A Monostable Multivibrator is a circuit that produces an output pulse of fixed duration each time the input of the circuit is triggered. This is useful in producing timing signals. We can use 555 as monostable multivibrator by arranging the 555 timer as shown at picture below.
When the trigger input is slightly less then +Vcc/3, the lower op amp has a high output and reset the flip-flop. This cuts off the transistor allowing capacitor to charge. When the threshold voltage is slightly greater than +2Vcc/3, the upper op amp has a high output, which sets the flip-flop. As soon as Q goes high, it turns on transistor and quickly discharge the capasitor.
The output pulsewidth can be calculated by this formula
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