555 Timer Monostable Calculator
Enter R and C for the pulse width, or enter a target time and capacitor to get a standard resistor.
Step-by-step
- Enter valid values to begin.
Model: ideal one-shot with the threshold at 2/3 VCC, T = ln(3) × R × C = 1.1 RC. The trigger input must fall below 1/3 VCC and return high before the timing interval ends, otherwise the output stays high until the trigger is released. The initial error of about 1%, 50 ppm/°C and 0.1%/V figures are typical bipolar NE555-class datasheet values, the 200 mA output rating is the typical bipolar limit (CMOS parts source and sink much less), and the 10 MΩ resistor ceiling, 5-time-constant recovery and 50 Ω discharge-transistor resistance are engineering estimates, not guaranteed limits. Check the datasheet of your exact part. E12 values are from the IEC 60063 series.
555 Monostable Timing: One Resistor, One Capacitor, One Pulse
Core Engineering Principles
In the one-shot configuration a falling trigger below one third of VCC sets the output high and releases the capacitor. It charges through a single resistor toward the supply and the comparator ends the pulse at two thirds. Charging from 0 to 2/3 of the final value takes ln 3 = 1.0986 time constants, which we round to 1.1. VCC cancels again, so the pulse width moves only about 0.1% per volt of supply change.
The trigger rules cause most of the field failures. The trigger must go low, below VCC/3, and then return high before the pulse ends. If it stays low longer than T, the output stays high until it is released, and your pulse width is whatever the trigger is. A trigger during the interval is ignored, and one right after it can arrive before the capacitor has discharged, giving a short pulse. For delays of minutes, remember that an electrolytic’s leakage current fights the timing resistor: use film capacitors at a few microfarads and resistors below about 10 MΩ, or let a counter divide a faster 555.
Trigger < VCC / 3, pulse width < T • Spread = tolR + tolC + IC error
NEC & Standard References
The NE555 / LM555 datasheets from TI and ST give T = 1.1 R C, the trigger threshold of one third of VCC, a typical initial timing error of about 1% and a drift near 50 ppm/°C, plus an output rating of about 200 mA for the bipolar part. TLC555 and LMC555 are the CMOS equivalents with much lower supply current and lower output drive. IEC 60063 provides the E12 preferred values used to round R, and IEC 60384-1 is the generic fixed-capacitor specification covering tolerance and leakage classes. Confirm figures against the exact part’s datasheet.
1. T = 1.1 × 100 kΩ × 10 µF = 1.100 s.
2. Worst-case spread = 1% + 10% + 1% = 12%, so T is 0.968 s to 1.232 s.
3. Trigger threshold = 12 / 3 = 4.00 V; 1 V is safely below it and the 10 ms pulse is far shorter than T.
4. Recovery ≈ 5 × 50 Ω × 10 µF = 2.5 ms, so the maximum trigger rate is about 0.907 Hz.
5. The 20 mA load is well within the 200 mA output rating.
- Keep the trigger short. A switch held for longer than T makes the output follow the switch, so condition it with a differentiating RC network.
- Watch the output current. The bipolar output is rated to about 200 mA, but the high level droops under load; a CMOS 555 cannot drive a relay coil without a transistor.
- Add a flyback diode. A relay coil on the output generates spikes that can destroy the timer.
- Electrolytics stretch timing. Leakage steals charging current, so intervals longer than a few minutes need film capacitors or a counter.