Intro: How to Make a Simple Temperature Control Fan Speed
Step 1: Electronic Parts You Need
1 x 10K thermistor
1x IRF540n or n channel MOSFET
1 x 50 kilo-ohms potentiometer
1 x 2.2 uf 16 volts at least capacitor
Step 2: Connect All Parts As the Image Above
This circuit uses a thermistor as a sensor, the thermistor resistance decrease when a temperature increase.
When the temperature increases the thermistor resistance decrease until the voltage set point that adjusts by potentiometer higher than Vgs threshold, MOSFET change to ON state and make a 12 volts fan start and rotating speed faster when a temperature hotter.
The capacitor that parallels with a thermistor help fan starts instantly at lowest speed when power on, The 12 volts DC fan make noise and not rotating without the capacitor(maybe a cheap Chinese power supply cause it).
I set the threshold point at 25 C, The fan start at lowest speed @ 25 C and rotating fastest @ around 38 C (I touch it and wait few munites to see the result)