A monostable multivibrator, commonly known as a one-shot circuit, is a type of electronic circuit that generates a single pulse output in response to an external trigger or input signal. It is called "monostable" because it has one stable state and one unstable state. The circuit remains in its stable state until triggered, and then it switches to an unstable state for a specific duration before returning to its stable state automatically. The output pulse width (the time it spends in the unstable state) is determined by the circuit's design and external components.
The basic components of a monostable multivibrator circuit are a trigger input, a timing resistor (R) and timing capacitor (C) network, and an output stage. The operation of a monostable multivibrator can be explained in the following steps:
Initial State (Stable State):
When no trigger signal is applied, the circuit is in its initial stable state.
The output is in a defined logic state (either high or low, depending on the circuit design).
The timing capacitor (C) is discharged and holds zero voltage.
When a trigger signal is applied to the trigger input of the circuit, the monostable multivibrator is activated.
The trigger signal can be a positive or negative pulse, depending on the circuit design.
The trigger signal causes a temporary change in the circuit's state, forcing it to switch to the unstable state.
The timing capacitor (C) starts to charge through the timing resistor (R) connected to it.
The timing capacitor charges exponentially, following the RC time constant (τ = R * C), until it reaches a certain threshold voltage level.
During the charging of the timing capacitor, the output remains in the opposite state from its initial state (e.g., if the output was initially high, it becomes low during the unstable state).
The output stage is driven to this opposite state while the capacitor charges.
Stable State Restoration:
Once the timing capacitor reaches the threshold voltage level, the output stage quickly returns to its initial state (the stable state).
The timing capacitor now discharges rapidly, which resets the circuit.
Output Pulse Duration:
The duration of the output pulse is determined by the RC time constant (τ = R * C).
A larger value of R or C will result in a longer pulse duration, while smaller values will produce shorter pulses.
Ready for Next Trigger:
After the output returns to the stable state, the circuit is ready to be triggered again.
Monostable multivibrators are widely used in electronic circuits for various applications, such as pulse generation, time-delay circuits, and debouncing switches, where a clean, single pulse is required in response to an input event.