A monostable multivibrator contain one (mono) permanent stable state. The circuit remains
in this permanent state if left undisturbed. On the application of an external trigger input, the circuit comes out of this permanent state and suppose the other possible state for an amount of time 'T' dictated by an external RC circuit, after which it goes back to its permanent state. A monostable multivibrator, therefore, generates a 'single pulse' of duration 'T' in response to a short (narrow) trigger pulse and is also termed as a 'one-shot'.
'One-shots' may be implemented in a variety of ways by using BJTs, inverters, op-amps and
IC Timer and are also available in IC from, for example. 74123. In the following, we illustrates how a 'one-shot' can be realized form a 555 Timer.
Consider the circuit shown in Figure
Figure: Monostable Multivibrator using 555 Timer
The permanent stable state of the circuit is Vo3 = LOW, so that pin 7 is SHORT and it maintains the capacitor shorted to zero volts.
Though, even if the circuit accidentally starts from V03 as HIGH, eventually after some time, it will switch to LOW and would continue to remain so until triggered.
The circuit operation may now be explained as follows At t = 0, a narrow pulse (negative going) with amplitude lower than 1/3 Vcc is applied at Vtr.