Local onlyThe math runs in JavaScript in this tab. Your figures are remembered in this browser's local storage, and nothing is uploaded.
The 555 is the chip that launched a thousand projects, blinking LEDs, making tones, timing relays and triggering one-shots. Its timing comes from a resistor or two and a capacitor, and this works it out: the frequency and duty cycle when it free-runs as an oscillator, or the pulse length when it fires a single shot.
Astable: it runs forever
In astable mode the 555 oscillates on its own, charging the capacitor through R1 and R2 and discharging it through R2, over and over. The frequency is 1.44 divided by R1 plus twice R2, all times C. Bigger resistors or a bigger capacitor slow it down; smaller speed it up. This is the mode for a blinking light, a tone, a clock pulse or anything that needs a steady beat, and the frequency can run from a fraction of a hertz to hundreds of kilohertz.
Why the duty cycle is lopsided
Because the capacitor charges through both resistors but discharges through only R2, the output stays high longer than it stays low, so a basic 555 astable always has a duty cycle above 50 percent. Make R1 small against R2 and it approaches 50, but never quite reaches it with plain resistors. To get an even square wave or a short pulse, add a diode across R2 so charging bypasses it, or use a different arrangement. The calculator shows the real duty cycle for your resistors.
Monostable: one pulse on demand
In monostable mode the 555 sits idle until it is triggered, then puts out a single pulse of a set length and returns to rest. The pulse is 1.1 times R times C, from just one resistor and the capacitor. This is the mode for a timed relay, a debounce, a missing-pulse detector, or any one-shot where a trigger should produce a fixed delay. Pick R and C for the pulse you want and the calculator gives the length.
Keep the timing honest
The timing is only as steady as the parts. Use a stable capacitor, film or good ceramic, for the timing element, because electrolytics drift with temperature, age and their wide tolerance, which shifts the frequency or pulse. Very large resistors can make the chip sensitive to leakage and noise, so keep them reasonable. For precise or temperature-stable timing, the CMOS 7555 draws less and behaves better than the classic bipolar 555, with the same formulas.
More MakerKiln tools
- rc timeRC Time Constant CalculatorTime constant and cutoff of an RC pair, or the part for a target cutoff.→
- op-amp gainOp-Amp Gain CalculatorInverting and non-inverting amplifier gain from the two resistors, in V/V and dB.→
- led resistorLED Resistor CalculatorThe series resistor for an LED from supply voltage, forward voltage and current.→
Put this calculator on your site
Free to embed, and welcome. It runs the same way, in your visitor's browser, and it sizes itself to fit. Paste this where you want it, and it stays current on its own:
Prefer to host it yourself, offline or on an intranet? Download the file above. Keep the MakerKiln credit and it is free to use.