Calculator KB-42
NE555 Calculator: Oscillator
Radio hobby classics: blinkers, clock generators, and simple PWM.
Result
In brief: NE555 astable oscillator calculation: frequency, period, pulse and pause durations, and duty cycle based on R1, R2, and C. Example: Frequency: 6.871 kHz.
Calculation example
| R1 | 1 kΩ |
|---|---|
| R2 | 10 kΩ |
| C | 10 nF |
| Frequency | 6.871 kHz |
|---|---|
| Period | 146 µs |
| Pulse duration | 76.2 µs |
| Pause duration | 69.3 µs |
| Duty cycle | 52.4 % |
How it is calculated
tL = 0,693 · R2 · C
f = 1 / (tH + tL)
The capacitor charges through R1 and R2 from 1/3 to 2/3 of the supply voltage, and discharges only through R2. So the pulse is always longer than the pause, and the duty cycle is over 50%. For a lower duty cycle, shunt R2 with a diode.
What to look for
- The frequency is almost independent of the supply voltage, but depends heavily on the capacitor's tolerance.
- CMOS versions such as the LMC555 are more efficient and operate at higher frequencies.
Frequently asked questions
How do you calculate the frequency of a NE555 oscillator?
f = 1 / (0.693 · (R1 + 2 · R2) · C). With 1 kΩ, 10 kΩ, and 10 nF, the frequency is about 6.87 kHz at a 52.4% duty cycle.
How do you get a duty cycle below 50% with a 555?
Put a diode in parallel with R2: the capacitor charges through R1 and the diode, and discharges through R2. This lets you set the pulse and pause durations independently.
What's the maximum frequency of a NE555?
Typically, a few hundred kilohertz is a reliable limit. CMOS versions, such as the LMC555, work up to about 3 MHz.
