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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

About LEAP#541 Oscillators
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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, 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!).

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