555 timer LED heart beat

LED fading in and out using 555 timers.

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This a demo that show how to produce a heart beat LED fading in and out using 2 555 timers.

There is a kind of running gag in comments on hackaday blog. It goes like this, someone present a project using a MCU and a commenter says let's do it with a 555.

Usually one use a MCU to fade a LED. I say, let's do it with 555 timer.

Two of them are required though.  Here the schematic.

How does it works

This is based on frequencies difference.  U2  frequency is adjusted by RV1 to be slightly off to that of U1. The LED is connected between the outputs of the two timers. It can light up only when U1 pin 3 is high while U2 pin 3 is low.  Since the two frequencies are slightly different the phase between the two slip continually so that the time the LED is on is a fraction that is variable. The video below show the circuit in action as well the output of both 555 on oscilloscope. One can see the slipping of the blue trace compared to yellow trace.

  • sculpture

    Jacques12/25/2018 at 14:59 0 comments

    I decided to make a sculpture with that demo. I mounted the circuit on a microscope slide. Here the result.

    An a video

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Cees Meijer wrote 10/29/2020 at 07:25 point

"Usually one use a MCU to fade a LED. ".

That's really funny. 20 years ago that would have been an insane statement..;-)

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