Using electronic diagram below can be designed a speed regulator for DC motors. The circuit use pulse phase modulation (IC1 rectangular pulse modulator always engages the motor to 0.5 ms).
The final speed is influenced by frequency of the pulse repetition. The space between pulses, which is the time of discharge of C4, can be set between 1us and 14 ms with R2 and P1.
T1 can switch up to 6 A, with condition to be mounted on a heat sink no lower than 20 ° K / W.
The circuit uses a 555 timer and a few classic components. The motor used must have a maximum voltage of 12V and a maximum current of 6A.