DC Motor Speed Controller

Required devices:

1. 3v - 12v dc motor

2. 100k potentiometer

3. MOSFET TO-220AB

4. 12v battery

How to connect?

Connect Drain (D) of MOSFET and right end of potentiometer (P) to the +ve terminal of battery. Connect Source (S) of MOSFET to the one end of motor. Connect another end of motor and another end of 'P' to the –ve terminal of battery. Finally, connect the middle point of 'P' to the Gate (G) of MOSFET. Now you can control the speed of motor by rotating the knob of potentiometer.

NOTE: 
The MOSFET should be N channel enhancement  MOSFET (IRF540, IRF3205,...).
The supply voltage depend on MOSFET and the motor. Example: If the MOSFET is designed for 12v and capable of 50v. The supply voltage should be 12v.