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Project Notes

#676 LED-LDR Ring Oscillator

Building a simple ring oscillator using a light dependent resistor to couple the LED stages. From a circuit published in Elektor 2012/07.


Here’s a quick demo..



The LEAP#164 Watson LED Ring demonstrated the typical LED ring oscillator with BJTs to couple stages.

This circuit from Elektor 2012/07 is even simpler, using LDRs to couple the stages:

  • the LDR triggers the capacitor discharge (turning off the LED)
  • and the feed resistor on the anode determines the charge time to turn on the LED in combination with the capacitor

The LDR is pointed at the previous LED in the ring:

  • the circuit is enabled when ambient light is low (when too bright, all LEDs are effectively bypassed by the LDRs).
  • as an LED turns on, the light will strike the LDR of the neighboring LED, turning it off
  • when the next LED turns off, it allows the subsequent LED to turn on

So the net effect is:

  • in bright light, all LEDs off
  • in low light, all LEDs want to be on, but an “off LED” will chase around the ring

Circuit Design

I’m using LDR 5528 which is specified as dark: 1MΩ, light: 8-20kΩ.



Breadboard Test


Freeform Build

Why not?

Just a quick bit of freeform wire sculpting…



Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.