# What's the third wire on a piezo buzzer?

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

What's the third wire for?

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I accepted stevenvh's answer because it makes it clear to me. Don't let this stop you from posting other answers, however, possibly giving more detail. I might change my mind :-) –  Federico Russo Aug 14 '11 at 14:10
It's a good answer. Nice and clear and to the point. –  Russell McMahon Aug 15 '11 at 8:03

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

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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