A monostable multivibrator or one-shot circuit is a type of electronic circuit that has one stable state and a single triggered unstable state. In its stable state, the circuit is in a high or low output state. However, when triggered, it temporarily enters an unstable state and then returns to the stable state after a specific time period. This type of trigger circuit is used in pulse generation and timing applications.

Block Diagram of Monostable Multivibrator

A monostable multivibrator consists of a switching circuit and an external timing network consisting of resistors ( R ) and capacitors ( C ). The switching circuit can be realized using transistors or operational amplifiers. The time constant of the RC network determines the ON-OFF duration of the trigger pulse. As this single triggering pulse is sufficient to get back to the stable state, the frequency of the output pulse is the same as that of the input triggering pulse.

Working Principle

Initially, assume that the output voltage V₀ is at +V_sat, i.e., in its stable state. The diode D₁ conducts and the voltage across the capacitor C, V_c, rises to 0.7 V. The voltage at the non-inverting terminal, V₂ is controlled by the voltage divider circuit comprising R₁ and R₂ and is given by:

Now, a negative trigger voltage V_T is applied to the non-inverting terminal, making the effective voltage at this terminal less than 0.7 V. This results in a change of op-amp output state from +V_sat to -V_sat. The diode D₁ is now reverse-biased and the capacitor starts charging exponentially to -V_sat through R. Now, the voltage at the non-inverting terminal is -βV_sat. A slightly more negative voltage than -βV_sat across the capacitor will result in a change of state in op-amp output to +V_sat. The capacitor now charges towards +V_sat through R until the voltage across it reaches 0.7 V.

Waveform of a Monostable Multivibrator Circuit

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