Op-Amp Circuit: Thermostat Fan

This is an op-amp circuit that turns on a fan when the temperature goes above a certain threshold. The threshold can be adjusted with a potentiometer.

The Circuit

R2 and the thermistor creates a voltage divider that will output a voltage based on the temperature. The opamp compares this voltage with the voltage from the potentiometer. If the voltage from the potentiometer is higher, the output is low. If the voltage from the thermistor is higher, then output becomes high.

When the output of the opamp is high, the transistor turns on, which turns on the fan.