This PWM Controller circuit is ideal for controlling small motors with 2A maximum current consumption. For higher currents you need additional cooling for T2 and increase C2 value. This pwm motor controller is using the fact that the small dc motor has the revolution speed depending on the power supply voltage.
In this circuit the discharge time can be adjusted between 0.2ms and 25ms. When the P1 resistance in at minimum value, the motor speed is maximum.