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 stable or standby mode the output of the circuit is approximately zero or at logic-
low level. When an external trigger pulse is obtained, the output is forced to go high upto V CC ). 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
circuit has only one stable state (output low), hence the name monostable. Normally the output of the
Monostable Multivibrator is low. When the power supply V CC is connected, the external timing
capacitor ‘C” charges towards V CC with a time constant (R A +R B ) C. During this time, pin 3 is high
(≈V CC ) as Reset R=0, Set S=1 and this combination makes Q =0 which has unclamped the timing
capacitor ‘C’.
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