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555 Timer Calculator

Calculate frequency, duty cycle, and timing for 555 timer circuits.

Frequency
481.00 Hz
Period
2.08 ms
Duty Cycle
66.7%
T_high / T_low
1.39 ms / 693.00 µs
f = 1.44 / ((Ra + 2·Rb) · C)
T_high = 0.693 · (Ra + Rb) · C
T_low = 0.693 · Rb · C

Design 555 timer circuits with precision

Enter resistor and capacitor values to calculate frequency, period, and duty cycle for astable oscillators, or pulse width for monostable one-shots.

Frequently asked questions

What is astable mode?
In astable mode, the 555 timer oscillates continuously between HIGH and LOW, generating a square wave. The frequency and duty cycle are set by resistors Ra, Rb and capacitor C. Formula: f = 1.44 / ((Ra + 2×Rb) × C).
What is monostable mode?
In monostable (one-shot) mode, the 555 produces a single pulse of a fixed duration when triggered. The pulse width T = 1.1 × R × C.
Why can't duty cycle go below 50% in astable mode with standard wiring?
In standard astable wiring, both Ra and Rb charge the capacitor but only Rb discharges it. This means the HIGH time always exceeds the LOW time. To achieve ≤50% duty cycle, add a bypass diode across Rb.

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