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Actually i can easily describe the transistor circuit...
Take an NPN transistor, connect the collector to +12v (assuming fan runs from 12v), connect the emitter to the fan (+) terminal, connect the fan (-) terminal to ground. Connect a potentiometer between +12v and ground (the two ends, not the wiper arm), connect the wiper arm to the base of the transistor.
That's the simplest version i can come up with. Uses one NPN transistor that can handle the current and power and one potentiometer. The NPN has to be able to handle 1.2 watts if the fan draws 200ma at 12vdc so it probably needs a small heat sink like the small ones made for the TO220 package. Highest value for the pot will probably be 5k, although a 10k may work with a high enough gain transistor. A power transistor that comes in a TO220 package would probably be a good idea to use with a small heat sink made for that package. If the gain of the transistor happens to be lower than usual we would have to lower the pot value to maybe 2.5k or 2k, or possibly even 1k.
It is also possible to use a PNP if the collector is connected to ground and emitter to the fan (-) terminal and the fan (+) terminal to +12v, with the pot connected the same as with the NPN, with wiper to base and ends to power supply plus and ground.
The LM317 version would just be the same as the voltage regulator circuit using that chip, just make sure the output can get at least up to 10v or higher if possible. The only problem with this circuit is that we would be lucky to be able to get 10.5v output as it wont go up to 12v. The transistor circuit will reach up as high as 11.3v or so, which isnt too bad, but then again if we wanted to run the fan at full speed we wouldnt need a controller anyway