555 timer calculator
Frequency, high and low time and duty cycle of an astable 555, or the pulse width of a monostable one, from the NE555 datasheet equations.
- High time tH
- 831.6 µs
- Low time tL
- 311.9 µs
- Period T
- 1.143 ms
- Duty cycle (output high)
- 72.73 % Always above 50 % in this circuit, since tH includes RA.
How this is calculated
Astable: tH = 0.693 · (RA + RB) · C tL = 0.693 · RB · C T = 0.693 · (RA + 2RB) · C f = 1 / T (the datasheet rounds this to 1.44 / ( (RA + 2RB) · C )) Duty = (RA + RB) / (RA + 2RB) Monostable: tw = 1.1 · RA · C
- Supply
- NE555 recommended VCC is 4.5 to 16 V (SE555 to 18 V). Timing is independent of VCC as long as it is steady.
- Limits
- RA + RB (RA alone in monostable) at most 3.4 MΩ at 5 V and 10 MΩ at 15 V; the calculator applies 3.4 MΩ to any supply below 15 V. Astable below 100 kHz; above that the datasheet points to the TLC555. Monostable pulses at least 10 µs.
- Duty
- This is the output high fraction. In the standard astable circuit it is always above 50 %, because C charges through RA and RB but discharges through RB only.
- Tolerance
- The result is only as good as C. Use C0G or film capacitors for timing; high-k ceramics lose capacitance with voltage and temperature.
- Texas Instruments SLFS022K, xx555 Precision Timers (NA555, NE555, SA555, SE555) datasheetRevision K, March 2026, sections 5.3, 6.3.1 and 6.3.2
Questions
These are the component values of the typical astable waveforms in the TI datasheet (Figure 6-6). f = 1 / (0.693 × 11 kΩ × 0.15 µF) = 874.5 Hz (with the rounded 1.44 the datasheet prints, 872.7 Hz), with tH = 0.832 ms, tL = 0.312 ms and a 72.7 % duty cycle.
Not with the standard astable circuit: the duty is (RA + RB)/(RA + 2RB), which approaches 50 % only as RB gets much larger than RA. With RA = 1 kΩ and RB = 100 kΩ it is 50.2 %.
1.1 × R × C. 91 kΩ with 10 µF gives 1.0 s, and 9.1 kΩ with 10 nF gives 100 µs.
No. The thresholds sit at 1/3 and 2/3 of VCC and the charge rate scales with VCC, so the timing cancels out. The resistor limit does depend on VCC: 3.4 MΩ total at 5 V.