Temperature-Triggered Fan Switch (Thermistor)

Circuit Diagram

How to Build It

1. Place the NTC thermistor on the breadboard, in series with a fixed resistor, forming a voltage divider.
2. Connect the divider's junction to a BC337 transistor's base, through a resistor.
3. Connect the transistor's collector to one fan terminal (or fan relay), the other terminal to the positive rail.
4. Connect a 1N4007 diode across the fan (protection).
5. Connect the emitter to GND.
6. Gently warm the thermistor (with your fingers) — the fan should switch on.

Bill of Materials