This speed controller is mainly designed for small 12V motors that do not absorb more than 2A. Many regulators limit the current through the motor, which reduces torque. The control circuit is based on a CMOS inverter, IC1 (type 40106). The motor is switched by a darlington with discrete components, T1-T2.
IC1a inverter operates as an oscillator whose duty cycle (the T2 leads, so the motor receives energy) is determined by R2-C2-D3. Gave the inactive signal oscillator (when T2 is blocked) is determined by R3-P1-C2-D4, and can thus be varied by P1.
Inverters connected in parallel, and IC1c IC1b, forms a buffer between the oscillator and power Darlington.
Motor and controller power is provided by a network transformer, whose secondary voltage is half-wave rectified by D1 and D2.
An indicator LED current limiting resistor is connected to the motor terminals.
The transistor T2 to be mounted on a heat sink.