Calculator KB-43
NE555 calculator: timer, monostable
A short pulse at the input produces a single pulse of precise duration at the output.
Result
In brief: NE555 monostable mode calculation: pulse duration based on R and C, and selection of an E24-series resistor for the desired delay. Example: Pulse duration: 1.1 s.
Calculation example
| R | 100 kΩ |
|---|---|
| C | 10 µF |
| Desired time (select R) | 500 ms |
| Pulse duration | 1.1 s |
|---|---|
| Trigger no more often than | 0.7576 Hz |
| R for desired time | 45.45 kΩ |
| Nearest E24 value | 47 kΩ |
How it is calculated
After triggering, the capacitor charges through R to 2/3 of the supply voltage, and the output stays high the whole time. Pulse duration is almost independent of the supply voltage.
What to look for
- The trigger pulse must be shorter than the output pulse; otherwise, the output will stay high longer than calculated.
- For accurate, long delays, a film capacitor is a better choice. Electrolytic capacitors have wide tolerances and leakage.
Frequently asked questions
How do you calculate the timing of a 555 monostable?
t = 1.1 · R · C. 100 kΩ and 10 µF give a 1.1-second pulse.
What resistor do you need for a 500 ms delay with 10 µF?
R = t / (1.1 · C) = 45.45 kΩ. The nearest E24 value, 47 kΩ, gives about 517 ms.
What's the maximum delay you can get with a 555?
Minutes with large R and C values, but electrolytic capacitor leakage and capacitance tolerance reduce accuracy. For long delays, a CD4060 counter or a microcontroller is a better choice.
