anuary 1986 ©!®[kito# Rs. 7 50 @i@dtr©ries 0 0 0 o j surface-mount devices selex-8 8-bit I/O bus lithium batteries elekter Volume 4 - Number 1 news & views Editorial electronics technology Surface-mount technology Channel multiplier for flat TV panel Microprocessor navigation Protecting computers from fraud projects Jumbo clock Phase-corrected cross-over filter Graphics card — 4 Telephone exchange Dissipation limiter information This month's cover of a typical surface-mount assembly shows clearly that this has a rather different appearance than conventional printed-circuit boards. Although they differ ex- ternally from their cur- rent counterparts, surface-mount devices are internally basically the same, except that they are generally of better quality. Although surface-mount tech- nology is undoubtedly the assembly technique of the future, largely re- placing printed-circuit boards within the next decade, it is still in its infancy and dynamic development is likely to continue for some years. Year index 1 985 Appointments Index of advertiser selex-8 digi-course II (chapter-2) semiconductors transistors resistance bridges resistance decade box eiext@? Publisher: C.R. Chandarana Editor: Surendra Iyer Editorial Assistance: Ashok Oongre Advtg & Admn: J. Dhas Production: C.N. Mithagari Address: ELEKTOR ELECTRONICS PVT LTD. Elekiuur B.V. Peter Treckpoelstraat 2 4 6191 VK Beek — 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 Raedersdon Elektor Verlag GmbH 5133 Gangelt — Postfach 1150 West Germany Editor: E J A Krempelsauer 16673 Voula - Athens - Greece Editor: E Xanthoulis Elektor JCE Via Rosellihi 12 20124 Milano Italy Editor: 0 Fumagalli Ferreira £r Bento Lda. R.D. Estefania. 32-1° 1000 Lisboa Portugal Editor: Jorge Goncalves £r technical manager: KSM Walraven Copyright ® 1 986 Elektor B.V The Netherlands January 1986 Communicating electronics Electronic is the world's No. 1 industry. In India, too, it has emerged as the most important one and is on the verge of dwarfing such giants as steel, chemical, automobile, et. al. Such is the impact that electronic has made on the society: it has brought about progressive sophistication and auto- mation; progressive reduction in cost without compromise in quality; and progressive increase in speed and accuracy of operation . Indeed, electronics is progressively leading us to become a high level information society. At the beginning of this century who could have foreseen such a remarkable metamorphosis in scientific technology. And the developments that have taken place in the last decade are a pointer to things to come in the 21st century. If the progressive and pragmatic policies adopted by our country are any indication, the year 2000 will usher in a century of changes thereby leading us to enjoy convenience and comfort to the full in every aspect of life. On this cheerful note, elektor, the magazine that communi- cates electronics, enters the New Year The elektor group comprises a team of 150 professionally skilled and dedicated people working in Holland as well as in many countries of the world with a common goal — of th. owing light on the exciting world of electronics on a continuing basis. The group has made a dynamic success story over last 25 years producing a diverse range of electronic designs for textiles, telecommunication, cars, computers, stereos, ships, and many more. This tremendous growth stems from: a keen awareness of the need to move with the times; high-reaching ideals; and a strong will towards constant innovation and improvement. Watchwards these that will carry us to even greater heights in 1986. CONSIDER COST OF OWNERSHIP OVER SEVERAL YEARS RATHER THAN INITIALLY CHEAP PURCHASE PRICE. 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Some are totally nonavailable. □ Repair vary difficult on PTH PCB puts DMM out of use in few months. anemd TAN DELTA' d & Glue Melting. □ t Plastic Melting. □ Solder Melting [ Impregnating □ Dip Coating, p Attractively styled Rugged Construction. GxSSt 3® ELECTRIC ——A MELTING Offers you A Microprocessor System Analyser • Develop & Diagnose Your Hardware/Software with this Versatile Instrument. • Microprocessor System Analyser finds application in R & D. Production, Testing & Field Servicing. Compatible both on 8085 and Z-80 accord electronics MEASUREMENTS Measurement of INDUCTANCE." CAPACITANCE. RESISTANCE are greatly simplified by VLCR 7. Connect the component to the terminals. VLCR 7 gives you directly the digital reading of value and its loss factor simultaneously. 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Toggle-Push Button-Rotarv-Micro-Centrifugal Foot-Rocker-Slide- Limit-Cord- Proximity and other switches. Indian Engineering Co.. Post Box 16551. Worli Naka. Bombay-400 018 (India) Phone: 493-9544 & 493-5565 Cable : AMPVOLT Telex :11-75217 IERG IN Only m (jor tSwikhes l UCJ »/25 Yeors 1060 — 1 985 | For further details contact : KOENIG ELECTRONICS & INDUSTRIES (P) LTD. I 54, INDUSTRIAL AREA, PHASE-II, CHANDIGARH-160002. Always a move ahead tance Measurement Is it because we've used solid state circuitry, with an indicator that is both shock and vibration proof, instead of clumsy clutch mechanism of conventional tes- ters? Or is it our hand driven generator which is smooth, stable, silent, and long lasting — alternatively battery operated? Could it be because of our taut band suspension rr 1 “ For further details w THE MOTWANE MANUFACTURING COMPANY MOTWANE PVT, LTD., at Gyan Baug. Nasik Road 422 101 Tel. 61297/ 61084 Telex: 752-247 MMPL IN Grams: MOT- WANE or Gyan Ghar. Plot 434 A, 14th Road. Khar, Bombay-400 052. Grams: MOTESTEM. I.C.'s : JTL CMOS. MOS, LSI, Microproccessor. Micro computer etc. Zener Diodes : 400 mw & 1 Watt s : Red, Green, Yellow in 5mm and 3mm dia 1C Sockets : SMK & Memorex make Trimpots : Multiturn Bourn's, VRN & Beckman make Single Turn cermets : EC as well as imported Floppy Discs : 8" as well as mini floppy of memorex, & dyson make zqosot ELECTRONICS CORPORATION Pushpdant Niwas 3rd Floor, 3, Chunam Lane. Dr. D. Bhadkamkar Marg Bombay- 400 007. Phone : 5137225. 5135845 WE OFFER FROM STOCK © Dantes Elsotrcntes Pts. ltd ■MM €L€CTRONIC €DUCHTIONflL 61 T€STING D€VIC€S we stock ir TELEDYNE ■c IZUMIYA 1C INC. P.C.B. Drafting Aids Magazines RAYTHEON Semiconductors Trimming Potentiometers Texas Instruments Semiconductors Semiconductors RS 232 Connector Panels Ribbon Cable Connector Panels Semiconductors 101 Kitchener Road *02-04, Singapore Electrical, Electronics and Hardware Centre, Singapore 0820. Telex : DEVICE RS 33250 FOR ENQUIRIES CALL: 298 6455 {4 lines) mam r 1 3 I | WHS 1.15 el ectronics scene Sales Tax Slashed Television sets used to cost more in Bombay and other parts of Maha- rashtra compared to the rest of the country. A stiff sales tax imposed by the state government had upset the manufacturers, dealers and buyers as well. The government of Maha- rashtra, after a persistent demand from all sections, has decided to reduce the sales tax on electronic goods All manufacturers of electro- nic goods, components and periphe- rals and computer companies have welcomed this move. The sales tax reduction, establish- ment of three7 electronic industrial estates, single window clearance and setting up of a high-level Maharashtra Electronics Develop- ment Agency will go a long way in achieveing the target of Rs 3,000 crore in electronics goods by 1990 according to the Bombay Chamber of Commerce and Industry. The All India Radio and Electronics Association, hailing the sales tax cut as a badly needed boost to the industry urged the government to levy a uniform four per cent tax on electronic goods, irrespective of the place of production. The Maharashtra TV manufacturers' association has expressed the hope that the production of TV sets in the state would increase manifold and assured the government that the concession would be fully passed on to the consumer. The All India Association of Industries said the sales tax cut would encou- rage entrepreneurs to set up new units in the state. The government has taken a step in the right direction by this announcement. TV manufacturers, true to their word seem to have already implemented their decision to pass on the conces- sion to consumers as advertisements have been appearing announcing a reduction in prices. Exporl Zones The chairman of the Electronics Commission, Dr. M S. Sanjeevi Rao, has urged the foreign firms to consider relocation of electronic units in India from export processing zones in other countries. The wage rates in countries like Hong Kong, Korea and Singapore, where foreign subsidiaries have been set up to cater to the export market, have increased considerably compared to the wages prevailing in India. It would be profitable for those foreign firms to locate their units in Indian export processing zones. Dr Rao said while inaugurating a seminar on "Export processing zones as an instrument of export promo- tion". organised by the Trade Development Authority, recently Exports from the Santa Cruz Electro- nics Export Processing Zone crossed Rs. 1.000 million last year and by 1990. the exports would touch Rs. 3.500 millions. Currently. 65 units function in SEEPZ, Bombay. Value and output Electronics will become a forgotten industry unless a demand is created among consumers, according to Mr. S.R. Vijaykar, Secretary to the department of electronics. Research and Development facilities in electronic units should improve not merely to indigenous the imported equipment but also to understand the suitability and adaptability of foreign goods to Indian conditions. Mr. Vijaykar said while delivering the key-note address at a workshop on "Banking requirements of the Elec- tronics Industry" The seventh plan target of Rs. 10,000 crores of output set for the electronics industry should be viewed not in terms of value but in terms of output, he opined. To meet the objective of modernisation, accepted by the plan- ners, the electronics industry would have to become internationally com- patible both in quality and in price. Mr Vijaykar termed the plan alloca- tion of Rs. 4.000 crores to the communication sector as meagre and said at least double the amount was needed to achieve the pro- gramme of producing 100,000 micro- computers by 1 990- Dr. C. Rangarajan. Deputy Governor of the Reserve Bank of India, who inaugurated the workshop said the banks should take maximum care in financing electronic products as these faced a high degree of obsole- scence. He desired the development of a new financial norm to ensure that both the promoters and finan- ciers shared the risk equally. Mr. D.N. Ghosh, Chairman of the State Bank of India said the aim of the workshop was to understand the emerging financial needs of the electronics industry particularly in view of the envisaged massive expansion of investment under the new economic policy. High-Tech MECCA The Media Laboratory, a new experi- ment which began early in 1985 at the Massachusetts Institute of Tech- nology, is backed by more than 40 giant corporations with an invest- ment of 40 million dollars for the building and computers alone with another four million dollars for operating expenses every year. Media Lab's mission is to explore what computers could be doing ten or 20 years from now. It has diverse and -mind-boggling projects that range from talking computers to electronic newspapers, from compu- ter programmes for kids to computers The founders of the lab, Nicholar Negroponte. an MIT professor of media technology and Jerome Wiesner, president emeritus of MIT and science adviser to President Kennedy, have convcived a lab different from others. Here re- searchers buy off-the-shelf equip- ment and programme them for new uses in creative fields like broad- casting. publishing, motion pictures, music and theatre. None of the lab’s work is proprietary. The corporate sponsors can visit the lab for five years and learn anything they want of the various projects. Marvin Minsky of MIT who is called the dean of artifical intelligence, using computers to try to emulate human thought. Seymour Papert, the math and education professor of MIT. who developed the computer lan- guage. Logo, used by school children all over the world, Negroponte himself, an architect and pioneer in computer-aided design technology, are among the stafffers of the Media Lab. The Media Lab believes executives will find computers more friendlier in offices of the future. Instead of wrestling with keyboard and enig- matic codes the businessmen will be able to run the computers by talking to it. pointing to ir or even by glancing One of the research scientists, Richard A. Bolt wearing special glasses, sits facing as many as 40 television pictures, all running si- multaneously on a giant screen. Sensors on the glasses track where his eyes are looking and that information goes by cable to a central computer. If his gaze rests on a single picture, the other 39 recede and that one fills the screen. This exciting project is named Gaze Orchestrated Dynamic Windows project. The Conversational Desktop is Media Lab s talking computer. It can per- form secretarial tasks as making phone calls and reminding the boss of important meetings. Scientist Bolt has a wrist band with a magnetic sensor that can move an image from one location on a computer screen to another. As Bolt's hand moves through a magnetic field, sensors on the writsband sends signals by cable to the computer, relaying where on the electronics scene screen he is pointing. His eye. hand and voice systems could possibly enable a businessman to glance at a screen and have it display a report while talking on the phone. Another section of the Media Lab is busy programming a computer to play a synthesiser, following the tempo set by the conductor. When the conductor slows his baton, a sonar sensor following his hands tells the computer to have the synthesiser play more slowly. The synthesiser can react as quickly as a live musician and plays in nearly perfect synchronisation with a vio- linist and a flutist. Marvin Minsky wants to study what goes on in the mind when someone writes music or listens to it as he believes that understanding how people think about music will ulti- mately lead to smarter machines. Patrick Purcell, an associate pro- fessor of computer graphics, is developing a suit that emits infrared signals to be read by four light sensors connected to a computer which then generates a stick figure on its screen, copying every move- ment. The stick figure could be dressed in any way one likes. NHK, the Japanese counterpart of the BBC, has already picked up this technology and created a computer- animated host for a new show on the 21st century. Enter Teletext A beginning has been made in modernising visual communicatin medium, with the introduction of teletext service by Delhi Doordershan. The service will give information to viewers on finance, railway, airline timings, weather, major events in India and abroad and so on. This service is available from 9 a.m. to 2 p.m. and from 3 p.m. to 10 p.m. only. Criticisms have been made on the ground that this facility is not available to the black and white TV sets and even in colour TV sets, the viewer has to purchase a special decoder. Moreover, the decoders are now i mported and one does not know whether all the aspiring colour TV owners have been able to get the decoders. An additional query is on the choice of second channel for telecasting this service. Why not show it on the first channel itself and even TV authorities have no answer for this. Viewers in other cifi.es have been promised that this facility will be extended to them in a phased manner and one hopes that by that time the teething problems will be overcome. Electron Microscope A research team of Tottori university in western Japan has developed the world's most powerful scanning eletron microscope, with a resolution of up to five angstroms. An angstrom is one hundred'millionth of a centi- This microscope will provide a better view of the microcosm of antibodies which lend immunity to the human biological system, the genes and enzymes with their contents like Deoxyribonucleeic acid (DNA) and so on. The microscope is capable of magnifying object 800.000 times and researchers can see the rugged surfaces of ultra micro organism in three dimensions. This instrument, developed in colla- boration with Hitachi Ltd., can be used to examine precisely finished microchips, leading to the possibility of even higher intergration of Very Large scale integrated circuits. The high resolution of this new electron microscope has been at- tained by combining an electric on gun which shoots a very fine electron beam and a method to place samples in a special magnetic lens called high excitation objective lens. The pre- viously most powerful scanning electron microscope, also developed by the Tottori university, provided a resolving power of up to 15 ang- stroms, thereby yielding magnifi- cation by 350.000 times. New Entrants Murugappa Electronics Ltd., a new company has entered the capital market and its project is located at Hebbal industrial area in Mysore district. MEL is setting up a project to manufacture 2,350 million metres of 3.81 mm width high quality audio magnetictapes.lt has also registered for manufacturing 23 million audio tapes, using its own production. The cost of the project is Rs 530 lakhs. The company is expected to go into commercial production by April. 1986 and in the first full year of operation it hopes to market about five million cassettes, increasing to about 12 million at the end of the third year of operation. Computer Point (India) Ltd. which opened the first retail shop for all computer related items under one roof, has completed one year and is now entering the capital market. The compnay has retail outlets in Bombay Bangalore and Madras. It has com- menced a comuter education service called "Chip club". A new chain of computer retail shops called "Computer Shack" will soon come up in Bombay and other metropolitan cities, following in the foot steps of Computer Point. Ac- cording to Mr. Ashok Someshwar, 1 director of Computer Shack the initial project cost was around Rs. 60 lakhs. The unit would stock microcomputers in the price range of Rs. 5,000 to Rs. 2 lakhs. It would also sell packaged and custom-made software, com- puter media, peropherals, computer books and magazines. Snookered — by a robot The world's first snooker-playing robot is expected to be in action in about a year's time. The robot is the brainchild of scientists from Bristol University and London's Imperial College. They say that the robot will initially have fairly limited skills at the game, but there are plans to develop it further so that it will be capable of taking on top human players such as Steve Davis. The metal maestro of the green baize table will be programmed to learn the rules of the game and to study the position of the balls so that it can work out the best shots. It will then propel itself around the snooker table on wheels to play. The robot will have TV camera eyes and the sight will be fed into a computer which will operate as the robot's brain. Snooker is particularly well suited to robotic research because the game requires a high level of co-ordination between hand and eye. Scientists have long been trying to build such skills into robots for industrial applications. Ham Directory A Directory of Licenced Amateurs in India, popularly known as the Indian Callbook. updated till August 1985, has now been published. The last edition having been published in 1982. this new edition containing over 2000 entries will be found useful by all amateurs (Hams) and Shortwave Listeners (SWLs). The cover price is Rs. 10/- post free. If supply is required by registered post, Rs. 4/- should be added irrespective of number of copies in the order. The Callbook is available against prepayment only from RADIO. 3 Thiru-Vi-Ka Road, Post Box 725, Madras 600 006, to whom remittan- ces should be sent by Money Order or Bank Draft. This digital clock makes use of the giant displays published in the August/September 1985 issue of Elektor India, It has a face of 720 mm wide by 280 mm high, which makes it readable at distances of up to 100 m. The display alternately shows the time and the ambient temperature. JUMBO CLOCK by A Sevriens The circuit diagram in Fig. 1 shows that the clock is designed around well-tried ICs. The clocking frequency is derived from the mains: T, is provided with part of the sec- ondary voltage of Tr x and converts this into a suitable rectangular signal. Low-pass filter /?, -C 9 and monostable /C 2 , ensure a noise- free 50 Hz signal. This signal is divided by 6 in IC A , then by 5 and 10 in IC S , and finally by 10 in /C 6 . The signal at pin 12 of /C 6 is. therefore, 1/60 Hz, which is 1 pulse per minute Circuit /C 7 functions as a frequency con- verter: signals of 8V4 Hz, 1% Hz, and 1/60 Hz are applied to its D 4 , D 2 , and D 0 inputs respectively. When S 3 is in the centre — NORM — position, control inputs A, B, and C of the IC are logic low, and D 0 is then con- nected to output W. The clock is then sup- plied with normal minute pulses and operates normally. The clock may be set by switching S 3 to FAST or SLOW as the case may be. Clockwork The clockwork is formed by the chain con- sisting of four-bit synchronous counters IC n to IC X3 . The 0-outputs of these circuits give the counter position in 11-bit digital code, where Qa of IC U has the lowest value bit. Connections between the outputs and gate N 7 are so arranged that when the counter position reads 10111111111 (decimal 1439), the three ICs are reset to 0 This counter pos- ition corresponds to 23 hours 59 minutes. The clock can also manually be set to 00 hours 00 minutes with the aid of reset button ■Fa- Thermometer Circuit /C| 8 is the temperature sensor. Its temperature-dependent current causes a voltage drop across X n , which, after ampli- fication in A,, is supplied to digitizer IC W Provided P { and P 2 are adjusted correctly, the 0-outputs of /C| 4 have logic levels cor- responding to the temperature. The digitizer is clocked via gate N t . Decoding The 11-bit digital information as to time and the 8-bit data on temperature are applied to the A and B outputs of multiplexers IC a . . ./C 10 respectively. The signals at the A/B input of these three ICs determine whether the time or temperature infor- mation is provided to their outputs. The signals at the A/B inputs are derived from the clock oscillator, and arrange for a regular change-over at three-second intervals. The output signals of the multiplexers are simply used as addresses for EPROMs IC IS and IC,. Two EPROMs provide 16-bit data, and, since four digits are used for the clock, these are divided into four groups of four bits. At each address in the EPROMs now exists the relevant BCD code for controlling each of the four clock digits. Display How the outputs of the EPROMs control the individual display boards is shown in Fig. 2. Each of the display boards has a BCD to seven-segment decoder — see Fig. 4. This decoder converts the BCD codes into con- trol voltages for each individual LED element of the display in accordance with Fig.3. The RBI input of the left-hand display board is connected to the D 6 output of /C 16 : if this is logic low, the display cannot light (so that zeros are not shown in this position). The colon required for time indication is switched off by T t when temperature is displayed. The degree symbol is obtained by making the B-inputs of IC i0 logic high. This results in segments b, f, and g of the right-hand dis- play board being activated via the BCD code, and segment a via T 3 and the Z ter- minal. Connections to the collectors of T% and T 3 must still be made, because they were not provided for in the jumbo displays article in the August/September issue. These displays have a number of advantages: ■ they are entirely solid state, which prevents segment failure since the life of LEDs is much longer than, for instance, that of incandescent lamps: ■ they do not need intricate reflector con- structions: ■ if any one LED fails, they remain fully legible by virtue of the speciaal segment construction; ■ they are easily arranged in a variety of colours — red, green, blue, yellow, orange; ■ they work from 24 V with relative high efficiency, which keeps heat dissipation © © © It may be said that the large number of LEDs required is a disadvantage, but. in our opinion, this is largely negated by the advantages. The seven-segment display, shown in Fig- ure 4, is based on a type 74LS248 decoder, which has the same features as the well- known type 74LS47/247, but has in addition internal pull-up resistors and inverted output signals, so that external transistors can be used to cope with the large currents drawn by the segments. The inputs and outputs to the decoder, the read-outs, and the additional functions are correlated in Figure 3. All input and output controls have been arranged external to the decoder, so that they can be used in the same way as with normal displays. Wire link R-S serves to interconnect the earths of the + 5 V and + 24 V supplies. At the output of the decoder there is a switching stage for each segment that switches the relevant segment on or off. Each segment consists of four parallel groups of eight or nine LEDs in series with 1 , 20 . 1 . Fig. 4. Circuit diagram o a seven-segment display a current limiting resistor. The displays can be powered from a non- stabilized 20 ... 24 V supply. The current drawn per segment varies from SO mA to 100 mA. Figures lb and lc give the diagrams for displays with a ”1” and a respectively. Both can be used for a 12-hour clock. The ”1" display has provision for a lamp test (LT); open inputs are considered active, ie., the display lights. This is in contrast to the seven-segment display which treats inputs that are not connected as logic high, that is, inactive. As mentioned earlier, read-out boards con- sisting of several figures may be composed by mounting a number of displays side by side on a flat base. The whole may be pro- tected by translucent red perspex: this also acts as a light filter, which improves the legibility considerably. As you need a large number of LEDs, shop around for these because many dealers are prepared to allow a quantity discount. Uniformity of brightness of these diodes is not so important for this application, because at the distances for which these displays are intended, differences in brightness do not shown up. Power supplies Fig. 1 shows that the temperature process- ing circuits have their separate power supply, +5 A, provided by an additional voltage regulator Type 7805. This arrange- ment is necessary to prevent the analogue circuits being affected by the digital pulses in the remainder of the unit. The displays have their own power supply — see Fig. 5, which is not regulated. The sec- ondary voltage of Tr a was chosen at 2 x 18 V for red LEDs and good brightness. If green or yellow LEDs are used, or the displays need not be so bright, a secondary output of 2x15 V at 1.5 A will suffice. Construction and setting up The only problem in the construction is like- ly to be a wandering of concentration, since there are no fewer than some 2500 soldering joints to be made. It is, therefore, all too easy to make a dry joint. The clock itself needs no adjustments, but the temperature circuits need to be set up as indicated below. The temperature sensor is not yet fitted at this stage: in its place, connect a variable power supply (+ to the cathode terminal). The output voltage of the supply should be monitored with a digital voltmeter. As the LM355 provides a voltage of 10 mV/K, the voltage should be set to 2.53 V to simulate a temperature of —20 °C. 1.21 £ 06003000. ( ^j O ^ BO onnnonf i rp ooooooomn r .ooo onoo o. lOOQ O O g^ ftoooonoonof j Next, connect the digital voltmeter across pins 6 and 7 of /C„. Set the voltmeter to its most sensitive range and adjust P 2 so that the meter reads exactly 0.000 V. Then, set the power supply to 3.230 V, and measure and note the voltage now pertaining across pins 6 and 7 of /C 14 . Finally, connect the voltmeter between pin 9 of IC U and earth and adjust P x so that the meter reads exact- ly half the voltage noted before. Greater accuracy may be obtained by con- necting P t as shown in dashed lines in Fig. 1, and adjust this preset with the aid of dishes of water at exactly 0 °C and +50 °C. Now, fit /C| 8 into place. Finally, connect an analogue voltmeter between +5D and pin 3 of IC 2l , and adjust P 3 so that the meter reads about 300 mV. The clock should then operate normally. BBBBBBDB zero-modem connector ^ There is probably no other interface that gives so many problems as the RS232. Quite a few connecting wires are needed to ensure the correct links for all possible applications. The reason for this lies in the number of possible handshakes pertaining to the RS232 protocol. With modem equip- ment, many of these handshakes are no longer strictly necessary, so that a much simpler connection will suffice. At the same time, many idiosyncrasies of various com- puter manufacturers can be circumvented. The zero-modem connector proposed is based on the idea of reducing the hand- shakes. Each equipment provides its own handshake, while the connector looks after the interconnections of the data lines. There is then, of course, no longer any control between the individual units, but there is a correct data link. Normally, a cable is needed for connecting, for instance, a DTE (data terminal equip- ment) to a DCE (data circuit terminating equipment); the pins of,the connectors ter- minating the cable are then linked direct, i.e., there are no cross-connections. When two computers are inter-connected (DTE to DTE), some cross-connections are required: a few examples of these are shown in Fig.l. Fig.ld is the cross-connection used in the present zero-modem connector. It is, therefore, possible to interconnect two com- puters with the cable shown in Fig.la and the zero-modem connector. Construction Two D-2S shells are needed, of which the 113333 r I 15513233 The alarm described has been in continual use for over a year at a temperature of around —18 °C, which is, of course, quite normal for a deep-freeze alarm. Its function is to indicate an accidental rise in tempera- ture. There are, of course, indicators pro- vided on the deep-freeze unit, but as these are mains-operated they are of not muc'' use in case of mains failure! The principle of operation is quite simple: a green LED lights as long as the temperature stays within limits defined by the user, while a red LED shows when the temperature has risen above a critical level. Since operational amplifier IC, is arranged as a differentiator, two possible states ensue: (a) the output voltage is positive as long as the potential at the non-inverting input is higher than that at the inverting input, and (b) the output voltage is negative when the input levels are reversed with respect to those in (a). The voltage at the non-inverting input is derived from potential divider y? 2 -/? 3 -P| and is set by the user. The voltage at the inverting input varies with tempera- ture. The sensor is formed by the base- emitter junction of n-p-n transistor T,. which can be almost any type. The value of resistors J? 3 and P, depends on the transis- tor used. The values stated in the circuit diagram pertain to a 2N1711, a threshold tem- perature of —IS °C, and a supply voltage of ± 4.5 V. If more than a visual indication is required, the circuit may be used to control an additional audible alarm. When D 2 lights, transistor T 2 is saturated, so that its collector is nearly at earth potential. This transistor can, therefore, operate a small buzzer or deep-freeze alarm siren, or, indeed, anything else convenient C Sadot to you. The additional alarm must be con- nected between S+ and S— . If you are happy with the LED indication, transistor T 2 may be omitted and resistor R$ replaced by a wire link. If only periodic checks are to be carried out, the circuit may be supplied from two 4.5. . .9 V dry batteries via a spring-loaded push-button switch. Where permanent monitoring is desired, however, it is advisable to use two 6. . .9 V rechargeable NiCd batteries (without a switch). The cur- rent drawn by the buzzer or siren should not exceed 500 mA. 1.25 by J Steeman 8-bit I/O bus Robots are now being used in a variety of industrial processes, but they are very unlike their namesakes appearing in science fiction fantasy. Robotics is by now a generally accepted science, and constitutes an interesting meeting point for applied electronics, mechanics, and computer programming. To control a simple robot, a computer will need input and output channels, so there's another reason to build this expansion for the universal I/O bus. This circuit, together with the analogue computer input featured in the June 1985 Applications issue of Elektor India opens the Have your computer perform a useful task world of control and measurement tech- instead of playing games. Have it guard and nology. It enables the measuring and log- control all of your domestic appliances like ging of eight analogue and eight digital heating, telephone, aquarium, slide projec- channels, as well as the control of eight tor, etc. You may also build a computer digital outputs. All physical quantities, con- measuring device to check your loud- verted to electric signals by sensors, may be speakers or the entire hi-fi installation, measured by the computer, which checks These examples, of course, require a certain the results and takes corresponding action, amount of software to handle the data. The or corrects an occasional system fault via nucleus of such a program is the correct the eight outputs. This procedure is called supply of control data to the input and out- interactive control. put channels. BASIC programs with PEEKs There is a constant exchange of data and POKEs will be quite adequate for the between the measured quantities, the com- selection of these channels. The eight out- puter, and the controlled systems. The soft- puts are thoroughly buffered and may ware enabling such interactive procedures switch up to 50 V at 0.5 A by means of a may be written in BASIC. ULN 2803. 1 .26 elo The circuit The ULN 2803 provides an ideal interface between TTL levels and relays, electro- magnets, stepper motors, etc. with its high- current Darlington transistor arrays, which allow a peak current of 500 mA. All outputs are of the open collector type and diodes for momentary suppression of inductive surges are fitted internally. The maximum voltage is 50 V, so a variety of relays may be used to increase switching currents and voltages (e.g. 240 V). As can be seen in the circuit — Fig. 1 — , gates N 7 . . ,N lA are driven by two bistable ICs which latch the output data. The bit combination at outputs Q„. . .Q 7 is retained until: 1. the_RESET button is pressed. Bus signal NRST goes low, is inverted by /V 6 , and clears any programmed data in IC Z and IC 3 via their CLR inputs. 2. new output data is loaded by aPOKE. This involves bus signals SS, R/W, and 2. When the board has been selected by SS and R/W is on write, data from the databus is read into the bistables IC Z and IC 3 during a 2 cycle. 1 3. the mains supply fails, or the computer is switched off. IC 6 is selected by gates W, . . .W 4 . A low level at the G, and G2 inputs enables data transfer to the bus. This low level exists for the duration of a 2 cycle (high level), when the board has been selected^ by the SS (Slot Select) signal and when R/W is in the read mode (ie. high). The inputs of driver IC 6 have pull-up resistors and accept TTL levels. Construction If the ready-made PCB, available through our PCB service, is used, the construction of this I/O unit will present no difficulties. Moreover, no adjustments are required. More information on the I/O bus can be found in the June 1985 issue of Elector India. The robot There are many applications for interactive control, but undoubtedly the most appeal- ing is robot control, for the very reason that one sees what happens. We already have 1. Another circuit for universal I/O bus. It /ides control of eight 1 .27 the controlling elements such as computer, and down through an angle of thirty I/O bus. analogue input, and the present cir- degrees. At the same time, it can revolve cuit. Only the robot is missing. A prototype around its own axis, thus creating a cone- robot shown in the photograph was built shaped working area: Fig. 4. from Fischer Technik parts. Its movements The robot is able to move small metal may be limited, but it is eminently suitable objects to and from different locations for demonstration purposes. These mechan- within its working area, by means of a small ical parts may also be used to build other electro-magnet at the end of its arm. Two devices like a lathe, elevator, antenna rotor, motors move the arm: one takes care of the sorting machine, pantograph, or a solar cell movement in the vertical plane, while the tracking system. other revolves the arm around its axis. A set The robot's arm (Fig. 3) may be moved up of gears enables the spindle of poten- tiometer 1 to turn in line with the first motor, while potentiometer 2 is driven direct by the axle of the arm. Both potentiometers are connected between the +5 volts supply and ground, so that the voltages at their wipers vary between these potentials, according to the position of the arm. These two voltages may be read by the computer via its analogue input, and thus provide information about the position of the arm. A level of 5 volts is translated to the binary value 1111 1111 = 255 by the A/D converter. Thus, 2.5 volts equals decimal 128; 1.75 volts equals decimal 64; etc. These values are subsequently read by the computer with a PEEK command for comparison with the position to which the arm is to be moved. Motor control is effected by four output channels of the present I/O unit. Outputs 1 and 2 move the arm up and down, whereas outputs 3 and 4 move it in the horizontal plane. As far as their power consumption is concerned, the motors could be connected direct to the outputs, but this would preclude the possibility of reversing their direction of travel. For this purpose, a bridge circuit such as the one published in the August/September 1984 issue of Elektor India, page 8-98, will be needed. The electro-magnet is switched by output bit 8. Table 1 shows the eight output bits and their corresponding functions. Switching the electro-magnet on and off is achieved by addition or subtraction of decimal 128 to or from the decimal value of the lower four bits. The wiring of the robot is evident from Fischer Technik's documentation. Fig. 5 shows the interconnections between robot, computer I/O, and analogue input. Any other connection results, of course, in differ- ent bit combinations. For other purposes, more I/O units or analogue inputs may be added onto the bus. Finally, although the Fischer robot is an interesting demonstration model, clearly showing the workings of simple robotics, it is simply too small, too light, and lacking in precision for any useful applications. And yet, it offers a worthwhile comparison with an industrial robot, because technically it hardly makes any difference whether one moves tiny parts or heavy loads from one place to another. H hi I in pT" | | j | it 0 L 1 1 (X. a |0 jo to jo [0 [o [o jo = magnet off, arm holds POKE XXXX. 128 = |l jo jo jo [0 jo jo jo = magnet on, arm holds POKE XXXX. 134 j Zero phase shift between the drive units in an active loudspeaker unit I has been the goal of virtually all designers and constructors ever , since the first multiple speaker unit was conceived. For a long time, j it has been like trying to achieve a perpetuum mobile, but now it has become reality! phase-corrected cross-over filter -*»n 1. The simplest form Fig. 2. New techniques of laving only two RC sec- ions provides a 12 dB / ictave profile and ibviates a number of It is well known that the loudspeaker is the weakest link in an hi-fi chain: it is the final factor that determines how the hi-fi instal- lation sounds. The most serious problem is that presented by the processing of the wide range of frequencies. As long ago as the thirties, designers have tried to solve this problem by sub-dividing the audible fre- quency range and using a separate drive unit for each of the resulting bands. In the simplest case, this means that a bass speaker (woofer) is used for the low audio frequencies, and a so-called tweeter for the high audio frequencies. Right up to the 1960s, the network that divides the frequency ranges consisted of a passive filter constructed from chokes and capacitors. When semiconductors became less expensive, designers began to use active filters and to provide each separate drive unit with its own power amplifier that is fitted inside the speaker enclosure. Such active systems are generally better than passive ones, but they are also more expensive. But whether active or passive, filters create problems of their own. Problems with filters The simplest two-way dividing filter consists of a choke in series with the bass speaker and a capacitor in series with the tweeter — see Fig. 1. At the cross-over between low and high frequencies, both drive units are fed with the same signal, the level of which is about 3 dB below that of the nominal out- put at the input to the filter. Moreover, the 0 ." • signal at the bass speaker lags that of the input signal by 45°, while that at the tweeter leads that of the input signal by 45°. Because of the phase difference of 90° between the signals at the two speakers, the air pressures produced by them are added geometrically, so that the overall sound is as if caused by a signal that is identical to the original input signal to the filter. Provided, that is, that everything is ideal. Tolerances in the components, differences in the drive units, and effects of the enclosure prevent such an ideal state being attained. Even tolerances of 10 per cent can alter the situation quite a lot. If, for instance, the capacitance is 10 per cent smaller, and the choke 10 per cent larger, the levels at the two drive units are almost 0.5 dB lower than in the ideal case: —3.444 dB. The phase difference is also larger: 95.5°. The result is that the overall signal is almost 0.9 dB lower than the original signal. This may not seem serious, until the considerations concern higher-order filters that give a Bessel, Butter- worth, or Chebishev response. Such filters have a much steeper cut-off profile. Even small component tolerances then cause a reduction of a few dB in the available gain. The phase characteristic in these filters also has a steeper roll-off. Component toler- ances may cause such a large phase shift that the gain is reduced by another few dB. Finally, the loudspeaker characteristics themselves should also be taken into account. The (larger) bass unit is inherently somewhat slower in action than the (smaller) tweeter: they have different rise times. The difference between these times manifests itself at the cross-over frequency as an additional phase shift. Odd-order filters have a phase difference of about 90°. In a two-way system with a cross-over frequency of 1 kHz and a difference in rise times of 100 iis (a typical, practical value), there is an additional phase shift of 36°, resulting in a total of 126°. Even if all other parameters of the network are one hundred per cent cor- rect, such a phase shift results in a 2 dB loss. In even-order filters, the situation is somewhat better: here an additional phase shift of 36° causes only 0.5 dB loss. Solution The requirement is, therefore, for a filter that produces no phase shift between the loudspeakers, is not affected by component the filter. Note, however, that its phase characteristic has the form of that of a high- pass section; that is, the phase of leads that of the input signal. This means that there is no phase difference between the high-pass and low-pass branches over the entire frequency range. The dividing filter of Fig. 2 is a two-way ver- sion. It has a 12 dB/octave cut-off profile. Normally, such second-order filters have two RC networks in both the low- and high- pass branches. Problems may arise then if, owing to component tolerances, the time constants in the two branches are not the same. These problems are negligible in the set-up of Fig. 2, because, due to difference amplifier A,, the sum of outputs B and D is always the same as input A, irrespective of component tolerances. The cross-over frequency. f a , is defined as that frequency where both the low- and the high-pass output a re attenuated by 3 dB. This happens when aiJtC-\, whence f a =\/2nRC [Hz] (6) where R is in ohms and C in farads. Practical filter The foregoing considerations lead to a prac- tical filter, the block schematic of which is given in Fig. 4 and the circuit diagram in Fig. 5. It concerns a three-way version with a 24 dB/octave cut-off profile: its response resembles that of a critically coupled net- work, ie., there is no tendency of overshoot. The filter has two low-frequency outputs, which are inverted with respect to one another. This offers a simple push-pull amplifier for the bass drive unit, since this often requires more power than the middle- and high-frequency speakers. The input signal — Fig. 4 — is applied via a buffer to a four-stage RC high-pass section, | and is then available at the high-frequency Hl]0000}p* " ■ 00^C? (00000’ ea-p — ^o» t[ E 0SjT"" output. Two of the RC stages are designated a; the other two, b. This is done to clarify that the phase shift in all-pass section AP a is identical to that in the HP a sections; the phase shift in AP b is the same as that in sec- tions HP b — more about this later. The difference amplifier forms from its two input signals a low-frequency output, whose half power frequency, f a — see Eq. (6), is the dividing frequency between the high- and middle-frequency branches. This signal is fed to another four-stage RC high-pass sec- tion, and is then available as the middle- frequency output. The low-frequency out- put is obtained in the same way as the middle-frequency output. Circuit description Opamp A, is the input amplifier, whose low-impedance output provides the audio signals for the remainder of the circuit. Opamps A 2 . . .A s and A 9 . . .A )2 are buffers that decouple successive high-pass sec- tions from one another. Opamps A 6 , A 8 , A i3 , and A k are connected as all-pass sections with a leading phase shift characteristic. Opamps A 8 and A 15 are arranged as differ- ence amplifiers, while A !6 functions as an inverter. Construction There is no ready-made printed-circuit board available for the filter, but it should fit on a vero board, or similar, the size of half a Eurocard, i.e., 80 x 100 mm. Before the con- struction is started, the two cross-over fre- quencies should be decided. The relevant component values in Fig. 6 result in cross- over frequencies of 570 Hz and 3800 Hz. That is a frequency ratio of 1:6.7 — about two and a half octaves, which is a convenient value. The frequency ratio should not be allowed to be less than 1:4. The cross-over frequencies, f a , are calculated from Eq. (6). The next aspect to be looked at is the impedance of the RC networks. To ensure low thermal noise and minimum delays, all resistors should have values between 10 k and 27 k. As the opamps also contribute to noise (see, for instance Intuitive IC Opamps by T M Frederiksen, published by National Semiconductor), the TL 074 should be preferred to the TL 084. Capacitor values are calculated from Eq. (6) once the resistor values have been determined. Where absolute accuracy is desired, one per cent resistors should be used: these are much cheaper and more easily obtained than close-tolerance capacitors. However, in most cases five per cent resistors are per- fectly all tight, but it is preferable that resistors with the same letter indices, for instance, R ia and R^, or R vlg and R Mg , have identical values. It is, therefore, more econ- omical to buy, say, fifty 5 per cent 18 k resistors than thirty-two 1 per cent ones, and, with the aid of a digital multimeter, sort out equal-value ones: four groups of three- and eight sets of two identical resistors are required. Capacitors can be sorted in a similar way — see Fig. 6. Connect one of the capacitors to : TL 074 lithium batteries by Ernst Krempelsauer For more than ten years, lithium cells and batteries have been used in digital watches, pocket calculators, and pace- makers, but now they appear to be on the verge of coming into much wider use in other electronics equipment. Air iy, they are used for direct mou lting onto printed-circuit boards, and some ICs have already had them embedded inside their dual in-line packages. Because of their small size and extremely high energy density, lithium batteries are eminently suitable for use in miniaturized electronic equipment. They operate over a wide range of tem- peratures (typically —20 °C to +50 °C), have an exceptionally low rate of self-discharge, and maintain their e.m.f. within tight tolerances with normal loads throughout their life. Unfortunately, at present lithium bat- teries are not really suitable as direct replacement for conventional dry bat- teries: apart from their higher price, they do not stand up well to short cir- cuits, and are also easily mechanically damaged. Under certain cir- cumstances, they have a tendency to catch fire or even explode. None the less, as already stated, there are econ- omically priced, perfectly safe versions for use in a variety of small electronic equipment. Types of lithium battery Lithium, a silver-white metal that tar- nishes rapidly in air and reacts with water, halogens, nitrogen, and hydrogen, is. with a density of only 0.531, the lightest alkali metal. It is of particular interest as the anode material in a voltaic cell, where it pro- vides a higher e.m.f. (3.020 V) than other materials. Because of its reaction with water producing explosive hydrogen lithium poses problems, since it is not easy to produce an electrolyte that is completely devoid of water. Some- times acetonitrile. CHjCN, is used. This is a poisonous liquid, prepared from ethyne and ammonia. The choice of cathode material also has a decisive effect on the cell. This is the reason for the multiplicity of available lithium-based batteries and the differences in e.m.f. and energy density between the various versions. Basically, there are two main types of lithium battery: one uses a liquid or gaseous cathode, such as sulphur dioxide, S0 2 , or thionyl chloride, SOCI 2 ; the second has a solid cathode, typically manganese dioxide, Mn0 2 , and polycarbonmonofluoride, (CF)n. In general, solid cathodes are used in small to medium capacity batteries required to deliver relatively low load currents, while the other type of cathode finds application in larger capacity batteries that provide rela- tively high load currents. It is worth noting that the data sheets of all manufacturers given even more stringent warnings against misuse and abuse of batteries with liquid or gaseous cathodes than those given with lithium cells generally. SAFT, for instance, warns specifically of the danger of explosion and the pro- duction of poisonous gases in the use of liquid- or gaseous-cathode bat- Construction and properties Externally, lithium batteries resemble NiCd batteries rather than conven- tional dry batteries. Both spiral-wound and pressed electrodes are found, again as in NiCd cells. Spiral-wound electrodes have a larger operating sur- face and are, therefore, able to provide a higher current than pressed elec- trodes. On the other hand, cells using pressed electrodes generally have a larger capacity- to-volume ratio. Fig. 1 shows the construction of a typical, sealed, cylindrical cell with solid (CF)n cathode. Lithium batteries are available in cylin- drical, button, or special shape; the latter, for instance, as a nylon-enclosed memory back-up cell for direct mount- ing onto a printed-circuit board. It should be noted that there are appreciable differences in the characteristics of the same type of lithium battery produced by different manufacturers, and also between the various types. For instance, the data sheets of a number of manufacturers give low operating temperatures vary- “TT7 .35 S Fig. 10. Output voltage vs discharge time curves of a 9 V lithium thionyl Lithium thionyl chloride. LiSOCI 2 Liquid cathode; e.m.f. 3.5 V; universal type for load currents up to 2 A; operating temperature range — 40 °C to +75 °C; capacity up to 18 Ah; intended for use in measuring instru- ments and communications equip- operating under difficult conditions. Application and use It appears that not all types of lithium battery are suitable for general use yet. This has not so much to do with the price as with the care that needs to be taken by the user. Although short cir- cuits do not necessarily cause an explosion, account must be taken of the tremendous rise in temperature (to well over 100 °C). Under these condi- tions, pressure inside the cell will rise, causing the safety valve to open and health damaging gases to escape. Bat- teries (consisting of cell) are normally protected against short circuits by a fuse or series resistance. Lithium cells used as back-up for memory ICs should be provided with a protection diode to prevent any tendency to charge and also to avoid large discharge currents. Soldering direct to the battery ter- minals is not permissible. Many lithium bai batteries are, however, provided with soldering tags at their terminals, but even these should not be subjected to soldering heat for more than 10 seconds. The very low rate of self-discharge allows small charge and discharge cur- rents — of the order of a few jtA — so that these batteries may be charged from solar cells. At the time of writing, it is not known whether these batteries will become chloride cell at differc available in higher capacities and for higher load currents. Many lithium batteries can cope with the momentary short circuit during dip soldering, but normally require hours to recover their e.m.f. If a battery falls into the soldering bath, it may explode: it is, therefore, essential that it is securely fastened to the board or equipment being soldered. State of the art SAFT have produced a replacement for conventional 9-volt PP3 batteries that demonstrates the advantages of lithium batteries in an impressive man- ner. This new battery consists of two button cells of the LiSOCI 2 type, and thus provides an e.m.f. of 7 V. Since this voltage remains stable during the operational life, the battery is perfectly suitable as a substitute for a PP3. In contrast to most other lithium bat- teries, this new type stands up very well to short circuits; its temperature rise during such conditions rises only moderately. From a technical point of view, this battery would have to be recommended for any application requiring a 9-volt source. Unfortu- nately, at a retail price of well over £10, it is not going to replace too many PP3s just yet. Characteristics Nominal capacity Output voltage Recommended load current (for 50% of nominal capacity at 20 °C> Weight Operating temperature range Rechargeable lithium batteries Although much research has been ci ried out, a number of patents ha 1.36 been registered, and several pro- discharge cycles the capacity shows totypes have been publicized, there is, no signs of deterioration. at present, only one rechargeable lithium battery in production. This is a Characteristics Panasonic lithium carbon type, first Nominal 1 mAh introduced in early 1984. Production capacity (at 2 . - . 3 V) models are expected to become Nominal voltage 3 V available in Europe during 1986. Recommended 1 pA to 5 mA The most important characteristics are load current shown in the table below and in the Charging 1.5 to 3.0 V (con- accompanying charge and discharge voltage stantlwith current curves. These curves show the truly limiting resistance amazing property of this battery, Life expectancy At least 2000 which enables the output voltage to be charge-discharge freely chosen between 1.5 and 3.0 V cycles dependent upon the charging voltage. Diameter 10 mm As regards the life-span, the makers Thickness 2 mm claim that even after 2000 charge- Weight 1 .9 g surface-mount technology substantially reduced parasitic parameters, which is of particular importance in high-frequency circuits. Reliability is of prime concern to any engineer. Most component manufac- turers have run extensive reliability pro- grammes, which show that the re- liability of SMAs is better than that of conventional PCBs. Some hidden difficulties It must be pointed out that there are also unforeseen difficulties with surface-mount assembly, but only in existing system designs. These designs are often poorly partitioned for surface-mount realization, and this is hampering the introduction of surface- mount techniques by original equip- ment manufacturers (OEMs!. The difficulties originate in the current practice of including a mixture of con- trol and associated higher dissipation interface circuits on PCBs. As these higher power components are not yet available in surface-mount, the designer is left with the choice of either using the unpopular mixed print board with a combination of through- hole and surface-mount devices, or repartitioning the design into surface- mount and through-hole boards. Such repartitioning represents a major design investment, which is difficult to justify for an existing system and, therefore, tends to restrict the use of surface-mount techniques to new systems. chnology uses com- of the printed-circuit board, the holes, much smaller than and the cost of plating, together with es. These SMDs the increase of the board density and evices) have no or complexity, is the key factor of :ting terminals since surface-mount technology. This in to be soldered direct spite of the fact that SMDs are cur- ks of a circuit board, rently still more expensive than con- within five years half ventional components, cuits will use SMDs, SMDs have connecting terminals of her five years it will 1 mm or less, whereas conventional current type com- components require at least 2.5 mm. This means not only a 70 per cent srs engaged in the saving on board space (National Semi- y are already firmly conductor figure), but also one third of jrface-mount tech the internal semiconductor leads, ome about not only These two factors together result in Advantages of SMA (surface-mount assembly) Currently, the cost of an electronic assembly and not by the components. This has come about because compo- nent manufacturers have continuously invested heavily in the development of better, smaller, and cheaper compo- nents, whereas equipment manufac- turers have hardly changed their production methods since PCB assembly was automated. Relatively speaking, therefore, assembly costs have continued to rise, while compo- nent costs have become lower. The reduction in the number of layers Surface-mount devices The photographs accompanying this article show that SMDs, compared with conventional components, have a rather different appearance: miniature blocks and cylinders, tiny ICs with very short pins, and other unfamiliar exteriors. As far as their interiors are concerned, however, there are no basic differences other than that SMDs are generally of better quality than their conventional counterparts. An important aspect is that SMDs are designed to withstand immersion into molten solder. Rather than ask which components are already available in surface-mount technology, ask which ones are not yet available, because about eighty per cent of conventional components have a surface-mount counterpart, be they resistors; ceramic, electrolytic, or tan- talum capacitors; diodes; transistors; ICs; even inductors and LEDs are already available in surface-mount. Passive components Surface-mount resistors are available in values from 1 ohm to 10 megohms, with tolerances of ±5%; +10%; and +20%. The construction of such a resistor is shown in Fig. 2a. It consists of a rectangular ceramic carrier onto which a layer of resistive material is deposited that is cut to its exact value by a laser. The whole is glazed for pro- Ceramic capacitors are available in surface-mount in values from 0.47 pF to 1 p F. The value affects the dimen- sions, of course, and there are, therefore, quite a number of formats. All types have the same working voltage: 50 V (IEC standard). A typical construction is shown in Fig. 2b. Screened electrodes are pressed onto ceramic wafers, after which the wafers are pressed together, protected by foil, and then cut into small blocks. Finally, the two terminations are attached. Electrolytic capacitors - see Fig. 2c — come in values from 0.1 pF to 22 pF and working voltages from 6.3 V to 63 V. They are constructed from etched aluminium foils, which are separated by paper impregnated with electrolyte. The tubular aluminium case is provided with a polythene sleeve. The bevelled edge identifies the anode ( + ). These capacitors come in Tantalum capacitors come either as chips or in moulded form; values of the former range from 0.1 pF to 100 pF, with working voltages of 4 V to 50 V; the latter are available in values from 0.068 pF to 100 pF, and working voltages of 3 V to 50 V. A typical moulded tantalum capacitor is shown in Fig. 2d. Chip types have body coats of tough epoxy resin. Active components Virtually all current transistors and diodes can be produced in surface- mount technology without any diffi- culty: too many to describe in detail. There are general purpose, switching, high-frequency, low-noise, field-effect; and high-voltage transistors. Diodes are available from zener, Schottky, and 4 Fig. 4. Already there is ample choice of surface-mount ICs. Shown here is a typical example in a SO-14 package. switching to variable capacitance types. Various types of case are shown in Fig. 3: (a) SOT-23; (b) SOT-89; (c) SOT-143; and (d) SOD-80. integrated circuits Many familiar ICs are already available in surface-mount technology: digital as well as analogue; TTL as well as CMOS. Inverters; multivibrators; buf- fers; decoders; multiplexers; shift registers; voltage comparators and regulators; timers; phase-locked loops; video amplifiers; digital-to-analcque converters; stereo decoders; IF amplifiers; and many more. As far as cases are concerned, there are two basic styles, the lengt i of which varies with the number of pins. Fig. 4 shows an SMD-IC in a 14-pin 1.39 The quality of SMD-ICs is, like that of transistors and diodes, at least as good as that of DIL types, since the same type of crystals are used. The smaller package results in a lower permissible dissipation, however. Note that Hall-effect SMD-ICs are also available. Mounting SMDs For mass production purposes, SMDs are packed in blister tape — see Fig. 5. The tape protects the components and ensures that they are efficiently pro- cessed by the automatic mounting equipment. Mounting of SMDs in quantity production is, of course, fully automatic: that was the whole idea behind the new technique. It is beyond the scope of this article to give other than a brief description of this auto- matic process. The most commonly encountered pro- cess uses droplets of thermal harden ing epoxy glue which holds the SMDs temporarily in position. The glue may be applied to the substrate (the circuit board) or to the components. After the glue has been hardened, the compo- nents are attached to the board by two stages of wave soldering: one to ensure that all metal surfaces are pro- vided with sufficient solder, and the second to remove any excess of solder. Home constructors, of course, have no access to automatic mounting equip- ment and wave soldering baths, and they will, therefore, have to mount SMDs with small pincers and a mini soldering iron. None the less, it is advisable to glue the components in place prior to soldering. Glue should be applied with the sharp end of a pin. Soldering should be done very care- fully, and the tip temperature should be electronically controlled. A special surface-mounting solder cream is pro- duced by the Indium Corporation of America and is available in the UK from Dage (GB) Limited. SMDs may also be attached to the board with special conductive epoxy, which is, however, quite expensive. Where this epoxy is used, it should be hardened at 150 °C for not more than 60 minutes. A termination material that will not oxidize, such as gold plating, should preferably be used if the epoxy forms the electrical connec- tion between the component and the board. The future It is clear that surface-mount technology is not a whim that is forgotten tomorrow; it is the assembly technique of the future. It is also still in its infancy, and dynamic development will no doubt continue for some years. At present, the technology is really only suitable for automatic mounting equipment in factories, but, no doubt, equipment for the smaller producer Fig. 5. For mass production. SMDs are delivered in blister tape, i.e., a series of compartments separated by a thin polythene tape. The tape can be fed into auto- matic mounting equipment. SMDs are. however, also available in different packing. will become available in the Surface-mounting solder cream foreseeable future. It is to be hoped available from: that in the further development, the Dage (GBI Ltd • Intersem Division one-off producers, the hobbyists, will • Rabans Lane • Aylesbury • not be forgotten and then for them, Bucks HP19 3RG • Telephone (0296) too, there will become available 33200 suitable tools for use with SMDs. Sources: Transistors it diodes for surface mounting (ITT) SMD Technik (Siemens) Opperviaktemontage (Philips) Surface-mount Components (Sprague) Fig. 6. A surface-mounted board has a different appearance than a conventional PCS. This sample board houses a simple flashing-light circuit. 1 .40 elek.oc in battery fitness centre DC DC converter direct-voltage doubler economical power supply lead-acid battery charger mains interface mains power supply with prir mains voltage monitor negative supply converter power supply sequencing for simple zero crossing detector variable 3 A power supply voltage frequency converter 8 20 8 22 8 82 8 43 8 84 8.73 8.75 8 20 8 72 8 34 8 58 generators and oscillators dock oscillators (design ideas) programmable band-rate generator rectangular pulse generator The XR 2706 m the function genera function generator CMOS function generator HF electronic VHF/UHF aerial switch . light powered radio NAVITEX receiver RATTY calibration indicator RTTY/CW filter rend receive ident simple field strength indicator spot frequency receiver VLF converter 7HF/UHF TV modulator 8.48 5.18 8.44 8 36 8.79 8.66 8.77 8.58 2.26 hobby and car absorption-type metal detector 8.53 automatic car alarm 9 01 bicycle lights and alarm 8 31 digital joystick interface 9 05 qarage stop light 9 01 K I T T scanner 4 20 metal detector 8 35 model aircraft monitor 8 56 model railway monitor panel 8 94 remote model control with a microprocessor 3 32 revolution counter 5 20 service interval Inner 7 35 7400 Siren 1 58 simple sound effects 11 .48 burglar deterrei CH-boiler contr flashing light w four position to nlra red movement detector ntruder alarm LED direction indicator mams voltage monitor mams wiring locator metal-pipe detector programmable timer smoke and gas dector temperature regulator w zero crossing switch twin bell-push 904 827 8 28 8 93 8 50 8 50 8 32 7 18 10 16 8 97 8 72 8 59 8 28 6 20 8 95 8 27 8 75 8 43 8 29 840 miscellaneous and design ideas clock oscillators (design id« combining digital circuits designing a low noise amp DIY connector electronic pantograph fast opto coupler fast opto isolator gyroflash lumbo displays LEO direction indicator miniature running lights overload protection for elec play ball with Elektroi power supply sequencing f 8 80 8 70 3 39 7 42 8 66 8 98 3 20 8 88 8 97 12 56 8 23 8 97 12 40 1242 8 58 7 55 1022 controlled slide fader long interval tii multipurpose ti reading in bed X-Y graphic plotter economical crystal lime t temperature to voltage ct 4 44 5 24 2 59 6 52 1 .42 ele 1985 electronic VHF UHF aerial s< 'ight powered radio NAVTEX receive' RTTY calibration indicator send-receive ident Simple lieid strength mdicatr spol frequency receiver VLF converter VHF /UHF TV modulator 1 2 GHz input stage (March 85 p. 3.24) anodizing aluminium (Oct. 84 p. 10.46) aviary illumination (June 84 p. 6.36) daisywheel typewriter printer interface (July 84 p. 7.32) - ■ • • • • - • digital bandpass filter (Aug/Sept 84 p. 8.42) direct reading digitzer (Aug/Sept. 85) elabyrinth (April 84 p. 4.30) EPROM copier (June 84 p. 6.48) .... the first cuckoo in spring (May 85 p. 5. 35) floppy centering unit (Aug/Sept. 85) funny bird (Aug/Sept. 84 p. 8.82) a new keyboard for spectrum (April 85 p 4 38l RlC meter (March 86 p 3 501 speacli for microcomputer lApril 85 p 4 34) Reading in bed limiter (May 85 p b 53) versatile counter circuit (April 85 p 4 44) VI F converter (Aug/Sept 85 p 8 58) PL 301 (Oct 19851 0>gile< (Oct 1985 musical door bell (Aug/Sept 84 p 8 80) PARSER (Aug/Sept 84 8 94! capacitance meter (March 84 p 3 42l FM pocket radio (Aug/Sept 84 p 8 53) vaive amplifier (Oec 84| burglar deterrent (Dec 84 1 2X81 cassette pulse cleaner (November 84) real time analyser Ipart 1) (April 84 p how to recycle dry batteries (Nov 84 p 1 1 58 digital tachometer (Oct 84 p 10 35) •lash meter (Oct 84. p 10 30) informative articles artificial intelligence batterries and the envi capacitors and resistor digital graphic equalizer LMC 835 electric motors, drives, and controls electronics and medicine grand unification for and against moves towards a cashless society output amplifier 1C LM 1875 shmmg a light on new technology stepping motors the changing face of communications 2000 kilowatts under the sea viewdata m Britain voice recognition and speech processing wide band amplifier for satelhete TV receive give your soldering tip a longer Me with a pencil point toroidal transformers COS MOS digital ICS throwing some light on LEDS soldering aluminium out pul power nomogram selea 3 soldering techniques digi-course (chapter 31 4 5V battery eliminator computer tomography batteries m senes connection dry battery charger eaperiments with batteries 1.43 check list for electronic fault finding or 'where and how to look for what that doesn't' Before soldering in components ■ Check that the components agree with the parts list (value and power of re- sistors, value and voltage rating of capaci- tors, etc . . .)• If in any doubt, double-check the polarized components (diodes, capaci- tors, rectifiers, etc . . .). ■ If there is a significant time lapse between last reading an article and building the circuit, take the trouble to re-read the article; the information is often given in very condensed form. Try to get the most important points out of the description of the operation of the circuit, even if you do not understand exactly what is supposed to ■ If there is any doubt that some compo- nents may not be exact equivalents, check that they are compatible. ■ Only use good quality IC sockets: ■ Check the continuity of the tracks on the printed circuit board (and through- plated holes with double-sided boards) with a resistance meter or continuity tester. ■ Make sure that all drilling, filing and other ‘heavy’ work is done before mount- ing any components. ■ If possible keep any heat sinks well isolated from other components. ■ Make a wiring diagram if the layout involves lots of wires spread out in all directions. ■ Check that the connectors used are compatible and that they are mounted the right way round. ■ Do not reuse wire unless it is of good quality. Cut off the ends and strip it anew. After mounting the components ■ Inspect all solder joints by eye or using a magnifying glass and check them with a continuity tester. Make sure there are no dry joints and no tracks short circuited by poor soldering. ■ Ensure that the positions of all the components agree with the mounting diagram. ■ Check that any links needed are present and that they are in the right position to give the desired configuration. ■ Check all ICs in their sockets (see that there are no pins bent under any ICs, no neighbouring ICs are interchanged, ■ Check that all polarized components (diodes, capacitors, etc . . .) are fitted correctly. ■ Check the wiring (watch for off-cuts of component leads); at the same time ensure that there are no short circuits between potentiometers, switches, etc . . ., and their immediate surroundings (other components or the case). Do the same with mounting hardware such as spacers, huts and bolts, etc . . . ■ Ensure that the supply transformer is located as closely as possible to the circuits (this could have a significant influ- ence in the case of critical signal levels). ■ Check that the connections to earth are there and that they are of qood quality. ■ Check that any pins, plugs or other connectors used are making good contact. ■ Make sure the circuit is working correctly before spending any time putting it into And if it breaks down . . . ■ Recheck everything suggested so far. ■ Reread the article carefully and clarify anything about which you are doubtful. ■ Check the supply voltage or voltages carefully and make sure that they reach the appropriate components especially the pins of the ICs (test at the pins of ICs and not the soldered joints! ). ■ Check the currents (generally they are stated on the circuit diagram or in the text). Don’t be too quick to suspect the ICs of overheating. ■ If possible check the operation of the circuit in separate stages. As a general rule, follow the course of the signal. ■ Check the contents of any PROMs or EPROMs fitted. ■ While checking voltages, currents, fre- quencies or testing the circuit with an oscilloscope, work systematically and take notes. ■ It is always a good idea to do any fault finding as a combined operation with a friend, two heads are better . . . ■ Be wary of ‘red herrings' when fault tracing. Do the simple checks first. ■ Finally, remember our constant com- panion Murphy is looking over your shoulder. If that part of the circuit cannot possibly be wrong and you haven't checked it - that’s where to start looking. ■ ... And don’t forget to switch the power on and check the fuses! 1 .44 eteklo Channel multiplier for flat TV panel Scientists of the Philips Research Lab- oratories in Redhill, Surrey, have achieved a flat cathode-ray tube with a picture diagonal of 12 inches and nor- mal TV resolution. The depth of this tube is less then 3 inches. The first flat, sealed-off monochrome tubes have been made. The problems of gain stability have been overcome and an acceptable operating life can now be obtained. The feasibility of achieving full colour has been demonstrated. For the near future applications are expected to be in professional use System The flat cathode-ray tube (see Fig.1) consists of an electron gun, deflection plates, an electron multiplier array, a phosphor screen, and a faceplate that is vacuum sealed in a metal can. Because of the electron multiplier, the electron beam can be of both low cur- rent (less than 1 v A) and low energy (400 eV). The electron beam travels down the back of the tube to a revers- ing lens where it is turned through 180° into the front section. A central partition carries a series of frame deflection plates which create a field to turn the beam forward on to the multi- plier. The current from the gun is amplified several hundred times by the multiplier before the beam is accelerated to the screen. Because of the low primary beam energy and cur- rent, the scanning system can be un- orthodox. Vertical scan is achieved by progressively ramping the potentials on the frame plates. Electrostatic deflectors near the gun provide the line scan. State of the art Much progress has been made con- cerning the picture area and resol- ution. The spacing between the multiplier channel centres has been reduced to 0.55 mm, providing appr. 170 000 channels in the 305 mm diagonal display. The resolution of the screen image and the grey scale capability is appropriate for TV appli- cations. The spot size is such that the resolution of the tube is limited by the pitch of the channels in the electron multiplier. The main factor which determines the life of the flat display tube is deterioration of multiplier gain. Multiplier tests show that after 7500 hours of continuous operating the gain falls to 63% of its original value. Colour is important for many pro- fessional applications and several methods have been studied. The presence of the electron multiplier poses problems which are very differ- ent from those of a shadowmask tube. Colour selection can be carried out either before or after the multiplier. If the selection process takes place before the multiplier then one channel must be dedicated to each primary of a colour triad. This limits the maximum colour-display resolution to one third of its monochrome resolution. The Philips Research Laboratories are studying methods in which a system of electrodes on the output of the multiplier directs the emerging elec- trons onto phosphor of the desired colour. The ultimate tube design has one gun, and sequential colour selec- tion is therefore needed. The low deflection voltages and the high picture brightness make the tube particularly suitable for this mode of operation. Two methods are being studied, the dots-and-rings method and the deflection method. Dots-and-rings method The electron source inside each multiplier channel is a ring which is imaged on to the screen. A system of dynode-like electrodes at the multiplier output can be made to act as a lens of variable focal length, enabling the size of the image to be altered. The phosphor triads on the screen consist of concentric patterns in the three primary colours, which are aligned with the multiplier channels. The emergent electrons from each channel can be focused into a spot exciting the red phosphor, a ring exciting blue, or a larger ring exciting green. Deflection method With the deflection method, the screen consists of a pattern of phosphor strips in the three colours. A positive voltage applied to a dynode- like extractor electrode is used to draw the electrons from the final multiplier stage. They are then deflected on to the desired colour by pairs of strip elec- trodes located between adjacent rows of channels. The strip electrodes and the extractor electrode form an asym- metric lens which causes the electrons to be focused on the screen as an elongated spot. The results of both methods obtained so far are close to the requirements for various professional applications (such as data display) where flat screens are important; the possibilities for dom- estic applications are being studied. The practical work is carried out in demountable vacuum systems with small-area multipliers (2 by 2 inches). They now need to be developed into a large-area technology. The results described here refer purely to laboratory research; they in no way imply the manufacturing or marketing of new products. Electron gun Microprocessor navigation by Kevin Desmond The miniaturization and computeriza- tion of electronic navigational aids can only benefit today's motor yachtsman as he voyages through increasingly crowded waters and marinas. His means of knowing his speed, direction, and the depth have been made simpler, largely thanks to the sophistication of microprocessor con- trolled alphanumeric displays, which are easy to recall and read and are small enough to fit into a handbag. Take, for example, the Triton F.15 as developed by Baron Instruments (’). Here is a 15 function yacht instrumen- tation system, fitted into a console measuring a mere 234x171 x 50 mm. It is designed with the express purpose of easing the congestion of a whole array of instruments. The functions are boat speed; velocity made good; total log; resettable log; depth in feet; depth in metres; depth in fathoms; true wind speed; apparent wind speed; true wind angle; apparent wind angle; elapsed time; countdown timer 110 min); real time in hours, minutes and seconds; date (day / month). Digital display These functions may be recalled on a large digital display which makes use of a six digit, 25 mm, liquid crystal system. This is backlit for night illumi- nation and also fitted with an anti glare window. Another 14 light emitting diode indicators in traffic light red are positioned immediately beneath the display to denote the particular func- tion selected. Below that again are the function switches themselves, placed over touch sensitive membrane devices. Using the latest generation 8031 micro- processor, Triton F.15 works off a 12 or 24 V supply, consuming only 300 mA at 12 V. When the power is turned off, the clock calibration and alarm settings are retained by an internal nickel cad- mium battery, rechargeable off the ship's supply to give up to six months' back-up. Existing Baron water speed and depth transducers can be used with the Triton F.15 system. As with the voice advice system used on the Austin Maestro and other cars, so has Seafarer I 2 ) made it possible for the yachtsman to be "told" his depth, enabling him to focus his eyes on other tasks in hand. The Echovox talking repeat meter can be used in conjuction with the 110 m Seafarer 5 and the 183 m Seafarer 700 echo sounders. Clear voice Metric and imperial versions are available, offering presentation of soundings both through a numerical digital display and a variable volume, synthesized, clear English voice. Soundings are given in units and decimal parts of a unit in depths of less than 10 m. The voice repetition rate increases in shallowing water and a shallow water alarm signal can be pre- set to any depth up to 10 m. Echovox operates from a ship's supply of 12 V dc and measures a mere 158x168 x 75 mm. If you do not like voice synthesis, there is the Navsounder. as developed by Stowe Marine Equipment’ 31 . This is a microprocessor controlled digital depth sounder, with alarm settings that can be selected individually at either station - so that the yachtsman does not venture into water that is too deep or risk running aground. The liquid crystal display gives a digital presentation to a range of 100 m. In this particular unit, the eight-bit micro- processor, with 2 K of fixed random access memory, gives special capacity to selectively process and interpret acoustic signals. Secondary acoustic signals caused by turbulance, debris and fish are rejected and do not appear on the display. Deep and shallow alarms can be set to the nearest foot, fathom or metre, and allow for keel offset. Audible deep and shallow alarms are distinguished from each other by tone, with dashes for deep, and dots for shallow. When selected, the "anchor watch" function continually monitors depth and warns of abnormal changes. Universal sensor Navsounder operates from a ship's supply of 10 to 12 V and 80 mA, with 60 mA lighting when required. It measures 110x110 mm. Moving from depth to direction, devel- opment of the age old compass has certainly not stood still. This is evident in the Meteor Digitrac electronic compass system produced by Marinex 141 . The detection of the earth's magnetic field with a solid state, electronically damped universal sensor, enables up to five remote compass displays, both analog and digital, to be actuated without any extra circuitry. Among these displays are an analogue pointer display; an analogue head-up display unit with rotating card or grip pointer options; a digital display unit; and a microprocesssor controlled tape repeater with liquid crystal display. The Meteor system can also be used for satellite navigation, automatic direc- tion finding, and autopilot interface. The all-important universal sensor unit offers automatic compensation for changes in the horizontal field strength, and maintains absolute voltage control outputs. It is also con- stant over angles of dip of up to 80 degrees. This aids helmsmen of both power and sail boats to maintain an accurate course, even in very rough conditions. By allowing analogue and digital displays to be inter-wired, both helmsman and navigator can see what is happening, on different types of compass. Ship to shore Apart from human-controlled direc- tion, there is automatic control. With the Wheelpilot 4000, developed by Navico 161 , the functions of rudder ratio, sea state, and trim are entered via keyboard control. Each and every com- mand is confirmed on the liquid crystal display and by a bleep. There is also an off-course alarm, a port and starboard dodge facility, and a highly efficient gearbox. During the design stage, Navico researchers paid a great deal of attention to waterproof- ing, so that special seals and double gaskets were incorporated to function even in the wettest and roughest of sea conditions. Last but not least, the ability to com- municate ship to ship and ship to shore must be regarded as a vital aid to navi- 1 .46 eleklo gation. Navico has also come up with the first totally British designed and built mircocomputer controlled VHF radio telephone for yachtsmen. With its black casing 190 mm wide by 89 mm high, the Navico RT 6100 is also of handbag size. There is a scan of up to ten channels, each easily pro- grammed via a large keypad, with entry confirmed by both an alpha- numeric liquid crystal display and a bleep. There are also six private chan- nels, a fist microphone or telephone handset option, and a selective calling system option. These and other electronic navi- gational aids under development by small and enterprising British companies inevitably lead one to either dream of, or dread, the time, in the not too distant future, when computer- ized, voice synthesis, navigator robots will do all the work, leaving the motor yachtsman the uncluttered leisure of such pastimes as fishing, photo- graphy, and even sunbathing. (LPS) 7. Baron Instruments Ltd, 6 West Wycombe Road, High Wycombe, Buckinghamshire. England, HP11 2LG. 2. Seafarer Navigational International Ltd, Fleets Lane, Poole, Dorset, England, BH1 5BW. 3. Stowe Marine Equipment Ltd, 1 Bowes Hill, Rowlands Castle, Hampshire, England, P09 6BP. 4. Marinex, 77 Balena Close, Creekmoor, Poole, Dorset, England, BH17 7DB. 5. Navico, 49 Harbour Parade, Ramsgate, Kent, England, CT11 8LJ. Protecting computers from fraud by Cheryl F Williams In the enthusiastic rush to gain the full benefits of computers, few companies seemed to have thought of the disad- vantages and, in particular, the prob- lems of computer security. Computer crime is a new growth industry of the 1980s, with rich pick- ings being available. Many frauds are never made public and the increasing use of computers in routine commer- cial work offers increased scope for this type of crime. Even the most soph- isticated computer lacks the human attribute of commonsense and so any transaction that conforms to the com- puter's rules will be processed. Computer frauds require a knowlegde of the system's characteristics and could involve tampering with data, programs or software. The per- petrators vary widely from trainees to senior management, and the sums involved range from tens to millions of pounds. Heavy losses A study of computer crime by an American Bar Association committee says that organizations often did not know who had committed a crime; many did not know when a computer crime had taken place and could not monitor their systems to detect it. In a survey of 283 large corporations and government agencies about 48% reported some form of computer crime in the last year with losses conser- vatively estimated at between $145 million and $730 million. ' A recent "Washington Post" series on computer crime suggested that annual losses may be between $100 million and $3000 million. No one knows with any certainty how many millions are going missing, and the problem is snowballing. Open Computer Security 111 com- ments: "For every one computer crime that is reported it is estimated that probably a further 20 go completely undiscovered. Such frightening figures indicate two things. Coded messages "First, without the proper safeguards almost every computer is prone to this type of attack and, second, there are more individuals than you would imagine who have the required degree of technical knowlegde to carry out armchair robberies. Indeed, computer crime has become so rife and so lucrative that the head of the Scotland Yard specialist section in London recently predicted that, by the end of the decade, all cases of fraud would involve a computer." Open Computer Security goes to great lengths to ensure that its systems are secure. It says: "Due to the extremely high levels of encryption and verifi- cation which are built into our systems, not even we are able to over- come the security procedures inherent to our finished and installed product. Apart from putting a message into coded form there are special authenti- cation codes built into our security "This means that the host computer will accept instructions only from another computer that has previously been given clearance. Also, if the tamper-proof box is opened the memory is instantly wiped clear and the system goes into alarm. Every message that goes through the system automatically has an authentication code attached in it which tallies with the contents of the message. "This means that an instruction to transfer $1000 cannot be changed to $1000 000 — either by operator intervention or a fault on the line — without the change being high- lighted." Automated security Open Computer Security has designed and manufactured the authentication equipment for CHAPS, the Clearing Houses Automated Payment Scheme in London. CHAPS will replace the physical carrying of large cheques about the City of London by messen- gers. Approximately $37 000 million a day is handled between different clear- ing banks, the clearers and the Bank of England. CHAPS uses the data encryption stan- dard (DES) as its basic scrambling device and the session key, which is changed daily, is held in a tamper- resistant module designed by Open Computer. The module fits into the authentication unit attached to the tandem gateways in each of the clear- ing banks. The key itself is a series of random numbers produced by electronic noise and no one, neither the users nor Open Computer staff need know what it is. CHAPS has a way of identifying each bank's module to prevent substitution. The authentication unit is designed to detect and reject any message that has been interfered with. Many specialists believe that it is one of the most secure systems in the world and it is becoming a model for other financial institutions. One expert has commented: "CHAPS is more Award winner Open Computer Security has won the British Computer Society/Computing Applications Award for its Padlock system which was designed to prevent software piracy. Padline 7, a more recent development, is operative at all seven levels of communication as defined by the International Standards Organisation in its OSI model. This allows the unit to be used in virtually all computer networks whether public - such as X25 - or private - such as SNA. From the most basic physical layer - level 1 - Padline will perform through X25 layers right up to level 7. Here, encryption takes place under the con- trol of the user's actual applications software making this, the manufac- turer believes, the most secure method yet devised. This "end to end" level gives total network transparency under all communications protocols. Data encryption is via the accepted DES algorithm. Special circuitry in Padline allows the generation of truly random keys whilst the RSA public key provides a secure method of transporting the keys over non-secure networks, so that the user can confidently design a complete key management system. The codes used in Padline 7 are housed within one of the machine's microprocessors. To prevent the possibility of these codes being misused, the entire cabinet is designed as a totally sealed unit. Padline is without vulnerable air vents and, if it is tampered with, the memory is instantly wiped clear and alarms are triggered. Data can be loaded into the unit via its cassette input socket so the user can upgrade Padline on site — perhaps to operate on a different communications level — from a supplied tape. Double locked For security reasons, field upgrades can only be carried out in the presence of both keyholders. Additionally, the tape (which is prepared to order) is programmed to match only the par- ticular Padline for which it was Racal-Milgo 121 has designed the Datacryptor II which operates by rear- ranging the digital bit pattern of infor- mation into an indecipherable stream. This is achieved with the DES algor- ithm. By using this in a single bit cipher text feedback mode, Datacryp- tor II achieves a high degree of secur- ity with true protocol independency. Both asynchronous and synchronous | protocols can be managed by Data- cryptor II operating in full or half duplex modes over point-to-point and multidrop networks. The unit will also operate over leased line or dial-up cir- cuits. A point-to-point link merely The Ri involves two Datacryptor II units — one at both central and remote sites. Each unit has a small, hand held, removable memory device known as a key transport module (KTM). The KTMs are normally double locked inside the Datacryptor II units and without them the system will not operate. Master key The Datacryptor II has randomly generated 64 bit keys. The user does not see these keys or have any influence over their generation. The initial or master key is generated at the central site Datacryptor and is loaded into two KTMs. One KTM is then transported and loaded into the remote site Datacyptor and the other is left at the central site unit. The central Datacryptor then generates a working key which is down-line loaded to the remote site. This operation is quickly and simply carried out at the central site Datacryp- tor II front panel. Working keys may then be changed as often as required - at both ends of the link — simply by pushing a button at the central site. The master key is also easily and quickly changed simply by generating a new key for loading into central and remote sites. Down-line key loading provides an additional degree of security against the presistent line monitor attempting to determine the working key. This -dual level of keys therefore ensures a higher level of security for sensitive data networks and avoids costly and time consuming procedures of con- stant transportation of keys. All key management and control func- tions are performed at the central site Datacryptor where key generation and down-line loading are also carried out. All controls and the KTM are double locked behind a front panel which can only be accessed by the operation of two security key locks. Information rejected Once accessed, the controls consist of three pushbuttons, one to initiate the master key, one to copy this infor- mation into the second KTM, and one to generate down-line loaded working keys. The front panel LED indicators display transmission and security status. Test indicators allow checking of keys and the data link. Datacryptor II is housed in a robust and secure package. Should any unauthorized access to the unit be attempted, the anti-tamper switches will automati- cally erase the keys, disabling the system, and maintaining the integrity of the data network. In the event of a power failure, the working key and module security code are protected by a nickel-cadmium bat- tery for a minimum of 1000 hours. As a standard feature, Datacryptor can be securely mounted on a desk top or in a rack. Removal requires the use of two keys. Datacryptor can be en- hanced by the expansion module option which provides a module security code. This allows the user to provide each remote ’ site with an individual address so that, unless the correct KTM is received, the key infor- mation is rejected. 1. Open Computer Security Ltd, 31-32 High Street, Dorset Place, Brighton, East Sussex, England, BN2 1RP. 2. Racai- Mi/go Ltd, Richmond Court, 309 Fleet Road, Fleet, Hampshire, England, GU13 8BU. - 4 After the preliminary and theoretical con- siderations of the past three months, this fourth article in the series deals with the’ construction of the main printed-circuit board. The board is Euroformat (100 x160 mm), double-plated, and has through-plated holes: clearly not a card for home production. It is not absolutely necessary to have read through the three previous articles, but it does help. It should be noted, however, that the circuit diagram was published in Part 3 high-resolution colour graphics card (December issue), and this, of course, is essential knowledge. The construction of the card is not all that difficult, particularly since there are no adjustments or cali- bration. None the less, a beginner in this type of work will almost certainly experience difficulties if it comes to fault- finding. A really good soldering iron is required, preferably with a temperature- controlled tip, which should not be heavier than indicated in the photograph. Because of the thinness of some of the tracks, the card may easily be damaged beyond repair if too much heat is applied to it. The ICs may be mounted in good-quality sockets, but, at least as far as the dynamic RAMs are concerned, it is better not to. If any faults manifest themselves, do not immediately suspect the ICs: experience shows that in the initial stages most faults are not caused by faulty components, but rather by suspect workmanship. If, in spite of all this, it is found that an IC is at fault, just cut off the pins, remove the body, and then unsolder the pins from the holes in the board. If you have a desoldering device available, so much the better. Although the board is a very reliable com- ponent, it often pays to inspect it carefully (and possibly with a magnifying glas) for hairline breaks in the tracks. This can save a lot of tedious work later. Fitting the components First, fit the five wire links. Since these carry a reasonable current, they should be made of relatively heavy insulated copper wire. The GDP (graphics display processor) is best fitted in a really first-class socket. DIL switches 5, . . .5 8 are soldered direct to the board. Make sure that the quartz crystal used is housed in a HC18U or HC25U case, and that its frequency is suitable for the GDP used (14 MHz for the 9365 or 9366, and 12 MHz for the 9367). Where available, 7?, . . .i? 8 , 7f 9 . . ,7? 15 , and T? 27 . . .7? 34 should be bought as ready-made networks, which are easier to handle. Capacitors C l0 . . .C m are not yet fit- ted: more about this later. Connector K, may also be omitted for the moment, as it is not required until the colour extension is added. Movable wire links A-B, C-D, and I-J are best made with PCB pins and 2.54 mm matrix shorting sockets. Links K-G, K-H. E and F, on the other hand, consist of stout wire or of normal soldering pins — see also Table 8. Once this done, a first test should be made to verify that the supply voltage is present at the IC sockets or relevant soldering ter- minal on the board. Note that in the case of IC V . . IC U the +5 V line is connected to pin 8, and the return (earth) line to pins 1 and 16. Next, oscillator /C 28 and the address de- coding ICs (/CV-./Cy should be fitted. When the supply is connected, pin 8 of /C 28 should produce a clock signal of 12 MHz or 14 MHz, depending on the crystal. When the decoding address for the graphics card is known, write to it the highest-value byte with the aid of 5, . . .S a . In case of address range ElXX hox , the situation shown in Fig. 19 then pertains. As soon as an address from this range appears on the address bus, output P = Q (pin 19) of /C, goes low. Pin 9 of IC 2 must be low at address XXSft while pin 10 should be active at address XX69 As these addresses are present on the bus of the microproces- sor for very short times only, it is impossible to detect them with an oscilloscope. It is, therefore, better to construct a small instruc- tion loop to produce the wanted addresses cyclically. When the address decoding has been tested in this way, buffer /C, and registers IC 6 , IC„, IC a . and /C 13 can be fitted. It is, of course, not permissible that these ICs affect the computer that controls the graphics card. Next, /C 29 , !C X (programmed PROM), and /C 16 should be fitted. After this it shou l d be verified whether signals STR, RAS, CAS, CK, LD. and A7X are present at by P Lavigne £f D Meyer 1.49 .51 the output of /C !6 and the inputs of the rel- evant ICs — particularly pin 1 of IC 5 (which has not yet been fined). The timing diagram of these signals is given in Fig. 20 (see also Fig. 16 in the November issue). The card is now in such a stage that the most important components can be fitted: graphics display processor IC, and memories /C| 7 .../C 24 . as well as all the other ICs, such as PROM IC„. Before any further progress can be made, a monitor is needed. If this has a separate SYNC (link A) or SYNC Oink B) input, carry on. If, however, it needs a composite video signal, it is necessary to first build the mixer stage of Fig. 21. When the supply is switched on (power-on reset), nothing will initially be visible on the screen, because the image depends, among others, on the decoupling capacitors on the power lines to the dynamic RAMs. These capacitors, C 10 . . . C u are not fitted on the component side of the board, but direct at the supply connections (pins 8 and 16) of the ICs as shown in Photograph 1. It is very important that the leads of these capacitors are properly insulated. Without these capacitors, the + 5 V line would be badly affected by the current pulses which are so typically produced by the RAMs. There are, therefore, sound reasons for fit- ting the capacitors at the track side of the When, after the capacitors have been soldered in place, no image at all appears on the screen, this is normal. It may also be that there are some vertical lines visible. All that is not so important; what is, however, is that the screen image does not change after the power has been switched on. It is also advisable to check the supply to the control computer before and after the graphics card has been switched on. The current should be of the order of 4S0 mA, but may vary widely from this figure, depending on the screen image. The only IC that may get slightly warm is the GDP: all the others should stay fairly cool. When power is switched on, the registers of the GDP may be read; they should show: XX50: 07, 05, or 0D XX51: 00 XX53: 03 in hexadecimal, of course. It is now possible to carry out a simple test: • enable the write mode of the screen memory by writing 00 to addresses XX66 and XX64. • Next, write 0C to XX50 in the command register of the GDP. • If the contents of that register is 03 at XX53, the screen will go white. The screen is cleared by writing 01 to XX64 and giving the GDP the command 0C at XX50 If everything is in order, the screen should now turn black. Note that the com- mand 0C at XX50 cannot be read, since at this address of the graphics processor writing accesses the command register; and reading, the status register. If nothing happens on the screen, either the graphics processor has not received any instructions, or the logic levels on the WRlS and DIS lines were not corr ect fo r a write operation. In either case, the WRIS line must be logic 0 to enable the memories to be accessed, while DIS must be logic low to light the pixels, and logic 1 to quench them. It rpay be that the output-register has not received signals HCK and SH/I required for its proper operation. It is also nece ssary to verify signal STR at IC 7 , signal RAS for col- lective accessing at pin 14 of 7C| 0 , and signals RAS for individual accessing at pins 1 ... 7 and 9 of IC m . The slightest short- circuit or bad contact at one of the signal lines can disrupt or even disable the whole If something has gone wrong, a systematic search and verification of the various signals will soon show where the fault lies. For instance, if signal CK is not present at pin 1 of /C 6 , the device wil l not work. The same applies if signals RAS, CAS, o r HCK a re no t present. If one of the signals RAS0. . .RAS7 is missing, one eighth of the screen does not function; the remainder operates nor- mally. If one of the DAD lines is short- circuited, the card will only function partly: the extent of the malfunction depends on the binary loading of the relevant address The information given here, particularly that in Table 7, will enable anyone with some 1 .52 elekto experience in electronic construction to build the card satisfactorily. The colour extension will be dealt with in a forthcoming article. Remember that plug K, will provide the connection between the black-and- white card and the colour extension. The wiring to this connector is shown in Fig. 22. Until the colour extension is there, the plug is useful in providing test signals. Choosing the GDP and memory ICs As the EF9365, EF9366, and EF9367 cost roughly the same, it is best to buy the EF9367. This is the latest model and also the most efficient; moreover, it can be used in interlaced as well as in sequential scanning. In the present graphics card, it permits the following modes of operation: 512 x 256 (sequential scanning), and 512x512 (interlaced scanning) It also offers the possibility of providing 1024x512 pixels, but this is not used in the card, mainly because this mode of oper- ation demands a very high quality monitor, particularly as regards bandwidth and res- olution. The EF9366 is also an excellent device, but it cannot work in interlaced scanning mode and its resolution is, therefore, limited to 512 x 256 pixels in the present card. In most cases, this is, however, perfectly satisfactory. It should be noted in this context that a ver- tical resolution of 512 pixels in interlaced mode requires a good-quality colour moni- Choosing the correct memory ICs is also important. In theory, they should be fast, say, 150 ns access time or better, so that the RMW mode functions properly. Practical experience with the GDP has shown, how- ever, that in many instances access times of 200 ns or even 300 ns do not pose particular problems. Moreover, these ICs do not get a refresh pulse on A 7 and A, 5 , and pin 1 (which is earthed) does not receive a refresh clock pulse. Table 9 shows all suitable types as far as they are known to us (column YES); those in column NO are definitely NOT suitable. The types in brackets in the YES column have an access time that, theoretically, is too long for the RMW mode. Photo 2 shows ICs of the Japanese firm Fujitsu as used in one of our prototypes. M .53 telephone exchange by J Steeman Nowadays, there is a variety of inexpensive, yet sophisticated, telephone sets on the market. Not all of these are permitted to be connected to the British Telecom network, however. None the less, two or more of such sets may be used to form a simple, but effective, internal telephone system for the home, an office, or anywhere where a number of people want to communicate from different locations within the same building. The proposed system may, of course, also be built around British Telecom approved sets. Note that the system is intended for up to eight sets each of which generates a pulse code when a number is dialled or keyed in. Facilities A telephone exchange does, of course, more than just connecting one set to another. In fact, this is about the only thing it need not do, because the set with which communication is required is already accessed by the pulses generated when the relevant number is dialled or keyed in. What the exchange is required to do is: • to decode and process the pulses generated by the telephone sets; • to generate a dialling tone; • to generate and pass on a ringing tone; • to interconnect sets as soon as the receiver is lifted; • to prevent a third set listening in; • to generate and pass on to a third set an engaged tone. In addition, the system allows communi- cation between two sets to be established in two ways: • by dialling or keying in the required number and waiting till the other set responds; • semi-automatic: when the receiver of one 1.54 elekto set is off the hook, and the receiver of another set is lifted, the two sets are inter- connected, even when no number has been dialled. The exchange is provided with LEDs that show at all times which of the sets, if any, are engaged. A ninth LED indicates whether the exchange is engaged or not; this only goes out when the communication has been ter- minated, i.e., when the two relevant receivers have been replaced on their rests. All sets are powered from a common source via the standby and speech lines; the con- nection between each of the sets and the exchange is, therefore, in many cases poss- ible, via two lines only. The bell voltage is placed on the speech line via a relay. Calling one set from another is done by simply dialling or keying in the number of the wanted set. ie. 1 ... 8. Circuit description Since the telephone sets can only be con- nected to the exchange — see Fig. 1 — via the interface shown in Fig. 2, it is important to know how many sets the exchange will control before all the parts are bought. If, for example, only three sets are envisaged, the relevant part of the circuit in Fig. 2 needs to be built only three times. If, however, the full capacity of the exchange is used, eight interfaces are required. As soon as the receiver of a set, say, number 1.55 1, is taken off the rest, the relevant transistor, here T x , is switched on, so that the output of N x goes high. After a time determined by the RC network at the input of N z this gate toggles and its output goes low. If now from set 1 a number is dialled, the output of JV, will toggle in rhythm with the pulses produced by the telephone set. Because of the RC network at its input, gate JV 2 will not follow suit: its output remains logic low during the dialling of a number. The low logic level indicates to the exchange that the receiver is off its rest. As soon as the receiver of one of the other sets is also taken off its rest, the output of com- parator /C I8 goes low, which renders all other sets inoperative. How this is achieved will be reverted to later. The pulses generated during the dialling of a number trigger monoflops MMV, and MMV, via one of the lines D, D 8 . and also serve as clock signal for /C 5 , a counter with ten outputs. The contents of this counter, i.e., the dialled number, is only accepted by bistables FF X ...FF 9 if two conditions are met: (a) only one receiver is off its rest, and (b) FF-, is not generating a ringing tone. As long as pulses keep arriving at pin 11 of MMV |, the 0 output of this monostable will remain high. When this pulse train comes to an end, a short pulse is provided at the 0 output (pin 6) of MMV,. This pulse sets FF a (which generates the ringing tone) and clocks bistables FF . . ,FF a , depending on the output code of IC S . The wanted set is then connected to the speech line via its associated relay. At the same time. N zo (an oscillator with a long T and a short ‘0’) inter- mittently connects the bell voltage onto the speech line via contact re 9 (see Fig. 2). The wanted telephone will then ring until its receiver is lifted. To ensure that a third set cannot listen in, the logic levels at the Q outputs of FF . . .FF S are held: this is done by making both the set and reset inputs of these bistables low when the receivers of two telephone sets are off their rests. The set inputs are made low via /C 18 : the output of this opamp is low when two telephones are interconnected. The output of Schmitt trigger N zx is then high, and since this output is connected to FF, ...FF e via NOR gates N,, N x , the set inputs of the bistables are low. As long as two receivers are off the hook, the output of N 39 is logic high. The output (pin 2) of N 23 , and consequently the reset input of bistables FF X . . .FF t , is then low. An engaged tone, generated by gates N, q and N 2 6 in combination with transistor T l0 , and applied to the wait line, indicates that the exchange is busy. This tone generator, as well as the dialling tone generator con- sisting of N n , JV IB , and T 9 , is actuated by FF X o as soon as the receiver of any one of the sets is lifted. The dialling tone generator is provided to indicate that the exchange is processing a number: is has no connection with the actual dialling pulses. In fact, as soon as a dialling pulse appears on one of the D lines, the dialling tone generator is switched off immediately by FF I0 . Semi-automatic operation is achieved as follows. As stated, bistables FF, . . ,FF a are rendered inoperative when two telephone sets are communicating. When only one receiver is off the hook, the output of N 2X is low, and the bistables can still be accessed. When in that condition a second receiver is lifted, it takes a second before the bistables are really inoperative, and the two telephones are interconnected. Note that it is not necessary in this case to dial a number. Power supply The + 15 V power supply is provided to the exchange via the speech and standby lines. The bell voltage — here chosen at 2 x 18 V — is also applied to the speech line, but in this case via relay Re a . Transistors T lx and T 22 ensure a high supply impedance to prevent attenuation of the speech signed. Construction As no presetting or alignment is necessary, the exchange may be fitted in a suitable enclosure as soon as the wiring of the PCB shown in Fig. 3 has been completed. The telephone sets are connected to terminals a, . . .a 8 and b, . . .b 8 on the board respect- ively. Note that British Telecom approved sets need a four-wire connection to the exchange, because their bell circuits need to be connected separately to the standby and speech lines respectively. The bell wires in these sets are coloured red and green, while the other two are blue and white respectively. Finally Because of the RC network between gates N x and N 2 (or AT 357 and A r , 68 ) in the inter- face circuit of Fig. 2, the bell rings briefly when the wanted extension picks up the receiver. This could have been eradicated, but it was not thought that the cost of the additional electronics required was justified by this very minor flaw. K In a stabilized power supply the dissipation in the stabilizer may become very high when the difference between input and output voltages is large. This phenomenon occurs particularly in stabilized mains supplies, and can be remedied by lowering the secondary | voltage before the stabilizer. The suggested circuit here does this in a neat manner by making it possible to select either the full or half the I secondary voltage. And that with only a few components! dissipation limiter switches transformer secondary Z. Paskvan Figure 2. This diagram shows the effect on the voltage applied to the rectifier circuit when a base voltage. Ug. is applied to T2. Figure 3. To get the full secondary output voltage a voltage of 1 . . .10 V must be applied to the base of T2. Rectifiers D1. D2, Thl. and Th2 are ther connected in a bridge configuration. The stabilizer in a power supply may get very hot indeed when the output current is high and the output voltage is low, because it alone has to dissipate the dif- ference between the input and output voltages. It should therefore not come as a surprise when the device gives up the ghost. Some supplies are fitted with a switch that enables the lowering of the secondary transformer voltage, u s = Ui + u 2 , in such situations, so that the stabilizer need not dissipate so much power. The proposed circuit shown in figure 1 also makes it possible to lower the sec- ondary transformer voltage but in a rather special way. With a centre-tapped trans- former it is possible to halve the output voltage, u Q , by switching the rectifiers from full-wave to half-wave rectification. The switching is effected without the use of a mechanical switch: all that needs to be done is to give the correct instruction and even that could be automated! From full-wave to half-wave and vice versa The two parts of the secondary winding of the transformer must be in series to give u s = U) + u 2 . All that is necessary to ensure this is to apply a voltage of 1 ... 10 V to the base of T2. Both that tran- sistor and T1 then conduct: silicon- controlled rectifiers (SCRs) Thl and Th2 are consequently switched on via Rl, Rl', D4, and D5. The SCRs together with D1 and D2 now form the wanted full-wave rectifier in a simple Graetz circuit as shown in figure 3. Diode D3 ensures that in this configuration the secondary wind- ings are not short-circuited via the centre tap and one of the SCRs. When it is required to halve u s , the base voltage of T2 should be made zero. The SCRs are then off and rectification is car- ried out by D1 and D2 only: this is half- wave rectification. This situation is shown in figure 4 and illustrated in figure 2. You will see that the peak value of u s rises appreciably when a base voltage is applied to T2, and drops to u, = u 2 when the base voltage is removed. No values have been given in this article because these will of necessity depend on the type of supply used. All compo- nent ratings, particularly those of D1 and D2, must, of course be chosen to comply with your particular requirements. 1 i — | i 1 <3) -T ©*- LFJ -0-04 Mr ™.T» TIC 1060 II™, ■ 5A TIC 1 1SO liman ■ 8 A TIC 126D llmii ■ 12 A jjisk&ss |aaA Aa .. j — 1,„". 3 f ! _ 4 i i y. y Q Pi 4* 1 .60 elector India January 1986 selex 8 Digi-Course II Chapter - 2 In the last chapter of Digi-Course II we have seen how two NAND gates can be connected to make a simple R-S Flipflop. The circuit and its truth table is again reproduced here for reference. For complex memory applications like data storage, data processing, calculations which require large quantities of data to be handled, we require extremely large quantities of storage cells like Flipflops. Each Flipflop contains one bit-which is either zero or one. Integrated Circuits which serve this purpose are commercially available. One such 1C is the popular 6116 memory 1C, called a Static RAM. This Chip contains more than 16 thousand Flipflops. These Flipflops are internally arranged in such a way that they can be accessed externally using just a few pins. (16 thousand pins are not requiredl) Nowlet us see another variation of our basic Flipflop. Connect two more NAND gates as shown in figure 3. As we have already seen, the last line of the truth table is ambiguous. Its relation is not defined in isolation, but the previous history of the outputs is involved in deciding which of the outputs will be 1 and which will be 0. The gate which had a ' O’' input prior to going on "1 "Retains a "1" at the output. By placing a "0" on theS input (set input) the Cl output LED is turned on. By placing a "O" on the R input (Reset input) the Q output LED is turned Off. A practical application of this simple Flip flop circuit is the game of skill called "Old Shatterhand". t+iis game tests how steadily you can move your hand? Our R-S Flipflop works as an impartial judge of this game. The basic idea is very simple. A metalic ring is passed over a complexly bent wire such that the ring surrounds the wire but does not touch the wire at any place. If the ring touches the bent wire at any moment, that player has to drop out. Whether the ring touches the wire or not. is faithfully recorded by the R-S Flipflop. The bent wire is connected to the ground terminal on the Djgilex board and the ring is connected to the input S (Pin K1 3) At the beginning of every round, the R input (Pin L 10) is momentarily shorted jo the ground terminal. This gives ”0" input to R, thus resetting the Flipflop. and turning off the Q output LED. Now the player engages the ring aound the bent wire at one end and starts moving it towards the other end, without touching the wire with the ring. Even if the ring touches the wire just for a fraction of a second, the Flipflop is immediately set and the Q output LED glows, announcing the player to be an "Old Shatter hand". The simple Flipflop circuit that we have just used can be said to have stored the information that the particular player has touched'the wire with the ring. In short, the Flipflop is a memory device which stores the information last received. The information stored is always in form of zeroes and ones. This arrangement is called a controlled or clocked R-S Flipflop. The Flipflop of this circuit can change its state only when a "1" appears on the control or clock input C. D 0 Q 1/0 1/0 1/0 The C input thus behaves like a "Store" command input. The input conditions present when this input gets the "Store" command (logical "1" on C) are allowed to set or reset the Flipflop and this condition at the output is retained till a new "Store" command appears at pin C. One disadvantage that still remains in this circuit can be removed by modifying it as shown in figure 4. Now we have only one input D for Set/Reset and one input for the control command "Store". 4 The restriction of having to use a ze 3 and 3 is now removed. The pin D accept either OUTPUT And using the counterpart of the NAND gate, namely the NOR gate, we can construct a Flipf lop as shown in figure 7 below. The functioning of this circuit can be summed up briefly as follows. ”1 " appearing on D sets the Q output to 1 " "0" appearing on D resets the Q output to "0" provided a '71" was present on C. When a "0" is present on C, input D becomes ineffective. As we have already seen, truth tables in case of Flipflops have ambiguous entires due to the time dependent nature of Flipflop operation. A better way to understand Flipflop operation is the use of timing diagrams. OUTPUT OUTPUT The NOR-Flipflop functions a bit differently from the NAND-Flipflop. Now that you have studied the NAND Flipflop in detail, it will not be difficult for you to prepare the truth table for the NOR-Flipflop as well, alongwith the timing diagrams! A simplification of the circuit of figure 7 is also possible, and it is given below for reference. OUTPUT OUTPUT These timing diagrams clearly illustrate the following properties of the Flipflop. 1. When C=1 (section a), whatever appears on D also appears on output Q. 2. When C-0 (section b), output retains its last level when C changed from 1 to 0 which means that the condition of output Q which was present at the moment when C changed from 1 to 0. is stored in the Flipflop. This is called the falling edge of the input 3. Now that we have seen that only the following edge of input on C is effective as a "Store" command, we can easily understand the significance of points C and in the timing diagrams. How long the level at c remains ”1" is not important, what is important is only the moment it falls to "O" whatever state Q has at this moment is then retained The circuit of figure 4 can be further simplified as shown in figure 6. 1.62 know different word for resistors?" selex "Semiconductors!” "Why do you say that? Resistors are not semiconductors" Semiconductors "Why not? If something has almost no resistance, we call it a conductor. If something has such a high resistance that it does not allow any flow of current, we call it a non-conductor. So if something has a resistance in between these two we must call it a semi-conductor I" "No, it is not so. The resistors are not semi- conductors. The word semiconductor has a different meaning. "Semiconductors are materials which sometimes conduct and sometimes don't. When they conduct and how much they conduct depends on various other factors." "Then these semi conductors must be some sort of switches I "No, these are not switches either, you are somewhere near the truth. We shall see a semiconductor with an example. You have seen a diode this is a semiconductor. It has two leads, and its symbol is like an arrow head and a bar. A current can flow through the diode if it has a direction which coincides with that of the arrow. In the other direction, no current can flow © ► ►! © © M © For simplicity let us say that when the plus pole of the applied voltage is behind the arrow, a current can flow, however when the plus pole is in front of the arrow, the bar between the plus pole and the arrow head can be imagined as blocking the current flow." "In that case, we can call the diode as an electrical 'One Way street I" "Exactly that is what a diode is! It acts as a conductor for current trying to flow in the direction of the arrow and it acts as a non-conductor for current trying to flow in the opposite direction. This is the reason why it is called a semi conductor. We can also compare the diode with a unidirectional valve which opends only in one direction". "But how do you cmpare this with a switch?" "Let us again take the example of a diode. t£7 t L ±i K f selex Transistors simplify the matters we can say that the current flowing into the Base terminal, takes along a current from the collector and comes out through the Emitter terminal." " What are transistors ?" "Transistors are also semiconductor devices like diodes. We have seen how diodes function like one way valves. Transistors also behave like valves, but with a difference. Transistors are valves for electrical currents which can be regulated. With transistors, we can make currents flow with greater or lesser intensity." "They are the water taps of electronics" "Right!" "But then the current can also be regulated with a potentiometer." "A potentiometer has to be turned mechanically. It has a rotating spindle and a knob. The transistor has no such mechanical parts, it functions fully electronically. Most of the transistors are quite small and they have three leads coming out at the bottom. "Three terminals? Then certainly it has some kind of a potentiometer inside?" "No, transistors and potentiometers have absolutely nothing in common. The -transistors have two very intelligently designed diodes inside them, as you can see in the illustration given here." Collector Current "Unbelievable I Do you mean to say that a current flowing into the Base can make the upper diode conduct a current from Collector to Emitter even with a plus pole connected to the collector?" "That is exactly what happens inside a transistor. However when there is no Base current, the upper diode can block the current and no current can flow from Collector to Emitter." "This means that the transistor is woking like a remote control switch, which is switched ON and OFF by the current flowing through the Base." "Yes. it can be used as a switch controlled by the Base current. Incidentaly, as the transistor is not just two diodes connected together, it has been given a different symbol" Current Base "How can you control current with two diodes?" "No, it is not at all possible just with two diodes. But it can be done with a transistor. The illustration we just saw is nothing but a simplified view and not an physical combination of two ordinary diodes "I don't understand!" "This is how it happens: When a positive voltage is applied to the base, a cuttent flows through the diode between the Base and the Emitter." (Illustration) "Now, if we apply a negative voltage to the Collector current is also allowed to flow through the diode between the Base and Collector". (Illustration) "That is right, and with a plus pole connected to the collector " the upper diode will block the current I What is the use of all this?" "Just wait. It is certainly true that the upper diode will block. But as I have already said, these are just ordinary diodes. These diodes are designed in such a way, that the upper diode becomes conductive due to the current flowing through the lower diode. To "Does the transistor work only like a switch? Then what does it do in an amplifier?" "The Collector current is not only switched ON and OFF by the Base current. It can also be regulated by the base current. As in case of our water tap, the quantity that can flow through is adjustable. The Collector current can be as strong as 500 times the Base current. A Base current of just two microamperes can cause a Collector current of about one milliampere, which is quite a substantial amount in electronic circuits". "This means that a transistors can also be called an amplifier?" "Yes, and the ratio of the Collector current and the Base current can be said to be the amplification factor of that transistor" "Do the Hi-Fi Stereos also function in the same "You are right! However, a large number of transistors and many other components are necessary to obtain the Hi-Fi quality Stereo amplification." "Has nobody ever thought of building an amplifier from water taps? We could call it an Under-water Hi- Fi". branches made up of R , R-. and R,, Ri are nothing but voltage dividers. The meter connected between these two branches at the junctions between R , R : and Ri. Rj measures the difference between the two junction points. In the circuit of figure 2. the lift branch R,. R ; divides the voltage into 2V on RI and 3V on R2. The right branch divides the 5V supply voltage into 2.5V and 2.5V across Ri & Ri. The difference voltage thus becomes 3V - 2-5 V = 0.5 V which is measured by the meter and indicated. If we now reduce RI to 2600 instead of 4000, the voltage across R2 will increase by about 0.5V. This will increase the reading on the meter across the junction points by 0.5V. The meter connected directly across R2 will also show an increase in reading by 0.5 V. A very important fact can be observed here that for a rise of about 17% in the voltage across R2 there has been a rise of 100% in the difference voltage across the junction points of the bridge, which can also be called as the bridge voltage. The above observation shows how sensitive the bridge voltage is. in relation to any change in the individual branch voltages. Even the slightest change in branch voltages will be reflected as a large deviation in the beidge voltage. A simple applications of this bridge circuit is shown in figure 3. which works as a light intensity meter. One of the resistors in the left branch of this bridge is an LDR. The Resistance of an LDR in darkness is very high, whereas its resistance in light falls as the intensity of light increases. Thus the voltage across RI brcomes dependant upon the intensity of light falling on the LDR. the potentiqmeter PI can be adjusted to compensate for voltage across RI so that selex the meter reading becomes zero. If the potentiometer knob is provided with a dial, it can be calibrated to> read the light intensity. This circuit can work even without the resistance R2 (IKli) shown in dotted lines, but to protect the LDR from excess current when the potentiometer is in the extreme lower position, R2 should be included in the The potentiometer position required, to obtain zero reading on the meter when the LDR is in total darkness, can be marked as zero intensity. The light can then be increased in known steps of intensity level and every time the position of potentiometer can be marked with the known value. This circuit can be easily assembled on a small SELEX board and calibrated using a standard light intensity meter as a reference. For a resistance bridge, the individual values of the resistances are not important. The operation depends only on the resistance ratios of the individual branches. When the resistance ratios of the left and right branch become equial. the bridge voltage becomes zero, irrespective of the individual values of resistors and supply voltage. Standard configuration of a bridge circuit. The individual elements can be any type of impedances, or even rectifier diodes. Figure 2 : The meter connected across the junction points measures the voltage difference between those two points. The sensitivity of this measurement is much more than that of a meter directly connected across one of the voltage divider resistor. Figure 3 : A simple light intensity meter. The potentiometer is used to balance the bridge in such a manner that the voltage across the shunt arm of the bridge in Figure 4 : The circuit of figure 3 can be assembled on a small SELEX PCB. Resistance Decade Box Even though the resistance decade is not a very sophiticated circuit, it has very great practical utility. The resistance decade box can be used for experiments, testing, bridge balanceing, voltage dividing and many other priactical applications. The values available being adjustable from 1011 to more than 1 Mil. comprise of effectively 5 different resistance values selected by the 5 switches, as shown in figure 3. The individual values of resistors used in each group are selected in such a way that each switch represent a decimal place in the final effective combination of the five groups. The circuit is shown in figure 2. This is a selectable series connection of total 50 resistors. Individual resistors are added in to the series combination by selecting the switch positions. Each switch selects the number of resistors from a group of 10 resistors from a group of 10 resistors of equal value. The total resistance of equal value. The total resistance The series combination of five groups R1 R5, added up to make the total resistance Rg = R1 + R2 + R3 * R4 + R5. lies between the external sockets A and F of the decade box. It is not essential that sockets A & F must be used as the two ends of the effective resistance from the decade box. Any pair of 1.67 Available from: miHHHlfttt Chhotani Building, 52-C, Proctor Road. Grant Road (East), Bombay 400 007 we can use only the sockets relating to the switches of interest. When using the circuit as a voltage divider, any one of the sockets B, C, D, E can be used as the output socket for the divided voltage and the switches can be set to suitable positions to achieve the desired voltage division. A more useful application of this circuit is as a decimal voltage divider. For this the switch settings must be in the sequence 9, 9, 9, 9, 10 on the five switches from left to right. The effective distribution of resistors is as shown in figure 5. By conecting a voltage V across sockets A and F we are able to get the following voltage outputs on the sockets A = V B = V/10 C = V/100 D * V/1000 E = V/ 10000 F = 0 This has been achieved by the fact that the resistance between the pairs of sockets are distributed as follows. R (A-F) = 1 Mil = Rg R (B-F)= 100 KO = Rg/10 R (C-F)= 10 Kfl = Rg/100 R (D-F) = 1 Kfi = Rg/1000 R (E-F) = 100 ft = Rg/ 10000 Using this circuit it is possible to obtain very small voltages. For instance, a 4.5V battery connected across sockets A and F will give a voltage of 450 micro volts at socket E. A good sturdy sheet metal enclosure gives a professional appearance to the decade box. The front panel graphics has been designed for ease of understanding the operation. precious ELECTRONICS CORPORATION | Figure 2 : The resistance decade contains 50 resistors. They are divided into 5 groups. Each group has one selector switch for selecting the individual digit value of that decimal place. Figure 3 : The effective combination of the resistances. R1, | R2 R5 are selected by the switch positions. j A.B.C F are the sockets on the front panel. Figure 4 : Inside view of the assembled decade box. The switches are mounted on front panel. I Figure 5 : I Use of the resistance decade box as a decimal | voltage divider. Voltage divisions available are from I 1 /10 »n 1 /1 000 The Digilex-PCB is now available! The Digilex-PCB is made from best quality Glass- Epoxy laminate and the tracks are bright tin plated, the track side is also soldermasked after plating. Block schematic layout of components and terminals is printed on the component side. Price: Rs. 85.00 + Maharashtra Sales Tax. Delivery charges extra: Rs. 6.00 Send full amount by DD/MO/PO. new products SOUND WAVE GAUGE Anushya have developed (compensated ultrasonic tire nique) instrument tor measi travel time and velocities i.. Based on NGRI's Patent No 4 DEL/83, the solid-state digital inst ment uses a direct pulse tor opening electronic gate from a compensating of no delay acoustic transducers, and another delayed pulser from a second ~et of acounstic transducers to close the DIGITAL TACHOMETER Spectrum Services offer the digital tachometr Model AT- 100 to measure rotational surface speeds, over a range of 0 to 10,000 RPM, to an accuracy of I RPM A memory circuit facilitates measurement of speed at inaccessible places The memory i9 actuated by a touch switch. Storage is indicated by LED. The built-in auto shut-off circuit switches off power after 35 seconds if left unattended. The instrument works on 6V DC off four pencil cells. new products DIGITAL PANEL METER The Die DI-80 3Vj-digit panel meter, suitable for OEM production of digital instrument, . . housed in a 68 x 28 mm panel cut-out metallic box. It is available in two basic ranges of 200 mV and 1 .999 V DC operating at 5 or *5 V for a range of parameters. Also available are DPMs for measuring high DC and AC voltages, ohms and DC/AC current. The DPMs are provided with auto polarity, over-range indication and program- mable decimal indication Other options mains operation, multiplexer of parallel BCD outputs. Hold condition, use of For further information contact: Digital Instruments Corporation N.B. Chamber. Opp. Canara Bank. Sajpur Bogha. Ahmedabad 382 345 SOLDERING IRON SOLDRON have recently introduced a 50 Watt/230V soldering iron. This new model has features similar to the 25 Watt quick heating light weight solder- ing iron. Due to its high efficiency, the 50 Watt soldering iron can replace the conventional 65 Watt soldering iron and the resulting saving is electrical consumption is claimed to be about 20%. The iron comes with a 'Cool Grip' polypropylene handle. 5mm spade and 5mm chisel type slip on bits are available Bombay Insulated Cables S Wires Co.. 74, Podar Chambers, S.A. Brelvi Road, Bombay 400 001. DISPOSABLE DISC WITH HOLDER Imexu (India) have developed a disposa know-how from the Fibral Organisation of UK. The disc is suitable for applica- tions such as cleaning, surface prepara- tion and conditioning, micro-deburring, polishing, decorative finishing, de- nibbing, and de-fuzzing in a variety of industries— metalwroking, electronics, woodworking, aerospace, automotive, ceramic, metal and non-metal. The 75- inm dia disc is made from low density, three dimensional impregnated abras- sive with controlled cutting action. It fits positive mechanical locking in a fraction of a second. The holder has a 6-mm dia For further information write to: Imexu (Indial 6-A Vaibhav Industrial Estate. Saki Vihar Road. Saki Naka. Bombay 400 072 LIGHTED PUSH BUTTONS Efficient Engineers have developed a new series of lighted push buttons and indicators with rectangular bezel of 48 x 24 mm Two independant lamp circuits and one or two pole self cleaning and snap acting microswitches in momentory or maintained action are offered Any legend can be engraved on the front facia These push buttons are suitable for process control instrumentation, supervisory remote control systems, data acquisition, control systems, sequencing logic controls, hierarchy controls and turn key instrumentation. For further information, write to: Sai Electronics Thakore Estate, Kurla Kirol Road. Vidyavihar (West; Bombay 400 086 QUARTZ DIGITAL STOP-WATCH ION Electricals have developed a quartz digital stop-watch, which features the Lap function (useful in sports events), an annuracy of 0.001% at 25 C. and a resolution of 0.01 second (for readings up to 30 minutes). The full range covers 1 2 hours. The stop-watch has two modes of operation— Start/Lap/Stop and Start/Stop. The ergonomically designed case fits perfectly in the palm and operation is controlled by two thumbswitches. On the switch being pressed, a Beep sound indicates that the switch has made proper contact. The switches are positioned such that left or right handed operation is equally easy. The stop-watch works on a single penlite cell with a service life of about two years. It measures 6.5 x II x 3 cm and provided for safe handling Unique House. 5th Floor., 25 S.A. Brelvi Road. Bombay 400 001 . 16 PIN 1C CLIP Comtech have introduced theirT-16 1C clip for 16 pin DIL packages. It provides easy access to ICs through non- shorting electrical contacts with posi- tive mechanical clamping. Selectively gold plated spring brass contacts are designed for wiping action. The con- tact-comb with its insulating barriers provides easy positioning and prvents accidental shorting of adjecent leads. For further information, write to: Component Technique 8. Orion Apartments, 29/a, Lallubhai Park Road, Andheri (West), Bombay 400 058. TEXONIC YOUR DEPENDABLE NOT JUST ANOTHER DATALOGGER... ...IT'S UNIQUE ND IT'S FROM APLABI THE ONLY MULTIMETER WITH PROMPT SERVICE AFTER SALES ACCURATE! ROBUST! E CON OMICAL! AVAILABLE AT ALL COMPONENT SHOPS APLAB 9640 Universal Datalogger/Controller Automatic control of the process is a unique feature of 9640. This means, your process industry can go on without your continuous supervision. Amazing. Isn't it? Yes— With 9640. your constant intervention with the controls is no longer required. You just leave it to 9640 which can manage it alone. ■ Maximum of 96 inputs channels in 19" rack construction. ■ Software linearisation. ■ Wide range of sensors. ■ Built in cold junction compensation. ■ Alphanumeric printer with real time clock. ■ Analogue or Digital Input. APPUED ELECTRONICS LTD Aplab House, A-5/6 Wagle Industrial Estate Thane 400 604. India. Phone: 59 18 61/2/3. 50 73 18. Telex: 011 71979 APEL IN, Cable: Aplabthana. MANUFACTURERS : ELECTRICAL INSTRUMENT LABORATORIES, 339/68. RAJESH BUILDING, LAMINGTON ROAD. 1986 1 .75 0 DATA BOOKS Texas Instri Zilog HC/1 f int 1 S,EMENS SANYO □ Se N v a , l c°e g s SGS i § PHILIPS | SPRRGUE | TELEDYNE SEKERAl ^ ELECTRIC HEWLETT-PACKARD ERA COOKBOOKS. 0S80RNE BOOKS. SYBEX BOOKS. TAB BOOKS- SAMS BOOKS. TOWER BOOKS. ELEKTOR BOOKS. APPLE COMPUTER BOOKS ANO MICROPROCESSOR COMPUTER BOOKS etc EACH NEW DAWN BRINGS THE POSSIBILITY OF A NEW PROGRESS. • ICs- TTL CMOS. MOS. LSI LINEAR. MICROPROCESSOR. MICRO COMPUTER etc. • 1C SOCKETS: MOLEX & SMK MAKE INCLUDING MOLEX PINS & TEXTOOL O INSERIION SOCKETS. • TRIMPOTS MULTI TURN AS WELL AS SINGLE TURN OE BOURNS. VRN & OAKEMAN MAKE • DISPLAYS: 3". 5" & 1" IN COMMON CATHOD & COMMON ANODE INCLUDING 3v, DIGT LCD TANTALUM CAPICITORS Of AU. VALUES. ZENER DIODES SIUCON DIODES POWER 1RANSISTORS. PHOTO TRANSISTORS PHOTODIOOES. LED S CRYSTAL, etc. INDENTOR FOR ALL TYPES OF ELECTRONIC COMPONENTS. ' INSTRUMENTS. GREEN PHOSPHER AS WELL AS RGB MONITORS ETC. LEOffiC^ ELTEK BOOKS-N-KITS 6 RITCHIE STREET. MOUNT ROAD MADRAS-600 002. S. INDIA GRAM "ELTEK’PHONE: 844405 TELEX4I-6734 TEX IN classified ads. INDEX OF ADVERTISERS ACCORD ELECTRONICS AFCO I. & C. LTD APEX APLAB APPOINTMENT ATRON ELECTRONIC COMPONENT TECHNIQUE . DEVICE ELECTRONICS ECONOMY ENGINEERING .. ELCIAR ELTEK BOOKS N KITS GLOLITE ELECTRICALS IEAP INDIAN ENGINEERING KEJRIWAL KIRLOSKAR ELECTRODYNE KOENIG ELECTRONICS .... LEADER LEONICS LOGICS MINI CIRCUITS MODI ELECTRONICS MOTWANE ONGC OSWAL ELECTRONIC CO. . PADMA ELECTRONICS ... PECTRON PIONEER PLA PULSHECHO RAJASTHAN ELECTRONIC . 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Advertisements, together with remittance, should be sent to The Classified Advertisement Manager For outstation cheques please add Rs. 2.50 course on -BASIC ELECTRONICS Golden opportunity to learn from experts. Write to: ELECTRONICS CENTRE. 21 Shastri Nagar. Jaipur - 302 016. READ all ELECTRONICS BOOKS at 1/3 ed of original price. For details contact: SRI KRISHNA ASSOCIATES. B/25. Mahesh Co-operative Housing Society. Dattapada Road Bombay - 400 066. Write to: DATA BANK, g. No. 3, Flat No. 17. Marol Maroshi Road, ombay 400 059. 1 .8 2 elektor In R.N.N©: 39881 m3 MH/BYW-228 LIC. No 9 1 WE WISE you A VECr HAPPy NEW yEAR RUSH YOUR ORDER TILL STOCKS LAST !!! LED Digital Clock Kit Using IC MM 5387 Digit size 0.5 inch. Ciystal controlled operation. Kit is supplied without cabinet. Rs. 235.00 With Calendar Function Using IC MM 7317 Digit size 0.5 inch. Crystal controlled operation. Kit is supplied without cabinet. Rs. 260.00 (Red colour display with '5" digi height, ciystal controlled operation) NEW ARRIVAL SOLDERLESS Bread board (small size) WBU-301 ideal for testing small electronic projects for Rs. 90.00 only SPECIAL OFFER Two Colour LED in 5 mm. Flat & 3 mm. Sizes. Rs. 3.00 each DEMANDED KIT LCD thermometer Complete kit now for Rs 600.00 only NOW AVAILABLE DIGILEX PCB Rs. 85.00 Only EX STOCK |u ^phit tquolUf*r an ds pfe*^jannel i ,c "Pons&~yeWiJo20KHz. wTth-®Ueontrols at zero) Range • 1 3 dB. Maximum output (0.1% distortion) 6 Volts. Maximum input 10 Volts. Input impedance 47 K. Output impedance 100 ohms. 1 7, Kalpana Building, First Floor 349, Lamington Road Bombay-400 007 Tele: 362B50, 363549