555 Timer Astable Calculator
Enter R1, R2 and the timing capacitor to find the 555 timer oscillation frequency, period, high and low output times and the duty cycle in astable mode. The classic NE555 astable always runs above a 50% duty cycle because R1 sits in the charge path only.
How to use the 555 timer calculator
- Enter resistor R1 in ohms (or kilohms).
- Enter resistor R2 in ohms (or kilohms).
- Enter the timing capacitor in farads (or microfarads).
- Read the frequency, period, high and low times and duty cycle.
Examples
1k, 10k and 1 microfarad
R1 = 1000 ohm, R2 = 10000 ohm, C = 1e-6 F
frequency = 68.57 Hz, duty = 52.38%
Lower frequency
R1 = 4700 ohm, R2 = 47000 ohm, C = 1e-7 F
frequency about 145.9 Hz
Frequently asked questions
What is the 555 astable frequency formula?
Frequency is 1.44 / ((R1 + 2 R2) C). For R1 = 1k, R2 = 10k and C = 1 microfarad the result is 68.57 Hz.
How is the duty cycle calculated?
Duty cycle is (R1 + R2) / (R1 + 2 R2) times 100. The output is high while the capacitor charges through R1 and R2, and low while it discharges through R2.
Why is the duty cycle always above 50%?
In the standard astable circuit the capacitor charges through R1 and R2 but discharges only through R2, so the high time is always longer than the low time. A diode across R2 can bring it near 50%.
What are the high and low times?
High time is 0.693 (R1 + R2) C and low time is 0.693 R2 C, where 0.693 is the natural log of 2. They add up to the full period.
Does this cover monostable (one-shot) mode?
No. This tool covers astable oscillator mode. A monostable pulse width is a separate formula, 1.1 R C, using a single resistor.
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