Friday, 14 October 2016

What's the third wire on a piezo buzzer?


Most piezos I've seen have just two connections, but this type has three.

3-wire piezo
What's the third wire for?



Answer



They're called self drive types, and they're meant to be used as part of the oscillator:


piezo self-drive


The piezo effect works both ways: if you apply a voltage the piezo stretches, but also if it stretches it creates a voltage. This principle is used to create a feedback signal which drives the oscillator.
The advantage of the self drive is that it will automagically work at its resonance frequency, where it produces the loudest sound. In 2-wire circuits the oscillator's frequency is independent of the piezo's resonance frequency, and it's the designer who has to make that they're close.


For the piezo of your picture:



"G" = black

"M" = red
"F" = blue



(I guess M, F and G stand for Main, Feedback and Ground, resp. CMIIW)


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