This DC Motor Temperature Controller Circuit Diagram is based on two transistors, used to control the speed of 12 DC volts fan depend on temperatures. A thermistor (R1) is used to sense the temperature. When the temperature increases, the base current of Q1 (BC547) will increase which in turn decreases the collector voltage of the same transistor. The collector in Q1 is wired to the base of Q2 (BD140), the decrease in collector voltage of Q1 forward biases the Q2 more and so do the speed of the motor.
The brightness of the LED will be proportional to the speed of the motor. It preferred to use 15K @20 celsius NTC transistor as R1 and 12V, 700mA fan motor. Use also 25V rated for all capacitors.