Schematic & Wiring Diagram

Here is a simple circuit based on two transistors that can be acclimated to ascendancy the acceleration of a 12 V DC fan depending on the temperature.A thermistor (R1) is acclimated to faculty the temperature. When the temperature increases the abject accepted of Q1 (BC 547) increases which in about-face decreases the beneficiary voltage of the aforementioned transistor. Since the beneficiary of Q1 is accompanying to the abject of Q2 (BD 140), the abatement in beneficiary voltage of Q1 advanced biases the Q2 added and so do the acceleration of the motor. Also, the accuracy of the LED will be proportional to the acceleration of the motor.

Notes.

• The R1 can be a 15K @ 20°C ,N.T.C thermistor.
• The M1 can be a 12V,700mA fan motor.
• The capacitor C1 must be rated 25V.
• The circuit can be powered from a 12V PP3 battery or 12V DC power supply.
• Assemble the circuit on a good quality PCB or common board.