Electronics · Free Calculator
555 Timer Astable Calculator
Find the output frequency and duty cycle of a 555 timer wired as an astable multivibrator.
FrequencyDuty cycleHigh and low timeNE555
Frequency6.87 Hz
Duty cycle52.4 %
Period145.6 ms
High time76.25 ms
Low time69.31 ms
How this result was calculated
- thigh = 0.693 × (R1 + R2) × C = 76.25 ms
- tlow = 0.693 × R2 × C = 69.31 ms
- f = 1 ÷ (thigh + tlow) = 6.87 Hz
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555 Astable Formulas
thigh = 0.693 (R1 + R2) C
tlow = 0.693 R2 C
f = 1.44 ÷ ((R1 + 2R2) C)
Duty = (R1 + R2) ÷ (R1 + 2R2)
tlow = 0.693 R2 C
f = 1.44 ÷ ((R1 + 2R2) C)
Duty = (R1 + R2) ÷ (R1 + 2R2)
Worked example
R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF: T = 0.693 × 21 kΩ × 10 µF = 0.146 s, so f = 6.87 Hz with a duty cycle of 52.4 %.
Key insight: in the basic circuit the duty cycle is always above 50 %, because C charges through R1 + R2 but discharges through R2 only; add a diode across R2 to get below 50 %.
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Frequently Asked Questions
What is the formula for 555 astable frequency?
f = 1.44 ÷ ((R1 + 2R2) × C).
How do I get a 50 % duty cycle from a 555?
Make R2 much larger than R1, or add a diode across R2 and set R1 = R2.
What is the maximum frequency of a 555?
About 500 kHz for the bipolar NE555; CMOS versions such as the 7555 go higher.
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