​What is Monostable Multivibrator?

 Monostable Multivibrators have only one stable state (hence their name: "Mono")  and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.

Consider the MOSFET circuit on the left. The resistor R and capacitor C form an RC timing circuit. The N-channel enhancement mode MOSFET is switched "ON" due to the voltageacross the capacitor with the drain connected LED also "ON".