Persistence of vision display using only 555 timers

I created this display for the 555 timer contest, a compact art piece that cries out in appreciation of the venerable 555 timer. It’s a persistence of vision display formed by 7 blue leds and 3 NE555 timers, which spells out 5 5 5 as it revolves.

Theory of Operation

  • The first 555 is setup as an astable oscillator, with the reset triggered by the tachometer output of the PC fan that the whole thing is mounted on. This timer generates the ‘horizontal’ parts of the 5.
  • The second 555 is positive edge triggered (using a transistor NOT and a capacitor to generate the level input the 555 wants), and generates the *right* edge of the 5 (kind of counter-intuitive, but it spins counterclockwise).
  • The third 555 is negative edge triggered (capacitor filter) and generates the left edge of the 5.
  • Three 5’s are created by tuning the first astable 555’s period to complete 3 cycles in 1/4 of a revolution (the tachometer is high for 1/4 rev, low for 1/4, high for 1/4, low for 1/4, so you get two copies per full revolution).

The original writeup for the compo is still available at