expressed as 7o=9.7x10»fs>lAs/Of5 [%][6] where 4 is the resonant fre- quency stated by the manufacturer; t /as is the volume compliance in litres; and Qe$ is the elec- trical Q(uality) factor. The values obtained with- formula [6] pertain to a hemispherical space sub- tended onto an infinite baffle (see Fig. 4). Typical values of a popular 25 cm drive unit are: 4=19 Hz; K«=310 litres; ©f*=0.28. Entering these into formula [6] gives an efficiency of 0.737 per cent. Calculat- ing this percentage in decibels (10logio0.00737)— the so-called electro- acoustic index— gives a value of —21.325 dB. The negative sign indicates a loss. The electroacoustic index added to the sfandard intensity level gives the Sound Pressure Level (SPL), so that in the above example SPL=112+(— 21.325)=90.675, or, rounded off, 91 dB W ’ m l Another example, a polypropylene drive unit with a smaller diaphragm, ! has the following characteristics: 4= 50 Hz; 14«=13 litres; Q«=0.93. j Entering these into formula [6] yields ?o=9.7x10 8 x50 3 x13/0.93 =0.169% Electroacoustic index is j 10!ogio0.00169= -27.7086 dB SPL=112+(— 27.7086)= 84 dB W ' m - ' (rounded off). A comparison ot these two examples shows how the i SPL varies with the dia- phragm area, when the electrical input is kept : constant. Here, the SPLs dif- fer by 7 dB, which is a power ratio of 5:1. The efficiency is no yard- stick for the maximum ob- j tainable loudness level (in phons), but the power handling capacity is. It is, however, necessary, to dif- j ferentiate between the electrical and mechan- ical power handling ca- pacities. The former indi- cates the maximum elec- trical power in watts that may be applied to the j speech coil before this burns out. The maximum loudness level, particularly from a bass unit, depends primarily on the ability of the unit to produce large cone displacements (am- plitudes), and this in turn depends on the construc- tion. The diaphragm should, of course, only move back- wards and forwards, not sideways, and stiff, hard materials are better in this respect than soft, pliable ones. However, if the cone material is too stiff, it may actually impede the free movement of the dia- phragm. As so often in life, a suitable compromise has to be arrived at. A further criterion is the difference between the length, h, of the speech coil and the height ot the annular air gap, He (see Fig. 2). In modern bass drive units this difference lies between six and ten Fig. 1. Intensity level ks fre- quency characteristic. Fig. 2. Cross section of a drive unit. Maximum speech coil displacement without distortion is equal to the difference between the length of the speech coil and the height of the air gap. 2 millimetres. Provided that the diaphragm is seated centrally, the speech coil can, therefore, move from ±3 mm to ±5 mm before it leaves the uniform field of the ring magnet. When high electrical power in- puts cause the coil to move outside the mag- netic field, distortion of the sound produced is the in- evitable result. Generally, the spider supports that maintain the speech coil at the centre of the gap allow a free movement ot about ±2 mm, outside which they decelerate the diaphragm. It is because of this that many woofers produce distortion at even medium input powers. Drive units with relatively small cone areas need a greater speech coil dis- placement to produce the same sound intensity as units with a larger dia- phragm. Clearly, these smaller units also reach the limits of their mechan- ical capabilities sooner. Such compression factors are among the most troublesome in the design of compact loudspeaker systems Moreover, frequency modulation occurs when a single diaphragm moves with large ampli- tude at low frequencies, while simultaneously radiating high fre- quencies which causes the high frequencies to be altered because of the Doppler effect. The following example makes this all a little clearer. The maximum intensity level, Lm pro- duced by a loudspeaker (fitted in a closed box) is Zm=112+10logiaPi [dB] [7] where Pi is as defined before (formula [1]), and may be calculated from Pi =50?^ [Wa] [8] where Zr is the radiation impedance and v is the root-mean-square dia- phragm velocity in m/s. The radiation impedance, Zr is calculated from Z,=2n e HPlc [Q] [9] where e and c are as designated in formulas [3] and [4] respectively; r is the effective radius of the diaphragm in metres; and t is the operating fre- quency in hertz. The diaphragm velocity, v is determined by v=Haf [m s ') [10] where Ha is the difference between the length of the speech coil, h, and the height of the air gap, He, in metres, i.e., Hd-h—He [m] [11] Again, the 25 cm and 13 cm drive units en- countered previously will be compared (fitted in a closed box). The operating frequency shall be 60 Hz throughout. The 25 cm unit has an ef- fective diaphragm radius, r, of 0.107 m; the length of the speech coil is 0.016 m; and the height of the air gap is 0.008 m. This gives a value for Ha of 0.008 m. elektor india July 1986 7.45 From [10]: v=0.008x60=0.48 m s '. From [9]: Z)=2x 3.142x1.205 x 0.107 4 x 60 2 /342.8=0.010415974 Q. From [8]: Pi =50x0.010415974x0.48 2 = 0.11999 Wo. This acoustic power is equal to an intensity level of 10logio0.11999=— 9dR From [7] : Zm=112— 9=103 dR The 13 cm unit has an ef- fective diaphragm radius of 0.05 m; the length of the speech coil, h, is 0.012 m; and the height of the air gap is 0.006 m. From [11] : #o=0.006 m. From [10] : v=0.006x60=0.36 m s" 1 . From [9] : 2>=2x3.142x1.205x0.05 4 x 60 2 /342.8=0.000496945 Q. From [8] : Pi =50x0.000496945x 0.36 2 =0.00322 Wo. This acoustic power is equal to an intensity level of 10logio0.00322=— 25 dR From [7] : £m=112— 25=87 dR The 25 cm unit requires a signal input of only 16 W to produce the maximum intensity level of 103 dR Any higher electrical input will lead to distortion. 7.46 elektor india jufy 1 986 None the less, this par- ticular unit is rated at 110 W by the manufac- turers. The 13 cm unit reaches its maximum intensity level of 87 dB at a signal input of only 2 W. From these considerations, it is clear that the mech- anical maximum power handling capacity of the 25 cm unit (a good-quality, reputable make) is about 16 watts at a frequency of 60 Hz, while that of the 13 cm unit (also from a first-class manufacturer) is of the order of 2 watts at 60 Hz. The maximim intensity level may be increased by the use of a bass reflex or horn enclosure. The bass reflex box increases the ef- fective diaphragm area, while a horn enclosure causes a substantial in- crease in the radiation im- pedance. A simple way of increas- ing the radiation im- pedance (and thus the efficiency) is placing the bass loudspeaker in a cor- ner of the room (see Fig. 3 to 6 incl.). In practice, this will only work well, how- ever, with loudspeakers that have a small elec- trical Q factor (Qfs). Such units have a high driving force which ensures that the frequency response rises smoothly into the middle frequencies as shown in Fig. 7. The best reproduction ot bass frequencies is achieved by the use of horn loudspeakers, but this is impractical for most in- door uses as these units are very large. Finally, a detailed example of a 38 cm loud- speaker intended for use in very large rooms or discotheques. This unit has the following character- istics. • h = 30 Hz • Qfs=0.43 • Qms= 2.3 (mechanical Q factor) • 6Jc=0.36 (total Q factor) • ,fc>=0.0780 m 2 • I6ts=330 litres • 6=0.014 m • //f=0.008 m • Af=250 W maximum To obtain the optimum overall quality factor, Qtc, of 0.6 in normal operation, the enclosure should have a net volume of not less than 160 litres. The res- onant frequency of the system then lies around 50 Hz. On the basis of these data, it seems natural to choose a bass reflex enclosure. It should be noted, however, that a net volume of 160 litres would give rise to a poor step response. A volume of about 250 litres is, therefore, chosen, which lowers the overall resonant frequency, fc, to around 33 Hz, and gives a clear Chebishev response, i.e. 0.26 dB ripple. The —3 dB frequency is 34 Hz. The reference efficiency of the drive unit, calculated from formula [6] is 2.01 per cent, equivalent to an electroacoustic index of —17 dR The SPL is thus 95 dB W-' itt 1 . The efficiency of the system at 33 Hz, 100 Hz, and 300 Hz will be differ- ent from the reference, because at 33 Hz the bass reflex enclosure will ef- fectively double the area of the diaphragm to 0.1560 m 2 . At 100 Hz, the ef- fective area, because of phase shift, is about 0.1170 m 2 . At 300 Hz, the reflex aperture has no ef- fect, and the system behaves as a closed box. At 33 Hz, the effective, radius is 0.22284 m, and the radiation impedance is 0.057974816 Q. The acoustic power is 0. 11364 Wa, and the maxi- mum intensity level is 102.6 dR Since the res- onant frequency (33 Hz) is very nearly the same as the —3 dB frequency of 34 Hz, the reference SPL at the resonant frequency is 95—3=92 dR The maxi- mum intensity level is thus 10.6 dB above the refer- ence level. The maximum power handling at 33 Hz is, therefore, 10 dB above 1 W, 1. e. 10 watts. At 100 Hz, the effective radius is 0.193 m, and Z- is 0.299549182 Q. The acoustic power is 5.39189 Wo, corresponding to a maximum intensity level of 119.32 dR which is 24.32 dB above the refer- ence level of the drive unit. To obtain 5.39189 Wo Fig. 3. A freely suspended of acoustic power, there- sound source propagates fore, an electrical signal the sound equally in all input of around 250 watts directions (spherical). is required, i.e. the maxi- mum rated power. Fig. 4. A sound source fit- At 300 Hz. the effective ted to a closed baffle pro- radius is 0.1576 m, and the pagates the sound henn- radiation impedance is spherically 1198688295 Q. The acoustic power amounts Fig. 5. A sound source to 194.1875 W a , which is located at the junction of equivalent to a maximum two baffles propagates the intensity level of 134.88 dB sound in the shape of a or very nearly 40 dB above quarter sphere. Certain the reference level of horn loudspeakers operate 95 dB To achieve this, the in this way. electrical signal input would have to be an enor- Fig. 6. The radiation lm- mous 10 000 watts. This pedance of a loudspeaker shows that at frequencies is increased by placing the above around 200 to unit at the junction of three 250 Hz the only limitation is baffles. the electrical power hand- ling capacity. Fig. 7. Typical frequency Music power handling is response of a bass drive of the order of 370 W, cor- unit with a strong driving responding to an intensity force (low Qes). level of 121 dB or some 26 dB above the reference • The efficiency cannot level of the unit. It should be improved by more be noted that the required than 6 dB however electrical power input as much the electrical in- calculated pertains only put is increased. The at one frequency. With the main reason for this is amplifier operating over that, particularly at low the whole audio range, it frequencies, the mech- has to provide higher anical power handling powers than calculated to capacity becomes the ensure faithful step re- limiting factor, sponse. • The electrical power handling capacity, because of modern , construction methods COndUSiODS and improved speech coils, has become one The foregoing consider- of the least important ations and calculations parameters of a loud- lead to the following con- speaker system, elusions. • High efficiencies are only possible with large effective diaphragm areas. • Large cone areas result in lower distortion than small diaphragms. elektor india July 1986 7.47 VHF/UHF - tv -modulator To illustrate the principle of the TV modulator it is useful to look at a typical video waveform and the corre- sponding modulated r.f. signal, both of which are illustrated in figure I . Figure la shows one line of a video waveform. The maximum positive excursion of the signal is known as white level, since it is the signal obtained from white areas of the pic- ture. Line sync pulses are, of course, present at the beginning of each line, and are distinguished from picture information by the fact that they are negative-going pulses from 33% of white level down to zero (sync level). Picture information, on the other hand, extends from 33% (black level) up to 100% (white level). This description of a video signal is necessarily rather brief, and the various levels, etc. for broadcast video signals are, of course, defined much more rigorously. An r.f. signal amplitude-modulated with this video signal is shown in figure lb. It will be noted that the type of modu- lation employed is negative modulation, i.e. minimum video signal level (sync level) corresponds to peak r.f. signal level and vice versa. This type of modu- lation is used in the practical modulator circuit, which means that it is unsuitable for use with British, VHF, 405-line TV sets, which use positive modulation. In the UK the modulator must be used with UHF, 625-line sets, which are designed for negative modulation. The VHF output capability of the modulator is principally intended for use in countries outside the UK which use VHF systems employing negative video modulation. In a broadcast TV transmitter great care is taken to ensure that the carrier is a pure sinewave, otherwise spurious signals could occur around harmonics of the carrier frequency. Steps are also taken to reduce wastage of transmitter power by partial suppression of the carrier, and one of the sidebands of the signal is also partially suppressed to minimise the bandwidth of the trans- mitted signal. This is illustrated in figure 2. In a TV modulator for domestic use none of these criteria apply, since the signal is not going to be broadcast (and care must be taken to ensure that it is This circuit will modulate a video signal onto an r.f. carrier to give a signal that may be fed direct to the aerial socket of a VHF or UHF television receiver. not broadcast). There is no need to suppress the carrier or one of the sidebands, and the presence of har- monics of the carrier frequency is a positive advantage since (if the carrier fundamental is in the VHF band) it allows TV sets to be tuned to these harmonics right through from the VHF band to the UHF band. This means that a single modulator can supply signals to both VHF and UHF sets and makes tuning easier, since the set can be tuned to a signal at one of several frequencies throughout its tuning range. Modulator circuit The fundamental carrier frequency is derived from a 27 MHz crystal in an oscillator circuit based onTl in figure 3. For domestic use, crystal stability is not always required. In that case the crystal, X 1 , can be replaced by a 1 0 n capacitor. The output signal of this oscillator is amplified by T2 and T3 and differen- tiated by the three RC networks C3/R4. C4/R6 and C5/(R9 + PI). The resulting waveform at the junction of R8 and R9 is a sequence of short spikes containing harmonic multiples of 27 MHz up to around 1 GHz The video signal is fed in via P2 and modulates the carrier by varying the forward bias on Dl and thus changing its impedance. This causes the level of the r.f. signal appearing across RIO to vary in sympathy with the video input signal, i.e. the carrier signal is amplitude modulated. The signal is coupled out via C7 to a coaxial output socket. RI3 matches the output impedance of the modulator to that of the coaxial cable. Potentiometer PI can be used to set the carrier level by varying the static forward bias on Dl. whilst P2 adjusts the video input level and hence the modulation depth. Construction and adjustment A printed circuit board track pattern and component layout are given in figure 4. This board is available from the F.lektor Print Service, EPS No. 9967. Two alternative mounting positions are provided for the crystal, allowing for two different pin spacings. 7.48 elektor irtdia july 1 986 Figure 1. a. One line period of a typical video signal, showing picture information and line sync pulses, b. An r.f. carrier modulated with the signal of la, using negative modulation. Figure 2. a. Spectrum of a broadcast TV signal with partially suppressed lower sideband and vestigial carrier, b. Spectrum of a TV modu- lator for domestic use, in which both sidebands and the carrier are retained. This spectrum is also repeated at multiples of the carrier frequency. Figure 3. Complete circuit of the TV modu- lator. The precise frequency of the crystal is not critical and any radio control crystal around 27 MHz will be suitable. Figure 4. Printed circuit board and com- ponent layout for the circuit of figure 3. (EPS 99671. ■i MM ' ' , L ■■ va / R9| rU I 22p 8p2 (V 1 p C6 1 ( t TK 1 _ i ol 1 X1 - * R3 [cl c iL R4 h / R6 1 X " 22P [ P 2T JjJ y'rMHz X. Jzop □ L t£L T1.T2 = BF 494 T3 = BFY 90 D1 = 1N4148 Because of the high frequencies involved the board is designed with a generous earth plane for stability. In addition a screening plate, made of tinplate or a piece of copper laminate board is con- nected between the oscillator and modulator. The completed board must be mounted in a metal box for screening, to avoid the possibility of stray radiation. The modulator may be powered from + 12 V to +15 V unstabilised DC supply, which is stabilised at +5 V by the IC regulator on the board. Alternatively, the unit may be powered direct from an existing stabilised +5 V supply, in which case IC1 should be omitted and the holes in the board for its two outer pins should be bridged by a wire link. Setting up the modulator is extremely simple. Connect the modulator to the aerial input of the TV set using 75 £2 coaxial cable, then switch on the modu- lator and the TV set. Set PI to its mid- position and tune the TV set to one of the harmonics of the carrier. This will be around channel 7 (189 MHz) in the VHF band and at a number of fre- quencies in the UHF band. When the carrier is picked up the screen of the TV set will darken and noise (snow- storm effect) will disappear. A video signal may now be fed in, and elektor india july 1986 7.49 Parts list to figure 2. Resistors: R 1 = 33 k R2 = 22 k R3.R9 = 470 a R4 = 1 k R5 = 220 n R6 = 270 H R7 - 150 n R8 = 6k8 R10,R1 1 = 100 il R1 2 = 1 k5 R13 = 68 PI = 2k5 (2k2) preset potentiometer P2 = 1 k preset potentiometer Capacitors: Cl ,C7 = 33 p C2 = 1 20 p C3,C4,C5 = 8p2 C6 = 22 p C8 f C9 = 1 /i/16 V tantalum Semiconductors: T1 ,T2 = BF 194, BF 195. BF 254. BF 255, BF 494, BF 495. T3 = BFY90 D1 = 1 N4148 IC1 = 7805 (see text) Miscellaneous: LI = 1 txH XI = crystal, 27 MHz approximately, (or XI = 10 nF, see text) 4 P2 should be adjusted so that the video signal level does not exceed 3 V peak- to-peak at its wiper. The TV set may now be tuned to the sideband which gives the best picture. If tuned to the wrong sideband the picture will tend to appear negative. If the picture lacks vertical synchronis- ation (i.e. rolls) it will be necessary to adjust PI until it stabilises. P2 is used to adjust the contrast by varying the video input level, but should not be turned up too much or the modulator will overload, causing the picture to appear negative on highlights. Finally it should be noted that, when using the modulator, the output should always be connected direct to the TV set via a length of coaxial cable and must never be connected to any un- screened wire or other conducting object that could act as an aerial. otherwise the user could receive an unwanted visit from the Post Office Radio Interference Officer! K 7.50 elektor india july 1 986 A compact radar for helicopters by A W Pressdee, BSc, CEng, MIEE The pilot of a Hiller UH12 helicopter engaged on crop-spraying in Lincoln- shire failed to see the spur of a power line to a farm. The helicopter hit the wires which damaged the con- trol rods and caused it to climb out of control until it crashed into a field. Over the last six years the number of United King- dom registered helicopters sustaining damage from hitting power cables has run into double figures. Helicopter accidents oc- cur during a diversity of taska Many happen while crop-spraying, but others happen on surveys, low- level photographic work, and military operations. Such examples of ac- cidents to helicopters are repeated in international air accident statistics. Wherever this type of air- craft operates at low altitude, it is vulnerable to the hazards of poor visi- bility and unseen ob- stacles, particularly power lines, or both. Even when power lines have been clearly visible to the pilot, accidents have happened because he has been unable to estimate ac- curately his distance from them. For the majority of helicopters, payload is of prime importance. For most small and medium types, weight and space considerations rule out the carrying of bulky radar or obstacle-detection equip- ment. Instead, the pilot has to rely almost entirely on his visual acuity and good sense. Accidents caused by collision with unseen or undetected obstacles are unfortu- nately common and, even if not fatal, are expensive in terms of repairs to the aircraft and compensation for the damage it causes. Electronic solution Such problems may be mitigated by a new radar system under development by Philips Research Lab- oratories at Redhill, Surrey. It Is small, light in weight, and compact. It is also ex- tremely accurate and has high definition, operating at millimetric wavelength, and employing a tech- nique known as frequency modulated continuous wave (FMCW). The ability of a radar system to detect an object depends directly on the "illumination" of that ob- ject by electromagnetic waves. Conventional pulsed radar systems employ high power pulses, at say 10 kV, of very short duration, perhaps one mi- crosecond. at a pulse rep- etition frequency of poss- ibly 1000/s, giving a rela- tively low mean target illumination of one micro- second pulse every thou- sand microseconds. An FMCW radar illuminat- ing the target continuously —for 1000 microseconds every 1000 microseconds —can achieve compar- able target illumination and hence equivalent or better target detection with considerably lower power. The low-voltage system enables millimetric wave solid-state oscillators to be used. Lower voltages and solid-state techniques mean considerable re- duction in space and weight. The FMCW radar operates at 94 GHz, about ten times the frequency of most standard radars, which enables a very compact front end unit to be assembled. This incor- porates a 10 mW bias- tuned Gunn oscillator for the transmitter and a balanced mixer in the re- ceiver, both items developed by Philips Research Laboratories. The aerial reflector dish is 300 mm in diameter and produces a beam width of 0.7 degrees. Fast frequency sweeps In the FMCW radar system the transmitter is modu- lated with a continuous linear sweep. Conse- quently, the frequency of a returning echo will differ from the instantaneous fre- quency of the transmitter by a beat frequency pro- portional to the target range. The use of fast fre- quency sweeps allows small range differences to produce large frequency differences so enhancing the range resolution. The received signal gener- ally will contain several frequencies, correspond- ing to targets at different ranges, so a means of fre- quency analysis is necess- ary. This is achieved by a mathematical technique known as fast Fourier transform (FFT). The tech- nique analyses data over a fixed period, made con- veniently equal to the transmitter sweep time, so frequency flyback does not affect operation of the system. In the past, FFT has re- quired several hundred medium scale integrated (MSI) logic chips, but the application of high speed very large scale inte- grated (VLSI) techniques to digital signal processing has increased the speed and reduced the size of such systems. For the FMCW radar, a Texas In- struments TMS 320 pro- grammable single chip processor implements a single Euro-card sized board comprising the FFT processor. A dual processor system is used to maximize the duty factor for each set of data samples. One processor is always inputting data for the next FFT and output- ting data for the previous FFT, while the other pro- cessor is executing the current FFT. At the com- pletion of this sequence, elektor India July 1986 7.51 the two processors ex- change functions. Field tests The specification of the FFT was determined by the maximum range and range resolution require- ment coupled with the bandwidth needed for the FMCW receiver. All the FFT software has been writ- ten with a high level as- sembler and, to maximize program execution speed, straight line coding was used; in other words, to remove all time consum- ing program loops, all ex- ecutable instructions were placed in consecutive lo- cations. The FFT has been connec- ted to an experimental display system and various field tests ot the develop- ment system conducted to obtain radar views of sites which would present haz- ards to helicopter oper- ation. Radar pictures in plan and elevation have been obtained of aerial towers and power cable lines. On the display used, which is colour-coded to show incremental height, strong signals were re- corded from the cables, the conductor spacers and the top of the sup- porting pylon. The droop of the catenary can be clearly seen. As the system can measure targets of approximately 20 m a at 400 m with signal-to-noise ratio of 34 dB, such results ! can be confidently ex- pected. It is evident from the tests that the viability of FMCW radar as a future aid to re- duce helicopter accidents has been established and that the system can be made sufficiently light and compact for smaller heli- copters As the system nears production model stage, careful consider- ation is needed for the development of a display system which will give the pilot the information he needs in an easily understandable form. An additional bonus of the FMCW system is the fact that it is very difficult for electronic systems to detect it and even then it exhibits an excellent elec- tronic counter-counter measures (ECCM) per- formance. There are uses for the system, or components of if, other than in helicop- ters. Its high target defi- nition, coupled with its compactness and port- ability, suggests a variety of possible applications such as in weapon guidance for small mu- nitions systems. (LPS) Philips Research Labora- tories Cross Oak Lane, Redhi/i, Surrey, England, RH1 SHA. Monitoring highways electronically by K W Dickinson Techniques for the auto- matic detection and counting of vehicles as they pass an overhead video camera, have been developed from research at the University of Shef- field!’) and the University of Manchester Institute of Science and Technology (UMIST)®. Individual ve- hicle speed and length can also be estimated from a sequence of video images. During recent years there has been a growing need for greater traffic monitor- ing. Traffic data can be col- lected automatically by equipment installed in the carriageway. However, such installations are gen- erally only suitable at per- manent or semi-perma- nent sites. Traffic planning and sur- veillance engineers are now becoming increas- ingly interested in the concept of a wide area vehicle detector, based for example on a video 7.52 elektor india july 1 986 camera rather than the present highway point sen- sors such as axle or induc- tive loop detectors. Although highway auth- orities are using more video cameras for single traffic surveys, manual analysis of the video tapes is time consuming. It can take up to five hours to analyse a 30 minute re- cording. Because of such problems there is an ob- vious need for a com- pletely automatic video system to monitor traffic at permanent sites and provide automatic data abstraction from video tapes of short-term traffic surveys. During the early 1980s several computer-based video image processing systems were constructed by engineers from Shef- field University and UMIST. Over the past three years the British Transport and Road Research Labora- tory(3i, as part of its research programme, has provided financial support for a project, the aim of which was to assess the feasibility of using image processing to collect traf- fic data for various pur- poses and later to de- velop techniques for traffic monitoring on motorways Accurate system This has resulted in the design and construction of the traffic research and image processing (TRIP) system, a flexible development tool based on a powerful Intel microcomputer linked to purpose-built hardware. The fully automatic system is capable of accurately counting vehicles, measur- ing their speed and length, and calculating lane occupancy and the gaps between the ve- hicles In its original version, TRIP takes the video signal from a solid-state camera, converts it to digital form, and presents the video picture to the computer system as a two dimen- sional array of numbers, each representing the average image brightness (grey value) in that picture element (pixel). Vehicles are counted by analysing the different shades of grey whithin each image and by filtering out ex- traneous non-moving fea- tures such as carriageway markings and parked ve- hicles. There are several ways ot interpreting a sequence of digitized images and detecting moving objects in the scene. The TRIP system uses both back- ground frame and inter frame differencing tech- niques. Essentially, background frame differencing is a method of storing a grey value reference image, which does not contain any vehicles, and subtrac- ting it from each incoming frame or image. This causes all non-moving features of the image to disappear, leaving mov- ing grey value objects which can be represented as binary images after applying a suitable thres- hold. Counting vehicles then becomes the simpler task of counting white shapes. However, since ambient light levels can radically change within seconds, it is periodically necessary with this method to update the stored background frame tor satisfactory detection of moving vehicles over a length of time. Overcoming daylight variations Inter frame differencing usually overcomes the problem of changes in ambient light. A back- ground frame is again subtracted from the in- coming image, but then the incoming frame be- comes the background for the subsequent frame. Such systems can suffer from problems associated with matching edges from frame to frame; stationary vehicles disappear and random noise in pairs of frames becomes cumulat- ive. Trials were undertaken dur- ing 1985 to evaluate the feasibility of the TRIP system to monitor traffic passing a point on the road network. During trials with an earlier system, a high threshold was ap- plied to overcome noise in the binary image caused by changes in lighting conditions. Therefore twelve per cent of vehicles were missed and, be- cause of limitations in the available computing power, the system was only able to capture Images at 4 (rames/s. Also, it was im- possible to estimate ve- hicle speeds because a fast vehicle might appear in one frame but move out of the scene before the next frame. Performance of the TRIP system has been improved by splitting each image and concentrating atten- tion on a few small but im- portant areas (windows) within each scene. This can be considered as a method of projecting simple light sensors on to the carriageway. Both background frame and inter frame differ- encing techniques can be applied to the whole im- age or windows within the image. If processing is confined to windows, the image frame rate can be increased, giving a better measure of the time at which an event occurs. Site trials During site trials, the solid state video camera was mounted at heights be- tween 8 m and 24 m above the carriageway and several windows were superimposed across the image of each traffic lane. Images of the scene were sampled at 50 frames/s and the time that each vehicle was observed/ detected at a window was later compared with its time of arrival at the next window. By knowing the space between the road elements corresponding to the window positions, an estimate of vehicle speed, vehicle length, and lane occupancy could be de- rived. Trials were carried out in a range of weather con- ditions and ambient light levels. The results indi- cated a miss of less than 1 per cent of vehicles. Estimates of individual vehicle speed were found to be within 10 per cent although no systematic error in speed was ob- served. It is now possible to use the TRIP development system to automatically detect and measure the speed of vehicles as they pass through a typical highway environment in which ambient light levels change. However, wider traffic engineering appli- cations such as surveil- lance throughout a 1 km stretch of motorway for automatic incident detec- tion purposes, vehicle tracking through a junc- tion, and classification of traffic by vehicle type are awaiting further investi- gation by the Sheffield University/ UMIST TRIP group. Nevertheless, much effort will be required to provide suitable applications soft- ware before general pur- pose traffic data collec- tion systems based on video image analysis are readily available. (LPS) 1. University of Sheffield, Depart- ment of Civil and Structural Engineering, Mappin Street, Sheffield, England. SI 3JD. 2. University of Manchester Institute of Science and Technology. Department of Electrical Engineering, PO Box 88. Manchester, England. M60 1QD 3. Transport and Road Research Laboratory. Highway Traffic Division. Old Wokingham Road. Crowthorne. Berkshire, England, RG11 6AU The author is a Senior Research Associate. Department of Civil and Structural Engineering, Univer- sity of Sheffield. Background or reference frame differencing images. Top left; input. Top right: reference. Bottom left: difference. Bottom right: binary. The role of plastics in communi- cations The use of plastics in the world's expanding infor- mation technology in- dustries will be examined by leading world experts at the Plastics and Rubber Institute’s fourth inter- national conference, to be held in London from 17 to 19 September this year. More than 100 delegates are due to attend from outside Britain, including a large delegation from Japan, which will be fielding six conference speakers. Leadings profes- sionals are also expected from the polymer and telecommunications in- dustries in the United States and Australia. The conference, to be held at the Institution of Electrical Engineers, will feature thirty-three lecture papers, reinforced by • displays in the conference hall. Apart from a detailed examination of the role of plastics in telecom- munications equipment, the delegates Will be briefed on the latest testing methods by British Telecom's Materials and Components Centre, which is organizing a reception for delegates aboard a motor vessel on the Thames. (LPS) °/astics and Rubber In- stitute It Hobart Place London SW1W OHL elektof india July 1 986 7.53 Output Inputs + 5V 84689 X-1 1 (Ground) TTL The TTL family is characterised by their type numbers starting with 74 The LS TTL family also has similar numbers starting with 74 LS. Both TTL and LS TTL families are very similar, most of the individual ICs are interchangeable, and pinto pin compatible. Only the output loading capacities differ. As LS TTL output should not be loaded with more than 5 TTL inputs. The 74 LS series ICs are as fast as the 74 series ICs but consume less current than the 74 series ICs, contrary to the concept that faster the circuit, greater is the current consumption. This has been made possible by the low power Shottky devices used in these ICs. selex-14 Digi Course II Chapter 8 In the last few chapters of Digi Course II we saw how combinations of individual building blocks available in form of integrated circuits are achieved. The basic idea is to use standard building blocks like the NAND gates, to create different logical functions. The ICs which can be combined in such a way without any problems of matching (compatibility) are said to be of the same family. ICs of the TTL and LS TTL families are examples of such grouping. 7.54 elekior india july 1 986 selex Outputs of these ICs can not be coupled arbitrarily. Only when the outputs are always logically identical, at the most 2 such outputs can be coupled together to enhance the output loading capacity. With how many inputs cart one output be loaded? This can be calculated from the specified loading factors of the ICs, which are given in the data sheets of the ICs. When connecting more than one inputs to an output of another 1C, care should be taken not to exceed the specified loading capacities. For example, consider a 7404 inverter which has an output loading capacity of 10. We can connect 4 clear inputs of 7476 ICs and 2 gate inputs of 7400 ICs. Without overloading the 7404 inverter. TTL and LS TTL ICs can be operated at 5V (I 0.25 V), This makes it difficult to adapt them into circuits operating from other sources voltages. To overcome this difficulty to some extent, some devices have been designed with open collector outputs as shown in figure 1. In case of these ICs; the output does not switch between 0 and 5 Volts but it has a built in driver transistor with emitter tied to ground and collector brought out on the output pin. Such ICs can work with voltages upto 30 V at the output pin which then switches between 0 and 30 (if the output is tied to 30 V through a pull up resistor). CMOS CMOS technology is totally different from the conventional TTL technology. Though the logic of all gates must be same whether they are TTL, LS TTL or CMOS, the electrical characteristics are different. CMOS ICs consume very low power, as the operating current currents are very low. However, the speed is sacrificed tosome extent. A TTL NAND gate draws almost 20 times more current compard to the CMOS counterpart. As the current drawn is dependent on frequency of switching, the CMOS operated at low speeds consumes still less power. The CMOS logic ICs are generally identified by type numbers starting with 40 or 4. A 4001 contains 4 CMOS - NOR gates. The 40 series ICs are not pin compatible with 74 series. The supply voltage for CMOS ICs can be between 3 and 15 Volts. The TTL and CMOS ICs are difficult an arrangement similar to the one shown in figure 2 does work. CMOS ICs may drive LS TTL inputs but never a TTL input. 2 84689X2 Another point to remember about CMOS is that no input pins should be left floating; to reduce effect of interference. Even CMOS ICs can be used onthe Digilex Board provided that all precautions are taken to avoid any damage to them. CMOS inputs are sensitive to electrostatic discharges and can be damaged even during handling. CMOS ICs are to be stored on conductive foam or in an aluminium foil. 74 HC A more advanced development in the Integrated Circuit Technology is the high speed CMOS ICs. These ICs combine merits of both the TTL and CMOS technologies. Their speeds are as fast as TTL and the current drawn, as low as the CMOS ICs. However, these are still out of the reach for the hobbyist due to their high cost. This family is characterised by type numbers starting with 74 HC With this chapter, our Digi Course comes to an end. The theme "Digital Technology"' is of course not finished — there is much more to learn. More about it in the next issue. 1 nS Nanosecond = 1 billionth of a second. elektor india July 1986 /.55 the diode cannot conduct even if the voltage across it has the correct polarity. Threshold Voltage and the LED. A Germanium diode requires around 0.2 to 0.4 V for conducting. This voltage does not fluctuate very much with change in current through the diode and can be used as a reference voltage. However, it should be remembered that the threshold voltage is dependent on temperature. Figure 2 shows how the voltage is distributed between the diode and the actual load. Figure 3 shows a simple arrangement to obtain a reference voltage of 0.6 V. The limiting resistor ensures that the diode current does not become too high. As the voltage across the diode is fixed, energy dissipated as heat in the diode is directly porportional to the current flowing through it. A diode which carries high currents must therefore be cooled by providing a heat sink. We already know that the diode conducts current only in one direction, like a "Value of current". Figure 1 shows the directions in which current flows or blocks. The direction in which current flows is called the forward direction. The blocking direction is called the reverse direction. Current flows when the Anode is more positive than the Cathode. In the diode symbol, the bar represents the cathode. Physically, the diode has coloured ring or a dot marked on the body to indicate the cathode. Diode can be compared also to a switch which depends on the polarity. Just as a mechanical switch requires pressure of our fingers to close it, the diode requires electrical pressure (potential difference). Only difference is that once the mechanical switch is closed it remains closed. Diode requires continuous energy to keep it conducting. This is more similar to a mechanical key switch, which remains closed only as long as the pressure is applied, and opens as soon as pressure is removed. Applying a pulling force instead of pushing does not close the switch. Electrical pressure (Potential difference) applied to the diode in the reverse direction does not force current through it. The energy required by the diode to keep it conducting appear as the loss of voltage across its terminals. This drop in voltage is about 0.6 to 0.7 V in Silicon diodes. This is also called the threshold voltage because the diode cannot start conducting unless a voltage more than this value is given in the forward direction across the diode. Below this threshold Forward Direction Blocking Direction Cathode Direction of Current LED The energy that is dissipated as heat in ordinary diodes has been exploited by the inventive scientists to design a very useful device called LED! Voltage Source Load 83701 2 Figure 1 Blocking and conducting directions of a diode. The cathode is marked as a coloured ring or dot on the body of the diode. Figure 2 There is a drop of about 0.6 V in the forward direction across the diode. The difference between the supply voltage and the diode voltage appears across the load. 7.56 eleklor mdia july 1 986 LED is a Light Emitting Diode, which is similar to an ordinary diode except that the energy required by the diode to keep it in conduction is given out in from of light rather than heat. LEDs are made of materials like Gallium Arsenide or Gallium Phosphide; and can have different colours depending on material. Threshold voltage for an LED can be between 1.6 V to 2.2 V (See table 1). The intensity of glow depends directly on the current flowing through the LED. Commercially available LEDs have current ratings upto maximum 50 mA. A safe value to operate the LED without damage is between 1 5 to 20 mA. The cathode of an LED can be seen through the transparent casing and is a broad dish shaped electrode. The cathode terminal is made shorter than the anode terminal as a physical indication of polarity. The current limiting resistor can be calculated using the Ohm's law, taking into consideration the threshold voltage. For example: A green LED being opeated from a 12 V supply and having a threshold voltage of 2.2 V will need a limiting resistor given by the following calculation. Parallel combination of LEDs is not a sensible application because depending on individual characteristics they will carry different currents and thus give varying intensities. The light emitting property of LEDs affects their blocking properties in the reverse direction. LEDs can tolerate at the most 3V in the reverse direction, and should never be operated with reverse polarity. The leakage current through an LED with reverse voltage can go upto 0.1 mA whereas a regular diode like IN 4148 typically conducts about 25 nA in the reverse direction (one nanoampere = One billionth of an ampere) when supplied with 20V reverse voltage. Figure 3 A reference voltage of 0.6 V canbe obtained from any standard voltage source. Figure 4: LEDs are always connected in the forward direction. The cathode canbe recognised by three features: the shorter terminal, flatterned body and wider of the two inside electrodes. Figure 5: The limiting resistor takes the difference between the supply voltage and the threshold voltage of the LED. Value of the limiting resistor decides the current passing through the LED. 83701-4 Figure 6: Threshold voltages add up in a series connection of LEDs. Figure 7: LEDs are available in various shapes, sizes and colours. A wide range of LEDs characterises the appearance of modern electronic apparatus. LEDs should be used only for indication purpose and not as ordinary diodes, so that their defective blocking properties do not become significant - LEDs are available in various sizes and shapes. The most commonly available colours are red yellow and green. Blue LEDs are also being offered by some manufacturers but they are very expensive. The nearest available standard value is 470 n , which can be used with LED When more than one LEDs are connected in series, their threshold voltages will add up. If we connect 4 LEDs in series, having an individual threshold voltage of 2.2 V then the total drop across them would be 8.8 V, thus leaving only 3.2 V to be taken by the limiting resistor. A resistor carrying 20 mA with 3.2 V across its terminals must be about 150 n 83701-6 Colour Red Red (High glow) Yellow Green Light Intensity LED Current (mA) Threshold Voltage (V) Beam Angle 3 j— Limiting Resistors (+) T f t ^ ■Sb MV 1 r\ 83701 3 5 12V nri Ur o R V i^j 2,2 V 1 1 & © 83701 5 leftmost switch controls the available capacitance at the leftmost pair of terminals which gives 0 to 680 pF. Remaining three switches control the capacitance value available at the second pair of terminals. The connection diagram is shown in figure 2. It can be seen from figure 2 that switch 2 gives capacitance values from 1 nF to 6.8 nF, switch 3 gives from 10 nF to 68 nF and switch 4 gives 100 to 680 nF. As all the three are You already know what a resistance decade box is. A capacitance decade is almost same - except for the fact that it uses capacitors! The circuit is very simple, and very useful for experiments. Our decade box has two pairs of output terminals, one for values from 0 to 680 pF and the other far values from 0 to 750 nF. A photograph of the decade box is shown in Figure 1. There are four rotary switches. The connected in parallel, what we get on the second pair of output terminals in the sum of the three capacitance values selected by S 2 S 3 and S 4 . Figure 3 illustrates the exact operation of these three switches. It is possible to get a combination of maximum 3 capacitors from the three individual groups and the result is their sum, because of the parallel combination. If any one or two switches are used, it will either give Figure 1 : A sturdy housing with a properly laid out front panel gives a professional look and ease of operation. Figure 2 : Total 24 capacitors are divided into 4 groups with 6 pieces each. Switch SI controls the first group of 6 switches S2. S3. S4 control the remaining three groups. 7.58 elektor india july 1 986 selex 3 6 O Figure 3 : Various combinations of effective capacitance are possible. Figure 4 : Capacitors are directly soldered on to the lugs of the switches. Those who have already constructed the resistance decade can use its rear panel to serve as the front panel of the capacitance decade. Figure 5 : All soldering of capacitors should be carried out before mounting the switches on the front panel. Including the Zero position, the switch requires 7 positions. one capacitor across the ouput or a sume of two capacitors which are selected. If S2. S3, S4, are all closed to select a capacitance, the effective value is CE = C2 + C3 «■ C4 The characteristic of a parallel combination of capacitors is that the individual values add up to give the effective value. This is in contrast to the characterise of resistors in parallel combination. In case of resistors the series combination gives the total of individual resistance values. We shall take a few examples to see the usefulness of the decade box. Say, we need a capacitor of 270 nF. We can set S4 to 220 nF and S3 to 47 nF so that the result is 267 nF. It is certainly not exactly 270 nF, but if we consider the tolerance range of capacitors available, the value of 267 nF calculated theoratically will fall within the tolerance range of + 10%. We could have used switch S2 at 3.3 nF to get the effective value of 270.3 nF, but practically it would make no difference. Now, let us take another example where we need 39 nF. In this case we set S2 to 6.8 and S3 to 33 so as to get 6.8 + 33 = 39 8 nF. Using SI we can obtain 0 to 680 pF and using S2, S3, S4 we can obtain 0 to 754.8 nF. If we need a value little higher than 650 pF, we can connect both pairs of terminals in parallel to get a maximum addition of 750 nF to the 680 pF capacitance. Construction While assembling this useful decade box. the main job consists of soldering the 24 capacitors in place over the roatry switches, as shown in figure 4 and figure 5. elektor mdia july 1 986 7 . 5 9 selex 6 capacitors are soldered on the lugs of the rotary switch SI as shwon in figure 5. The free ends of the capacitors are connected together and then to one of the output terminals of the left most pair. The centre pin of the switch (the common point connected internallyto the wiper contact) is connected to the remaining output terminals. S2, S3. S4 are also connected similarly as shwon in figure 2. All free ends of the 18 capacitors coming to one of the output terminal and all three centre; pins of S2. S3. S4 coming to the remaining terminal. Once all 4 switches are soldered they canbe fitted onto the front panel. The type of capacitors to be used depends on your application, required accuracy and your budget. High accuracy will always cost more. Application This capacitance decade box can be used with the resistance decade as a very useful test unit for R.C. cirucits. When working with AC cirucits, various RC combinations can be used as filter cirucits. High-Low or Band Pass filters. These are nothing but frequency dependent impedances. A High Pass' filter is one which allows only frequencies above. the desired limit to pass i through. A Low Pass' filter allows frequencies from 0 upto the desired limit to pass through. A Band Pass' filter allows frequencies between a lower and a higher limit to pass through. In case of a practical filter cirucit, even frequencies which are not intended to pass through are allowed to pass through the filter but they are considerably attenuated. Figure 6 shows three basic filter circuits using RC. network. Figure 7 shows how you can start experimenting with the capacitance decade box. The cirucit shown is an astable multivibrator which is "almost'' complete — just a capacitor between points A and B is missing. You can use the decade box to connect different values of capacitors across A B and observe the effect. With change in capacitance value | across A B, the frequency and duty cycle of the astable multivibrator changes. First use the leftmost pair of output terminals to select a capacitor from 100 to 680 pF Then use the second pair to select 1 nF to 750 nF. You will get a complete series of audio frequencies. 0 o - — o . a rt - L i ¥ V - " — O o— -o Figure 6 : A resistance decade and a capacitance decade together can create an RC circuit which plays an important role in AC applications. Three basic types of RC filter cirucits are shown here: (a) High Pass, (b) Low Pass and (c) Band Pass. Figure 7 : By connecting the capacitance decade between points A and B you can create different audio frequencies by changing the effecting capacitance values. 9 V The Digilex-PCB is now available! Price: Rs. 85.00 + Maharashtra Sales Tax. Delivery charges extra: Rs. 6.00 Send full amount by DD/MO/PO. Available from: precious ° ELECTRONICS CORPORATION Journal Division 1 1, Shamrao Vithal Marg (Kiln Lane) Off Lamington Road, Bombay-400 007. 7.60 elaktor india july 1 986 selex High current and magnetic fields Electric current and magnetic fields are very closely related. Any current flowing through an electrical conductor produces a magnetic field around itslef. The higher the current flowing, the higher is the magnetic field surrounding it. Ordinary electrical wiring carrying currents also has a magentic field surrounding itslef, but as the currents are not very high, the magenetic field will need very sensitive equipment to detect it. Existance of such a field can be proved with a simple experiment, by producing a very high current. For the experimental construction, about 1 .5 meter insulated thin copper wire, an electrolytic condensor of 4700 pF/25 V, two 9V batteries and a changeover switch are required. The copper wire is suspended as shown in figure 1 without tension. The distance between the outgoing wire and the i returning wire should be as small as possible, not more than one or two millimeters. One end of the suspended wire is connected to the negative terminal of the battery and the capacitor. The changeover switch is connected as shown in figure 2. The experimental set up is now ready. As the switch connects the capacitor across the battery, the capacitor gets charged to 1 8 Volts. Once the capacitor is charged the switch is thrown over to the other position, where it connects the wire directly across the capacitor. This forces the stored charge in thie capacitor to discharge quickly through the wire, producing a very high current for a moment. This momentary current goes almost upto 45 A in the above set up. This high current flowing through the conducting wires produces a magnetic field around itself. This creates a momentary jerk, which can be observed. The duration of this jerk can become more visible if larger values of capacitors or higher charging voltages are used. (Be careful about the the rated Votage of the electrolytic condensor.) Figure 1 : The conducting wire loop must be suspended without tension and the gap between them should be very small. Figure 2 : The circuit of the experimental set up. The electrolytic capacitor is charged from the batteries to 18 V. After the changeover switch is thrown to othe position, the capacitor quickly discharges through the wire, driving a very high current through the loop. 2 eiektor india jufy 1 986 7.61 DIGITAL MEGGER ARUN has introduced a Digital Megger for measurement of insulation resistance. This is designed for 0.1% linearity throughout the range. The instrument has various ranges, which are manually selected The test voltage is generated with the help of a DC to DC convert or eliminating the need for a hand driven mechanism This model has four ranges viz 20,200,2000 and 10000 M ohms with test voltages of 250 V DC. 500V DC and 1000V DC. The unit operates on 230V DC. For further details contact: M/S ARUN ELECTRONICS PRIVA TE LIMITED B-1 25/ 1 26 Ansa Industrial Estate. Saki Vihar Road. Saki Naka. Bombay 400 072 Phones: 583354/581524 CABLE-TIES Novoflex have introduced RE- USABLE CABLE TIES which provide positive holding and permanent locking. The Ties remain securely locked until released by pushing the projection hear the locking head. Available for cable bundle diameter upto 106 mm. They maintain holding strength over a temperature from -40°C to +135°C. WALKIE - TALKIE/ TRANSCIEVER KITS Fiji Electronics offer complete know how and literature of assembling a walkie- talkie/Transceiver set with indigenous components The literature includes an application form and procedural details on how to obtain an experimental licence from the government to operate the transceiver. The kit supplies PCB and only those components for the transceiver which are usually difficult to get like quartz crystals, filters, IFT, hardware etc. Contact for further information:- M/S FIJI ELECTRONICS. Mail Order Sales. (WIT) Puthencurichy. Trivandrum - 695 303 FIBERSCOPE Mecord introduces Industrial Fiberscopes of Japan make. The Fiberscope transmits a bright and clear image over a long distance in vivid colours. The images can be observed not only by the naked eye but also through a TV camera. The probe is flexible and small in diameter permitting inspection of normally difficult areas. For Details Contact MECORD MARKETING PVT. LTD 304. Hill View Industrial Estate. Off L B S Marg . Ghatkopar (W) Bombay-400 086 TWILIGHT SWITCH 'lEC' Twilight Switch is an automatic light control device for switching lights "ON" automatically when the intensity of light falls below a preset level and conversely it switches "OFF” when the ambient light intensity exceeds a preset threshold. The Twilight Switch is available in two models: OUTDOOR and INDOOR, with a choice of load ratings varying from 10 A to 60 Typical applications can be found in street/factory/courtyard lighting, housing societies, railway stations, airport runways, neon displays, cinema houses, etc. For further information. Contact INDIAN ENGINEERING COMPANY. Post Box 16551. Worli Naka. Bombay 400 018 PRINTER DATE SWITCH A new data switch, introduced for the first time in India by Kit Enterprises, allows one Centronics-type printer to be switched between two computers or one computer between two printers. The switch supports the 36 pin Centronics Parallel interface used by many leading printer manufacturers. It is equipped with three 25 pin D shell connectors and is operated via a single front-panel switch. Applications: (a) To switch one computer between two printers. For example, to alternate a computer between a Daisy wheel (letter quality) and a Dot Matric printer (for high speed printing). (b) To share one printer between two computers so that while one computer is printing the other can be used to run a different programme. It eliminates the need to plug and unplug delicate printer cables which may cause carnage to connectors if done too often. For enquiries contact Kit Enterprises 18. Rebe/lo Road. St. Sebastian Colony. Bombay ■ 400 050. Tel: 642 9064 COUNTING DIALS * 10-Turns Counting Precision Dials * 100 dial div per turn * Shaft diameter Vi" (Vs" and 3/32” - non-standard) available upon request. * Two types 1 'A" and 1 7 /e" outer diameter * Available both locking/non- locking type These dials are claimed to be highly accurate and easy to read with a long life They can be mounted directly to the potentiometer shaft. Dial backlash has been eliminated by locking the knob to the shaft and non-rotating base to the panel. For further details please contact: UNIROYAL 15. B/6 Silver ia house. L.J. Road. Mahim Bombay - 400 016 7.62 elektor india july 1 986 SPIRALWRAP MICROSIGN Spiralwrap (Flexible Protective Sheath) provides abrasion protection to wires • cables and tubings Available in five different sizes it can be quickly installed and is re- useable too. It is used to bundle, harness, protect or • nsulate wires-cables tubings or nouse Versatile by design and •material it can be used in many applications and can be reused as and when revisions are required for further information contact '.'ICROSIGN PRODUCTS \4ehta Terrace " S ityanarayan Road Snavnagar 364 00 1 SERVO CONTROLLED VOLTAGE STABILIZER The Jivan is a Stepless Servo Sontrolled Voltage Stabilizer with output accuracy of + 1% gainst wide input range. The jtput level can be set between 220 to 240 V or 400 to 420 V Vanual operation is also : ossible. High and low voltages i'e idicated by the neon c ovided on the front panel. VCB is provided to protect --camst over load (Upto 10 KVA, 1 phase & 30 KVA, 3 phase). It 5 recommended for computer ?. stems. laboratories, various -dustries. communication -• stems, CNC Machines etc. For further details write to : — M S JIVAN ELECTRO 'JSTRUMENTS. 394. G.I.D.C. Wakarpura. BARODA - 390 010. 'JTERC ONNE C T/ON GUARD ’• ectar E lectronics offer -oulded polythene Guard for nterconnections. The guard can be assembled with the connector or detached from it for tests and repairs with the "^Ip of nuts and bolts. The assembled guard improves reliability and appearance of electronic equipment. The number of interconnections and the pitch between them can be selected to suit various types of standard electronic connectors For more critical requirements, further protection from environmental effects caused by moisture, gases, vapours, fine dust particles or living organisms etc can be provided by using more complex guarding systems incorporating rubber gaskets, grommets and other types of sealing For further information, please write to NECTAR ELECTRONICS PB No 5009 GPO Bangalore Karnataka 560 001 DIGITAL FREQUENCY COUNTER VDC18 VDC18 is the smallest size over made in India. Features include BATTERY OPERATION cum mains operation through adaptor, 7digit 0.5 inch LED display,30 MHz frequency range, light weight, resolution selection etc. etc. VDC18 incorporate latest L S I. circuitry Model VDC19 has frequency range upto 500 MHz and PERIOD. FREQUENCY modes * * * s * For further information, please contact: - ASIA ELECTRIC COMPANY Katara Mansion 132 A. Dr A B Road. Wor/i Naka. Bombay 400 01 8 DIGITAL TACHOMETER RC offers digital tachometers in two models DT 4 (0-9999 RPM) and DT 5 <0-19999 RPM). At an accuracy of * 1 count and speed of one reading per second, it works within the temperature range of 10° C to 50° C. It has input impedence of 100 K Ohms Readings are displayed on seven segment display Measurement methods are magnetic pick-up (variable reluctance method, non contact type) and optical pick-up (contact type). Magnetic pick-up models have an input voltage protection for ♦ 250 Volts. For further details, contact: - RC IN FORM A TION TECHNOLOGY SYSTEMS PVT LTD.. 1413. Dalamal Tower. Nariman Point. Bombay 400 021 . P CLIPS for details contact: VASA VI ELECTRONICS (Marketing Division) 630.Alkarim Trade Centre. Raniganj Secunderabad-500 003 Phone: 70995 STRIP CONNECTORS IEC Strip Connectors are available in wide range, from 5 Amps to 30 Amps in 12 ways, moulded in Bakelite & PVC The metal parts are made of brass and screws of M S duly plated to prevent corrosion. The strip connectors are tested to withstand High Voltage for 2 K.V. This product can be used to secure a wide range of cables diameters because of its adjustability Apart from this, a range of sizes is available in adjustable and non adjustable forms The P-Clips are made from Nylon and all expected to withstand temperature from 35° C to 1 35°C. insulation and hence find particular use in instrumentation and electronic equipment & appliances. For further information, please contact: - STARLITE ENTERPRISES 1 24B. Vivekananda Road. Calcutta - 700 006. PLASTIC INSTRUMENT BOX FOR BACK MOUNTING Comtech T-77' is an elegently designed plastic moulded instrument box suitabre for back mounted instruments such as Timers. & various other control instruments, having overall dimensions of 1 10 mm. I x 77mm W x 100 mm. B. It consists of a moulded box, a cover. & a M S plate for back mounting. The box has an inside space of 73 mm. x 71 mm. for various components. A six way terminal strip fixed at the top & bottom, infornt of the box provides a easy access for the terminals. The cover can accommodate a PCB of 77 mm x 72 mm. from inside & has a 1 2 mm deep recess in front to take an Aluminium plate of 65 mm x 66 mm. for control indications. The box offered in Black & Grey colour with either Glossy or Matt finish, is most suitable for small instruments to be mounted side by side from the back, like e g. counters, controllers & timers etc. For further details contact COMPONENT TECHNIQUE 8. Onon Appartment 29 A La/lubhai Park Road. Andheri (West) Bombay - 400 058. elektor india july1986 7.63 Plug-in modem MULTIMODEM puts com- prehensive data communi- cation within the reach of C64 and C128 owners. BABT approval is expected shortly. Miracle Technology (UK) Ltd. St Peters Street Ipswich !P1 1X8 for Com- modore 64 and 128 computers Telex: 946240 CWEASY G 19002985 (3417:4:F) Miracle Technology Ltd have introduced their new 64 MULTIMODEM, which gives Commodore 64 and 128 owners access not only to Prestel, Micronet, Microlink and viewdata services, but also to databases, bulletin boards, electronic mail, telex and user-user com- munications. The modem features autoanswer, autodial, and on-board software in ROM. Menu- driven and multi-speed, it supports the CCITT V21 / 23 and Bell 103 standards, handling baud rates of 300/300, 1200/75 and 75/1200. Functions include save and print frame, automailbox with edit and save and telesoftware downloading. The unit fits in the computer's car- tridge port, and has only one external connection — the telephone lead. At £98.50 exc. (£116.15 inc VAT & UK delivery), the 64 A safe and highly reliable DMM Harris Electronics have an- nounced the availability of a new hand held Digital Multimeter, de- signed with safety and re- liability in mind. TMK model G44 is housed in a rugged plastic case with integral tilt stand and safety sockets. A large 0.5 inch LCD dis- play clearly indicates the 0.25% basic DC voltage accuracy to 1000V and AC voltage range trom 100 M V to 750 V. AC and DC current ranges are speci- fied as 100mA to 20A and resistance can be measured from 100 milliohms to 20 megohms. The G44 gives approxi- mately 2000 hours ot use when fitted with a single 9V alkaline battery. A low battery warning has also been incorporated, along with full overload protec- tion on all ranges. The G44 is priced at £49 and comes complete with battery, safety style test lead set and a com- prehensive operator’s manual. Quiswood Ltd 21 Eastbury Court Lems/ord Road St At bans Herts. AL1 3PS (3417:10:F) New lithium battery Venture Technology has launced a new 3-volt lithium-manganese diox- ide battery, the LiM3512E. This battery has a ca- pacity of 27 000 mAh when discharged at 275 mA at 25 °C It has a diameter of 35.8 mm and a height of 128 mm. Venture Technology Limited 18 Nuffield Way Abingdon Oxon OX14 1TG Telephone: (0235)20502 Telex: 837887 (3382:13:F) New versatile switching regu- lator chip National Semiconductor has recently introduced the LM1578, a switching regulator that generates a positive or negative voltage from one positive supply. The LM1578 can be set up tor dc-to-dc voltage conversion circuits such as the step-down, boost, and inverting configurations. The output can switch up to 750 mA while output pins tor its collector and emitter have been pro- vided to promote design flexibility. Also the LM1578 has a 1% on-chip oscil- lator and an external cur- rent limit terminal. National Semiconductor (UK) Ltd. 301 Harpur Centre Horne Lane Bedford MK40 1TR. Telephone (0234) 47147 Telex 8 26 209 (3417:7) 7.64 elekior india july 1 986 TESTICA T-3 ONLY THE ONLY MULTIMETER WITH PRO MPT S ERVI CE AFTER SALES ACCURATE! ROBUST! E CO N OMICAL! AVAILABLE AT ALL COMPONENT SHOPS MANUFACTURERS: ELECTRICAL INSTRUMENT LABORATORIES, 339/68, RAJESH BUILDING, LAMINGTON ROAD, BOMBAY-400 007. PHONE-36 07 49. ELECTRONICS (BOM GANESH NIWAS, 11, LANE, BOMBAY- 400 TEL: 363592/356529 RESI: 624931 TELEX: 01 Two latest, indispensable aids for the industry— FULLY ELECTRONIC THE SOURCE OF PERFECTION IN SOLDER BATHS: V The Solder Baths are specially designed to meet the needs of todays growing electronic industry. The baths are available in following standard Models. All these types are provided with an over- load fuse, indicating lamp. These baths are designed to provide satisfactory working condition. Temperature control baths are also available. h Models available - TC 175, TC 350, Su P erb Products TC 500 & B 350, SB 175 — Precious Electronics Corporation SUPERB SPIDER 52 C Proctor rood. Grout Rood (I) — *■ Bomboy-400 007 Pfc 357459. 369478 DIGITAL I j 3T CLAMP TESTER |r^rp!jS DIGITAL I tS« iSi MULTI METER • Large LCD for easy reading. msco msTRumEms private ltd. Bharat Industrial Estate. T.J. Road. Sewree. Bombay-400 015 Phones 413-7423. 413-2435. 413-0747 Telex: 011-71001 MECO IN for Precision, Accuracy & Reliability. elektor mdia july 1 986 7.71 classified ads. CONDITIONS OF ACCEPTANCE OF CLASSIFIED ADVERTISEMENTS 1 ) Advertisements are accepted subject to the conditions appearing on our current rate card and on the express understanding that the Advertiser warrants that the advertisement does not contravene any trade act inforce in the country. 2) The Publishers reserve the right to refuse or withdraw any advertisement. 3) Although every care is taken, the Publishers shall not be liable for clerical or printer's errors or their consequences. 4) The Advertiser's full name and address must accompany each advertisement submitted. The prepaid rate for classified advertisement is Rs. 2.00 per word (minimum 24 words). Semi Display panels of 3 cms by 1 column. Rs. 150.00 per panel. All cheques, money orders, etc. to be made payable to Elektor Electronics Pvt. Ltd. Advertisements, together with remittance, should be sent to The Classified Advertisement Manager. For outstation cheques please add Rs. 2.50 ZX-Spectrum users contact for exchange of softwares: M S. Bhatnagar, 45 A, Paloground, Udaipur, Rajasthan- 313001 High-power AF amplifier — 1 In this issue Resistor R 58 should be a 27 k, 1% type, as indicated in the circuit diagram Fig. 3. Telephone exchange (January 1986) Capacitors C 21 and C 22 have been shown with the wrong polarity in the component overlay. Fig. 3. Also R6 in the parts list should read R62. Advertisers Index ATRON 7-10 APEX 7-71 CREATIVE DATA SYSTEM . . . 7-09 COMPONENT TECHNIQUE . . .7-10 CTR 7-11 CYCLO 7-73 CONNECTRONICS 7-73 DEVICE ELECTRONICS 7-13 DYNATRON 7-65 ELECTRONICA 7-06 ECONOMY ENGINEERING ...7-12 K L INDUSTRIES 7-66 7-67 IEAP 7-70 LEADER ELECTRONICS 7-12 MECO INSTRUMENTS 7-71 OSWAL ELECTRONICS 7-02 PHILIPS 7-04 7-05 PECTRON 7-06 PLA 7-07 PIONEER 7-14 PRECIOUS 7-68 7-72 RAJASTHAN ELECTRONICS . . 7-02 S.S. INDUSTRIES 7-06 SUCHA ASSOCIATES 7-12 SUPERB PRODUCTS ... 7-14 7-71 SAVLA 7-73 SAINI 7-68 TESTICA 7 _7i TEXONIC 7-68 YABASU 7-10 VASAVI 7-14 VISHA 7-75 Ron,- I, Rubric -C R uhMRkrlsi Pipe Road leaver Parel Bombay 400 0 1 3 7.74 elektor iodia jufy 1986 R N No 39881/83 MH/BYW-228 LIC No 9 1 DO IT YOURSELF LEARN-BUILD- PROGRAM The Junior Computer book is for anyone wishing to become familiar with microcomputers, this book gives the opportunity to build and program a personal computer at a very reasonable cost. The Indian reprint comes to you from eletate? Send full payment by M.O./I.P.O./D.D. No Cheque Please. Packing & Postage free to eIeIctor eIectronics PVT ItcI. 52-C, Proctor Road, Grant Road |E), Bombay-400 007.