EMBEDDED TOOL
555 Timer Frequency Calculator
Calculate astable frequency/duty cycle or monostable pulse width for a 555 timer.
Frequency
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Period
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Duty cycle
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About this tool
Calculate the output frequency, period, and duty cycle of a classic 555 timer astable oscillator from R1, R2, and C — or the single-pulse width of a monostable (one-shot) configuration from R1 and C.
Frequently asked questions
- What are the astable formulas?
- Frequency f = 1.44 / ((R1 + 2·R2) · C), and duty cycle = (R1 + R2) / (R1 + 2·R2) — the classic 555 astable output is always above 50% duty cycle with this basic (diode-less) configuration.
- What's the monostable formula?
- The output pulse width is T = 1.1 × R1 × C, triggered once per falling edge on the trigger pin, regardless of how long the trigger stays low.
- Why is duty cycle always above 50% in astable mode?
- The capacitor charges through R1+R2 but discharges only through R2, so the high time is always longer than the low time in the standard circuit — a diode across R1 is a common trick to get closer to 50%, which this basic calculator doesn't model.