555 Timer Astable Calculator
Design a 555 oscillator from component values.
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555 Timer Astable Calculator by TheCalcTools
How the 555 Timer Astable Calculator works
Enter R1 and R2 in kilohms and the timing capacitor in microfarads.
The capacitor charges through R1 + R2 and discharges through R2.
That sets the high time, the low time and so the frequency and duty cycle.
The formula
Worked example
Using the default values shown in the calculator:
| R1 (kΩ) | 1 |
|---|---|
| R2 (kΩ) | 10 |
| C (µF) | 10 |
- R1 + 2R2 = 1 + 20 = 21 kΩ.
- f = 1.44 ÷ (21,000 × 0.00001) = 6.857 Hz.
- Duty cycle = 11 ÷ 21 = 52.4%.
Result:
| Frequency (Hz) | 6.857 |
|---|---|
| Period (ms) | 145.83 |
| High time (ms) | 76.23 |
| Low time (ms) | 69.30 |
| Duty cycle | 52.4% |
Frequently asked questions
Why is the duty cycle always above 50%?
In the basic astable circuit the output is high while charging through R1 + R2 and low while discharging through R2 only, so the high time is always longer. Making R1 small gets close to 50%.
How accurate is it?
The 1.44 constant is approximate (1 ÷ ln 2), and component tolerance and temperature affect the result by a few percent.
Sources and further reading
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