The 555 as a Monostable Multivibrator

A monostable multivibrator, often called as 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 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 (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.

Monostable Operation:

According to Figure (b), initially when the output is low, that is, the circuit is in stable  state, transistor Q1 is on and capacitor C is shorted out to ground.

However, upon application of a negative trigger pulse to pin 2, transistor Q1 is turned off, which releases the short circuit across the external capacitor C and drives the output high. The capacitor C now starts charging up toward V_CC through Ra. However, when the voltage across the capacitor equals 2/3V_CC, comparator 1's output switches from low to high, which in turn drives the output to its low state via the output of the flip-flop. At the same time, the output of the flip-flop turns transistor Q1 on, and hence capacitor C rapidly discharges through the transistor. The output of the monostable remains low until a trigger pulse is again applied. Then the cycle repeats. Figure (c) shows the trigger input, output voltage, and capacitor voltage waveforms. As shown here, the pulse width of the trigger input must be smaller than the expected pulse width of the output waveform. Also, the trigger pulse must be a negative-going input signal with an amplitude larger than 1/3V_CC.

The time during which the output remains high is given by;

t_p = 1.1R_AC        seconds

where R_A is in ohms and C is in farads.

Once triggered, the circuit's output will remain in the high state until the set time tp elapses. The output will not change its state even if an input trigger is applied again during this time interval tp. However, the circuit can be reset during the timing cycle by applying a negative pulse to the reset terminal. The output will then remain in the low state until a trigger is again applied.