Most piezos I've seen have just two connections, but this type has three.
What's the third wire for?
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
G stand for
Ground, resp. CMIIW)
Here is a good in-depth explanation of buzzers including self-driven ones + some usefull schematics: Piezoelectric Sound Components Application Manual (812kb).
Excerpt (from p.5):
Method Fig. 9 shows a typical application of the self drive method. The piezoelectric diaphragm provided with feedback electrode shown in Fig. 9 (i) is involved in the closed loop of a Hartley types oscillation circuit. When the frequency is closed to the resonant frequency, the circuit satisfies oscillating conditions, and the piezoelectric diaphragm is driven with the oscillating frequency. Fig. 9 (ii) shows a simple oscillating circuit consisting of one transistor and three resistors. In general, the node support shown in Fig. 3 (a) is popular in the self drive method. Proper resonance of the piezoelectric diaphragm by the node support provides stable oscillation with high mechanical Qm of vibration but also a single high pressure tone.
protected by Dave Tweed♦ Aug 26 '15 at 11:35
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?