Last updated: 2026-06-16

555 Timer Astable Calculator

The 555 timer is one of the most widely used integrated circuits ever made, and its astable configuration turns it into a free-running square-wave oscillator. With just two resistors and one capacitor it produces a continuous train of pulses whose frequency and duty cycle are set entirely by those three components. This calculator takes $R_1$, $R_2$, and $C$ and returns the output frequency, period, duty cycle, and the high and low times of each cycle.

Formula

The capacitor charges through $R_1$ and $R_2$ while the output is high, and discharges through $R_2$ alone while the output is low. With $\ln 2 \approx 0.693$:

Because the charge path includes both resistors but the discharge path includes only $R_2$, the high time is always longer than the low time, so the duty cycle of the basic circuit is always above 50%.

Worked example

Take $R_1 = 1\ \text{k}\Omega$, $R_2 = 10\ \text{k}\Omega$, and $C = 100\ \text{nF}$.

The output is high for about 0.76 ms and low for about 0.69 ms in each cycle.

Component selection

Why it matters

Astable 555 circuits appear everywhere — LED flashers, tone generators, PWM dimmers, clock sources for logic, and metronomes. Knowing the exact frequency and duty cycle before building lets you pick standard resistor and capacitor values that land on the behaviour you want, instead of trimming components on the bench. For a near-square output, add a diode in parallel with $R_2$ so charge and discharge use separate resistors.