This circuit is a fan controller which is using the pulse width modulation method. It is tiny and easy to build.
For load currents up to about 600mA a 2N2222A NPN transistor is recommended. It comes in a TO-18 metal can.For higher loads go for a darlington power transistor such as the TIP120, 121 or 122, rated to 5A, or a power mosfet. The IRF530 is easy to find, not expensive, and can carry up to 14A. Providing you take the usual precautions for handling CMOS, static electricity is not going to zap it. Most n-channel MOSFETs will do, look for a low RDS(on) and adequate current-handling ability. Both darlingtons and mosfets are in the TO-220 case.
Using the 2N2222A bipolar transistor you might lose 200-400mV from the 12v supply to the fan, double that for one of the darlington types; with the IRF530 I measured the loss at only 40mV with a 200mA fan.
Check the transistor or mosfet pin-outs, base or gate to R9, emitter or source to ground, collector or drain to the fan negative. A heatsink is not necessary at moderate loads.
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