555 Timer Monostable

Formula

Description

In monostable (one-shot) mode, the 555 produces a single output pulse of a precise duration each time it receives a trigger input. The output goes high for exactly 1.1 × R × C seconds, regardless of the trigger duration, then returns low and waits for the next trigger. The factor 1.1 comes from the natural logarithm of 3 (the ratio of Vcc to the 1/3 Vcc threshold), which is ln(3) ≈ 1.0986. Monostable mode is used for debouncing switches, generating fixed-duration timing pulses, and missing pulse detection.

Variables

• t — Output pulse duration (s)
• R — Timing resistor (Ω)
• C — Timing capacitor (F)

Practical Notes

The monostable is not retriggerable in the standard 555 configuration. For retriggerable operation, use a 74HC123 or similar dedicated monostable IC. For very long timing intervals (minutes to hours), consider a microcontroller timer instead, as large RC values suffer from leakage current and tolerance issues. The timing accuracy of the 555 is typically 1-2% with good quality components.