In this circuit, open loop control is used. Speed variation in open loop systems is not as accurate as closed loop systems, but will be good enough providing the motor load is constant. Instead of a constant current to the motor, this circuit uses current pulses and varies the duration of the pulse to control motor speed. This technique is known as pulse width modulation (PWM), and is more energy efficient than using a constant current supplied to the motor.
The circuit uses the common 555 timer IC configured as an astable pulse generator, but with variable mark and space ratio. The timing period is set by C3 and R1, but the non symmetrical on and off times (mark and space ratio) controlled by 10k variable resistor VR1.
The output pulses are then amplified by the BD139 power transistor which controls the motor current. The direction switch is a DPDT switch and LED's D3 and D4 are there to indicate direction. C4 is there to help electrical noise and suppression.
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