Small DC motors are used extensively in the electrical and electronics fields, but they have one common shortcoming. As the applied voltage to a DC motor is reduced to reduce the speed (which is necessary for many applications), the motor loses its torque. At low voltages, DC motor operation becomes erratic, or it might fail to start at desired speeds. The circuit in Fig. 13-12 solves that problem. The principle behind the operation of this type of motor control is called pulse width modulation (PWM). The amplitude of the pulses applied to the motor always stays the same, but this circuit varies the duty cycle (on time versus off time) of the pulses.

The motor cannot respond to the rapid voltage changes, but it integrates the duty cycle into an equivalent “power” value. The speed of the motor will vary in accordance with any changes in duty cycle, but the torque characteristics are much improved. Q1 is a Darlington transistor providing the additional current gain needed for this circuit. IC1C and IC1D are paralleled to provide more current drive for Q1. Any type of DC motor, drawing up to 2 amps, can be speed-controlled. Depending on certain variables, you might have to experiment with different values of P1 for best results. Start with a 10- K ohm potentiometer as a baseline. The NTE4049 is a CMOS device, so use appropriate precaution during construction.