555 Timer Monostable Calculator

How it works

pulse width t = 1.1 × R × C (exactly ln 3 × R × C ≈ 1.0986 RC) · R for a target time = t ÷ (1.1 × C) · maximum trigger rate ≈ 1 ÷ t

In monostable (one-shot) mode a falling edge on the trigger pin (pin 2) sets the 555 output high and lets the timing capacitor charge through R. When the capacitor reaches two-thirds of the supply the threshold comparator resets the output and the discharge transistor empties the capacitor. Because the trip point is a fixed fraction of the supply, the pulse width ln(3)·RC ≈ 1.1RC does not depend on the supply voltage. Electrolytic capacitor tolerance (often ±20%) and leakage dominate the real error on long delays.

Worked example

With R = 100 kΩ and C = 10 µF the pulse lasts 1.1 × 100,000 × 0.00001 = 1.1 s, so the circuit can be retriggered at most about 0.9091 Hz. For a 5 s delay with the same capacitor you need R = 5 ÷ (1.1 × 10 µF) = 454.5 kΩ; the nearest E24 value, 470 kΩ, gives 5.17 s.

Sources: Texas Instruments — NE555 precision timer datasheet (monostable operation) · Wikipedia — 555 timer IC

Frequently asked questions