555 Timer Astable Calculator
Get the frequency, period and duty cycle of an NE555 astable oscillator.
Waveforms
Formulas
tH = 0.693·(R1+R2)·C, tL = 0.693·R2·C, f = (1.44)/((R1 + 2R2)·C). In the basic circuit the duty cycle is always above 50%. For 50% or less, add a diode across R2.
Try it: 555 waveforms from power-on
CH1 is the output (pin 3), CH2 the capacitor (pins 2 and 6). The trace starts the moment power is applied.
- Frequency f
- 6.87 Hz
- Period T
- 146 ms
- High time tH
- 76.2 ms
- Low time tL
- 69.3 ms
- Duty cycle
- 52.4%
- First high time after power-on
- 121 ms
At power-on the capacitor is empty, so during the first high time it charges all the way from 0 V to ⅔VCC. After that it only swings between ⅓VCC and ⅔VCC: up through R1 + R2 while the output is high, down through R2 alone while it is low. Each swing takes ln 2 ≈ 0.693 time constants, and the first one ln 3 ≈ 1.10.
Because the capacitor charges through R1 + R2 but discharges through R2 only, the high time is always the longer one and the duty cycle stays above 50%. A small R1 with a large R2 brings it close to 50%. VCC changes the levels but not the timing.
Worked example: a 1 kHz oscillator
- Choose R1 = 1 kΩ and C = 100 nF.
- Solving f = (1)/(0.693 (R1 + 2R2) C) for R2 gives R2 = ((1)/(0.693 f C) − R1)/(2) = 6.72 kΩ
- With the nearest E24 value, 6.8 kΩ, the actual frequency is 988 Hz
- f = (1.44)/((R1 + 2R2) C) gives 986 Hz. The two only differ by rounding 1.44 and 0.693 (about 0.2%); it is the same formula.
555 part values by frequency
| Target | C | R2 (E24) | Actual | Duty |
|---|---|---|---|---|
| Target1 Hz | C100 µF | R2 (E24)6.8 kΩ | Actual988 mHz | Duty53.4% |
| Target10 Hz | C10 µF | R2 (E24)6.8 kΩ | Actual9.88 Hz | Duty53.4% |
| Target100 Hz | C1 µF | R2 (E24)6.8 kΩ | Actual98.8 Hz | Duty53.4% |
| Target1 kHz | C100 nF | R2 (E24)6.8 kΩ | Actual988 Hz | Duty53.4% |
| Target10 kHz | C10 nF | R2 (E24)6.8 kΩ | Actual9.88 kHz | Duty53.4% |
All with R1 = 1 kΩ.
Using it in practice
Step by step
- Enter R1 (between VCC and the discharge pin), R2 (between discharge and threshold) and the timing capacitor.
- Read the frequency, the high and low times and the duty cycle.
- Adjust R2 to move the frequency; adjust R1 to change mainly the high time.
Common mistakes
- Trying to get a duty cycle below 50%. In the standard astable circuit the high time always includes R1, so it is always longer than the low time. Add a diode across R2 (or use a different topology) for shorter high times.
- Making R1 tiny. R1 limits the current into the discharge pin when it switches on. Keep it at 1 kΩ or more.
- Trusting the frequency to a few percent. Electrolytic timing capacitors are often ±20%, and the formula ignores the chip's own delays. Use a film capacitor and a trimmer if you need accuracy.
Practical tips
Put a 10 nF capacitor from the control pin (pin 5) to ground and a 100 nF capacitor across the supply right at the chip; the bipolar NE555 draws a big current spike on each transition and can reset itself without them. For battery projects, a CMOS version such as the TLC555 or ICM7555 uses far less supply current and allows larger timing resistors. Above a few hundred kilohertz the formula drifts; use a dedicated oscillator there.
Frequently asked questions
Where do 0.693 and 1.44 in the formulas come from?
0.693 is ln 2. The capacitor charges from ⅓ to ⅔ of the supply (or discharges back) in 0.693·R·C. And 1.44 is simply 1 ÷ 0.693.
Can I get a 50% duty cycle?
Not with the basic circuit, because charging goes through both R1 and R2, so the duty cycle is always above 50%. Put a diode across R2 so the capacitor charges through R1 only and discharges through R2 only, then make R1 equal to R2 for close to 50%.