Work out 555 timer monostable instantly with clear inputs, formula shown and shareable results.
One-shot pulse width is 1.1 RC, the time for the capacitor to charge from zero to two thirds of the supply — that is -ln(1/3) = 1.0986 time constants. Because the threshold is a fixed fraction of the supply, timing is independent of supply voltage.
555 monostable
t = 1.1 x R x C, from -ln(1 - 2/3) = 1.0986 time constants
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.
The threshold is derived from an internal divider, so it tracks the supply. The ratio the capacitor must reach stays at two thirds regardless of voltage.
Capacitor leakage and the timer's own input current. Above roughly 100 seconds a CMOS version and a low-leakage capacitor are needed.