Astable mode: a steady square wave
In astable mode the 555 switches on and off by itself, for blinking an LED or making a tone. With resistors R1 and R2 and a capacitor C:
- Time high = 0.693 × (R1 + R2) × C
- Time low = 0.693 × R2 × C
- Frequency = 1.44 ÷ ((R1 + 2 × R2) × C)
- Duty cycle = (R1 + R2) ÷ (R1 + 2 × R2)
With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF: high for 76 ms, low for 69 ms, about 6.9 Hz with a 52% duty cycle.
In this standard circuit the output is always high for longer than it is low, so the duty cycle stays above 50%. A diode across R2 lets the capacitor charge through R1 alone, which allows lower duty cycles.
Monostable mode: one pulse
In monostable mode a trigger produces a single pulse of fixed length: t = 1.1 × R × C. With R = 100 kΩ and C = 10 µF, that is 1.1 seconds, a typical delay or debounce time.
Component tips
- Electrolytic capacitors can be 20% or more off their marked value, so the real timing may differ from the formula. Use a film capacitor where accuracy matters.
- Keep R1 at least 1 kΩ, so the discharge pin does not carry too much current.
The 555 Timer Calculator works out both modes. If the output drives an LED, size its resistor with the LED Resistor Calculator.