how to analyse the Pulse Width Modulator with Motor c Designing a PWM Circuit The concept of PWM inherently requires timing. Two 555 timer ICs and some potentiometers can be used to generate a PWM signal, and since PWM provides a digital, on/off signal, it is also easy to use a PC or micro-controller to create the signal; however this is beyond the scope of this article. The circuit in figure 2 uses two 555 ICs and is actually a combination of two types of circuit. The first is a free running multivibrator (astable) with an adjustable frequency around 30Hz. The output of this circuit then triggers a pulse shaping (monostable) circuit which adjusts the width of the pulse. The circuit produces a duty cycle in the range of approximately 0.3% to 97%.
The speed of the motor is controlled with a single potentiometer (variable resistor). It is possible to run a Meccano M5 motor to test the circuit, and it will run from dead still to full speed using the potentiometer speed control and a 6V battery as the sole power source. If you have a 12V motor, you can of course use a 12V power source. The motor is switched on and off via a TIP31C transistor (shown in figure 3) which can handle motors rated up to 3A at 100V, or a total power of 40W. If you are using a high power motor, make sure there is a heatsink bolted to the transistor.