This circuit can vary the speed of a low to medium power motor using PWM (Pulse Width Modulation). This is a very efficient method to control the speed - better than a variable voltage supply

There are 2 methods of connecting power, shown below.
Use the dual supply if motor voltage less than 9 volts.

Circuit Voltage: 9 to 18 volts

Triangle Astable:
Set maximum frequency by turning PR1 fully clockwise.

Comparator Switch:
Set Schmitt option to OFF. VR1 alters the duty-cycle of the output, or the effective speed of the motor.

The Darlington will drive up to 3A, enough for most small motors and bulbs, The heatsink may get hot for higher loads and can give minor burns.

The Triangle Wave Astable continuously produces a signal that connects to the input of the Comparator Switch. The wave smoothly ramps up and down between 2 voltage levels creating a triangle shape.

When the input voltage rises above Vref, set by VR1, the output from the Comparator Switch turns on. When the input voltage falls below Vref, the output turns off. As Vref is increased, the time the triangle wave is above Vref is reduced, also reducing the time the output is on, in turn reducing the duty cycle

When the Triangle Wave Astable frequency is increased, the rapid switch action of the supply appears as an average voltage to the load. When the mark/space ratio is even this appears as 50% power.

Replace the motor with a bright LED (and resistor) and reduce the Triangle Astable frequency to minimum. By changing Vref on the Comparator, the duty cycle can be seen. A light bulb can also be used, but the cool down time reduces the effect if demonstrating the principle.

• Lamp dimmer
• Low power heater
• Fan speed controller
• Speed control for a motorised display

More info: Speed Controller Circuit