Permanent magnet DC motor speed control circuit

The circuit shown in the attached figure is a permanent magnet DC motor speed control circuit. The sawtooth voltage is compared by IC1 (1/2 unit of LM358 dual op amp integrated circuit), and a square wave signal is output to drive VT3 to control the switch tube VT4. Adjusting the potentiometer RP can change the given voltage, thereby adjusting the duty cycle (width) of the output square wave to change the switching time of the switch tube, thereby achieving the purpose of adjusting the motor speed.