Sunil Saharan

555 Timer Calculator (Astable and Monostable)

Get the output frequency, duty cycle and timings for a 555 astable oscillator, or the pulse width of a monostable.

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Formulas used

  • Astable: f = 1.44 / ((R1 + 2 x R2) x C)
  • High time = 0.693 x (R1 + R2) x C
  • Monostable: T = 1.1 x R x C

Frequently asked questions

Why is the duty cycle always above 50 percent?

The capacitor charges through R1 and R2 but discharges only through R2. A diode across R2 removes that limit.

What are good component ranges?

Keep R1 at 1 k ohm or more and total resistance below a few mega ohms.