555 one-shot timer

A trigger starts the stopwatch. The capacitor charges until the timer says ‘enough’, then the output falls.

t = ln(3) × R × C ≈ 1.1RC

Ideal monostable charging from zero to ⅔ of supply. Uses ln(3), the basis of the datasheet’s rounded 1.1RC. NE555 needs 4.5–16 V, suitable triggering and a trigger returning high at least 10 µs before pulse end. Leakage and tolerances matter at long times.