Monostable Multivibrator: A One-Shot Pulse Generator

A monostable multivibrator is a relaxation oscillator that delivers exactly one output pulse of a chosen duration each time an external signal triggers it. It has one stable state and one unstable state, returning to the stable state automatically once the pulse ends. That duration follows from the RC time constant of the circuit, which you can adjust by changing the resistor or capacitor value.

Monostable multivibrators are also known as one-shot, single-shot, single-swing, delay, or un-vibrator circuits. Timing applications rely on them heavily: timers, delay circuits, frequency dividers, pulse generators and pulse shapers all use their fixed-duration output.

How Does a Monostable Multivibrator Work?

A monostable multivibrator can be built from transistors, op-amps, or 555 timer ICs. Here, we explain the working principle using two bipolar junction transistors (BJTs).

The circuit includes two BJTs (Q1 and Q2), a capacitor (C), and four resistors (RC1, RC2, R1, and R2). The output is taken from the collector of Q2.

Initially the circuit rests in its stable state: Q1 off and Q2 on. The output sits low because the collector of Q2 is effectively tied to ground. The right plate of capacitor C rests near 0.7 V at the base of Q2, while the left plate charges gradually toward VCC through R1.

A positive trigger pulse T applied to the base of Q1 turns it on, and it conducts current through RC1. The collector of Q1 and the left plate of C drop toward ground. Because the voltage across a capacitor cannot change instantly, the right plate swings sharply negative, and that negative voltage reverse-biases the base-emitter junction of Q2, switching it off.

The circuit now sits in its unstable state: Q1 on and Q2 off. The output rises high because the collector of Q2 is pulled up to VCC. Meanwhile C recharges through its resistor until its right plate climbs back above about 0.7 V.

The moment the base-emitter junction of Q2 becomes forward-biased again, Q2 switches on. Its collector and the right plate of C drop to ground, and C begins charging through R2 in the opposite sense, driving Q1’s base low until Q1 turns off as well.

The circuit then returns to its stable state: Q1 off, Q2 on, output low. Capacitor C keeps charging through R1 until it reaches 0.7 V again, ready for the next trigger.

How long does the output stay high? The pulse width comes straight from the RC time constant:

Where T is the pulse width, R1 is the resistance in ohms, and C is the capacitance in farads.

The trigger rate sets how often pulses appear. Two conditions apply: each trigger pulse must be shorter than the pulse width T, and enough time must separate consecutive triggers for capacitor C to recharge fully. Otherwise the output pulse may fall short of full amplitude or duration.

Applications of Monostable Multivibrators

Monostable multivibrators are useful for generating fixed-duration pulses that respond to some external event or signal. Some examples of their applications are:

• Timers: Monostable multivibrators can create timers that produce a delay or timeout after a trigger signal. For example, they can switch a device off after a set period or raise an alarm after a preset interval.
• Delay circuits: Monostable multivibrators can introduce a lag between an input signal and the matching output. Digital systems use them to align signals; analog systems use them to schedule events.
• Gated circuits: A monostable stage can enable or disable an output based on a trigger. Examples include gating an oscillator or counter on for a fixed window or switching between two signals.
• Frequency dividers: Triggering a monostable faster than its pulse width lets it swallow extra input edges, dividing the input frequency. Cascaded stages divide a clock by two or more.
• Pulse generators: Monostable multivibrators produce single pulses with precise widths on demand, handy for sampling signals or exercising circuits under test.
• Pulse shapers: Feeding any ragged input into a monostable yields a clean fixed-width output, sharpening slow edges or swallowing noise spikes and glitches.

Advantages and Disadvantages of Monostable Multivibrators

Monostable multivibrators have some advantages and disadvantages compared to other types of multivibrators or oscillators. Some of them are:

• Advantages:
  • They are simple in design and inexpensive.
  • They produce clean square-edged output pulses with fast transitions.
  • They have high noise immunity and stability.
  • They can operate over a wide range of frequencies and voltages.
  • They can be easily triggered by various types of signals.
• Disadvantages:
  • They require external triggering for their operation.
  • They have limited duty cycle range and accuracy.
  • They have low power efficiency and high power dissipation.
  • They may have false triggering due to stray capacitance or leakage currents.
  • They may have variations in their output pulse width due to temperature changes or component tolerances.

Summary

A monostable multivibrator is a type of relaxation oscillator that produces one output pulse each time an external signal triggers it.

One state is stable, one is not, and the circuit returns to the stable state automatically once the pulse ends.

The RC time constant sets the output pulse duration, which you can adjust by changing either the resistor or the capacitor value.

Monostable multivibrators are widely used in timing applications, such as timers, delay circuits, frequency dividers, pulse generators and pulse shapers.

Simple and inexpensive they remain, though duty-cycle range, accuracy, power efficiency and false triggering impose real limits.