I prefer to use discrete designs whenever possible.
Nothing much to it, just a PNP differential amplifier direct coupled to an NPN astable multivibrator, driving a miniature power transistor to control fan speed with a pulse width modulated low frequency.
All common silicon transistors. Economical, flexible design, can be modified to use PTC thermistors and semiconductors for heat sensing. Easy to adapt for other applications.
A change in temperature causes the differential amplifier to go out of balance, which can shut the fan off completely, run it at full speed 100% duty cycle or any speed in between.
The frequency is adjusted by changing the value of the timing capacitors or common emitter resistor of differential amplifier.
The temperature is adjusted by changing the value of resistor in series with NTC thermistor.
