the monostable multivibrator (one-shot)
A monostable multivibrator, nicknamed a one-shot, has exactly one resting state and stays there until you poke it. Give it a trigger and it produces a single pulse of a precise, preset length, then returns to rest and waits for the next trigger. Picture a spring-loaded door that you push open: it stays open for a fixed time, then swings firmly shut and stays shut. Monostable means one stable state.
In a 555 wired as a monostable, the output normally sits low. A brief trigger (the trigger pin pulled below one-third of the supply) flips the output high and lets a single resistor R start charging a capacitor C from zero. The output stays high while the capacitor charges; the instant the capacitor reaches two-thirds of the supply, the chip snaps the output back low and dumps the capacitor, ready for the next trigger. The pulse width is a clean 1.1 times R times C, independent of the supply voltage. For example R equal to 100 kilohm and C equal to 10 microfarad gives a pulse of 1.1 times 100000 times 10 microfarad equals 1.1 seconds.
One-shots are the standard way to turn a messy or momentary event into a clean, fixed-length pulse: stretch a too-short button press into a usable signal, light a stairwell lamp for exactly 30 seconds after a press, or generate a precise timing window. The honest caveats: the basic 555 monostable is not retriggerable in the simple sense, a new trigger during the pulse is ignored, and if the trigger input is held low past the end of the timing, the output stays high, so the trigger should be a brief pulse. As always with RC timing, the pulse length is only as accurate as your R and C.
For a stairwell light that stays on after a button tap, pick R equal to 100 kilohm and C equal to 10 microfarad. The pulse is 1.1 times 100000 times 10 microfarad equals 1.1 seconds; scale C up to 270 microfarad for about 30 seconds. The brief tap triggers it; the timing does the rest.
One trigger, one clean pulse of width 1.1 times R times C, then back to rest.
The basic 555 one-shot ignores new triggers during its pulse, and if the trigger is held low past the timing end the output will not go low, so feed it a short pulse. The pulse length depends on RC tolerance, not a precise reference.