The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. I designed this circuit to use 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.
A litle link for al of you
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Step 1: Creating a Small Suport for De Circuit
I made maind oute of a CD cover!! and glued some screws tu hold the circuit!
Step 2: The Circuit Board
I uset to controle my pc case fan's
Step 3: Aplication