Hi, I designed (but never used) this timer circuit to control the fans in my PJ.
I'm just putting it here in case it's of some use to someone.
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R1 consists of a variable resistor and a fixed resistor. And allows you to control the time period the fans stay on for. The circuit is triggered when it's own power supply is connected.
To determine your timing components (R1 and C1) use this formula:
T (Time period) = R1 * C1
Remember, R1 is actually the VR + the Fixed Resistor value added.
