555 Pulse Width Modulator (Voltage Controlled Duty Cycle) (1)

This is one variant of Pulse Width Modulator using the 555 timer. This circuit configures the 555 in monostable multivibrator mode periodically triggered by a rectangular signal. Control of the period of the monostable pulse is through the voltage applied to the timer's CONTROL pin. The 10 kOhm modulation resistor attenuates the modulating signal by a factor of 4. When the modulating signal is at 0 V, the control voltage is at one half the supply voltage (Vcc/2). For a +Vcc/-Vcc modulation signal the control voltage varies between Vcc/4 and 3Vcc/4. Currently the carrier signal is set to 87.5% duty cycle so that a narrow pulse (the remaining 12.5%) will be applied to the TRIGGER input of the 555. This prevents high frequency oscillation from occurring when the timing capacitor voltage is near the control voltage. When the carrier has a large space width, the true solution is to employ an RC differentiator in the TRIGGER input of 555. My objective is for this circuit to be opened and simulated in Free Subscription version of Multisim Live, due to Components per circuit limitation, I simply used the trick above to make the circuit work.