555 astable: frequency and duty cycle
In astable mode the capacitor charges through R1 + R2 and discharges through R2 alone, between 1/3 and 2/3 of the supply. That gives the classic formulas from the NE555 datasheet:
t(high) = 0.693 × (R1 + R2) × C
t(low) = 0.693 × R2 × C
f = 1.44 ÷ ((R1 + 2 × R2) × C)
Duty = (R1 + R2) ÷ (R1 + 2 × R2)
Worked example
R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF: high time 0.693 × 11,000 × 0.00001 = 76 ms, low time 69 ms, period 146 ms, so f ≈ 6.87 Hz with a 52.4% duty cycle: a nice fast LED blinker.
555 monostable
As a one-shot, a trigger pulse makes the output go high for a fixed time t = 1.1 × R × C. 100 kΩ and 10 µF give 1.1 s.
- The basic astable circuit can't go below 50% duty; add a diode across R2 to get shorter high times.
- Keep R1 at 1 kΩ or more (a common rule of thumb) so the discharge transistor isn't overloaded.
- Electrolytic capacitors have wide tolerances (often ±20%), so expect the real frequency to differ.
Frequently asked questions
What is the formula for a 555 astable frequency?
f = 1.44 ÷ ((R1 + 2·R2) × C). With R1 = 1 kΩ, R2 = 10 kΩ and 10 µF it is about 6.9 Hz.
How do I get a 1 Hz blink from a 555?
For example R1 = 1 kΩ, R2 = 68 kΩ and C = 10 µF give about 1.05 Hz.
How do I get less than 50% duty cycle?
Put a diode across R2 so the capacitor charges through R1 only; then the high time is about 0.693 × R1 × C.