Subject: Liner Integrated Circuits

Topic: Special Purpose Integrated Circuits

Difficulty: High

555 Timer as PWM Generator

A 555 Timer can be used with variable "control" input to create a pulse width modulation (PWM) generator with its digital duty cycle varying based on an analog input voltage.

The 555 IC is wired in monostable mode of operation. In this mode the output is LOW (0V) when there is no triggering, when it is triggered via 2nd pin, the output goes HIGH (Vcc) for some time. This time period is determined by the expression T=1.11 R _AC Trigger is applied via a differentiator circuit to make sharp pulses. The resistor of differentiator is connected to Vcc to generate negative trigger pulses and the diode avoids positive spikes. And now this output is modulated using the input voltage applied at the control pin of the IC. So whenever the trigger pin pulses become low, the output of the IC switches to high and as a result the discharge transistor (internal to the 555 IC attached to the 7th pin) is disabled. So C charges through R _A. This capacitor keeps on charging until the voltage is above the input control voltage, at which the IC changes its state. Now the output is low which makes the discharge transistor activated thereby discharging the capacitor C. Hence the output pulse width is determined by the control voltage. This process continues and we get a continuous stream of pulses which can be used for motor control, driving LED’s, transmitting servo signals for remote control applications etc.