Home › Lessons › Oscillators › 555 astable (LED flasher)

555 astable (LED flasher)

Oscillators · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

C charges through R1 + R2 up to ⅔ V_CC, then the 555 shorts DIS to ground and C discharges through R2 alone down to ⅓ V_CC, and it repeats. t_high = 0.693(R1+R2)C, t_low = 0.693·R2·C.

555 astable (LED flasher): the circuit on the simulator's bench

Open this lesson in the labFree, in your browser, no account needed.

How it works

thigh = 0.693 (R1 + R2) C = 358 ms, tlow = 0.693 R2 C = 326 ms

f = 1.44 / ((R1 + 2R2) C) = 1.462 Hz, duty = 52.4 %

The scope’s 1 MΩ input across C leaks a little of the charge, and the discharge transistor bottoms out about 0.1 V above ground, which moves this to 1.444 Hz — the table allows for both.

Try it on the bench

  1. Change C to 10 µF: it blinks about five times faster.
  2. Make R1 much bigger than R2 to push the duty cycle up toward 100 %. A 555 astable can’t go below 50 % without a diode across R2.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Op-Amps & Analog ICs”.

More on oscillators