Temperature controlled DC fan

Posted on Feb 5, 2014

Here is a simple circuit based on two transistors that can be used to control the speed of a 12 V DC fan depending on the temperature. A thermistor (R1) is used to sense the temperature. When the temperature increases the base current of Q1 (BC 547) increases which in turn decreases the collector voltage of the same transistor. Since the collector

Temperature controlled DC fan
Click here to download the full size of the above Circuit.

of Q1 is coupled to the base of Q2 (BD 140), the decrease in collector voltage of Q1 forward biases the Q2 more and so do the speed of the motor. Also, the brightness of the LED will be proportional to the speed of the motor.

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