A monostable multivibrator, often called a one-shot multivibrator, is a pulse-generating circuit in which the duration of the pulse is determined by the RC network connected externally to the 555 timer. In a standby state, the output of the circuit is approximately zero or at logic-low level. When an external trigger pulse is applied, the output is forced to go high. the time the output remains high is determined by the external RC network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low stable state. The output stays low until the trigger pulse is again applied. Then the cycle repeats. The monostable cirucit has only one stable state (output low) hence the name "monostable".