555 Timer Calculator
Calculate frequency, duty cycle and pulse width for astable and monostable 555 circuits.
Component Values
Results
Enter valid component values.
f = 1.44 / ((Ra + 2·Rb) × C)
How does the 555 timer work in astable mode?
In astable mode the 555 timer has no stable state — it oscillates continuously between high and low, producing a square wave at the output. The capacitor charges through Ra and Rb, then discharges through Rb only. This asymmetry means the high time is always longer than the low time, so the duty cycle is always above 50%.
Frequency and duty cycle depend on three components: Ra, Rb, and C. To lower the frequency, increase any of them. To get close to 50% duty cycle, make Rb much larger than Ra — the Ra contribution becomes negligible. For a true 50% duty cycle you need either a diode across Rb or a different circuit topology.
Frequency
f = 1.44 / ((Ra + 2·Rb) × C)Duty Cycle
D = (Ra + Rb) / (Ra + 2·Rb)Key Points
- Duty cycle always > 50% (Ra always in charge path)
- Frequency: f = 1.44 / ((Ra + 2·Rb) × C)
- To lower frequency: increase Ra, Rb, or C
- Minimum supply voltage: 4.5V (5V typical)
Applications
- LED blink circuits
- Clock signal generation
- PWM motor speed control
- Tone and sound generation