555 Timer Calculator
Astable and monostable modes — enter R and C values to get frequency, period, duty cycle, or pulse width.
Frequency
68.7 Hz
Period
14.6 ms
Duty cycle
52.4%
High time
7.62 ms
Low time
6.93 ms
Formula
f = 1.44 / ((R1 + 2·R2)·C)
Astable mode. Monostable pulse width t = 1.1·R·C.
The two 555 modes
In astable mode the 555 free-runs, producing a continuous square wave — useful for clocks, blinkers and tone generators. The mark/space ratio (duty cycle) is set by R1 and R2; the animated waveform above tracks it.
In monostable (one-shot) mode it outputs a single pulse of a set width each time it's triggered — handy for debouncing, timeouts and delay circuits.
Design notes
- Astable duty cycle is always >50% in the classic circuit because R1 is in the charge path but not the discharge path.
- Add a diode across R2 to reach (or go below) a 50% duty cycle.
- Avoid very small capacitors (<1 nF) and very large resistors (>1 MΩ) where leakage makes timing unreliable.
- The 7555 (CMOS) version runs at lower current and higher frequencies than the classic NE555.
Where it's used
LED flashers, PWM dimming, simple oscillators, servo-pulse generators and timing delays in hobby and teaching circuits.
Frequently Asked Questions
What is astable mode?+
The 555 oscillates continuously between high and low, producing a square wave. Frequency = 1.44 / ((R1 + 2×R2) × C).
What is monostable mode?+
A single trigger produces one timed output pulse. Pulse width = 1.1 × R × C. Used for debouncing, time delays, and one-shot events.
What is duty cycle?+
The percentage of each period that the output is HIGH. In standard astable mode the duty cycle is always above 50% (R1 adds to the high time only).