555 Timer Calculator
Enter R1, R2 and the timing capacitor to get the frequency, period, high and low times and duty cycle of a 555 astable oscillator, or a resistor and capacitor for the monostable pulse width. You can also design backwards from a target frequency to standard resistor values.
Astable mode: the 555 runs as a free oscillator. R1 sits between VCC and pin 7, R2 between pin 7 and pins 6/2, and C from pins 6/2 to ground.
Waveform
Monostable (one-shot) mode: a falling edge on the trigger (pin 2) makes the output go high for a fixed time t = 1.1 × R × C.
Waveform
Pick a capacitor and enter the frequency and duty cycle you want. The calculator finds R1 and R2, then snaps them to standard E12 or E24 resistor values.
How to use the 555 Timer Calculator
- Choose the mode: Astable for a continuous square wave, Monostable for a single timed pulse, or Design to start from a frequency.
- Enter the resistors and capacitor and pick the unit for each (kΩ and µF are the defaults).
- Press Calculate, or just type: the frequency, period, high and low times and duty cycle update instantly.
- Check the waveform to see the capacitor charging between ⅓ and ⅔ of the supply and the output pulses.
- In Design mode, enter a frequency, duty cycle and capacitor to get R1 and R2, then press Use in astable tab to load the nearest standard values.
555 timer formulas
| Quantity | Astable | Astable with diode across R2 |
|---|---|---|
| High time tH | 0.693 × (R1 + R2) × C | 0.693 × R1 × C |
| Low time tL | 0.693 × R2 × C | 0.693 × R2 × C |
| Period T | 0.693 × (R1 + 2·R2) × C | 0.693 × (R1 + R2) × C |
| Frequency f | 1.44 / ((R1 + 2·R2) × C) | 1.44 / ((R1 + R2) × C) |
| Duty cycle | (R1 + R2) / (R1 + 2·R2) | R1 / (R1 + R2) |
In monostable mode the output pulse lasts t = 1.1 × R × C. The constants come from the capacitor charging curve: 0.693 = ln 2 (from ⅓ to ⅔ of the supply) and 1.0986 = ln 3 (from 0 to ⅔).
Worked example: LED flasher
With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF:
- R1 + 2·R2 = 1,000 + 20,000 = 21,000 Ω.
- Period T = 0.693 × 21,000 × 0.00001 = 0.1456 s, so f = 1 / 0.1456 ≈ 6.87 Hz.
- High time = 0.693 × 11,000 × 0.00001 = 0.0762 s; low time = 0.693 × 10,000 × 0.00001 = 0.0693 s.
- Duty cycle = 11,000 / 21,000 = 52.4%.
Typical component values
| Target | R1 | R2 | C | Approximate result |
|---|---|---|---|---|
| 1 Hz blink | 10 kΩ | 68 kΩ | 10 µF | 0.99 Hz, 53% duty |
| Audio tone 1 kHz | 1 kΩ | 6.8 kΩ | 100 nF | 0.99 kHz, 53% duty |
| 38 kHz IR carrier | 1 kΩ | 18 kΩ | 1 nF | 39 kHz, 51% duty |
| 10-second one-shot | R = 910 kΩ (monostable) | 10 µF | 10.0 s pulse | |
The E24 series values in each decade are 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2 and 9.1. E12 uses every other one.
Frequently asked questions
What is the formula for 555 astable frequency?
f = 1.44 / ((R1 + 2·R2) × C). The 1.44 comes from 1/ln 2. For R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF, f = 1.44 / (21,000 × 0.00001) ≈ 6.87 Hz.
How do I calculate the duty cycle of a 555 timer?
Duty cycle = (R1 + R2) / (R1 + 2·R2) × 100%. The high time is 0.693 × (R1 + R2) × C and the low time is 0.693 × R2 × C, so the output is always high for slightly more than half the period.
How can I get a duty cycle below 50% with a 555?
Put a diode (such as a 1N4148) across R2, with its anode on pin 7. The capacitor then charges through R1 only, so the high time becomes 0.693 × R1 × C and the duty cycle is R1 / (R1 + R2). Tick the diode option in the calculator.
What is the monostable pulse width formula?
t = 1.1 × R × C. The capacitor charges from 0 to ⅔ of the supply, which takes ln 3 ≈ 1.0986 time constants. 100 kΩ and 10 µF give a pulse of 1.1 seconds.
Why should R1 be at least 1 kΩ?
When the 555 discharges the capacitor, pin 7 is shorted to ground and current flows from the supply through R1. A very small R1 lets too much current flow through the discharge transistor and wastes power.
What is the maximum frequency of a 555 timer?
A standard bipolar NE555 is reliable up to roughly 100 to 500 kHz. CMOS versions like the TLC555 or LMC555 can run into the low megahertz range.
Does the supply voltage change the 555 frequency?
Ideally no. The thresholds are fixed fractions (⅓ and ⅔) of the supply, so the timing depends only on the resistors and capacitor. In practice component tolerances, especially of electrolytic capacitors, matter far more.