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
Set maximum frequency by turning VR1 fully clockwise.
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.