Working of a Monostable Multivibrator:

The free running oscillator (stable) can be changed into a switching circuit having one stable and one quasi stable state. The resulting mono stable circuit is widely used in industry for many different timing application. The normal mode of operation is to trigger the circuit into its quasistable state where it will remain for a predominant length of time. The circuit will then switched itself back (regenerate) into its stable state, where it will remain until it receive another input trigger pulse.

Since it has only one stable state the circuit is characterized by the term monostable multivibrator or simply monostable.

The input is labeled trigger and the output is Q.