Frequency dependent potential divider with low pass filter
close:- Frequency dependent potential divider AS low pass filter.
which FET should I use?
Any one that will take at least 200mA (most will) and is a logic level FET. The FET will get hot because you are burning the excess voltage off inside it when it is not full on. The power dissipated will be with half the voltage dropped across it at what ever current the motor runs with only half the voltage across it. You might need a heat sink.