555 Timer Calculator
Calculate 555 astable frequency, period, and duty cycle, or monostable pulse width, from resistor and capacitor values.
555 timer formulas
Astable (oscillator)
f = 1.44 / ((R1 + 2·R2) · C)
t_high = 0.693 · (R1 + R2) · C · t_low = 0.693 · R2 · C
Duty = (R1 + R2) / (R1 + 2·R2)
In astable mode the 555 free-runs as a square-wave oscillator. Because the output charges through R1 + R2 but discharges through only R2, the standard circuit's duty cycle is always above 50%; add a diode across R2 for a 50% (or lower) duty cycle.
Monostable (one-shot)
t = 1.1 · R · C
In monostable mode a trigger produces a single output pulse of width t, then the output returns low until triggered again.
How the 555 works
Inside the chip, two comparators watch the voltage on the timing capacitor and flip an internal flip-flop when it crosses ⅓ and ⅔ of the supply. In astable mode the capacitor endlessly charges toward ⅔ V through R1 + R2 and discharges toward ⅓ V through R2, so it oscillates on its own. In monostable mode it sits idle until a trigger pulse starts one charge cycle, giving a single output pulse whose length depends only on R and C — not on the supply voltage, which is what makes the 555 such a stable, cheap timer.
Worked example
For a roughly 1 kHz tone with R1 = 1 kΩ, R2 = 10 kΩ and C = 100 nF, the frequency is 1.44 / ((1000 + 20000) × 100×10⁻⁹) ≈ 686 Hz at about 52% duty. Want a clean 1-second pulse from a button? Use monostable mode with R = 100 kΩ and C = 10 µF: t = 1.1 × 100000 × 10×10⁻⁶ = 1.1 s.
Practical notes
Keep timing resistors between about 1 kΩ and 1 MΩ, and add a small decoupling capacitor across the supply — the 555's output stage draws a current spike each time it switches, which can upset the rest of the circuit. For the resistor and capacitor values themselves, theE-series andcapacitor code references help you pick real parts.
Common questions
- What is the difference between astable and monostable mode?
- Astable free-runs: it produces a continuous square wave and never settles, which is what you want for a clock, a flasher or a tone. Monostable fires once: a trigger pulse produces a single output pulse of a set length, then it waits. If you are making something blink, you want astable; if you are making something happen once after a button press, you want monostable.
- Why can't I get a 50% duty cycle in astable mode?
- Because in the classic two-resistor circuit the capacitor charges through R1 and R2 but discharges through R2 alone, so the high time is always longer than the low time. The duty cycle is (R1 + R2) / (R1 + 2·R2), which approaches 50% only as R1 becomes negligible — and R1 cannot go to zero without shorting the discharge pin. Getting a true square wave needs a diode across R2 so charging bypasses it, or a different topology.
- What capacitor value should I choose?
- Start from the timing you need and pick the capacitor first, then solve for the resistors — that is usually the way round that gives sensible parts. Aim to keep resistors roughly between 1 kΩ and 1 MΩ: below that the chip struggles to sink the discharge current, and above it leakage and the chip's own bias currents start to shift the timing. For long periods use a larger capacitor rather than a huge resistor, but avoid electrolytics where accuracy matters, since their tolerance can be ±20% or worse.
- Does the 555 work at 3.3 V?
- The original bipolar NE555 is specified from 4.5 V upwards, so 3.3 V is outside its rating. The CMOS versions — commonly labelled 7555, TLC555 or LMC555 — run from around 2 V, draw far less supply current, and do not produce the large current spike on switching that the bipolar part is known for. If you are designing at 3.3 V or running from a battery, use a CMOS variant; the timing equations are unchanged.