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the base voltage of Q2 when switched on is about 1V. Note that the 1.5kΩ resistor between pins 6 and 7 of IC1 will need to be changed to a value of about 1MΩ to extend the time to measure this switching when using a multimeter. Take the coil out of circuit when doing this since it will draw excessive current due to the long ontime. DiscoLight double triggers I have a technical query regarding the DiscoLight which was described in the August 1988 issue of SILICON CHIP. I have just built one which works perfectly in all modes except it triggers twice on each bass beat. Strobe seems OK but the ALT and Chaser moves in two steps and as there are only four channels only two lights effectively operate. It triggers fine via the Oscillator. Are there any errata for the trigger or squaring circuit? Have you heard of this problem? Any suggestions? (S. S., Melbourne, Vic). • Double beating when used to trigger from sound is probably caused by there being too much signal. Try adjusting the sen­sitivity down. You may also find that decreasing the 1MΩ resistor between pins 8 & 10 of IC1c to 180kΩ will improve the result. This provides a much greater hysteresis on Schmitt trigger IC1c. Overheating Tarago I have been trying to overcome a long-standing heating problem in my Tarago van. After checking thermostat and repairing the radiator, the hoses, clamps and the head and having the auto transmission checked, all seems to be OK. But I still have an unexplained loss of water and elevated temperature after driving about 100120km. As a result I am forever topping up the water after every second trip and can never really have any peace of mind when travelling. I want some circuitry to monitor K-type thermocouples to sense the temperatures of the automatic fluid and engine coolant, both leaving and returning after cooling, as well as the engine oil and ambient air temperature entering the radiator. I’d like to display the readings in pairs using the alphanumeric LCD (May 1993, Poor gas mileage in the Kingswood I installed the programmable ignition system (March 1996) in my 1974 Holden HQ approximately four months ago. I though this would be the solution I was waiting for, to allow me to change my timing on the fly when changing from petrol to gas and vice versa. Overall, I am very impressed with its operation, and I have noticed a marked increase in power. But after playing with vari­ ous settings constantly for the last few months, I am still experiencing a fairly large drop in my gas fuel economy. Before installing the system, I could drive from Melbourne SILICON CHIP) and be able to recall the maximum reading later. I feel that by monitoring all the heat sources simultane­ously, I can observe the thermal runaway building and catch the offender. I think the trouble is linked to the way the auto is cooled. The engine radiator has a small heat exchanger for the transmission fluid. It seems that at a particular ambient air temperature the radiator cannot handle the load from the auto and this starts the heating cycle off. At least, I suspect that this is what is going on as it appears to occur during the warmer months of the year. I plan to transplant the system to a friend’s 4-wheel drive when I have fixed my problem. I will include the transfer case and both differentials as he does some very serious driving. (T. F., Bund­ a­berg, Qld). • We have not published any circuits to suit a K-type thermo­ couple and we do not have any plans to do so in the near future. However, it may be possible to modify the Digi-Temp, as featured in the January 1997 issue of SILICON CHIP. As published, this circuit will read the temperature at up to eight separate loca­tions at up to 99°C. We have spoken to the design­er, Graham Blowes, and he is confident that it could be modified to read temperatures to about 120°C. This could make it suitable for your application. However, while it is not our normal to Benalla and back on one tank of gas, a round trip of approximately 420km. Now, I find that I can only just make the one way trip. Anyway, I am hoping that someone can provide me with some suitable settings to allow for better timing for gas. Please! The price of gas in the country is double that of the city! (R. B., Melbourne, Vic). • Unfortunately we do not have any information on ignition timing for cars and especially not for use with gas. We can only suggest you take the car to a speed shop which has a dynanometer and exhaust gas analysis equipment. They should enable you to obtain the best compromise between power and economy for your car. province to give auto­motive advice, we think that you have a leak in the cooling system, not a mechanical defect which will be revealed by a temperature monitoring system. The reasoning is this: if the engine coolant is overheating and causing the radiator cap to vent, no fluid should be lost; it will all go into the overflow bottle. The fact that you have to top up the system frequently points to a leak. We think that the elevated temperature is caused by the loss of coolant, not the other way around. We strongly suggest that you take your car to the local Toyota service people for a thorough investigation. Leaving it un­repaired will eventually lead to a failure of the alloy head and that will be very expen­sive to repair. Notes & Errata Digi-Temp Digital Thermometer, January 1997: the designer of this project has advised that the pinout diagram for the DS1820 sensors is reversed; the GND terminal should be on the righthand side and the +5V on the lefthand side. No damage ap­pears to occur when this wrong connection is made. Smoke Alarm Panel, January 1997: one of the array of 100µF ca­pacitors on the circuit of page 29, January 1997 should be 10µF. The component overSC lays are correct. April 1997  93