Monostable means only one stable state. A monostable multi-vibrator is a pulse generator that has only one state stable. it is also known as a one-shot MV. It is basically a pulse generator circuit in which the pulse duration time can be determined by R and C.
This way the output remains low for the Delay Time and then becomes high and remains high, or low. Here is a cool IC555 Monostable calculator that helps to calculate R(Resistor) and C (Capacitor).
This output pulse will stay high based on its pulse width. This output pulse width sets the pre-defined time and as said earlier it can be set by selecting the correct values of Resistor R1 and Capacitor (C1) using the below formulae. A circuit and calculator are shown below for calculation purposes.
Output Pulse Width duration (secs) = 1.1 x R1 x C1
