A warning on using piezo buzzers/sounders directly driven
by a microprocessor pin. When piezoelectric material has a
voltage applied it changes shape (details unimportant here)
- the direct desired result of this in this application is that
a sound can be generated if the piezo is mechanically
coupled to a diaphram.

HOWEVER when a piezoelectric crystal is mechanically
manipulated it generates a voltage. The voltage generated
can depend on orientation of force, magnitude, .... .
In this application the implication is that if a piezo
transducer is struck, a voltage will appear on the microprocessor
pin. For transducers which are in a housing this will normally
not be a problem. HOWEVER, for the small disk type
configuration the piezo element is directly available for
mechanical "timpany". Striking this sharply with your
finger may generate a voltage far in excess of the tolerance of
the microprocessor. I have heard of instances (urban legends?)
where pressing on the top of the case has caused an internal
processor to expire.

Uncertain of the magnitude of the voltages involved? Connect
a pizeo sounder to an oscilloscope and try it. Alternatively,
hold onto the business end of a gas lighting wand or a piezo
gas lighter (ouch :-( ) - these, of course, are mechanically
optimised for that application.