555 Timer
555 Monostable Calculator
Find the output pulse width of a 555 one-shot from its timing resistor and capacitor, using t = 1.1 x R x C.
555 Monostable Timing
One output pulse per trigger, set by R and C.
Where the 1.1 Constant Comes From
In monostable mode the 555 rests with its output low until a falling edge arrives at the trigger pin. The output then goes high and the timing capacitor charges through the timing resistor. When the capacitor reaches two thirds of the supply voltage, the internal comparator ends the pulse, the capacitor is discharged, and the circuit waits for the next trigger.
The constant comes from the exponential charging curve. Reaching two thirds of the supply takes ln(3) time constants, which is approximately 1.0986, conventionally rounded to 1.1. Because the threshold is a fraction of the supply rather than a fixed voltage, the pulse width does not change appreciably when the supply voltage does.
Only one resistor and one capacitor set the timing, which makes this the simplest useful timing circuit in common use.
How to Use the 555 Monostable Calculator
Enter the timing resistor and the timing capacitor, choosing units from the selectors beside each field. Kilohms and microfarads are the usual working units, and both are handled directly.
Select Calculate. The pulse width is shown in a readable unit chosen automatically, then repeated in seconds, milliseconds and microseconds so you can pick whichever suits your notes. The RC time constant and the maximum retrigger rate are given alongside.
The waveform diagram shows the relationship between the trigger pulse and the output pulse, with the calculated width labelled beneath it.
Worked Example: A One-Second Output Pulse
With R = 100 kΩ and C = 10 µF:
One trigger produces a single output pulse lasting about 1.1 seconds, regardless of how long the trigger itself is held, provided the trigger is released before the pulse ends.
For a much shorter pulse, reduce the capacitor. R = 10 kΩ with C = 10 nF gives 1.1 × 10 000 × 0.00000001 = 110 µs, a typical figure for debouncing or for generating a short strobe. Doubling either component doubles the pulse width, so the two are interchangeable within their practical limits, which are roughly 1 kΩ to 10 MΩ and 100 pF to a few hundred microfarads.
Reading Your Monostable Timing Results
The pulse width is how long the output stays high after each valid trigger. It is set entirely by R and C and is unaffected by supply voltage or by trigger duration, as long as the trigger falls and rises again within the pulse.
The time constant, R multiplied by C, is the underlying quantity. The pulse is 1.1 time constants long, and knowing the constant makes it easy to estimate the effect of changing a component in your head.
Maximum retrigger rate is simply the reciprocal of the pulse width and represents an upper bound. In practice the capacitor also needs time to discharge fully between pulses, so the usable rate is somewhat lower.
Where 555 One-Shot Pulses Are Used
Switch debouncing is a common use. A mechanical contact bounces for a few milliseconds when it closes, producing multiple edges; a monostable set to around 20 ms converts that mess into one clean pulse.
Timed outputs are another. A single press producing a fixed period of illumination, a fixed run time for a small motor, or a fixed delay before a following stage all map naturally onto a one-shot. Two monostables in sequence create a pulse that begins after a delay, which is a standard building block.
The circuit is also used for pulse stretching, where a very short input event is extended into something long enough for a slower stage or a human to notice. A sensor producing a pulse a few microseconds wide can be stretched into a hundred milliseconds, which is long enough to light an indicator visibly or to be read reliably by a slow polling loop.
Missing-pulse detection is a related use. If a monostable is retriggered regularly by an incoming signal, its output stays high continuously; when the signal stops, the pulse finally times out and the falling edge flags the fault. That arrangement is the basis of simple watchdog and tachometer circuits.
Limits of the Monostable Timing Model
- The trigger pulse must be shorter than the output pulse. If the trigger is held low for longer, the output stays high until the trigger is released.
- Capacitor tolerance sets the real accuracy. Electrolytics can be 20% or more away from their marked value and leak measurably, which makes long timings unreliable.
- Very long delays, beyond roughly a minute, are better handled by a counter or a microcontroller than by a large electrolytic capacitor.
- Keep the timing resistor between about 1 kΩ and 10 MΩ. Below that the chip is loaded heavily; above it, leakage currents become comparable to the charging current.
Questions
555 Monostable FAQs
What happens if the trigger stays low longer than the pulse?
The output remains high until the trigger is released. For a fixed-width pulse regardless of input duration, differentiate the trigger with a small series capacitor and pull-up resistor.
Can I retrigger the timer before the pulse ends?
Not in the basic configuration; further triggers during the pulse are ignored. Retriggerable behaviour needs the reset pin driven or a dedicated retriggerable arrangement.
Why is my measured pulse shorter than calculated?
Usually capacitor tolerance and leakage, especially with electrolytics. Comparator offset within the chip contributes a smaller error. Film capacitors give much closer agreement.
Does supply voltage affect the pulse width?
Only slightly. The two-thirds threshold scales with the supply, so the timing is essentially independent of it across the chip's operating range.
Work the Trigger Circuit Out on the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.