Bulgarian fan controller…2nd attempt.

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A friend of mine asked me to look at a problem he was having with the cooling fans in his 1999 Chevy Malibu.   For some reason the fans would not come on when the engine reached a high temperature. The car ECU reads the ECT sensor to determine the temperature of the engine. The ECT is just a thermistor that increases in resistance as the temperature rises. I figured it would not be that hard to use a small microcontroller that will read the ECT sensor inline with the ECU. Thermistors generally operate on a non-linear basis. When a thermistor is read while in a voltage divider, they operate on a more linear format; one downside is the sensor reacts a little slower while in a voltage divider. The first version (left), was having a phantom 5v on the ECT pin. When connected to a voltage it read fine; I assume it has something to do with the surface mount parts. That is when I decided to rework the board using only thru-hole parts. The first attempt at etching the board failed. For some reason a pinkish film formed on the copper that slowed the etching process dramatically. The second time everything worked perfect. Once I had the power supply portion soldered, I usually test it to make sure everything is working correctly….well it wasn’t. Somewhere there is a short on the pcb from the 5v rail to ground. A quick scan with a jewelers loupe showed nothing. Hopefully I can find it later tonight.

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