pwm using 555 timer ic

Posted on Feb 7, 2014

The 555 IC is wiredin monostable mode of operation. Please read the article Monostable Multivibrator using 555 Timer for more details. In this mode the output is LOW (0V) when there is no triggering, when it istriggeredvia 2nd pin the output goes HIGH (Vcc) for some time. This time period is determinedby the expression T=1. 11 RC (R=R2 ; C=C2 in the diagram).

pwm using 555 timer ic
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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 outputis 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 C2 charges through R2. This capacitor keeps on charging until the voltageis 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 C2. Hence the output pulse width is determined by the control voltage. This process continues and we get a continuousstream of pulses which can be used for motor control, driving LED`s, transmittingservo signals for remote control applications etc.

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