555 timer calculator
Astable frequency, duty cycle and high and low times from R1, R2 and C — or the pulse length of a monostable, with warnings on the resistor range.
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Frequency
1.388 Hz
Computed as 1 ÷ (0.693 × (R1 + 2·R2) × C). Textbooks round 1/0.693 to 1.44, which is the same sum to within 0.3%.
Period
721 ms
Output high for
395 ms
Output low for
326 ms
Duty cycle
54.8 %
Always above 50% — the capacitor charges through R1 and R2 but discharges through R2 alone.
Tie pin 4 to Vcc and fit 100 nF between pins 8 and 1, right at the chip. A floating reset pin and a missing decoupling capacitor are the two reasons a 555 that should oscillate does not.
Where this comes from
The number is the easy part. If you have to explain it to a panel, read the write-up — it goes through the formula, the assumptions and the mistakes that make the answer wrong.
The 555 timer: astable and monostableThe pin-out, both standard circuits with their formulas worked through, how to choose R and C, and the four wiring mistakes that stop a 555 oscillating.Other calculators
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