The automatic fan controller circuit shown in the schematic has 2 comparators with different triggering points that are independently adjustable. LM135 or LM335 in a TO92 package is used as a temperature sensor.
how does the fan controller works?
The R5 resistor polarizes the temperature sensor. On the non-inverting input of TL072 is applied a voltage that varies according to the temperature with 10mV/K. R1, R2, P1 and R3, R4, P2 form 2 resistive dividers whose voltage is applied on the inverting inputs of both comparators. Adjustment of the threshold point is done with P1 and P2.
When the non-inverting input voltage exceeds the threshold point (set by P1 and P2) the TL072 goes from Low to High state. The first comparator drives the fan motor with help from 2 transistors (T1 and T2) connected in a Darlington configuration as long as the temperature read by the sensor is higher than the level set with P1.
how to adjust the threshold level
The threshold level of the automatic fan controller is made with a thermometer. Place the sensor on the surface which temperature you want to monitor. Read the temperature with the thermometer and adjust the threshold level of the fan with P1 at the desired temperature.
The second comparator goes from Low to High state when the threshold level adjusted with P2 is exceeded. You can connect a relay on the OUT output (using the circuit from figure 2) to disconnect or connect a second fan or other type of equipment.
You can connect a red LED in series with one 1kΩ resistor to signals when it reached the desired temperature.