#541 TL431 Multivibrator

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

Here’s a quick demo..

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:

Testing on a breadboard first:

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

Final 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

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

• TL431 Multivibrator - Elektor Magazine, issue 7/2009 on page 40
• TL431 datasheet - Programmable Precision Reference