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comparator to pin 5 of IC2 LM3914? (S. S., Adelaide, SA). • The knock sensor should operate on the Multi-Spark CDI if the ignition coil input is altered to handle the higher voltages. This would involve using 1W resistors for the 15kΩ values con­necting to pin 6 of IC1d. An alternative method of driving the knock sensor would be to use the ignition coil input from the tacho meter signal on the Multi-Spark CDI. Unfortunately, the signal will need to be in­ verted using another common-emitter amplifier in a similar ar­rangement to Q5. Connect a 10kΩ resistor from the collector of Q5 to the base of another BC337 transistor. Connects its emitter to ground and the collector to the 12V supply via a 10kΩ resistor. The collector output will be an inverted tachometer signal suit­able for driving the ignition coil input of the knock sensor. Remote control extender not happy with Mitsubishi I have built up three of your IR Remote Control Extenders, as described in the July 1996 issue of SILICON CHIP. I have found them to be very useful in most applications. However, when I use one of these units for controlling Mitsubishi TVs or VCRs then the only way they will work is if the actual remote control is within 1cm of the IR extender. I suspect the IR receiver in the circuit does not have sufficient bandwidth and/or sensitivity for the Mitsu­ bishi units. Are you able to help by recommending a circuit modification or by suggesting a better IR detector which I could use? (G. T., via email). • We don’t have any information on this problem. Does anyone know of a solution? Converting the insulation tester A few years ago I built the Insulation Tester described in the May 1996 issue but have not found it very useful as the minimum resistance it measures is 1.4GΩ. A reading of less than this implies a fault but in fact the AS3100 requires a resistance of >1MΩ so a reading of <1GΩ may be still be well within the standard. It was therefore with interest that I saw the article de­tailing a Megohm Tester in the January 1999 issue. I noted that there were some similarities between the two circuits and won­dered if it would be possible to convert the older design to read LED ammeter green LED always on I have a problem with the LED Ammeter described in the January 1999 issue of SILICON CHIP. Everything is fine with the circuit apart from the extreme RHS green LED being continuously on after power up. No amount of adjusting VR2 can change this. I have replaced IC1 but it made no difference. I also noted that the 10µF electrolytic capacitor’s positive electrode is connected to D1 in the overlay diagram but is connected to pin 4 of IC1 in the schematic. I oriented the capacitor this way but still no difference was evident. (N. P., via email). • The 10µF capacitor should have its negative electrode con­nected to pin 4. The circuit diagram is wrong. Thanks for bring­ing this to our attention. As far as LED1 is concerned, when pin 5 of IC2 is at 0V, LED1 should be in MΩ instead of GΩ. Is it possible, without great effort or expense, to do this conversion and if so, could you please tell me how? (D. R.,, Mansfield, Vic). • As you have noted, the circuits are quite similar in princi­ple. They differ only in the impedance of the leakage testing circuit. To make your Insulation Tester read in Megohms you need to replace the circuitry involving IC4 and IC5 with that involv­ing IC2b and IC3b in the Megohm Tester. It would not be necessary to change the op amps as the CA­3140s can perform exactly the same function as IC2b and IC3 in the Megohm Tester circuit. The LM­3915 circuits in both testers are identical. off. Adjust VR2 to get 0V at pin 5 of IC2. You can check this with your multimeter. Also check that -5V is present at pin 4 of IC1. Notes & Errata Digital Capacitance Meter, February 1999: On the circuit diagram on page 68, trimpot VR4 should be connected to the +5V rail, not 0V. This is correctly shown on the wiring diagram on page 70. Also, on the circuit, the resistor between pins 5 & 6 of IC1b is shown as 20kΩ but appears as 22kΩ on the wiring diagram; either value will work. Command Control Encoder, February 1998: Under some circuit condi­tions, the buffer involving op amp IC8a may act as an inverter and this upsets the circuit operation. To avoid this, connect a 1MΩ resistor between the +12V rail and pin 3 of IC8. No other circuit changes are necessary. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. JUNE 1999  93