The two classic 555 modes
The 555 timer is configured with a couple of resistors and a capacitor into one of two workhorse circuits:
- Astable — a free-running oscillator (a square wave). C charges
through R1 + R2 and discharges through R2, giving:
f = 1.44 / ((R1 + 2·R2)·C), duty = (R1 + R2)/(R1 + 2·R2). - Monostable — a one-shot: a trigger produces a single pulse of width 1.1·R·C, then it waits for the next trigger.
Why the duty cycle is always over 50%
In the basic astable, the charge path (R1 + R2) is always longer than the discharge path (R2 alone), so the output spends more time high than low. To get exactly 50% — or below — add a diode across R2 so charging bypasses it, or use a different topology. Making R1 much smaller than R2 pushes the duty toward, but never quite reaches, 50%.
Worked example
R1 = 10 kΩ, R2 = 100 kΩ, C = 1 µF: f = 1.44 / ((10 k + 200 k)·1 µF) ≈ 6.9 Hz, duty = 110/210 ≈ 52%. A slow blinker.
Design tips
- Choose C first (few standard values), then solve for the resistors — use the reverse solver above.
- Keep resistors between ~1 kΩ and ~1 MΩ; too low wastes current, too high makes timing sensitive to leakage.
- The bipolar 555 tops out around a few hundred kHz; the CMOS variants (7555/TLC555) go higher, draw far less current, and work at lower supply voltages.
Frequently asked questions
- Does the supply voltage change the timing?
- No — the thresholds are ratios of the supply, so frequency and pulse width are supply-independent (to first order). That's a big part of the 555's popularity.
- How do I get a 50% square wave?
- Add a diode in parallel with R2 (anode to the R1/R2 junction) so charge and discharge both go through roughly R2, then set R1 ≈ R2 — or feed an astable into a flip-flop to divide by two for an exact 50%.
- What capacitor should I use?
- For timing stability prefer film or C0G/NP0 ceramic over high-K ceramics or electrolytics, which drift with temperature and voltage. The reverse solver lets you try standard values.
- Need just the frequency and period?
- The frequency & period calculator converts between them and adds wavelength and cycle timing.