Fork me on GitHub

Project Notes

#541 TL431 Multivibrator

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

Build

Here’s a quick demo..

clip

Notes

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!

Construction

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

Schematic

Testing on a breadboard first:

TL431Multivibrator_bb_build

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

TL431Multivibrator_wip

Final build:

Build

Performance

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

scope

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

tl431_functional

Credits and References

About LEAP#541 Oscillators
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 my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.