This PC fan controller circuit is designed with discrete components and is used to control 12 V fans that have a current consumption lower than 200mA. With the components values presented in the circuit diagram, the voltage won’t drop below 7V. If the fan won’t start at 25°C then temporarily replace the temperature sensor KTY10 with a 1.8kΩ resistor and reduce the value of R7. If the PC fan has a speed too high then increase the R7 value.
T1 and T2 transistors compare the fixed potentional from R3-R4 common point with the temperature dependent potentional of R1-R2 common point. It may be useful to connect a 25kΩ potentiometer instead of R2 and to adjust it until the PC fan is at the correct speed and to measure its resistance and then replace it with a fix resistor with the same value.
Place the temperature sensor in the warm air of the fan. When the PC is connected the speed rotation of the fan will be pretty high because of C1 but will decrease at a minimum value. Measure with a thermometer the air temperature close to the sensor. When the temperature reaches 35°C the controller circuit will start to work. If this won’t happen modify the value of R2 or adjust a potentiometer placed instead of R2. When the temperature increases so will the rotation speed of the PC fan. The maximum speed will be lower than the one obtained without the control circuit.