Here is a simple scheme for automatically turning on the fan and controlling its speed depending on the temperature of the sensor. This simplest circuit, consisting of only two elements, can be used in your amplifiers, power supplies, computers, etc.
Operation of the automatic regulator circuit
The LM35 chip is used as a temperature sensor. She looks like a three-legged transistor. The output voltage of this sensor is linearly proportional to the temperature in degrees Celsius. That is - 10 mV / degree Celsius, for example, if the temperature is 45 degrees, then the voltage at the sensor output will be 450 mV or 0.45 V.
The base of the transistor BC548 NPN structure is connected to the output of the sensor. From the characteristics of this transistor it is known that it begins to open when the voltage between the base and emitter exceeds 0.4 V. Consequently, the fan will begin to rotate when the sensor temperature exceeds 40 degrees. And when the sensor temperature exceeds 60 degrees and a voltage of 0.6 V appears at the output, the transistor will open completely and the fan will rotate at full power.
This is such a simple job and such a simple scheme.
If the circuit operates unstable or intermittently, I recommend connecting a 0.01 µF capacitor directly to the power legs of the LM35 sensor.
Instead of a computer cooler, you can also turn on a low-power brush motor, but then you need to insert a resistor with a resistance of 10-50 Ohms in series with it.
