calculator
555 timer frequency calculator
The 555 is a chip that blinks. Two resistors and a capacitor set how fast. Enter yours and the breadboard below runs the real chip model, so you can compare the formula with what actually happens.
The answer
0.72 Hz — one blink every 1.39 s
- Charging (output high): 0.693 × (R1 + R2) × C = 699.9 ms
- Discharging (output low): 0.693 × R2 × C = 693.0 ms
- Period = high + low = 1.39 s
- Frequency = 1 ÷ period = 0.72 Hz (same as 1.44 ÷ ((R1 + 2·R2) × C))
- Duty cycle (how much of the time it is on): 50 %
Above about 30 Hz the LED looks steadily on to your eye — it is still blinking. Real capacitors are often ±20 % off their label, so expect the bench to disagree with the formula by that much.
simulating…
How the 555 makes a blink
Inside the chip are two comparators — parts that compare voltages — watching the capacitor. The capacitor charges through R1 and R2 until it reaches two thirds of the supply; the chip then flips its output low and drains the capacitor through R2 alone until it falls to one third. Then it flips back and the cycle repeats. Charging goes through both resistors and discharging through one, which is why the on-time is always longer than the off-time in this wiring.
Picking values
Big capacitor and big resistors = slow, visible blinks (10 µF with 100 kΩ gives about one per second and a half). Small capacitor = audio: 10 nF with the same resistors is about 700 Hz, a clear tone from a piezo. Keep R1 above 1 kΩ so pin 7 never shorts the supply, and keep R1 + R2 under about 1 MΩ so leakage doesn't swamp the timing.
Common questions
Can I get a 50 % duty cycle? Almost: make R2 much larger than R1 and the duty approaches 50 %. Exactly 50 % needs a diode across R2 or a different wiring.
Why does my blink rate drift? Electrolytic capacitors are loose tolerance and change with temperature. For accuracy use a film capacitor, or accept it — a blinker doesn't need to be a clock.
Where does the 1.44 come from? 1 ÷ 0.693 — the natural log of 2 governs how long an RC charge takes to go from one third to two thirds of the way.