#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:
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