555 Timer — Astable

How to use this calculator

In astable mode the 555 runs continuously, charging C through RA+RB and discharging through RB. The frequency is f = 1.44 / ((RA + 2·RB)·C) and the duty cycle is (RA + RB) / (RA + RB). Enter RA, RB and C and the frequency, period, duty, high time and low time all update live.

What it means in a real circuit

This is the classic blinker and clock. For RA = 10 kΩ, RB = 10 kΩ, C = 10 µF you get f ≈ 4.8 Hz (a slow blink) with a 67% duty cycle. Note RA must be greater than RB for a stable 50–100% duty range, and the duty cycle is always above 50% in this basic configuration (add a diode across RB to go below 50%). Use it to size an LED blinker, a buzzer tone, or a clock for a counter.