Monostable

A monostable, in electronics, is a circuit that has one stable state. A common form is the monostable multivibrator, or one-shot, which produces a single transient output pulse in response to a triggering input. After the pulse, the circuit returns to its stable state.

Operation: In its rest state the output is at a defined level. A trigger initiates a timing interval during which the output switches to the opposite level. The duration is determined by a timing network—typically RC components or a digital counter—so the pulse width is controllable.

Implementation: Monostables can be built with analogue components (transistors, diodes, capacitors) or with integrated circuits such as the 555 timer in monostable mode or the 74xx123 family. In a 555-based circuit, the pulse width is approximately 1.1 × R × C, with practical tolerances.

Variants: Non-retriggerable monostables produce a single pulse; retriggerable versions extend or restart the pulse if a new trigger arrives during the timing interval. Triggers are commonly edge-triggered, but some designs are level-sensitive.

Applications: Debouncing buttons, pulse stretching, timing and sequencing in control systems, and simple delay elements in digital circuits. Monostables are contrasted with bistable circuits (flip-flops) that latch states, and with astable oscillators that continuously generate pulses.