NE555 / LM555 Monostable Pulse Width Calculator
Use this tool to calculate output pulse width for a 555 timer in monostable (one-shot) mode using the standard equation T = 1.1 × R × C.
How the 555 monostable calculator works
In monostable mode, the 555 timer produces one output pulse each time it receives a trigger. The pulse length is set by an external resistor and capacitor. During timing, the capacitor charges through the resistor until it reaches the internal threshold level, and the output then returns to its stable state.
T = 1.1 × R × C
where T is in seconds, R in ohms, and C in farads.
This calculator converts your selected units automatically, computes the pulse width, and shows equivalent component suggestions when a target time is entered.
Practical design notes
1) Choose realistic component ranges
For a classic bipolar NE555, practical values are often:
- R: 1 kΩ to 3.3 MΩ (very high values can be sensitive to leakage and noise)
- C: 1 nF to 1000 µF (large electrolytics can have wide tolerance)
2) Account for tolerance and drift
The actual pulse width may vary because resistor and capacitor tolerances stack. A ±5% resistor and ±10% capacitor can yield noticeable timing spread. If precision matters, use tighter tolerance parts and stable capacitor dielectrics.
3) Keep trigger and power clean
False triggering is common in noisy environments. Good layout, decoupling capacitors near VCC/GND, and proper trigger conditioning improve reliability.
Example calculations
Example A: About 1 second pulse
Choose R = 10 kΩ and C = 100 µF.
T = 1.1 × 10,000 × 0.0001 = 1.1 s
Example B: About 50 ms pulse
Set C = 1 µF, solve for resistor:
R = T / (1.1 × C) = 0.05 / (1.1 × 0.000001) ≈ 45.5 kΩ
When to use a 555 monostable
- Push-button pulse generation
- Relay on-delay / pulse stretching
- Missing pulse detection front-end
- Simple watchdog and timing experiments
- Educational analog timing circuits
Troubleshooting checklist
- Pulse too short? Verify capacitor value and unit (µF vs nF mix-up is common).
- Pulse unstable? Reduce resistor value and check leakage currents.
- No output pulse? Confirm trigger goes below 1/3 VCC momentarily.
- Retrigger behavior unexpected? Review trigger pulse width and input conditioning.
- Large timing error? Measure real R/C values and include tolerance in design.
Summary
The 555 monostable remains one of the easiest ways to create a fixed one-shot pulse. Use this calculator for quick timing estimates, then validate the final design with real component tolerances and your actual operating conditions.