Monostables

A monostable multivibrator, or one-shot, is a circuit that has one stable state and one quasi-stable state. In its stable state the output rests at a defined level. When triggered, it temporarily switches to the unstable state and then returns to the stable state after a time interval. The pulse width is set by timing components, typically an RC network, and is largely independent of the trigger width.

Operation: A trigger input initiates a pulse; the output transitions to the active level for a period T determined by R and C. After T, the circuit spontaneously reverts to its resting state. Triggers can be edge- or level-sensitive depending on design. In classic non-retriggerable monostables, arriving triggers during the timing interval have no effect; retriggerable types can extend the pulse.

Construction: Monostables can be built from discrete components or integrated into ICs. The 555 timer in its monostable mode is the most widely used example: a negative trigger causes the output to go high for a time T ≈ 1.1 R C, with the capacitor charging through R until the threshold is reached. Other implementations include logic family devices such as the 74xx123 or monostable configurations in op-amp or transistor circuits.

Applications: Pulse generation and shaping, debouncing mechanical switches, timing delays, digital edge detection, and sequencing in control systems. They are used wherever a fixed-duration pulse is needed from a trigger event.

Design considerations: Key issues include component tolerances and temperature dependence of RC timing, supply voltage sensitivity, and triggering sensitivity. Some designs require retriggerable behavior or symmetric output pulses; care must be taken to ensure proper reset and to prevent multiple triggering from bounce.