555 Timer — Monostable Calculator

One-shot pulse width from R and C, or the R needed for a target delay.

What it does

In monostable mode a 555 produces one pulse of fixed length each time it is triggered: T = 1.1 × R × C. It can also find the resistor for a pulse length you need.

Where it's used

Switch debouncing, time delays, pulse stretching and “stay on for N seconds” circuits.

How to use it

Choose whether to calculate the pulse width or the resistor.
Enter R (or the target pulse width) and C.
Enter the supply voltage.

The formula explained

T: pulse width, in seconds
R: timing resistor
C: timing capacitor
1.1: from ln 3 ≈ 1.0986

Worked example

Finding R for a 5-second pulse with a 100 µF capacitor.

Inputs:

Calculate: Resistor for a target pulse
Target pulse width: 5 s
Capacitor C: 100u F
Supply Vcc: 9 V

The calculator gives:

Pulse width T: 5 s
Resistor R: 45.45 kΩ
Nearest E24 R: 47 kΩ

Tips and common mistakes

The trigger pulse must be shorter than the output pulse.
Electrolytic leakage makes long delays longer and less repeatable.
A standard 555 ignores new triggers until the pulse ends.

Questions people ask

What resistor gives a 5-second pulse with 100 µF?

R = T / (1.1 × C) = 5 / (1.1 × 100 µF) ≈ 45.5 kΩ. The nearest standard value, 47 kΩ, gives about 5.2 s — close enough for most delays, given the capacitor’s own tolerance.

How do I trigger the 555?

Pull the trigger pin (pin 2) below one third of the supply voltage for a moment. A push-button to ground with a 10 kΩ pull-up resistor to Vcc is the usual way; the pulse starts on the falling edge.

How can I get much longer delays?

Use a CMOS 555 (such as the TLC555), which works with resistors of several megaohms, together with a low-leakage film capacitor. For minutes or hours, a counter chip such as the CD4060 dividing a faster clock is more accurate.

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