Adjust VR1 with a screwdriver to the center position.
If the temperature you require is outside the range of VR1 on the Comparator Switch, then adjust VR1 on the Heat Sensor
The Schmitt setting should be selected by moving the jumper to the right position, creating hysterisis
As the temperature rises, the voltage from the Heat Sensor rises.
As the voltage rises above Vref on the Comparator Switch, the output turns on, switching the Darlington Driver on and the Fan.
As the temperature falls, the voltage from the sensor falls below Vref and the output turns off, switching the Darlington Driver off and the Fan.
There is some hysterisis in the switching meaning that the on temperature setting is slightly lower than the off setting. This prevents chatter (rapid switching) if the fan is too close to the sensor.