555 Timer — Astable Calculator

Frequency, duty cycle and high/low times of a free-running 555 oscillator.

What it does

In astable mode a 555 timer runs as a free oscillator. Two resistors and one capacitor set the frequency and the duty cycle.

Where it's used

Blinking LEDs, clock signals, tone generators and simple PWM.

How to use it

  1. Enter R1 (from Vcc to pin 7) and R2 (from pin 7 to pins 6 and 2).
  2. Enter the timing capacitor C (pins 6/2 to ground).
  3. Enter the supply voltage.
  4. Read frequency, duty cycle and the high/low times.

The formula explained

t_H
0.693 (R1 + R2) C — output high time
t_L
0.693 R2 C — output low time
f
1.44 / ((R1 + 2R2) C)
D
(R1 + R2) / (R1 + 2R2) — duty cycle

Worked example

A slow blinker on 5 V.

Inputs:

The calculator gives:

Tips and common mistakes

Questions people ask

What is the formula for the 555 astable frequency?

f = 1 / (ln 2 × (R1 + 2·R2) × C), usually written as f ≈ 1.44 / ((R1 + 2·R2) × C). The high time is 0.693 × (R1 + R2) × C and the low time is 0.693 × R2 × C.

How do I get a 50 % duty cycle?

Put a diode across R2 (anode at the discharge pin) so the capacitor charges through R1 only and discharges through R2 only. With R1 = R2 the duty cycle is close to 50 %. Without the diode, a large R2 compared with R1 also gets close.

How fast can a 555 run?

The classic bipolar NE555 is comfortable up to about 100 kHz. CMOS versions such as the TLC555 or ICM7555 run to around 1–2 MHz and draw far less supply current.

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