555 Timer Astable Calculator

Work out how fast a 555 timer oscillates in astable mode and how long its output stays high and low.

Frequency
Period
High time
Low time
Duty cycle

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How it works

In astable mode the timing capacitor charges through R1 and R2 and discharges through R2 only. The output is high while charging and low while discharging, so the high time is always longer than the low time.

Because of that, the duty cycle of the basic circuit is always above 50%. Adding a diode across R2 lets you get duty cycles below 50%.

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)

Worked example

R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF: f = 1.44 / (21,000 × 0.00001) = 6.86 Hz. The output is high for 76 ms and low for 69 ms, a duty cycle of 52.4%, a good LED blinker.

Frequently asked questions

Why can't I get a 50% duty cycle?

The capacitor charges through R1 + R2 but discharges through R2 alone, so high time is always longer. Make R1 much smaller than R2 to approach 50%, or add a diode across R2.

What's the minimum value for R1?

Keep R1 at 1 kΩ or more so the discharge transistor isn't overloaded when it shorts the supply through R1.

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