Monostable Multivibrator

Calculations & Observations


The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach + 2 / 3 VCC.
In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as:

tp = 1.0986 RAC

where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as:
vc = VCC (1 - e - t / RAC)

Substituting vc = 2 / 3 VCC in above equation we get the time taken by the capacitor to charge from 0 to + 2 / 3 VCC.
So + 2 / 3 VCC = VCC (1 – e - t / RAC) or t – RAC log 3 = 1.0986 RAC

So pulse width,
tP = 1.0986 RAC
= 1.1 RAC



Fig.1 Showing Triggering Signal and Output Signal.





Fig.2 Triggering Signal and MMV Output Signal