Wiring circuit

The speed of DC motors is relatively simple to control.
For  independently energized motors, the speed is, in principle, a linear function of the supply voltage. Motors with a permanent magnet are a sub category of independently energized motors, and they are often used in toys and models. In this circuit, the motor supply voltage is varied by means of pulse width modulation (PWM), which ensures good efficiency as well as a relatively high torque at low motor speeds. A single control voltage between 0 and +10 V enables the motor speed to be reversed and varied from nought to maximum in both directions. Astable multivibrator IC1 is set up as an 80 Hz oscillator, and determines the frequency of the PWM signal.

Current source T1 charges C3. The sawtooth , voltage across this capacitor is compared with the control voltage in IC2, which outputs the PWM signal to buffer N1-N3 or N4-N6. The darlington based motor driver is a bridge circuit capable of driving loads up to 4A, provided the run-in current stays below 6A, and sufficient cooling is provided for the power transistors Tz T5. Diodes D2-D5 afford protection against  inductive surges from the motor  winding. Switch S1 makes it 1 possible to reverse the motor direction instantly.