How to use
- Choose Astable for a free-running oscillator (blinkers, tone generators, clock signals) or Monostable for a single pulse after a trigger.
- Enter the resistors in kΩ and the capacitor in µF. For nanofarads type a decimal: 10 nF = 0.01 µF, 100 nF = 0.1 µF.
- Read the frequency, duty cycle and high and low times, or the pulse width.
Formulas
The 555 charges the timing capacitor between one third and two thirds of the supply voltage, so the times do not depend on the supply.
Astable
- high time = ln 2 × (R1 + R2) × C
- low time = ln 2 × R2 × C
- frequency = 1 / (high + low) = 1 / (0.693 × (R1 + 2R2) × C) ≈ 1.44 / ((R1 + 2R2) × C)
- duty cycle = (R1 + R2) / (R1 + 2R2)
Monostable: pulse width = 1.1 × R × C.
With kΩ and µF the product R × C comes out directly in milliseconds.
Worked example
R1 = 10 kΩ, R2 = 47 kΩ, C = 10 µF (the default values):
- high time = ln 2 × 57 × 10 = 395.0939 ms (ln 2 = 0.693147)
- low time = ln 2 × 47 × 10 = 325.7792 ms
- period = 720.8731 ms, so the frequency is 1.3872 Hz — an LED blinks a bit more than once a second;
- duty cycle = 57 / 104 = 54.81 %.
Change C to 0.01 µF (10 nF) with R1 = 1 kΩ and R2 = 4.7 kΩ and the same circuit runs at 13,872.0677 Hz, an audible tone.
Tips
- Keep R1 at 1 kΩ or more: when the discharge transistor switches on, R1 alone limits the current from the supply.
- To get close to 50 % duty cycle, make R2 much larger than R1.
- Use film or ceramic capacitors for stable timing; electrolytic capacitors drift with temperature and age.