555 Timer Monostable Pulse Width Explained

How monostable mode works

The circuit has one stable state: output low, timing capacitor held discharged. A negative-going edge on the trigger pin flips the internal latch, the output goes high, and the capacitor starts charging through R.

When the capacitor reaches two-thirds of V₊₊, the threshold comparator resets the latch. The output falls, the capacitor is discharged, and the circuit waits for the next trigger. This is why it is called a one-shot.

Where 1.1 comes from

The capacitor must charge from zero to two-thirds of the supply. Solving the exponential charging equation for that point gives ln(3) = 1.0986, rounded to 1.1 in practice. As with astable mode, the supply voltage cancels out — the pulse width is the same on 5 V or 15 V.

Choosing components

Practical limits

• R between 1 kΩ and 1 MΩ. Below that the discharge transistor struggles; above it, leakage dominates.
• Beyond a few minutes, accuracy collapses. Electrolytic leakage can prevent the capacitor from ever reaching the threshold.
• Use film capacitors where timing accuracy matters. Electrolytic tolerance alone can be −20/+80%.
• Round to preferred values and recalculate rather than chasing an exact resistance.

Triggering correctly

• Pin 2 must be pulled below ⅓ V₊₊ — 1.67 V on a 5 V supply.
• Hold pin 2 high with a 10 kΩ pull-up when idle; a floating trigger causes random firing.
• A 100 nF series capacitor with that pull-up turns any long low into a short edge.
• Tie the reset pin (4) to V₊₊ unless you are actively using it.

Retriggering

The standard circuit ignores triggers that arrive while the output is already high — it is non-retriggerable. Some applications want the opposite:

• Non-retriggerable (default) — suits debouncing and fixed-length pulses.
• Retriggerable — add a transistor to discharge the timing capacitor on each trigger, so the pulse restarts. Useful for motion-sensor lighting that should stay on while movement continues.

Common applications

• Switch debounce. A 10-50 ms pulse masks contact bounce entirely.
• Missing-pulse detector. Retriggerable mode with T longer than the expected interval.
• Timed relay. Drive the coil through a transistor, with a flyback diode across it.
• Delayed switch-off. Fan run-on, stairwell lighting, courtesy lamps.

Output current

A bipolar 555 sources or sinks up to 200 mA, which is enough to drive an LED directly — still through a series resistor, as covered in our LED resistor guide. For anything heavier, use a transistor or a relay.

Improving timing accuracy

The 555 itself is stable; the timing components rarely are. Where the exact period matters, work on those first.

• Choose the capacitor carefully. Film or C0G ceramic holds tolerance; general ceramic and electrolytic do not.
• Favour a larger R and a smaller C. Resistors are available at ±1% far more cheaply than capacitors are.
• Add a trimmer. A fixed resistor plus a small preset lets you calibrate the period once and forget it.
• Keep the timing node clean. Flux residue across pins 6 and 7 forms a leakage path that shortens long periods unpredictably.

When to use something else

For periods beyond a few minutes, or where accuracy must survive temperature changes, a microcontroller with a crystal or an RC oscillator calibrated in firmware is both cheaper and far more precise than a 555 with a very large capacitor.

Design routine

1. Choose C from the table nearest your target time.
2. Rearrange: R = T ÷ (1.1 × C).
3. Round to the nearest preferred value and recalculate T.
4. Add a 100 nF decoupling capacitor at pin 8 and 10 nF at pin 5.
5. Confirm the trigger is a short edge, not a level.

Verify the numbers with the 555 monostable calculator, and see the astable guide if you need a continuous train instead of a single pulse.