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ASK SIUCON CHIP Got a technical problem? Can't understand a piece of jargon or some electronic principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097. Multiple coils for electronic ignition I am interested in building the High Energy Ignition published in the May and June 1988 issues. What I wish to do is make an ignition system using a multiple coil setup similar to the one used on the VN Commodore. This car uses microprocessor controlled ignition but the multi-coil idea could be adapted to normal electronic ignition if I can find a way of alternating the signal from the ignition module from one coil to the other. I intend to use two coils (for a four cylinder car) and I need either an electronic or mechanical solution to the problem. What happens is that one coil fires two spark plugs simultaneously. One is on the firing stroke and the other on a harmless exhaust stroke. I know very little about electronics and was wondering if it would be possible to build a system into the SILICON CHIP ignition High energy ignition does not start I have built the High Energy Ignition project featured in the May 1988 issue of SILICON CHIP and have a problem. The unit is fitted to a Toyota Land cruiser FJ 45 1984 model. When the engine is being cranked over the unit will not fire but immediately the key is released, it will fire. When cranking the voltage drops to about 10.5 volts and I am wondering if the unit is sensitive to low voltage. The unit was built and fitted primarily for a better start. I have replaced the MC3334P IC and the two transistors (BC337 and MJ10012) but the problem still persists. Could you please shed some light on my problem 116 SILICON CHIP module that would alternate the signal between the two coils. If you think it would be possible could I impose on you to do me a diagram and instructions? There are alternatives to alternating the output electronically, so if it is too complicated, don't worry. Your help would be greatly appreciated. (J.K., Ashgrove, Qld). • There are two drawbacks to the system you propose. First, where are you going to obtain the coils which will fire two spark plugs simultaneously? As described in our article on the VN Commodore in the December 1988 issue of SILICON CHIP, this car uses special double ended coils with a high tension output at each end and the coils are integrated with the electronic ignition module. You might be able to obtain these coils as spares but that could be expensive. Second, there is no easy way of switching the output of the high energy ignition from one coil to another. It would be much easier and probably cheaper to simply have two separately controlled ignition modules. Why don't you just try building the Hall Effect ignition as described in our June 1988 issue? It is highly effective and very similar to that used on many four cylinder cars today. Sure, this solution does not get rid of the distributor as would your proposed system but it does have the big advantage that you can do it without too many hassles. Needs a Thorn colour TV circuit Can you help? I need a circuit diagram for a 14-inch colour monitor, model Thorntel PX14TU. I've been all around the traps and have come to a dead end. Any chance that you can assist me? flan Barrow, PO Box 64, Port Melbourne, Vic 3207). • We don't have access to commercial circuit diagrams of monitors. If we did, we could solve a a few problems of our own. Can any reader help? and also give a fix for the unit? (R.K., Darkan, WA). • In our experience, the High Energy Ignition will function well down to quite low voltage. We suspect that you have wired the unit up to one of the fuses coming off the accessory position of the ignition keyswitch. When cranking, power to all accessories is removed which means that your ignition circuit is not powered up during this time. Why are we so sure about this? Because the writer made exactly this same mistake some years ago when installing an electronic ignition system. It did the same thing - nothing while cranking and then it would burst into life as soon as cranking stopped. Optical tachometer doesn't count Could you please help me in regards to your Optical Tachometer [SILICON CHIP, May 1988). After building it, I first set it up to 6000 RPM as per calibration with a fluorescent light. I then put a photostat copy of the strobe pattern on my turntable but no way would it pick up any light or show a glimmer of movement on the meter. Since I wrote the first part of this letter I have tried again to get it working and found that if I put a finger on the circuit board in the area of the Q3 & Q4, I get the needle to FSD and in doing this I was able to set it to 900 RPM using the strobe. The electrolytic capacitors may 6V version of the brake lamp flasher Could you please tell me if the "Flasher Circuit for Auxiliary Brake Lights" featured in the April 1989 issue of SILICON CHIP can be modified for 6V operation? I wish to fit this device to · my 6V motor cycle as a conventional stop light fails to attract enough attention when stopping. If I lower the value of the Darlington transistor and halve the value of the resistors could 6V then drive the unit? Keep up the very good articles and information in your magazine. I'm teaching myself electronics and find your publication very helpful. (G.W., Ocean Shores, NSW). • The circuit itself will work quite happily from 6V without modification but the voltage drop across the BD645, typically about lV, will reduce the brightness of the brake lamp. not all be new but are the ones stated. As the unit works perfectly when tested with a fluorescent light, I am wondering what could be wrong. (R.W., Stuarts Point, NSW). • There is a quick and dirty test to see if the Optical Tachometer is working. Just point it at a light source or open window and then rapidly flick your fingers back and forth in front of the sensor, to simulate a light interrupter. If the circuit is workir..g, the pointer will move part way up the scale. From what you have said with regard to calibration from the fluorescent light source, we think the circuit is probably working correctly. When you are testing from the strobe pattern on the turntable, you need to hold the sensor very close to the platter - within just a few millimetres - otherwise there is too much light reflected from adjacent parts of the pattern. That is probably why you've managed to get the unit going by putting your fingers on the board. By doing this you have altered the r---t>-----<1.--- -YN1r----- -+6V ZD1 15V ,w ~ 6VREIAY I 5A CONTACTS In a 12V system you can live with the slight reduction in brightness but in a 6V system a 1V reduction will mean that the lamp will be too dull. In daylight, you might not be able to see it at all. The only way to overcome this problem is to use a relay to do the switching, as shown in the accompanying circuit. The relay should be fast enough to flash the lamp but if it is not, you may want to reduce the flash rate. This can be done by increasing the 0.15µF capacitor at pin 6 of IC2. The only problem which re- bias on the preamp stage and thereby made it a little more sensitive. However, we don't think the circuit needs modifying to increase its sensitivity. Info wanted on gate turn-off SCRs Do you have any information on the operation, application and availability of gate turn-off SCRs, as referred to by Bryan Maher in the August issue of SILICON CHIP? My reason for asking is that I am interested in building a chopper type speed control for an electric golf buggy. I have seen a commercial speed control which uses two MJl 1016 transistors but it seems to me that a gate turn-off SCR could offer a better solution. (J.E., Bull Creek, WA). • We do have limited information on GTO (gate turn-off) SCRs as made by Philips but no applications circuitry. They are not simple to design into circuitry, for two This circuit will let you adapt the auxiliary brake lamp flasher for 6V operation. It uses a 6V relay to switch the brake lamp instead of a Darlington power transistor. mains is where to get 6V relays. Most kitset and parts suppliers do not stock 6V relays with a contact rating of 5 amps more. You may get one as a motorcycle accessory. Alternatively, you can obtain them from Farnell Electronic Components Ltd, 72 Ferndell Street, Chester Hill, NSW 2162. Phone (02) 645 8888. Their relay Cat 176-696 or 170-191 will do the trick. Alternatively, you could try the Perth branch of Radiospares at 28-30 Northwood Street, West Leederville, 6008. Phone (09) 381 4799. Their 6V 16A relay, Cat 346-025, would be suitable. reasons. First, they have no reverse blocking capability as does a normal SCR and so series diodes need to be added for reverse blocking or parallel diodes added for reverse conduction. Second, the big disadvantage of GTOs is that their turnoff gain is not very high and can be as low as unity. This means that you need a pulse of current, as high as the load current, fed into the gate to turn it off. When first introduced, GTOs were touted as being suitable for high frequency inverters, power supplies and motor control. These days though, these applications seem to have been largely taken over by Mosfets. For really high power applications, such as motor control in rolling mills and locomotives, GTOs are still used although they are very large devices indeed. For lower power motors, high power Mosfets and IGBTs (insulated gate bipolar transistors) have largely taken over. 1§:;l DECEMBER1989 117