This is a small circuit for controlling small dc-motors. It is based on the popular 555 timer. With the use of a good heat sink for the TIP142, this circuit will be able to control the rotating speed of any small dc-motor up to 6A.
Theory of operation
The 555 is used as a multivibrator and produces a clock signal in the range of 60Hz-2KHz. During one cycle, C3 charges threw R3 and D1 and then discharges threw R4 and R2. Charging and discharging takes T1 and T2 time, respectively. T1 is preset to about 0.47ms and T2 is controlled from R4. During charging the clock goes on and during discharging the clock goes off.
Actually, the motor speed is controlled by adjusting how much time the clock remains at the off state during one cycle. T2 time adjustment from R4, has also a direct impact to clock’s frequency and duty circle.