555 Timer IC as Monostable Multivibrator

With only minimal changes in wiring, the 555 timer discussed in previous article can be changed from a free-running oscillator (astable) into a switching circuit having one stable state and one quasistable state. The resulting monostable circuit is widely used in industry for many different timing applications. The normal mode of operation is to trigger the circuit into its quasistable state, when it will remain for a predetermined length of time. the circuit will then switch itself back(regenerate) into its stable state, where it will remain until it receives another input trigger pulse. Since, it has only one stable state the circuit is characterzed by the term monostable multivibrator or monostable.

The standard logic symbol for a monostable is shown below. The input is labeled Trigger, and the output is Q. The complement of the Q output may also be available at Q’. The input trigger circuit may be sensitive to either a positive terminal or negative terminal. In this case, it is negative-edge triggered. Usually the output at q is low when the circuit is in stable state.

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A typical set of waveforms showing the proper operation of a monostable circuit is shown in figure b’ below. In this case, the circuit rests in its stable state. Once triggered, Q goes high and remains high for a predetermined time t and then switches back to its stable unit another NT appears at the trigger input.

555 timer ic as monostable multivibrator

555 Timer IC as monostable multivibrator

A 555 timer wired as a monostable switching circuit is shown in figure c’. In its stable state, the timing capacitor C is completely discharged by means of an internal transistor connected to C at Pin 7. In this mode, the output voltage at Pin 3 is at ground potential.

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A negative pulse at trigger input Pin 2 will cause the circuit to switch to its quasistable state. The output at Pin 3 will go high and the discharge transistor at Pin 7 will turn off, thus allowing the timing capacitor to begin charging towards Vcc.

monostable multivibrator operation timer ic 555

Monostable Multivibrator 555 Internal Circuitry

When the voltage across C reaches 2/3 Vcc, the circuit will regenerate back to its state. The discharge transistor will again turn on and discharge C to GND, the output will go back to GND, and the circuit will remain in this state until another pulse arrives at trigger input.

The output of the monostable can be considered a positive pulse with a width.

t = 1. 1RA. C

trigger input output capacitor voltage waveforms monostable multivibrator 555

Input output waveforms of Monostable Multivibrator

Input voltage at the trigger input must be held at +Vcc and that a negative pulse should then be applied when it is desired to trigger the circuit into its quasistable ot Timing mode.

See also: Astable Multivibrator Operation of 555 Timer