⏱️ 555 Timer Calculator
Compute the astable frequency, period and duty cycle, or the monostable pulse width of an NE555 timer. Everything runs in your browser — no data leaves this page.
What is a 555 Timer (NE555)?
The 555 timer is a classic timing chip in production since 1972 and one of the most widely used integrated circuits in electronics. Thanks to its two internal comparators, an RS flip-flop and a discharge transistor, it can act on its own as a square-wave oscillator, a time delay, a pulse generator or a PWM source. With this 555 timer calculator you turn the resistor and capacitor values around the NE555 into a frequency, period, duty cycle or single pulse width in an instant. Every calculation happens in your browser; no data is ever sent to a server.
Astable vs monostable mode
Astable mode is the oscillator mode where the output flips up and down on its own; it is used for blinking LEDs, generating tones or producing PWM. In this mode the capacitor C charges through R1 and R2 and discharges only through R2, repeating forever. Monostable (one-shot) mode produces a single HIGH pulse of a fixed length whenever it is triggered, then returns to rest; it is ideal for button time delays, debouncing or a single triggered event.
NE555 astable frequency and duty cycle formulas
In astable mode the output frequency is f = 1.44 / ((R1 + 2·R2)·C). The period is T = 1/f, the charge time (output HIGH) is t_high = 0.693·(R1 + R2)·C and the discharge time (output LOW) is t_low = 0.693·R2·C. The duty cycle is D = (R1 + R2) / (R1 + 2·R2). Monostable mode is far simpler: the pulse width is T = 1.1·R·C.
A worked Arduino blink-style example
Say you want to blink an LED at a visible rate (about 1.5 Hz, similar to Arduino's classic blink sketch). Choosing R1 = 1 kΩ, R2 = 47 kΩ and C = 10 µF gives f = 1.44 / ((1000 + 2·47000)·0.00001) ≈ 1.5 Hz and a period of T ≈ 0.66 s, so the LED toggles about one and a half times per second. This circuit reproduces in hardware what Arduino does with digitalWrite + delay — without a single line of code.
Tips: choosing R/C, duty cycle and limits
- Keep R1 at 1 kΩ or higher: during discharge the current flows through R1 into pin 7; too small an R1 exceeds the chip's current limit and can damage it.
- Duty cycle never drops below 50%: in the standard circuit the charge path (R1+R2) is always longer than the discharge path (R2), so the duty cycle stays above 50%. Add a diode in parallel with R2 to reach exactly 50% or less.
- Pick a stable capacitor: long timings use electrolytic caps, but their tolerance is poor; for accurate timing prefer film or ceramic capacitors.
- Accuracy drops at very high frequencies: the 555 is not ideal above a few hundred kHz; the MHz range needs a different oscillator solution.
Frequently Asked Questions
What is the 555 timer astable frequency formula?
In astable mode the NE555 output frequency is f = 1.44 / ((R1 + 2·R2)·C), where R1 and R2 are in ohms and C is in farads. The period is then T = 1/f.
How do I calculate the 555 monostable pulse width?
In monostable (one-shot) mode the output pulse width is T = 1.1 · R · C, with R in ohms and C in farads, giving the result in seconds. When triggered, the output stays HIGH for that duration.
What is 555 duty cycle and why can't it go below 50%?
Duty cycle is the fraction of the period the output is HIGH: D = (R1 + R2) / (R1 + 2·R2). In the standard astable circuit the capacitor charges through both R1 and R2 but discharges only through R2, so the duty cycle is always above 50%. A diode is added to reach 50% or less.
Why should R1 be at least 1 kΩ in a 555 timer?
During discharge, current flows from the supply through R1 into the discharge pin (pin 7). If R1 is too small that current exceeds the chip's limit and can damage the 555, so R1 is kept at 1 kΩ or higher in practice.