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Project Notes

#541 TL431 Multivibrator

An unusual multivibrator using a programmable reference with clean square wave output.


Here’s a quick demo..



I found an unusual multivibrator design by Gilles Clément in Elektor Magazine, issue 7/2009. It is similar to BJT designs, but uses a TL431 precision voltage reference instead. And it oscillates - who’da thunk!


Here’s my version of the circuit, available in EasyEDA:


Testing on a breadboard first:


Then transfered to freeform on a micro USB adapter for 5V power:


Final build:



With my selection of RC components, I’m getting a very nice 4Hz square wave driving the LEDs:

  • CH1 (yellow):, left, offset -1V
  • CH2 (blue):, right, offset -5V


How does it work?

The Elektor article explains in full, but there’s the essence as I understand it..

  • cathode voltage Vk swings between supply voltage and a lower limit 2v
  • internally, the TL431 essentially compares the reference voltage to an 2.5V reference
  • RC crossed multivibrator
  • capacitor can discharge via Vref to cathode when Vk is low


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.