555 Timer Monostable (One-Shot) Calculator
555 Timer Monostable (One-Shot) Calculator
Pulse width of a 555 one-shot from R and C — or the resistor or capacitor you need for the pulse width you want, rounded to a real E12 or E24 value, with the datasheet’s trigger rules.
555 monostable one-shot
R = 100 kΩ and C = 1 µF
555 monostable timing
- ln 3
- 1.0986 — the capacitor starts at 0 V and stops at ⅔Vcc, which is ln3 time constants up an exponential; the datasheet rounds it to 1.1
- R
- from Vcc to the threshold/discharge node
- C
- from that node to ground; the discharge pin shorts it out between pulses
- t
- how long the output stays high after a trigger, in seconds
Worked example
R = 100 kΩ and C = 1 µF
RC = 100 kΩ × 1 µF = 100 ms
t = ln3 × RC = 1.0986 × 100 ms = 109.9 ms
The datasheet's rounded 1.1 would give 110 ms, 0.13% longer — well inside the tolerance of any real capacitor
The capacitor has to reach ⅔ × 5 V = 3.333 V, and starts out charging at 5 V ÷ 100 kΩ = 50 µA
Pulse width for common R and C pairs
| C | R = 1 kΩ | R = 10 kΩ | R = 100 kΩ | R = 1 MΩ |
|---|---|---|---|---|
| 1 nF | 1.099 µs | 10.99 µs | 109.9 µs | 1.099 ms |
| 10 nF | 10.99 µs | 109.9 µs | 1.099 ms | 10.99 ms |
| 100 nF | 109.9 µs | 1.099 ms | 10.99 ms | 109.9 ms |
| 1 µF | 1.099 ms | 10.99 ms | 109.9 ms | 1.099 s |
| 10 µF | 10.99 ms | 109.9 ms | 1.099 s | 10.99 s |
| 100 µF | 109.9 ms | 1.099 s | 10.99 s | 109.9 s |
How the 555 one-shot works
In monostable mode the 555 sits with its output low and the timing capacitor held at 0 V by the internal discharge transistor. Pull the trigger pin below ⅓Vcc and the flip-flop sets: the discharge transistor releases, the output goes high, and C starts charging from 0 V through R toward Vcc. When C reaches ⅔Vcc the upper comparator resets the flip-flop, the output goes low and the discharge transistor empties C again, ready for the next trigger. An exponential rising toward Vcc covers two thirds of the way in ln 3 = 1.0986 time constants, which is the 1.1 in the datasheet’s t = 1.1 R C. The rounding is 0.13% long — nothing next to a ±10% capacitor, but this page uses the exact value so the arithmetic is the arithmetic.
Triggering. The trigger is level-sensitive, not edge-sensitive, and that catches people out. TI’s datasheet says a further trigger during the timing cycle has no effect “so long as the trigger input is returned high at least 10 μs before the end of the timing interval” — if the trigger is still low when the cycle ends, the output simply stays high. Couple a long or repeating trigger signal through a small capacitor and a pull-up resistor so the pin sees a short negative spike, and tie the reset pin to Vcc when you are not using it, which the datasheet recommends “to avoid any possibility of false triggering”.
Recovery. The chip is not ready for the next pulse the instant the output falls. C has to be emptied through the discharge transistor first, and until it is back near 0 V a new pulse will be short. The discharge is fast — the transistor’s saturation resistance is small — but with a large capacitor it is not instant, so leave a gap between pulses. At the other end, TI notes that the lower comparator’s storage time can be as long as 10 µs, “which limits the monostable pulse width to 10 µs minimum”; for shorter pulses use a logic one-shot or a microcontroller timer.
Choosing the parts. Fix the capacitor first and solve for the resistor: resistors come in E24 steps of about 5%, capacitors in E6 steps of about 20%. For a 1 ms pulse with a 100 nF capacitor the exact resistor is 9.102 kΩ; the nearest E24 value, 9.1 kΩ, gives 0.9997 ms — -0.03% off. Keep R between about 1 kΩ and 1 MΩ and the supply voltage out of it: the thresholds are fractions of Vcc, so the pulse width does not change with supply. For a continuous square wave rather than one pulse, use the 555 astable calculator; for the exponential behind the timing, the RC time constant calculator; to read the capacitor’s printed code, the capacitor code calculator.
Frequently asked questions
What is the 555 monostable formula?
t = 1.1 × R × C, where the 1.1 is really ln 3 = 1.0986 — the number of time constants an exponential takes to climb from 0 to two thirds of the way to the supply. 100 kΩ and 1 µF give 109.9 ms.
How do I choose R and C for a 555 one-shot?
Pick the capacitor from what you have, then R = t ÷ (1.0986 × C), and round to the nearest E12 or E24 value. For 1 ms with 100 nF that is 9.102 kΩ exactly, 9.1 kΩ in practice. Keep R roughly between 1 kΩ and 1 MΩ.
Why does my 555 one-shot stay high?
The trigger input is level-sensitive. If pin 2 is still held below ⅓Vcc when the timing period ends, the output stays high. Couple the trigger through a capacitor with a pull-up resistor so the pin only sees a brief negative pulse, and return it high at least 10 µs before the pulse ends.
Does the supply voltage change the pulse width?
No. The capacitor charges toward Vcc and the comparator trips at ⅔Vcc, so the fraction — and therefore the time — is the same at any supply between 4.5 V and 16 V.
What is the shortest pulse a 555 can produce?
About 10 µs. TI states that the lower comparator’s storage time can be as long as 10 µs when pin 2 is driven fully to ground, which sets the floor on the monostable pulse width.
Related calculators
References
- Texas Instruments. LM555 Timer, datasheet SNAS548. Astable: t1 = 0.693(RA+RB)C, t2 = 0.693 RB C, T = 0.693(RA+2RB)C, duty cycle (RA+RB)/(RA+2RB); monostable t = 1.1 RA C; supply 4.5–16 V, supply current 3–6 mA at 5 V.
- Texas Instruments. LMC555 CMOS Timer, datasheet SNAS558N (revised March 2024). “Fast astable frequency of 3MHz”; “Less than 1mW typical power dissipation at 5V supply”; specified down to a 1.5 V supply; f = 1.44/[(RA+2RB)C].
- Horowitz P, Hill W. The Art of Electronics, 3rd ed. Cambridge University Press, 2015. §7.1.3, “The classic oscillator–timer chip: the 555”.
- IEC 60063:2015. Preferred number series for resistors and capacitors (E6, E12, E24, E48, E96 and E192). International Electrotechnical Commission.