The 555 timer
The 555 is one of the most popular integrated circuits ever made. Since 1972, it has been used to generate clock signals, blink LEDs, drive buzzers, debounce switches and create delays. Two resistors and a capacitor set its timing.
Astable mode: an oscillator
In astable mode, the capacitor charges through R1 + R2 and discharges through R2, so the output switches continuously between high and low:
- high time: t_H = 0.693 × (R1 + R2) × C;
- low time: t_L = 0.693 × R2 × C;
- frequency: f = 1 ÷ (t_H + t_L) = 1.44 ÷ ((R1 + 2R2) × C);
- duty cycle = t_H ÷ (t_H + t_L) = (R1 + R2) ÷ (R1 + 2R2).
The constant 0.693 is ln 2: the capacitor swings between 1/3 and 2/3 of the supply voltage.
Example
R1 = 10 kΩ, R2 = 47 kΩ, C = 1 µF:
- high time: 0.693 × 57,000 × 0.000001 ≈ 39.5 ms;
- low time: 0.693 × 47,000 × 0.000001 ≈ 32.6 ms;
- period 72.1 ms, frequency ≈ 13.9 Hz;
- duty cycle ≈ 54.8%.
An LED on the output would flicker visibly, about 14 times a second.
Monostable mode: a single pulse
In monostable mode, a trigger pulse starts a single output pulse of fixed length:
t = 1.1 × R1 × C
With R1 = 10 kΩ and C = 1 µF, the pulse lasts 11 ms. Use it for delays, timeouts and switch debouncing.
Design tips
- In the standard astable circuit, the duty cycle is always above 50%, since t_H includes R1. For 50% or less, add a diode across R2 so the capacitor charges through R1 only. Then t_H ≈ 0.693 × R1 × C.
- Keep R1 at 1 kΩ or more to limit the discharge transistor current.
- Use resistors from about 1 kΩ to 1 MΩ. Very large values make timing sensitive to leakage currents.
- For long delays, prefer film capacitors. Electrolytic capacitors have wide tolerances (often ±20%) and leakage that lengthens the timing.
- Add a 10 nF capacitor from the control pin (pin 5) to ground for stability, and decouple the supply.
- The CMOS versions (such as the 7555 or TLC555) draw far less current and work at higher frequencies.
Real frequencies typically differ from the formula by several percent because of component tolerances. Use a trimmer potentiometer when precise timing matters.