This is a fan controller I worked on, based on the Temperature Controlled Fan I found online.
In this version, R1 is a 10K NTC thermistor (shown in schematic at 20C, which is 12.5k). Adjust R2 for the turn on temperature and R4 for the full speed temperature. Fan is a 12v PC fan that draws 0.11 amps.
Note that if you change R4 (full speed temp) you will need to re-adjust R2 (turn on temp). Due to this behavior, I suggest using trim pots for R2 (5k trim) and R4 (500R trim) and adjusting R4 (full speed temp) first and then R2 (turn on temp).
*Note: I have not actually built this circuit, only played with it in LTSpice.