⏲ 555 Timer Calculator
Astable oscillator and monostable one-shot pulse calculations for the NE555 — with the canonical schematic, the formula reference, and live-recalculating frequency, period, duty cycle, and pulse width.
Astable Circuit · tap a value to type, or drag its slider
Timing depends on R1, R2 and C only — Vcc sets the output amplitude, not the period.
Worked Calculation · live with your values
Live Results
Output Waveform · Pin 3
two cycles shown · live with R1, R2, C, Vcc📐 Formula Reference
Astable: tH = 0.693 × (R1 + R2) × C | tL = 0.693 × R2 × C | f = 1.44 / ((R1 + 2×R2) × C)
Astable + diode across R2: tH = 0.693 × R1 × C | Duty = R1 / (R1 + R2) — can be below 50 %
Monostable: tw = 1.1 × R × C
R in Ω, C in Farads for exact results. Calculator handles unit conversion automatically.
Quick experiments
- Break the 50 % duty barrier. The plain astable charges through R1 + R2 and discharges through R2 alone, so duty is always > 50 %. Tick Diode across R2 — the charge current now bypasses R2 (tH = 0.693 × R1 × C), so duty = R1 / (R1 + R2) and can drop well below 50 %. The default 10 kΩ / 47 kΩ lands near 17 %.
- RC sets the frequency. Double C and the frequency halves; double R2 and the frequency drops roughly by a third. Try R1 = 10 kΩ, R2 = 47 kΩ, C = 1 µF (the default) — you should land near 14 Hz.
- Audio-band oscillator. Drop C to 10 nF and the same network jumps to ~1.4 kHz — solidly in the audible range.
- One-shot pulse-width math. In monostable mode, tw = 1.1 × R × C. R = 100 kΩ and C = 10 µF gives a 1.1-second LED-blink pulse — long enough to use as a human-noticeable strobe.
555 timer — formula reference
The NE555 contains two comparators, an SR latch, a discharge transistor, and a resistor divider that sets thresholds at 1/3 Vcc and 2/3 Vcc. These thresholds drive both operating modes.
Astable mode (oscillator)
- Frequency: f = 1.44 / ((R1 + 2 × R2) × C)
- Duty cycle: D = (R1 + R2) / (R1 + 2 × R2) — always > 50 % without a diode bypass on R2
- High time: tH = 0.693 × (R1 + R2) × C
- Low time: tL = 0.693 × R2 × C
- With a diode across R2: charge bypasses R2, so tH = 0.693 × R1 × C and duty = R1 / (R1 + R2) — the only way to get a duty cycle at or below 50 %.
Monostable mode (one-shot pulse)
- Pulse width: tw = 1.1 × R × C
- Triggered by a negative edge on pin 2 (TRIG). Output stays HIGH for tw regardless of further trigger pulses.
Learn more → 555 Timer IC — Full Guide
Common mistakes
Leaving the reset pin floating.
Pin 4 must be tied to V+ for normal operation. Floating, it picks up noise and resets the timer at random — the most common cause of a 555 that 'works sometimes'.
Omitting the supply decoupling capacitor.
The 555 draws a large current spike as its output switches. Without 100 nF close to the chip that spike disturbs the threshold comparators and causes double-triggering.
Expecting exactly 50 % duty from a plain astable.
Charging goes through R1 + R2 and discharging through R2 alone, so duty always exceeds 50 %. A diode across R2 is needed to go below it.
Re-triggering a monostable during its pulse.
Trigger pulses arriving while the output is already high are ignored by the standard configuration. If each trigger must restart the timing, a retriggerable arrangement is required.
Driving an inductive load directly.
The output can source and sink 200 mA, but a relay or motor needs a flyback diode. Without it, the coil's collapse spike damages the output stage.
Frequently asked questions
How do I calculate 555 astable frequency?
Frequency is 1.44 divided by the quantity (R1 plus twice R2) times C. With R1 = 10 kilohm, R2 = 47 kilohm and C = 1 microfarad the result is about 13.8 Hz.
Why can't a standard 555 astable reach 50 percent duty cycle?
The capacitor charges through R1 plus R2 but discharges through R2 alone, so the high time is always longer than the low time. Duty cycle is therefore always above 50 percent unless the charge path is changed.
How do I get a duty cycle below 50 percent?
Put a diode across R2 so charging current bypasses it. High time becomes 0.693 times R1 times C and duty becomes R1 divided by R1 plus R2. With R1 = 10 kilohm and R2 = 47 kilohm that gives about 17 percent.
What is the difference between astable and monostable mode?
Astable free-runs as an oscillator with no trigger needed. Monostable produces one output pulse of width 1.1 times R times C each time it is triggered, then returns to its resting state.
Why does my 555 circuit reset or behave erratically?
The reset pin must be tied high rather than left floating, and the 555 draws large current spikes when switching. A 100 nF decoupling capacitor across the supply close to the chip fixes most unstable behaviour.
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