555 timer calculator (astable)
In astable mode a 555 oscillates on its own, charging the timing capacitor through R1 + R2 and discharging it through R2. Those two paths set the high and low times — and because charge always sees more resistance than discharge, the duty cycle is always above 50%.
Results
- Frequency
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- Period
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- High time
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- Low time
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- Duty cycle
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Results update as you type.
Formula
t_high = 0.693 × (R1 + R2) × C
t_low = 0.693 × R2 × C
f = 1.44 / ((R1 + 2·R2) × C)
duty = (R1 + R2) / (R1 + 2·R2)
How it works
With R in kΩ and C in µF the times come out in milliseconds and the frequency in the audio range for typical values. R1 and R2 of a few kΩ with a 10 nF–10 µF capacitor covers most blinker and tone circuits.
To get closer to a 50% duty cycle, make R1 much smaller than R2 — but R1 must stay above about 1 kΩ so the discharge transistor is not overloaded. A true square wave needs a diode across R2 or a different topology.
The 555 is not yet a first-class part in the palette, so you build the timing network from a source, resistors and a capacitor and reason about the thresholds. The timing maths here matches that circuit.