monostabil

Monostabil, short for monostable multivibrator, is a type of one-shot circuit that has a single stable state. When triggered, it briefly switches to a quasi-stable state and then automatically returns to its original stable state after a predetermined time. The duration of the output pulse is set by external components, typically a resistor and a capacitor, making the timing inexpensive and adjustable.

In common implementations, such as the 555 timer in monostable mode, a trigger event starts the timing cycle. The output goes high (or low, depending on configuration) and the timing capacitor begins charging through a resistor toward the supply voltage. The circuit remains in its active state until the capacitor voltage reaches a specified threshold (often two-thirds of the supply voltage), at which point internal logic resets the output and initiates a discharge path for the timing capacitor. After discharging, the circuit returns to its stable state, ready for the next trigger. In a fixed-ronge design the pulse width T is approximately 1.1 times the product of R and C (for the classic 555 configuration), though component tolerances and supply voltage variations can affect the actual duration.

Variants of monostable circuits include non-retriggerable and retriggerable types. In non-retriggerable designs, a new trigger during the output pulse is ignored, while retriggerable circuits can extend the pulse if triggers occur while the output is high, depending on the specific implementation.

Applications for monostable circuits include pulse stretching, debouncing of mechanical switches, timing control in digital circuits, debounced one-shot pulses for control signals, and simple delay elements. They are common in analog and mixed-signal designs, often implemented with the 555 timer, discrete RC networks with Schmitt triggers, or programmable logic.