The 20K variable resistor is the thermistor. The other var. resistor, in practise, sets the sensibility of the thermistor. The more you increase it, the more sensible it becomes. The lamp on the first transistor would be our fan, and the LED simulates the height of the temperature. You have also the possibility to turn off all the stuff with the SPST switch in top of the circuit. 9 Volts are supposed to be given by a battery, while 12V from an adapter.
