IC 555 as a Pulse Width Modulator
As the definition suggests, the circuit is intended for modulating the width of the input applied pulses at its output.
Referring to the diagram, the IC can be seen wired up in the usual monostable mode, however if the trigger input pin #2 is now fed with a train of pulses, the width of these pulses available at its output can be simply modified through a control voltage applied at pin #5 of the IC. This transforms the design into a standard IC 555 PWM circuit stage.
The above two modes of operation can be staged together for obtaining an ideal PWM controlled output, which can be implemented for many different practical applications, here it’s used as a DC motor speed controller circuit.
The figure shows the entire details of the wiring connections between the stages. The circuit functioning can be understood with the following points:
The AMV circuit wired up around IC1 generates a constant 100 Hz frequency, which becomes triggering pulses for the next PWM stage configured around another 555 IC2. This frequency also becomes the frequency of the PWM signals.
A continuously varying voltage is applied to the pin #5 of IC2 whose level directly corresponds to the width of the acquired PWM signals at its output.
The single PNP transistor and its associated components act as a current source, also forming sawtooth pulses across C3. This sawtooth waveform is compared against the applied control voltage at pin #5 for generating the PWM output signals.
The above PWM output is applied to a Darlington transistor which responds to these pulses and translates the information into varying speed levels of the connected motor.