5 parts + RS pc board. I used a 2n3904 driving a 12 volt relay and a 100K thermistor #271-110 from Radio Shack. Emitter to ground. Thermistor to +12 from the base of the transistor, 1-10K pot from base to ground. Small 12v relay from collector to +12 with small diode reverse polarity across coil to prevent inductive kick (all connections to +12 fused of course). Switched relay contacts hooked in series with +12 to fans. Adjust the pot until fans just come on at room temperature, then back it off until fans stop. Wedge the thermistor (or in my case the whole PC board assembly) between the heat sink fins. Thermistor resistance goes down with heat, raises base voltage, biases on transistor. I'm using this circuit to trigger two 12v small CPU fans mounted on the edge of an R100 heatsink. Very sensitive as even the sun shining on the heatsink will trigger the fans on. Had to put foam in path of cool air as unit would cycle on and off with cool air blowing across it. Can send a picture/schematic if wanted.
Art - KC7GF
Golden, CO
GROL
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