35 c2 OA 06 7^ c»\ ©8 3 A e2 38 Q>7 CZ)6 3f c4 COT'37 ^4 > ' 03 c4 3e ca Ad L CDt 3t ,3 ^ > $ 0)3 c2 ©e J 3 A e2 A® ®\ t C 1 ils ^t 35 3® E-4 — elektor march 1978 decoder elektor 35 decoder Volume 4 Number 3 Editor Deputy editor Technical editors Subscriptions W. van der Horst P. Holmes J. Barendrecht, G.H.K. Dam, E. Krempelsauer, G.H. Nachbar A. Nachtmann, K.S.M. Walraven Mrs. A. van Meyel International head offices: Elektuur Publishers Ltd. Bourgognestr. 13a Beek (L), Netherlands Tel. 04402-4200 Telex: 56617 Elekt NL U.K. editorial offices, administration and advertising: Elektor Publishers Ltd., Elektor House, 10 Longport Street, Canterbury CT1 1PE, Kent. U.K. Tel.: Canterbury (0227)54430. Telex: 965504. Please make all cheques payable to Elektor Publishers Ltd. at the above address. Bank: 1. Midland Bank Ltd., Canterbury, A/C no. 11014587 Sorting code 40-16-11, Giro no. 3154254. 2. U.S.A. only: Bank of America, c/o World Way Postal Center, P.O. Box 80689, Los Angeles, CA 90080, A/C no. 12350 04207. 3. Canada only: The Royal Bank of Canada, c/o Lockbox 1969. Postal Station A, Toronto, Ontario, M5W 1W9. A/C no. 160-269-7. Assistant Manager and Advertising : R.G. Knapp Editorial : T. Emmens ELEKTOR IS PUBLISHED MONTHLY on the third Friday of each month. 1. U.K. and all countries except the U.S.A. and Canada: Cover price £ 0.50. Number 39/40 (July/August), is a double issue, 'Summer Circuits', price £ 1. — . Single copies (incl. back issues) are available by post from our Canterbury office, at £ 0.60 (surface mail) or £ 0.95 (air mail). Subscriptions for 1978, January to December incl., £ 6.75 (surface mail) or £ 12.00 (air mail). 2. For the U.S.A. and Canada: Cover price S 1 .50. Number 39/40 (July/August), is a double issue, 'Summer Circuits', price $ 3. — . Single copies (incl. back issues) $ 1.50 (surface mail) or $ 2.25 (air mail). Subscriptions for 1978, January to December incl., $ 18. — (surface mail) or S 27. — (air mail). All prices include post 8t packing. CHANGE OF ADDRESS. Please allow at least six weeks for change of address. Include your old address, enclosing, if possible, an address label from a recent issue. LETTERS SHOULD BE ADDRESSED TO the department concerned: TQ = Technical Queries; ADV = Advertisements; SUB = Subscriptions, ADM = Administration; ED = Editorial (articles submitted for publication etc.); EPS = Elektor printed circuit board service. For technical queries, please enclose a stamped, addressed envelope or a self-addressed envelope plus an IRC. THE CIRCUITS PUBLISHED ARE FOR domestic use only. The sub- mission of designs or articles to Elektor implies permission to the publishers to alter and translate the text and design, and to use the contents in other Elektor publications and activities. The publishers cannot guarantee to return any material submitted to them. All drawings, photographs, printed circuit boards and articles published in Elektor are copyright and may not be reproduced or imitated in whole or part without prior written permission of the publishers. PATENT PROTECTION MAY EXIST in respect of circuits, devices, components etc. described in this magazine. The publishers do not accept responsibility for failing to identify such patent or other protection. National ADVERTISING RATES for the English-language edition of Elektor and/or international advertising rates for advertising at the same time in the English, Dutch and German issues are available on request. DISTRIBUTION in U.K.: Spotlight Magazine Distributors Ltd., Spotlight House 1, Bentwell Road, Holloway, London N7 7AX. DISTRIBUTION in CANADA: Gordon and Gotch (Can.) Ltd., 55 York Street, Toronto, Ontario, M5J 1S4. Copyright ©1978 Elektor publishers Ltd — Canterbury. Printed in the Netherlands. What is a TUN? What is 10 n? What is the EPS service? What is the TQ service? What is a missing link? Semiconductor types Very often, a large number of equivalent semiconductors exist with different type numbers. For this reason, 'abbreviated' type numbers are used in Elektor wherever possible: • '741 ' stand for juA741 , LM741 , MC641 , MIC741, RM741 , SN72741 , etc. • 'TUP' or ‘TUN’ (Transistor, Universal, PNPor NPN respect- ively) stand for any low fre- quency silicon transistor that meets the following specifi- cations: UCEO, max 20V • C, max 100 mA 1 hfe, min 100 Ptot, max 100 mW fT, min 100 MHz Some 'TUN's are: BC107, BC108 and BC109 families; 2N3856A, 2N3859, 2 N 3860, 2N3904, 2N3947, 2N4124. Some 'TUP's are: BC1 77 and BC1 78 families; BC1 79 family with the possible exeption of BC1 59 and BC1 79; 2N2412, 2N3251 , 2N3906, 2N4126. 2N4291. • 'DUS' or ‘DUG’ (Diode Univer- sal, Silicon or Germanium respectively) stands for any diode that meets the following specifications: DUS DUG UR, max 25V 20V IF, max 100mA 35mA 1 R, max IpA 100 mA Ptot, max 250mW 250mW, CD, max 5pF lOpF Some ‘DUS's are: BA127, BA217, BA218, BA221 , BA222, BA317. BA318, BAX13, BAY61 , 1N914, 1N4148. Some 'DUG's are: OA85, OA91 , OA95, AA116. • 'BC107B', 'BC237B', 'BC547B' all refer to the same 'family' of almost identical better-quality silicon transistors. In general, any other member of the same family can be used instead. BC107 (-8, -9) families: BC107 (-8, -9). BC147 (-8. -9) BC207 (-8, -9). BC237 (-8, 9), BC317 1-8, -9), BC347 (-8, -9) BC547 (-8, -9), BC171 (-2, -3) BC182 (-3, -4), BC382 (-3, -4). BC437 (-8, -9), BC414 BC177 (-8, -9) families: BC177 (-8, -9), BC157 (-8, -9), BC204 (-5, -6), BC307 (-8, -9). BC320 (-1.-2), BC350 (-1.-2). BC557 (-8, -9). BC251 (-2, -3). BC212 (-3, -4), BC512 (-3, -4). BC261 (-2, -3), BC416. Resistor and capacitor values When giving component values, decimal points and large numbers of zeros are avoided wherever possible. The decimal point is usually replaced by one of the following abbreviations P (pico-l = 10 n (nano) = 10 u (micro) = 10 m (milli-) = 10 k (kilo-) = 10 M (mega-) = 10' G (giga-> = 10' A few examples: Resistance value 2k7: 2700 n. Resistance value 470: 470 £1. Capacitance value 4p7: 4.7 pF, or 0.000 000 000 004 7 F . . . Capacitance value lOn: this is the international way of writing 10.000 pF or .01 pF, since 1 n is 10“’ farads or 1000 pF. Resistors are '/• Watt 5% carbon types, unless otherwise specified. The DC working voltage of capacitors (other than electro- lytics) is normally assumed to be at least 60 V. As a rule of thumb, a safe value is usually approxi- mately twice the DC supply voltage. Test voltages The DC test voltages shown are measured with a 20 kH/V instru- ment, unless otherwise specified. U, not V The international letter symbol 'U' for voltage is often used instead of the ambiguous 'V'. 'V' is normally reserved for Volts' For instance: U^ = 10 V, not Vb = 10 V. Mains voltages No mains (power line) voltages are listed in Elektor circuits. It is assumed that our readers know what voltage is standard in their part of the world I Readers in countries that use 60 Hz should note that Elektor circuits are designed for 50 Hz operation. This will not normally be a problem; however, in cases where the mains frequency is used for synchronisation some modifi- cation may be required. Technical services to readers • EPS service. Many Elektor articles include a lay-out for a printed circuit board. Some - but not all — of these boards are avail- able ready-etched and predrilled. The 'EPS print service list’ in the current issue always gives a com- plete list of available boards. • Technical queries. Members o* the technical staff are available to answer technical queries (relating to articles published in Elektor) by telephone on Mondays from 14.00 to 16.30. Letters with technical queries should be addressed to: Dept. TQ. Please enclose a stamped, seif addressed envelope, readers outside U.K. please enclose an IRC instead of stamps. • Missing link. Any important modifications to, additions to, improvements on or corrections in Elektor circuits are generally listed under the heading 'Missing Link' at the earliest opportunity. Playing games on a TV screen has become quite popular. The colour TV games circuit described this month offers on-screen scoring, sound and colour (either NTSC or PAL), and some interesting handicap features. The same circuit and printed circuit board can be used with more versatile chips when they become available. selektor In order to expand the memory capacity of microcomputers use is often made of plug-in memory cards. The 4 k RAM card described this month is primarily intended as an extension of the SC/MP system, however it can also be used with any other 8-bit microprocessor. experimenting with the SC/MP (5) H. Huschitt This part of the series introduces 'Elbug’, the monitor pro- gramme for the SC/MP system. It takes a close look at the various command routines provided by Elbug, as well as examining the software used to control the cassette interface. missing link SC/MP power supply Although this power supply is primarily intended for use with the SC/MP system, its general-purpose design renders it suitable for other microcomputer systems as well. car lights failure indicator colour TV games Formant, the Elektor music synthesiser, is now beginning to take shape. This month, the low frequency oscillators and noise generator are described. fun with a RAM — M. de Bruin electronic maze — D. Neubert Whether they are formed by garden hedges or walls, galleries of mirrors or simply lines on paper, mazes have always proven a popular pastime for all ages. The 'electronic maze' described in this article provides an extra 'twist' to the problem of finding the correct path through the maze formant - the elektor music synthesiser (9) C. Chapman voltage comparison on a 'scope — H. Spenn . . This simple circuit allows up to four DC voltages to be measured or compared by displaying them side by side on an oscilloscope. Without software, any computer is little more than an expensive heap of worthless elec- tronics . . . This month, 'experimenting with the SC/MP' takes on a new meaning with the publication of the 'Elbug monitor software. safety first The golden rule when confronted with mains powered elec- trical or electronic equipment is 'if you aren't 100% sure of what you are doing leave well alone'. In this article the steps taken to ensure the safety of home-built and commercial equipment are discussed, both from an electrical (shock hazard) and from a fire-risk point of view. market advertisers index 1 IS 5 | ■ '^1 fBSSSSSM [nr fllj selektor elektor march 1978 — 3-01 Laser glass Special laser glasses that could be used to develop the concept of laser fusion will be melted and tested by Corning. Corning scientists are to investigate techniques of melting and forming beryllium-fluoride glasses and other fluoride-containing glasses for use in extremely high-power lasers. Laser fusion is a potential process for harnessing the heat and power of thermonuclear microexplosions to gen- erate electricity in fusion reactors. Beryllium-fluoride glass has the lowest non-linear index of refraction of any known glass. Because non-linear index relates to the ability to propagate the passage of light energy through the glass, it is a significant power-limiting characteristic of high-power laser glass. Extremely intense light energies can cause changes in the refractive index of laser glass. The extent depends upon the glass’ non-linear index. In turn, changes in the refractive index alter the uniform- ity of energy flow through the glass, causing breakup of the beam as a result of a phenomenon called self-focusing. Thus, the lower the non-linear index of a glass, the more it is possible to increase the deliverable power of laser systems developed for fusion. The laser for powering an economically practical fusion reactor has not yet been invented. Scientists developing laser systems to demonstrate the feasibility of laser fusion are now using lasers that employ neodymium-doped glass components. These glasses fill the need of experimental systems that produce as much as 20 to 40 trillion kilowatts of power. However, new glasses with a lower non-linear index will be required in the decade ahead for proposed laser systems with about 200-trillion-kilowatt power output. The laser fusion concept is based on the use of high-energy, short-pulse laser beams focused on suitable fuel pellets. The fuel targets are made of a gaseous mixture of deuterium and tritium, Toxic batch materials for the beryllium- fluoride glasses being melted by Corning are mixed in sealed units by employees wearing rubber gloves. isotopes of hydrogen frozen into a hollow glass sphere less than one milli- meter in diameter. The pulses of light generated by the laser last less than one-billionth of a second, but they have such power that they heat and compress the fuel target to the point where a thermonuclear microexplosion occurs. The nuclei of the deuterium and tritium combine, releasing energy in the form of neutrons and alpha particles. In the process, a fuel pellet density of up to 10,000 times normal solid density is achieved, and temperatures up to 100 million degrees Kelvin are reached. In addition to generating large amounts of electrical power, laser fusion has other advantages. For one, the fuel supply is virtually unlimited. Deuterium abounds in sea water, and the supply is estimated to be sufficient to provide for the world’s energy needs for 350 million years. Beryllium-fluoride glasses were dis- covered a half-century ago, but they have not been commercially exploited because of the toxicity of batch materials. Corning has developed and built a specially designed beryllium-handling laboratory to allow safe handling of raw materials. Glass melting and grinding and polishing of finished pieces will be done in a controlled environment. Characterization of properties of various glass samples developed during the study will be carried out by Corning in conjunction with other major laser fusion laboratories. Coming Glass Works, Corning, NY 14830, USA (281 S) nP for traffic signals Now even the traffic signals at road junctions can be controlled by micro- computers. At the heart of Siemens new Series M signal control equipment is a fast micro- programmable control unit and extensive software, making the equip- ment extremely flexible. Its applications range from fixed-time or traffic- dependent control at individual crossings to complex control systems governed by a traffic computer. In addition to the electronic signal safe- guarding feature designed to detect signaling conditions which might endanger the traffic, the series M equip- ment is also able to make sure that the length of green phases and intermediate phases does not drop below the permissible minimum. The control equipment can be programmed on the spot. Large data blocks are entered by means of paper tapes or magnetic tape cassettes. Smaller program modifications are performed with the ‘terminal M’, a device rather like a pocket calculator. The work of 3-02 — elektor march 1978 selektor maintenance personnel is greatly simplified by the fact that the input and output procedures do not use the binary form but alphanumeric expressions, i.e. letters and digits, which are easy to understand. With l M’ equipment, not only the operation but also the design of large- scale signal networks is particularly rational and economical. A new trans- mission system makes it possible for this equipment to interoperate with traffic computers via a single conductor pair, so that the number of control wires normally required (often 20, 30 or more), has been drastically reduced. This permits multiple utilization of existing lines, resulting in considerable cuts in installation and maintenance costs of the cable network. Siemens AG Postfach 103 D-8000 Miinchen 1 West Germany (283 S) Sortie planning The Ferranti Autoplan 2081 systems will be used by RAF operational squad- rons to speed up the process of planning and producing on the ground the necess- ary detailed flight navigational infor- mation required by aircrew prior to a sortie. For example, a sortie involving 20 turning points can be planned in as little as five minutes including changing from an en route map to a 1 :50,000 scale map for the precise plotting of the initial point and target. The problem Pre-flight planning, especially of low- level combat missions, is a very demand- ing task — particularly for single-seat aircraft. Missions have to be planned taking into account evasive routing, constantly changing intelligence, and the need to avoid conflicts with other strike aircraft on similar tasks. Planning must be performed quickly and accu- rately often under difficult operational conditions, and may have to be repeated several times a day. Manual flight plan- ning ‘against the clock’ induces high stress and, with battle fatigue added, it becomes increasingly difficult to main- tain the required accuracy. It is to allevi- ate this problem that Ferranti developed Autoplan 2081 — a digitising and com- puting system used in conjunction with current military maps to produce navi- gational data accurately and quickly. It consists of a map digitiser table with associated cursor, a computer, a fast output printer and a control panel. Autoplan is rugged, inexpensive and easy to use, and no special training is required. Autoplan operation Autoplan may be used with Universal Transverse Mercator charts at scales between 1/50,000 and 1/25,000, and Lambert conformal charts at scales between 1/500,000 and 1/2,000,000. The map is placed on the digitiser table and the co-ordinates of two datum points are entered, in turn, on the Auto- plan keyboard and related to the corre- sponding positions on the map by placing the cursor over each one in sequence and operating a ‘record’ button. From this information the computer calculates chart scale, projection, and orientation on the board. Basic mission planning information is then entered via the key- board. This is done by a “question and answer” procedure with the printer producing the requests for information. The cursor is then moved around the required route from turning-point to turning-point. The computer performs the necessary navigational calculations and the results in the form of tracks to steer, time to turning-points or target, fuel states and so on are printed out in a standard form on a paper strip. Autoplan is a simple, fast, highly accu- rate operational data preparation system that provides a very consistent method of flight planning under difficult oper- ational conditions. Several routes may be planned on the same chart without the need to re-insert basic mission data. At the end of the planning phase the printer can produce as many copies of the flight plan as are required. Other equipment and devices may be used in association with Autoplan to extend its usefulness. One is a small (less than six cubic inches) solid-state portable data store (PODS). Flight planning infor- mation produced by Autoplan can be recorded directly in PODS which is then carried by hand to the aircraft and plugged into its digital navigation system. The pilot, therefore, does not have to spend valuable time inserting turning-point co-ordinates and, in addition, the risk of transcription errors occurring is eliminated. Furthermore the storage capacity of the aircraft’s auto- matic navigation system is enhanced. A minimum of 32 turning-points can be handled instead of the more usual eight or ten. The rapid and secure transfer of infor- mation from Autoplan via PODS can be taken one stage further for aircraft fitted with the Ferranti COMED (Com- bined Map and Electronic Display). In addition to the basic flight plan infor- mation, such systems can store current intelligence information on gun-de- fended areas, surface-to-air missile sites, forward edges of battle area (FEBA) and so on. This is displayed pictorially, on demand, to the pilot by electronic superimposition on a topographical map. This has tremendous advantages when the rapid turn-round of aircraft is required. Cockpit rebriefing of the pilot is possible, mission and intelli- gence information being stored in a replacement PODS which is handed to him. PODS may also provide in-flight storage facilities for reconnaissance data. On landing, this information can be trans- ferred to Autoplan and the latitude and longitude of significant locations printed out for intelligence purposes. Ferranti Limited, Silverknowes, Ferry Road, Edinburgh EH4 4 AD, England (280 S) experimenting with the SC/MP elektor march 1978 — 3-03 experimenting with . the SC/MPc^ El bug Elbug is the monitor software for the Elektor SC/MP microcomputer. By monitor software is meant a program- me, usually resident in ROM, which provides the user with the control func- tions needed to operate the system satis- factorily. A monitor programme typi- cally contains a number of routines which perform such chores as programme loading, debugging and general house- keeping. As has already been emphasised, before Elbug can be used, it is essential that all the system software which has been published so far should function without fault. A further preliminary to using Elbug is the transfer of the CPU card from page ‘lxxx’ to page ‘0xxx’ of memory. This is done by changing the position of the appropriate wire links on the memory extension board (see Elektor 33) to ‘CPU 0xxx’. The page address of the RAM I/O (as long as it remains in use) then becomes 1 000. The Elbug programme is fairly long, and occupies all the l'A k of memory which was reserved for this purpose. The mem- ory hardware consists of three EPROMs, two of which (IC3 and 1C2) are mounted on the CPU card, with the third (IC 1 4) on the memory extension card. The listing for the monitor programme is given in a condensed form in tables 1 , 2 and 3. Only the machine code is listed. The first column in each of the tables con- sists of addresses, whilst all the remain- ing figures represent data. For example, the data byte 08 is contained in the lo- cation with address 0000. The next fig- ure, i.e.C4, is the contents of the follow- ing address, i.e. 0001. The programme is written into the three EPROMs such that tables 1 and 2 represent the con- tents of 1C3 and IC2 respectively on the CPU card, whilst table 3 is written into IC14 on the memory extension card. In order to write data into an EPROM a special PROM-programmer is required, and, unfortunately, these are rather ex- pensive. There are firms who operate a PROM-programming service, however that of course involves the prospective user recording the programme in question on papertape, cassette or PROM before it can be sent. For this reason it is the intention to make pre-programmed This part of the series introduces 'Elbug', the monitor programme for the SC/MP system. It takes a close look at the various command routines provided by Elbug, as well as examining the software used to control the cassette interface. H. Huschitt EPROMs containing Elbug available direct from Elektor. In addition to the three PROMs, Elbug also requires a section of RAM (STACK) n which to store variables. The section of memory from address 0FC9 up to and including 0FFF is reserved for this purpose. The rest of RAM present on the CPU and memory extension boards (ad- dress 0C00 to 0FC9) and the V* k RAM on the RAM I/O card (address 1000 up to and including 1 0FF) are left available for the user’s programme. This amounts to a total of more than 1 k of RAM, which should prove more than sufficient for the ‘apprentice’ programmer. If desired, the memory capacity of the system can oe expanded by incorporating one or nore of the 4 k-RAM cards, details of which are contained elsewhere in this issue. However, before one sets about considerably expanding the memory of the system, one should ensure that the supply is capable of delivering sufficient current. A suitable power supply is also described elsewhere in this issue. How to use Elbug With the arrival of Elbug, the SC/MP system has acquired ‘intelligence’. How- ever before starting to use Elbug one must first know which command key initiates which control software routine and what the significance is of the vari- ous statements Elbug outputs on the displays. Elbug splits the displays into three separ- ate formats: • Displays 0 and 1 (those furthest to the right) are reserved for data (data field) • Displays 2 to 5 (the 4 middle displays) indicate addresses (address field) • Display 6 and 7 (the 2 left-hand dis- plays) indicate the instructions (com- mand field) The command keys enter the following control routines: Key C7 (code F0): RUN Key C6 (code E0): MODIFY Key C5 (code D0): SUBTRACTION Key C4 (code C0): CASSETTE Key C3 (code B0): BLOCK- TRANSFER Key C2 (code A0): CPU REGISTER Key Cl (code 90): DOWN Key C0 (code 80) : UP 3-04 — elektor march 1978 experimenting with the SC/MP The UP- and DOWN keys are not in fact genuine command keys, but rather suffix keys, as will become clear further on. MODIFY Once Elbug has been installed, and the power turned on, pressing the Halt/Reset key should result in the word ‘ . . Elbug ’ appearing on the displays. The two deci- mal points which should also light up indicate that the programme is waiting for one of the command keys to be pressed. Once the MODIFY key has been pressed the text ‘MO . . . . ’ will appear on the since it enables the user to examine the contents of any location in memory, and if so desired, to directly alter the contents of that location (assuming it is RAM of course). Once the MODIFY key has been pressed the text ‘MO . . . . ’ will appear on the displays, indicating that the programme is waiting for an address to be entered via the data keys (keys 0 . . . F). When the last (hexadecimal) digit of the address has been entered, the contents of that address will also appear on the displays. To give an example; if address 0CC9 is selected, the contents of which is Al, the displays will now read ‘M00CC9A1’. The user now has a number of different possibilities: *The contents of the addressed location can be altered by using the data keys to enter the desired data-byte. When the first key is pressed the corresponding digit appears on display 1 , the code gen- erated by the second key appears on dis- play 0. It goes without saying that the above procedure is invalid if the address in question is located in PROM or does not reference memory at all. In the latter case the data field will display ‘FF’, whilst (as their name suggests) the contents of a location in PROM can only be read, and hence cannot be altered by means of the data keys. * If the UP-key is pressed, the address and contents of the following location will appear on the displays. The contents can again be altered in the above-men- tioned fashion. If, whilst in the course of entering new data, a mistake occurs and the wrong key is pressed, this can be rectified simply by entering the correct byte immediately after the false data, since the latter is then lost. *The contents of the previous address can also be examined by pressing the DOWN-key. *The MODIFY routine can only be exited from by means of the NRST key. The word ‘ . . Elbug ’ will then reappear on the displays. Within the MODIFY routine it is only possible to access addresses on one page, i.e. if the UP-key is pressed at address 4FFF the programme simply returns to the top of the same page and address 4000 appears. In order to ‘turn the page’ one must press the NRST, re-enter the MODIFY routine, then enter the address of the next page (5000). Table 1. ©oo© ©8 c4 15 c 8 fl c4 e© c 8 fl c4 ©f c 8 f 2 c4 ffl© c 8 001 © e9 c 8 e9 9© 3d cffl e9 31 c© e5 35 c5 ©1 c 8 de c5 0020 ©1 c 8 db c5 ©1 37 c5 ©1 33 c5 ©1 36 c5 ©1 32 c5 003© ©1 c 8 c4 c5 ©1 c 8 cl c5 ©1 ©7 c5 ©1 ©1 c5 ©1 c 8 ©©4© b 8 C© b4 35 c 8 b9 c© b© 31 c 8 b5 b 8 ad c© aa 3f ©050 9© ©4 9© 4d 9© bf c 8 al c© a 6 33 c 8 9b c© a© 37 006 © c 8 95 c4 ff 31 cf fc c4 Of 35 cf ff ©1 cb ©3 ©6 ©07© cb 02 cl f 9 cb ©4 32 cf ff 36 cf ff c 1 f 8 cf ff 008© cl f7 cf ff c 1 fe cf ff c 1 fd cf ff 37 c9 ff cl 009© fe 33 c9 0 © a9 fa e 1 fb 9c ©4 c4 ff c9 fc 3f 9© ©Oa© b3 c4 © © 31 c4 ©7 35 c4 e© 32 c4 ©f 36 c4 2 f 33 ©Ob© c4 01 37 c4 ©8 ca ©b c7 ©1 cd ©1 ba ©b 9c f 8 c4 00 c© Oa ca Id c4 ©2 ca 1 c c4 ©© 37 c4 55 33 3f c4 8 © OOdO cd fd cd ff cd ff cd ff c4 0 © cd ff c 2 ©8 ©1 4© OOe© e4 e© 98 53 4© e4 f© 9c ®7 c4 ©1 37 c4 a© 33 3f ©Of© 4© e4 d© 9c ©7 c4 ©3 37 c4 ea 33 3f 4© e4 c© 9c © 1 ©© ©7 c4 02 37 c4 fl 33 3f 4© e4 b© 9c ©7 c4 ©5 37 © 11 © c4 49 33 3f 4© e4 a© 9c 88 c4 ©4 37 c4 35 33 3f © 12 © ©6 5b 4f 66 6 d 7d ©7 7f 6 f 77 7c 58 5e 79 71 ©© ©13© 3d 1 c 7c 38 79 8 © 8 © c4 5c c9 ©5 c4 54 c9 ©6 c4 0140 3e ca Id 3f c 2 ©1 33 c 2 ©2 37 c3 O© ca © © c4 a© ©15© ca Id c4 0 © 37 c4 55 33 3f c4 ©a ca la 3f c 2 ©1 ©16© 33 c 2 ©2 37 c 2 ©8 e4 80 98 ©a e4 8 © e4 9© 9c ©e 017© c7 ff 9© ©2 c7 ©1 33 ca ©1 37 ca ©2 9© c 6 c 2 ©7 0180 c9 ©0 c4 o© c9 ff c 2 ©9 1 e 1 e 1 e 1 e ©1 c4 0 © 37 019© c4 55 33 3f c 2 ©1 33 c 2 ©2 37 c 2 ©9 58 cb 0 © 9© ©la© a3 c4 5© c9 ©6 c4 1 c c9 ©5 c4 3e ca Id c4 ©© 37 01 b© c4 55 33 3f c4 ©a ca Id 3f c 2 ©1 33 c 2 ©2 37 c7 © 1 c© ff c4 5© c9 o© c4 1 c c9 ff 3f c4 ©f 37 c4 ff 33 Old© 3f c 2 15 1 c ca 14 c4 ff ©1 19 4© 94 ©2 9© f7 c4 Ole© ff ©1 c 2 14 ca ©a ba ©a 9c fc c4 ©8 ca ©8 c 2 15 ©If© ca 09 c4 16 8 f o© ba ©9 9c fc 19 ba ©8 9c ef c 2 Table 2. © 20 © 15 ca ©9 ba ©9 9c fc 4© 3f 9© c 6 c4 14 33 c4 O© © 21 © 37 c4 ©1 31 c4 ©7 35 c4 e© 32 c4 ©f 36 cl ©8 94 © 22 © fc 8 f 1 e cl ©8 ca ©8 d4 ©f ca ©9 ©1 cl ©8 94 ©2 ©23© 9© fa 8 f 1 e c4 If 31 c4 ©1 35 c 1 8 © ca ©7 3f c4 ©24© ©6 31 c4 ©7 35 c4 e7 32 c4 ©f 36 c4 ©4 ca f 9 c4 ©25© 55 33 c4 0 © 37 c4 ©a cb a 8 c4 ©2 cb a7 3f c4 e© ©26© 33 c4 ©f 37 c3 ©7 cd ff c4 ©o c9 ff c9 fe c9 fd ©27© c9 fc c9 fb c3 ©9 ce ff bb ©© 9c d3 c4 8 © c9 ff ©28© c9 fe c3 ©6 1 e 1 e 1 e 1 e ©1 c3 ©5 58 cb ©2 c3 ©4 ©29© 1 e 1 e 1 e 1 e ©1 c3 ©3 58 cb ©1 c4 0 © 37 c4 14 33 © 2 a© 3f c4 e© 33 c4 ©f 37 c4 e© 32 c4 ©f 36 c4 e3 31 © 2 b© c4 ©f 35 c4 ©3 cb ©f c 2 0 © d4 ©f cd ©1 c 6 ©1 1 c © 2 c© 1 c 1 c 1 c cd ©1 bb ©f 9c ee c4 If 31 c4 ©1 35 c4 © 2 d© ©6 cb ®f c 6 ©1 ©1 cl 8 © ca ©5 bb ©f 9c f 5 c4 0 © © 2 e© 31 c4 ©7 35 c4 ©6 cb ©f c 6 ©1 cd ©1 bb ©f 9c fe © 2 f© 9© a 8 c4 39 c4 c9 ©6 c4 5f c9 ©5 ©1 19 c4 ff ca ©© © 30 © c4 0 © 37 35 33 3f c4 5f c9 ©o c4 5e c9 ff c 2 ©31© ©8 e4 e© 9c 1 e c4 54 c9 0 © c4 5c c9 ff c4 3e ca ©32© Id 3f c 2 ©1 ca 15 c4 ©a ca Id 3f c4 5f c9 ©0 c4 © 33 © 5e c9 ff c 2 ©8 e4 8 © 98 2 c c4 3e ca Id 3f c 2 ©1 © 34 © ca ©b c 2 ©2 ca ©c 3f c4 ©a ca Id 3f c 2 ©8 e4 8 © ©35© 9c ©4 ca 0 © 9© ©f e4 8 © e4 9© 98 ©2 9© 5© c4 ©4 ©36© 37 c4 e3 33 3f c4 1 c c9 0 © c4 73 c9 ff c4 d© 33 ©37© c4 ©1 37 c 2 0 © 96 ©e 3f ca ©c 3f ca ©b 3f ca ©2 ©38© 3f ca ©1 9© ©4 3f 3f 3f 3f c4 2 © ca ©5 c4 ©© ca ©39© ©6 ©2 c 2 ©b 31 c 2 ©c 35 3f c9 0 © f 2 ©6 ca ©6 35 ®3a© e 2 ©2 9c 11 31 e 2 ©1 9c ©c 3f e 2 ©6 9c 21 c4 ©f ©3 b© 37 c4 ff 33 3f ©6 ©1 ©2 c 2 ©b f4 ©1 ca ©b c 2 ©c ©3 c© f 4 0 © ca ©c 4© ©7 ba ©5 9c c 8 3f e 2 ©6 98 ba c4 ©3d© ©1 31 c4 ©7 35 c4 0 © c9 ©4 c4 79 c9 ©3 c4 5© c9 ©3e© ©2 c9 ©1 c9 ff c4 5c c9 ©0 9© fe c4 6d c9 ©6 c4 ©3f© 76 c9 ©5 c4 3e ca Id c4 0 © 37 c4 55 33 3f c4 4© experimenting with the SC/MP elektor march 1978 — 3-05 Table 3. ©4©0 c9 ©© c4 ©-crmj , AAAA IC3 = mA 741C,MC1741CP1 (Mini Dip) A5 . . . A8 = IC4 = LM324 T2 = BC108C,BC548Cor equiv. D3 = 1N4148 D4 = LED formant 3-28 — etektor march 1978 Figure 2. Showing the phase relationship of the triangle, square and sawtooth waveforms. Since the sawtooth is derived by inverting alternate half-cycles of the triangle waveform, its frequency is twice that of the other waveforms. Figure 3. Printed circuit board and component layout for the LFO module (EPS 9727-1). setting of PI will be comparable with the input currents of IC1. This will result in unreliable operation of the oscillator at low frequencies. R16 should be chosen so that the minimum frequency of the LFO is about one cycle every three minutes, but the value of R16 should not be less than 1 0 S2. If it is not possible to obtain this low frequency then the input currents of IC1 may be too high, or Cl may be leaky. The maximum LFO frequency is about 20 Hz. reliably and the symmetry of the waveforms at low frequencies. output voltage to zero by discharging Cl through a 1 k resistor. Adjust P2 until the voltage remains stable at zero volts for a period of several seconds (without the discharge re- sistor in circuit). Repeat this adjust- ment, progressively switching the multimeter to more sensitive ranges until the drift is only a few hundred millivolts in several seconds. Selection of R16 The value of R16 determines the minimum integrator input voltage that can be set by PI, and hence the minimum frequency of the LFO. The value of R16 must not be chosen too high or the minimum LFO frequency will be too great. On the other hand it should not be chosen too low, or the integrator input current at the minimum Careful adjustment of the offset is vital, as it determines the minimum fre- quency at which the LFO will operate formant elektor march 1978 — 3-29 Parts list for LFO module Resistors: R1,R1',R2,R2',R19,R20 = 68 k R3,R3',R4,R4',R6,R6’,R8,R8\ R9,R9',R21 ,R22,R24,R30, R31 = 100 k R5,R5',R23 = 47 k R7,R7',R12,R12',R13,R13',R28, R29.R34 = 1 k R10,R10',R32 = 3k9 R1 1 ,R1 1',R1 5,R1 5',R33, R35 = 470 H R14,R14',R18 = 22 k R16,R16',R17 = 47 12 (see text) R26 = 4k7 Potentiometers: P1.PV.P8 = 100 k log P2,P2',P4,P4',P5,P6 = 10 k preset P3,P3’,P7 = 1 k preset Semiconductors: IC1 ,IC1 ’,IC3 = pA 741 C, MC 1741CP1 (Mini DIP) IC2,IC2',IC4 = LM 324 (DIP) T1.TV.T2 = BC 108C, BC 548C or equivalent D1 ,D1',D3 = 1N4148, 1N914 D2,D2',D4 = LED (e g. TIL209) Capacitors: Cl ,C1 ’,C2 = 1 p (polyester or polycarbonate) C3.C4 = 100 p/25 V Miscellaneous: 31-way connector (DIN 41617) 9 x 3.5 mm jack 3 x 1 3 ... 1 5 mm knobs formant elektor march 1978 — 3-31 Parts list for noise module Resistors: R1,R9.R10,R13 = 47 k R2 = 100 k (see text) R3.R7.R8 = 470 k R4 = 10 k R5 = 2M2 (see text) R6.R1 1 ,R1 8.R19 = 470 O R12 = 4k7 R14,R1 5,R16,R1 7 = 1 k R20 = 22 k Capacitors: Cl = 22 m/25 V C2 = 1 m/16 V C3 = 47 m/35 V C4 = 680 n C5 = 1 m (polyester or polycarbonate) C6 = 330 n C7= 100 m/35 V C8.C9 = 220 n CIO, Cl 1 = 10 m/25 V Semiconductors: IC1,IC2,IC3,IC4, = mA 741 C, MC 1741CP1 (Mini DIP) T1 = TUN (selected) D1 = 1N4148, 1N914 D2= LED (e.g. TIL 209) Potentiometers: PI = 100 k lin. ganged potentiometer P2 = 100 k preset Miscellaneous: 1 x transistor socket 1 x 31-way connector (DIN 41617) 3 x 3.5 mm jack sockets 1 x 13 ... 15 mm knob formant 3-32 — elektor march 1978 Adjustment of the LED indicator P4 should be adjusted so that the brightness of the LED follows the amplitude of the triangle output, i.e. the LED should be at minimum brightness when the triangle voltage is at its most negative, and at maximum brightness when the triangle is at its most positive. P4 should adjusted so that the LED brightness does not reach maximum before the peak of the triangle, but on the other hand it should not extinguish completely before the trough of the triangle. The noise module The complete circuit of the noise module is shown in figure 5. The noise is produced by the base-emitter junction of an NPN transistor Tl, which is reverse-biased to breakdown. The noise is amplified to a level of about 2.5 V peak-to-peak. This white noise output is fed out via C4 and R6. The white noise is also fed into a filter constructed around IC2, which has two frequency dependent elements in the feedback path. These two elements interact as follows. On its own, the feedback network comprising RIO, R12, R13 and C7 would produce a 6 dB/octave rise in the gain of 1C2, from 0 dB at zero Hz via 3 dB at 9 Hz to approximately 20 dB at 90 Hz. The feedback network R9, Rll, C6, on its own would produce a 6 dB/octave fall in gain from 0 Hz to 1 kHz, above which the gain would remain constant at 0 dB. The combined effect of these feedback networks is that below 90 Hz the 6 dB/octave rise and 6 dB/octave fall cancel out, giving a gain of 20 dB Above 90 Hz the gain falls at 6 dB/oc- tave to 0 dB at 1 kHz, above which it remains constant. The result is that the bass end of the noise spectrum is boosted, and ‘coloured’ noise is avail able at the output of IC2. The coloured noise output is taken from the junction of R14 and R15. The coloured noise output is also fed to a second filter built around 1C3. This is a 12 dB/octave lowpass filter with variable turnover frequency, which passes only the very low frequency components to produce an extremely low frequency ‘random voltage’. The fluctuation rate of this random voltage is adjusted by means of PI , which varies the turnover frequency of the filter Fluctuations of the random voltage are displayed on a LED indicator, which is identical to those used in the LFOs. Construction and adjustment of the noise module A printed circuit board and component layout for the noise module are given in figure 6, and the front panel layout is given in figure 7. As not all transistors are suitable noise generators, a socket should be fitted in the Tl position on the board so that different transistors may be tried. Measuring with a multimeter on a suitable AC voltage range at the white noise output, a voltage of 0.5 V to 0.8 V should be present. Alternatively, if an oscilloscope is used to monitor the output, a noise signal of about 2 V to 2.8 V peak-to-peak should be obtained. It may be necessary to try several transistors before a suitable one is found. Varying the value of R2 between 33 k and 1 50 k may also help. If the transistor produces too high a noise level this can be reduced by making R5 smaller, thus reducing the gain of 1C 1 . The amplitude of the coloured noise output should also lie in the same range as the amplitude of the white noise output. If it is too small then R7 should be reduced and if it is too large R7 should be increased. The random voltage output should vary between about +2.5 V and —2.5 V with PI in the ‘fast’ position. The final adjustment to the noise module is to set P2 so that the LED brightness indicates the amplitude of the random voltage output in a linear manner. This adjustment is carried out in exactly the same way as the ad- justment of the LFO indicators. H voltage comparison on a 'scope elektor march 1978 — 3-33 Voltage comparison on a 'scope There is frequently a need, when exper- imenting with circuits, to measure or compare several DC voltages at test points etc. Since most readers are un- likely to possess more than one multi- meter this can be rather tedious. Using this simple circuit, up to four voltages can be compared or measured on any oscilloscope that has a DC input and an external trigger socket. The circuit uses only three ICs, five resistors and a capacitor. The complete circuit of the voltage comparator is given in figure 1 . The four voltages to be measured are fed to the four inputs of a quad analogue switch IC, the outputs of which are linked and fed to the Y input of the ’scope. N1 to N3 and associated components form an astable multivibrator, which clocks counter IC3. This is a decade counter connected as a 0 to 3 counter by feed- back from output 4 to the reset input. Outputs 0 to 3 of the counter go high in turn, thus ‘closing’ each of the analogue switches in turn and feeding the input voltages to the ’scope in sequence. Output 0 of the counter feeds a trigger pulse to the ’scope once every four clock pulses, so that for every cycle of the counter the ’scope trace makes one sweep of the screen. A positive-going This simple circuit allows up to four DC voltages to be measured or compared by displaying them side by side on an oscilloscope. (H. Spenn) n *0o- *0o ?0o ES4 ES *<> ES 2 pg m -o^ ES1...ES4 = IC1 = 4066 N1 . N4 =IC2=4011 0=1-0 JT ^ T, '99 CD O 1 trigger pulse is available via R4, or a negative-going trigger pulse is available from the output of N4 via R5. The resulting display is shown in figure 2, four different input voltages being fed to the inputs in this case. The oscillo- scope timebase speed should be adjusted so that the display of the four voltage levels just occupies the whole screen width. The supply voltage +Ub may be from 3 to 15 V, but it must be noted that the input voltage should be positive with respect to the 0 V rail and not greater than +Ub- If voltages greater than this are to be measured then potential dividers must be used on the four inputs. Setting up To calibrate the circuit, simply feed a known voltage into one input and adjust the Y sensitivity of the ’scope to give a convenient deflection (for example one graticule division per volt input). The unknown voltages may then be fed in and compared against each other and against the calibration. The circuit can easOy be extended to eight inputs by adding an extra 4066 IC and connecting 1C3 as a 0 to 7 counter (reset connected to output 8, pin 9). Figure 1. The circuit diagram of the voltage comparator. Figure 2. An example of 4 random voltage levels displayed simultaneously on the 'scope. 3-34 - elektor march 1978 safety first Readers frequently ask why more explicit wiring details of the mains input side of mains powered equipment are not given in Elektor articles. There are several reasons for this seeming omission. In the first place, this would entail a great deal of repetition every month, thus wasting space that could be put to better use. In the second place, Elektor is read in many English-speaking countries where wiring standards are different from those of the UK, and what is correct for the UK market may not be correct for these other countries. In the third place, anyone who under- stands the principles involved can wire up a mains powered circuit quite safely without any further details, whereas anyone who does not understand what he is doing should not attempt to build mains-powered circuits, no matter how explicit the wiring details. The only exceptions to this last remark are kits supplied by a reputable manufacturer, where the mechanical construction and mains wiring can be closely specified, and an industrial situation where unskilled personnel carry out mains wiring under supervision and subject to quality control checks. The purpose of this article is to provide readers with the necessary information to enable them to wire and use mains- powered equipment safely. Safety hazards There are two principal hazards that may be encountered in electrical and electronic equipment, namely electric shock and fire hazards. The currents required to cause physical injury or death are quite small com- pared to the currents that flow in many electrical and electronic circuits, and the effects of a 50/60 Hz AC current flowing through the body from arm to arm are listed in Table 1 . These figures are, of course, typical and the actual effect of a particular current will depend on the victim’s state of health. A person with a weak heart is likely to succumb at a lower current than a healthy person. The two most dangerous paths for current to take through the body are from arm to arm and from left arm to The golden rule when confronted with mains powered electrical or electronic equipment is 'if you aren't 100% sure of what you are doing leave well alone'. In this article the steps taken to ensure the safety of home-built and commercial equipment are discussed, both from an electrical (shock hazard) and from a fire- risk point of view. Table 1 Current Effect Below 500 mA Little or no sensation 500 pA to 1 mA Tingling sensation 1 to 5 mA Slight pain 5 to 1 0 mA Severe pain 10 mA to 100 mA Muscular contraction (inability to let go) 100 mA to 2 A Ventricular fibrillation (uncoordinated con- traction of the heart muscle) — DEATH. Above 2 A Burns and physiological shock, but good chance of recovery. left leg, since in both cases the current passes directly through the region of the heart. A well-known safety rule is based on this: ‘When working with high voltages, always keep your left hand in your pocket — even if you are left- handed!’. Any current flow of more than a few milliamps through the body should be regarded as dangerous. Since the re- sistance of the body from arm to arm can be as low as 10 k if the palms of the hands are moist, this means that not only mains voltages, but also relatively low voltages can be dangerous. It is generally agreed that voltages in excess of 60 V DC or 42 V AC can constitute a shock hazard, since they could cause a current flow of some 5 or 6 mA through the human body. Equipment housing Electric shock can occur in a variety of ways. Current can flow through the body if contact is made with the supply and zero volt rails of a DC circuit or the two supply terminals of an AC circuit. This occurrence is prevented while equipment is in use by suitable physical construction, i.e. by a housing that prevents physical contact with any dangerous voltages. If equipment must be ventilated to avoid overheating and consequent fire risk, the ventilation apertures must be sufficiently small to preclude contact with dangerous voltages inside the housing. Housings should be proof, not only against direct finger contact with dangerous voltages, but also against indirect contact via such things as necklaces which may inadvertently dangle through ventilation holes, or knitting needles wielded by tiny hands. British Standard tests for the safety of housings involve the use of a jointed test ‘finger’ of very slender dimensions and a thin chain ‘necklace’. Only housings which have the ventilation holes covered with fine metal or plastic gauze can pass such tests. When testing or repairing equipment with the housing removed the possi- bility of electric shock still exists. Obviously, power should be applied to unhoused equipment only for test ’ purposes; for repair work the equip- ment should be unplugged, and in any case such work should be undertaken only by a competent person. Earthing and double insulation The second way in which electric shock can occur is by current flowing from a high-voltage point, through the [ body, to earth. In the U.K. mains system, the neutral lead of the mains wiring is at a voltage very close to true earth or ground potential (i.e. the ec -co at ire Je to ■ brown or red = Live blue or black = Neutral yellow/green or green = Earth ft fuseholder 3-36 - elektor march 1978 4 potential of the mass of the earth, which is taken as being zero volts) while the live lead alternates between positive and negative. If some part of the body is in contact with earth potential then electric shock may be incurred if another part of the body contacts the live lead. Electric shock from DC circuits may also occur in this way if the zero volt rail is tied to earth potential and the supply lead is in- advertently touched. If the equipment is properly housed it will not be possible to touch the live lead or other high-voltage leads. How- ever, if the equipment housing is of metal then an insulation breakdown within the equipment may cause the case to become live, as illustrated in figure 1. The occurrence of a shock hazard by this means can be prevented in one of two ways. The first method is to construct the housing of insulating material so that, even in the event of an insulation failure within the equipment, the insulated housing provides protection against electric shock. If any metal a. 9821 4 parts are used in the construction they should not pierce the insulation of the case. This type of construction is known as ‘double-insulated’, and it can be seen in many small domestic appliances where it is identified by the symbol 0. For the home constructor, this type of con- struction simply entails the use of plastic housings for equipment and the avoidance of metal hardware such as metal screws to fix components to the sides of the case. Items such as poten- tiometers with metal shafts and switches with metal fixing bushes or toggles should also be avoided. Where a metal case must be used the second method of avoiding a shock hazard must be employed. This involves connecting the case to earth via the earth lead of the mains wiring, and placing a fuse in the live lead, as illus- trated in figure 2. Should an insulation breakdown to the case occur, a large current will flow to earth, blowing the fuse and thus isolating the live lead. This method of construction, of course, means that a three-core mains lead must safety first Figure 4. A strain relief bush prevents damage to the cable, which could cause a safety hazard. Figure 5. I.E.C. connectors may be used to feed mains power to equipment. In addition to preventing strain on wiring inside the equipment, there is also the advantage that the mains flex can be removed for storage of equipment, and cables of different lengths may be used if necessary. Figure 6. Circuits powered direct from the mains supply (not via a transformer) must have current limiting resistors in series with any external connections. This is only feasible for very low current circuits. Note that in the ioniser circuit (referred to in the text) the voltage is so high that resistor R must be replaced by a chain of 10 series resistors. Figure 7. Low voltage circuits generally derive their power from a transformer which both steps down the voltage and provides electrical isolation from the mains input. be used with live, neutral and earth conductors. Earthed construction is frequently employed on larger domestic appliances such as washing machines and cookers. It is perfectly possible to use a metal housing and double-insulated construc- tion by having the electrical circuits surrounded by a second layer of insulation within the metal case. Even if there is a primary insulation breakdown this second insulation layer then prevents the case from coming into contact with high voltages. This type of construction can be found in small domestic appliances such as food mixers, electric drills and some types of hi-fi equipment. However, it does require extreme care to make this type of construction totally safe, and it is not really suitable for the home constructor. Care should also be taken to ensure that the mains wiring external to the equip- ment is safe. The mains lead used must be of a type having an outer sheath and two or three insulated conductors, depending on whether an earth lead is required or not. Twin core cable of the ‘bell-flex’ type, having only a single layer of insulation, should not be used for mains wiring. The mains plug should be wired in accordance with figure 3, which shows both a 13 A fused plug and a 5/15 A unfused plug. Care should be taken to ensure that the cable strain relief clamp is gripping the outer sheath of the cable, to avoid any strain on the conductors. Where the mains cable enters the equipment the hole should be bushed with a plastic or rubber grommet safety first elektor march 1978 — 3-37 to avoid chafing, and the outer sheath of the cable should be clamped to avoid strain. These two functions may be combined in a strain relief bushing, as shown in figure 4. Alternatively, connection to the equip- ment may be made via an I.E.C. chassis plug and cable socket, as shown in figure 5. In the event of the cable being tugged the plug and socket will simply disconnect. Equipment with external connections The discussion so far has assumed that the equipment was totally enclosed within its housing, with no part of the circuit being accessible from the out- side. However, there is frequently a need for a circuit to have external connections, for instance equipment such as power supplies, oscilloscopes and other instruments. Equipment in which no part of the circuit is accessible from the outside may or may not contain a mains trans- former, depending on the type of cir- cuit. For example, a domestic appliance such as a hairdryer or foodmixer operates direct from the mains supply, whereas a digital clock or mains- powered transistor radio would prob- ably contain a mains transformer to step down the voltage to that required by the electronic circuit. However, if a piece of equipment has external connections then precautions must be taken to ensure that no danger- ous voltage can appear on these connec- tions, or if high voltages do appear then steps must be taken to limit the available output current to a safe value. An example of the latter practice is the ioniser circuit that appeared in Elektor 30, October 1977. The ioniser needle operates at a potential of around 7.5 kV, but a chain of series resistors limits the current to a few hundred microamps if the needle is inadvertently touched. This principle is illustrated in figure 6. In a majority of cases, however, elec- tronic circuits are low voltage circuits that derive their power from a mains transformer that steps down the mains voltage and also provides electrical isolation between the low voltage circuit and the mains input, as illus- trated in figure 7. The transformer has independent primary and secondary windings, and there must be no direct electrical connection between them. Of course, the possibility of an insu- lation breakdown between the primary and secondary must be considered, and there are two ways of preventing a shock hazard from this source. The first method is to use a transformer which is double insulated. This type of insulation makes the possibility of an insulation breakdown between primary and secondary virtually imposs- ible. Transformers of this type generally have a two-section bobbin made of an insulating material, the primary being 6 9821 6 ' see text wound on one half of the bobbin and the secondary or secondaries being wound on the other half. An example of construction using a split bobbin transformer is shown in figure 8. Provided care is taken to ensure that no high-voltage part of the circuit can contact the low-voltage circuit, no earth connection is required. This can be ensured by a physical barrier between the high voltage and low-voltage sides of the circuit, which prevents contact even in the event of a mains wire coming adrift, or by double insulation of the high-voltage wiring, as shown dotted. If double insulated construction is employed on the high-voltage side of the circuit then a metal case may be used if desired. Some types of transformer do not employ split-bobbin construction, but have both windings on a single bobbin in layers. Since the insulation in this case consists only of the enamel on the winding wire and thin paper or plastic interleaved between the windings, this type of construction is much more prone to insulation breakdown. This type of transformer should be provided with an interwinding screen, which is a layer of metal foil between the primary and secondary winding layers. This screen is earthed, thus forming a barrier between the primary and second- ary windings. Any insulation breakdown will reach the screen before it reaches the secondary winding, causing a large current to flow to earth and thus blowing the fuse in the live lead. An example of this type of construction is given in figure 9. If a metal case is used it must also be connected to mains earth. This type of circuit must always be used with the earth lead connected to the earth terminal of a three-pin mains plug. Additional protection against electric shock due to the housing of equipment becoming live may be provided by having an earth leakage circuit breaker fitted to the house wiring. Any current flowing to earth will cause the circuit breaker to trip and disconnect the mains supply. To summarise,. electrical equipment can be made safe from a shock hazard point of view in several ways: 1. Any equipment in which high volt- ages are present should be suitably housed so that high-voltage points cannot be touched. 2. Protection against the housing becoming live should be provided in one of three ways: a. The housing may be of insulating material. b. The housing may be of metal, in which case it should be earthed. c. The housing may be of metal and not earthed, in which case the high-voltage sections of the circuit should be double-insulated from the case. 3. If the circuit has external connec- tions then the supply to the circuit must come from a transformer with separate primary and secondary windings. Care should be taken to avoid any high-voltage points coming into contact with the low-voltage section of the circuit. To this end a double-insulated transformer or a transformer with an earthed inter- winding screen should be used. The high-voltage wiring should also be double insulated or physically separ- ated from the low voltage circuits. If double insulation and a double- insulated transformer are employed then the case, if metal, need not be earthed. If double insulated con- struction is not employed then the case should be earthed, if metal. 7 ©- low voltage ►©- c circuit 1 -O external connections -O ' see text 3-38 — elektor march 1978 safety first s a Fire hazards Overheating of equipment can lead to fire hazards, since many of the plastics used as insulators in electronic com- ponents and assemblies are inflammable, as are wooden and plastic cabinets. Overheating usually occurs in one of two ways, either through inadequate ventilation under normal operating conditions, or through excessive current flow caused by a circuit malfunction or misuse. Overheating due to inadequate venti- lation is prevented simply by providing ventilation holes and/or cooling fans in equipment that produces a significant amount of heat. In commercial equip- ment, protection against excessive current flow may be provided in a variety of ways, but for the home constructor the humble fuse fulfils most needs. The simplest form of protection is provided by a fuse in the live lead of the mains input. This fulfils two func- tions. If a live to neutral short-circuit occurs at any point in the circuit after the fuse, or any other malfunction occurs which causes excessive current flow then the fuse will blow. If the equipment is in an earthed metal case the fuse will also blow in the event of a live to case short, thus preventing an electric shock hazard. Fuses may also be used in other parts of the circuit to protect particular sections (for example each channel of a hi-fi amplifier may have a fuse in the supply line) but these should be in addition to the mains fuse. The ideal place for the mains fuse is in the mains plug, since it then also pro- tects the mains lead to the equipment. However, this is not always feasible for Figure 8. If double-insulated construction is employed, no earth lead need be provided. Care must be taken to ensure that no high- voltage point can contact the low-voltage circuit. Figure 9. A single insulated transformer with interwinding screen requires an earth con- nection to the screen. Care should be taken to ensure that no high-voltage point can contact the low-voltage circuit. This can be achieved by physical separation of the high- and low- voltage circuits and clamping of leads to ensure that no contact can occur, by a physical barrier, or by double insulation of the high-voltage circuit. several reasons. Firstly, the domestic wiring may not be suitable for plugs of the 13 A fused type. Secondly, the minimum rating of fuse available for this type of plug is 2 A, which may be too high if only a low-power circuit is to be protected. In this case a fuse of suitable rating should be installed inside the equipment. So that the fuse does not blow during normal operation of the equipment, its current rating must be greater than the maximum current consumption of the circuit. The current rating of the fuse must not be too great or it may not provide adequate protection. On the other hand, if the fuse rating is too close to the normal current consumption of the circuit it will eventually fail in normal operation due to premature ageing caused by the heating effect of the current flowing through it. K market elektor march 1978 — 3-39 Multifunction calibrator The Rotek 610 offers the highest accuracy available in a multi- function calibrator. Based on the 600 Wide Band Calibrator the 610 shares the same full scale ranges: 10 mV . . . 1000 V AC and DC voltage, 10 mA ... 1 A AC and DC current with AC operation to 50 kHz and resistance from 1 fl to 10 MSI. The model 650 High Current Adaptor extends current output to 10 A DC and 50 A AC. The 610 has a specified accuracy of 30 ppm (0.002% setting + 0.001% of range) up to 500 V DC and 0.03% (0.025% setting + 0.005% range) on AC voltage up to 1000 V over the range 400 Hz ... 1 kHz for a minimum of 30 days. AC frequency may be selected to a 2 digit resolution and the 610 offers an unparalleled combination of Volt-Hertz product and output current even into capacitive loads. A full 1000 Volts is available from 40 Hz . . . 10 kHz de-rating to 500 V at 20 kHz and 100 V at 50 kHz. The 610 features Digital readout of error of the instrument under test to 0.001% — with a printer output for this information as an optional and very time-saving extra. Output values are settable to 5 decade resolution (0.10 V on the lowest range) with the deviation control providing vernier setting. Rotek have produced in the 610 an instrument which caters for almost every meter calibration situation — from Dynamometers requiring 25 mA drive at 1000 V to the latest 5 Vi digit precision source the stability of the 610 is more than adequate for calibration work of the ultimate precision. All the previous safety features of Rotek Calibrators, such as reversion to standby on overload, are retained and new ones added, as are operator instruction lamps. Front panel switching, mainly by push buttons for speed of operation, only operates logic to eliminate the problems of switch wear and contact resistance which arise if output signals are maually switched. As with the 600, the 6 1 0 may optionally be programmed either by parallel BCD (TTL level) signals or via the IEC 66 (IEEE 499-1975) Interface. Both programming options may be added subsequent to initial purchase at no cost penalty. Response to programme command is very rapid to allow full use to be made of Automatic Calibration when the situation justifies it. A universal calibration programme is available to enable users to calibrate any meter via the HP9825 Desk top computer and IEC Bus. Datron Sales Limited Penmark House Woodbridge Meadows. Guilford Surrey GUI 1BA, England (652 M) 2 • •■C/dc mmt ***<« New type of power semiconductor A new class of power semi- conductors known as transcalent devices has been introduced by RCA Electro-Optics & Devices. The new' small, lightweight devices feature integral heat pipes bonded directly to large silicon wafers capable of handling currents of hundreds of amperes. The fust devices available include rectifiers, thyristors and transistors. The integral heat pipes in the devices minimise thermal resistance and increase radiator fin efficiency, thereby permitting RCA to produce devices signifi- up to 50°C. The transcalent devices are appropriate for a w ide range of commercial and military applications involving fixed, mobile or aerospace equipment. Typical uses are in welders, electro-chemical platers, power conversion and distribution systems, motor-speed controls, military-vehicle drivers, radar power supplies and aircraft pow er systems. RCA Solid State -Europe Sunhury on Thames Middlesex TWI6 7 HV! England 1649 Ml «ATt L ca- CEMMiC TO MtT«. t SEAL ENVELOPE HEAT PiPE „ AnC* cantly smaller and lighter than conventional semiconductor/heat- sink assemblies of similar power ratings. Typical size reduction is by a factor of four and weight reduction by a factor of seven. Other advantages of RCA’s heat- pipe/integral-fin construction include improved resistance against overloads and high-current surges, and the opportunity either to reduce the silicon-junction temperature for enhanced reliability or to operate at full ratings at high ambient temperatures. Installation is also simplified by the elimination of separate heat sinks. The first RCA transcalent devices available are the P95000EB Series of 250 A. 500 W rectifiers with blocking voltages up to 1 200 V, the P95400EB Series of 400 A, 500 W thyristors w'ith blocking voltages up to 1 200 V, and the P95200EE4 100 A, 500 W NPN transistor. The rectifiers and thyristors use compact radiator structures resulting in a weight of only 340 g and a volume of less than 230 cm 3 . The transistor uses a different radiator structure with a dissipation capability of 500 W, yet the device weighs less than 1 kg and occupies a volume of less than 1 100 cm 3 . Any of the devices can be supplied with radiator structures to accommodate either air or liquid cooling. Thermal resistances are of the order of 0.1-0. 2 °C/W, and operating ambient temperatures at full ratings range Paper-tape A new paper-tape punch/readcr unit for converting selected KSR (keyboard-send/receive) printer terminals to ASR (automatic- send/receive) is available from the Engineering Division of The Exchange Telegraph Company Limited (Extel). Designated the RP-30, the paper-tape unit can be used with approved 30 charactcr- per-second terminals such as the DECwriter LA36. The RP-30 offers 20 mA or V24 input and output interfaces, and also the option of the Post Office approved output interface. The basic cost of the unit is £ 1050 plus V.A.T., and the PO-approved interface costs £ 45 plus V.A.T. The Exchange Telegraph Company Limited, 73-75 Scrutton Street, London, EC2A 4PB, England (679 M) 3-40 — elektor march 1978 market Timer-counter The TC321, a new universal timer- counter from Gould Advance Ltd., offers a comprehensive range of measurements including automatic and manual frequency measurement, manual and automatic multi-period averaging, single-period timing, two-line timing with mechanical start and stop facilities, pulse-width measurements, counting, totalising and counting events over a set period. The 5-digit instrument makes extensive use of low-power C-MOS and Shottky circuitry, thick-film resistor networks and open-plan construction to give easy access, reliability and low cost ownership. The TC321 incorporates large, clear 7-segment Beckman-type displays for ease of reading, and covers frequencies up to 35 MHz with a maximum sensitivity of 10 mV r.m.s. To complement the mechanical aspects of two-line timing on contact positions, a variable hold- off is provided to remove the effects of contact bounce. Hence mechanical measurements on relays or event timing can easily be carried out. Up to ten of the seven decade steps of multiple-period averaging can be measured over a range of 500 ns to 100 ms overspill. For automatic period averaging, the periods counted are automatically selected from one to 10 4 in decade steps for maximum display resolution. The high-impedance 10 mV input is enhanced by an automatic position for optimum triggering, as well as positive and negative slope selection with a variable sensitivity. Pulse-width measurements down to 2 ms can be made directly into input A. The instrument is housed in a rugged metal case measuring 88 mm high x 258 mm deep x x 280 mm wide, and weighs 2.3 kg without internal batteries. The battery-powered option uses five rechargeable nickel-cadmium cells giving 8 hours’ operation. The batteries are trickle-charged during normal AC operation. A temperature-controlled crystal option is also available, which increases the crystal accuracy from one part in 10 6 to one part in 10’ and the temperature stability from one part in 10 s (0-35° C) to two parts in 10‘ (0-50°C). Gould Advance Limited, Roebuck Road, Hainault, Essex, England (677 M) LCD multimeter Gould Advance Ltd. announces the DMM9, a new 4 Vi-digit multi- meter incorporating 7-scgment liquid-crystal displays, a 0.05% measurement accuracy and true root-mean-square measuring facilities. The DMM9 features 28 different AC and DC voltage, current and resistance measurement ranges, including a separate 10 A current range, and is also available with optional probes for temperature, radio- frequency and high-voltage measurements. The DMM9 has a maximum reading of 19999, and maximum resolutions on the current, voltage and resistance ranges of 10 mV, 10 nA and 100 mS2, respectively. The liquid-crystal display incorporates separate positive or negative polarity indication plus a decimal point. Overrange is indicated by a flashing display, while ‘battery-low’ indication is provided by blanking the least significant digit. An important feature of the DMM9 is the true r.m.s. sensing AC/DC converter, which can accept waveforms with a crest factor (ratio of peak to r.m.s. value) of up to 4 at full scale. A combined AC/DC facility is also available to measure AC waveforms with a DC content. Because true r.m.s. voltage measurement is the only accurate way of assessing the energy content of an AC waveform, the DMM9 is ideal for applications where power is the main parameter of interest, e.g. in the electricity supply industry or in applications involving thyristor control. The electronic method of r.m.s. measurement used in the DMM9 involves squaring, averaging and square-rooting the input voltage, and gives a dynamic range greater than 1000:1. Feedback techniques allow the r.m.s. function to be synthesised with a single square- law device. Options available include a temperature probe with a range of — 20°C to +120°C, an r.f. probe and detector, a high-voltage probe for measurements up to 40 kV, and a printer interface offering a parallel BCD output. The DMM9, which measures 227 mm x 72 mm x 260 mm and weighs 1 .9 kg, may be operated from 90-130 V or 180-260 V AC supplies (45-400 Hz), or from four rechargeable ‘C’ cells. Gould Advance Limited, Roebuck Road, Hainault, Essex, England Sprocket-feed printer A new version of the Extel M30/RO receive-only matrix printer, available from the Engineering Division of The Exchange Telegraph Company Ltd., incorporates a sprocket-feed system designed to ensure accurate registration when printing on labels, forms or preprinted stationery. The 30 character-per-second unit can handle roll or fan-folded paper or business forms. A choice of 8-level ASCII or 5-level CCITT No 2 is available, and character sets of 64, 96 or 128 characters, upper or lower case can be supplied. Printing is by a 5 x 7 dot matrix impact via a ribbon onto plain paper. Vertical spacing is six lines per inch and the horizontal format is 72 characters-per-line. The speed is switch-selectable, with three standard speeds, 10, 15 and 30 charactcrs/scc available. The line interface is serial asynchronous to EIA RS232C Standard. Selective-calling facilities are available, with or without positive/negative answer-back. The printer is a compact, typewriter-sized unit weighing only 16.2 kg and with a power consumption of between 25 and 65 W. Modular construction is used throughout. The Exchange Telegraph Company Limited, 73-75 Scrutton Street, London, EC2A 4PB, England market elektor march 1978 — E-15 8" digital display A new all-British range of large illuminated digit display modules is now available, the series 1620, intended mainly for numerical data display. However it also includes units with a partial alpha- betic capability. Typical appli- cations include data display on industrial processes; petrol price displays for filling stations; counting (and digital clocks); and scoreboards in sports stadia. The use of high-intensity lamps in an 8 inch high format gives clear viewing over long distances, with equally good visibility in areas of intense lighting such as TV studios. Series 1620 comprises four dis- play modules and six accessory packs. The 1620Aand 1620S modules are full digits with partial alphabetic capacity. The former uses a 4-line BCD input, the latter a direct 7 -segment input. The 1620B is a half digit with polarity indication, while 1620P is a punc- tuation module. Accessories include a pow'er unit, controller, pulse counter, timer and an 18-line driver. Interconnecting trunking is also offered. Newly-developed, low cost lamps, incorporating a vibration-resistant filament, have a minimum life of two years and are fitted with a high performance Fresnel lens. The lamps are easily replaced, being a push-in type, while access requires no tools, the front screen being simply slid upw ards. Display may be in red or white- on-black. Power supplies may be either 12 VDC or 16 VAC with no need for switchover. The system is therefore fully adaptable for mobile use. The result is a highly flexible system with low cost (both first cost and maintenance) and long life. Strainstall Limited Harelco House Denmark Road Cowes Isle of Wight, P031 7TB (645 M) Mini Printer G.M.T. Products Limited announce the availability of their low' power Mini Printer - the 402. This compact stand-alone numeric printer has been specifically designed for use with digital voltmeters, frequency meters, etc. and for peripheral use with instrumentation systems. Avadable as a standard 15 column parallel input unit, an interesting feature which greatly extends its capability is a range of low power board options which, used with a rechargeable battery power unit, allows the 402 to be ideal for electrically isolated, stand-by or portable applications. With manual controls recessed to avoid accidental operation the 402 stands only 107 mm high, 157 mm wide with an overall depth of 314 mm. Power input is AC 230 V or 115 V± 10% 48-62 Hz; DC 20-28 V. The 15 column parallel input boards offer independent loading of each of the two halves of a printed line (8 cols, and 7 cols.). A five digit counter is available for line/sample counting, batch counting, rate measurement etc. and is combined very handily on one board. Parallel inputs on all versions have the important feature of a built in buffer memory for all data. This enables the data to be loaded prior to, or at the start of, the print cycle and eliminates the need to hold the data constant while printing takes place. Also provided is a synchronising input which enables data to be loaded by an external command after the manual print push button has been operated. The ‘sync’ input is in addition to the normal remote print input facility. G.M. T. Products Ltd, Woodlands Road, Epsom, England. (687 M) Low profile keyboard Low profile keyboard switches manufactured by Osmor Moulded Products ltd., are available individually or can be supplied mounted as complete Keyboards custom-made to individual specifications. The chief advantage of this switch is its low' profile, 0.450" (11.45 mm). The keytops, which form an integral part of the switch, come in a wide range of colours. The switch contacts, S.P.S.T. N/O, are gold-plated phosphor bronze with cross-bar switching reliability and are rated at 2.5 w'atts (50 V 50 mA). These switches, with a life of 10 x 10 6 operations and their proven reliability, are particularly suitable for microprocessor keyboards, computer peripheral and calculating equipment. Osmor Moulded Products ltd., 75 Bensham Grove Thornton Heath, Surrey, England (680 Ml BOSS switches True, low-cost programming of solid state equipment is offered by Binary Option Selection Switches from Molex Electronics’ BOSS family. Designed for applications where manual programming is required, these switches can also be used to eliminate point-to-point hand soldering of wires on pc boards. Switch features include base ribs which raise it from the board so facilitating cleaning of flux residues. A low-stress, high-force contact is achieved by means of a double-lever design for the contacts. The switching interface provides a butting/wiping action. This minimises wear and, at the same time, ensures maximum contact integrity. Brief electrical specifications include switching rating at 30 V DC (open circuit) of 50 mA maximum, a non-switching rating of 100 mA rms at 50 V DC (maximum). Contact resistance, measured at a current flow of 10 mA is 100 milliohms. SPDT and DPST versions are available. Housing of both the standard and low-profile models is of polyester material providing both excellent mechanical and electrical characteristics. Terminals are of phosphor bronze, whilst finish is gold-alloy inlaid. Up to ten circuits can be handled according to style ordered. Molex Electronics Ltd., 1 Holder Road, Aldershot, Hants, GU12 4RH, England (684 Ml TIL. 74 I.C.'s By TEXAS, NATIONAL, ITT FAIRCHILD Etc. SEMICONDUCTORS by MULLARDTEXAS, MOTOROLA, SIEMENS, ITT. RCA SPECIAL OFFER DL 707 DISPLAYS 65 p EACH XEROZA RADIO 306, ST. PAUL'S ROAD, HIGHBURY CORNER, LONDON, N1 Telephone 01 '226-1489 Emy access to Highbury via Victoria Line I London Transport) British Rail 1200 »F 63V . .2 for Cl. 00 2200. F 63 V 2 for Cl .SO 2300 .F 63V . 1 lor £1.60 MULLARD C280 T.T.L. OFFERS PLEASE NOTE ALL PRICES INCLUDE POSTAGE ANO V.A.T. AT 8 OR 12“.% AS APPROPRIATE OF R AU S Tvpp K r S ,°mfiiS o^-io A 7°» R r. TUBES ' S,NGLE AND MULTIPHASE HIGH CURRENT RECTIFIER STACKS, CAPACITORS PARTRPnR iwni.c5hVA D , ^cnPI?X LASH AN ° M0T0R START, TV TUNERS ALSO SOME HIGH TECHNICAL EQUIPMENT AND PARTS FOR INDUSTRIAL USERS AND SCHOOLS FOR PERSONAL CALLERS manufacturers (large «nd small! we welcome your enquiries, overseas buyersagents etc. let us know your requirements.