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> you see those musical greeting cards with piezo elements that
> appear to be producing a wide range of frequencies at loud
> volumes

If you do want to have a go with piezos -

You'll find that drive in a greetings card is by reciprocating outputs,
which is in effect a 3V p-p signal from a 1.5V cell. If you were to do
this with a PIC the piezo would be between two I/O pins, not one
pin and Vss. ie when one pin is '1' the other is '0'

Be aware that piezos are capacitors, ISTR in the 1nF - 20nF range.
(1) this may tax I/O pins current-wise (2) vibration of the piezo (eg
being struck or the simple act of it returning to its rest position after
being driven) will cause it to generate a voltage high enough to quickly
and easily fry an I/O pin, so it would be better to use a couple of
transistors to isolate the PIC

Watches use a parallel inductor (several mH) to increase the volume,
particularly at a resonant frequency. This may work OK if tunes
were restricted to an octave or so. I doubt piezos have anywhere
enough travel to create enough sound pressure for good bass

Attached are a couple of driving/voltage boost examples. The parallel
1k resistor increases volume by discharging the C of the piezo. You'll
find it's very quiet otherwise. The 22mH resonating inductor increases
volume in low-voltage circuits. 50V may be too high for some single
piezos, you can load the circuit or change the duty cycle to produce
25V - 30V

Joe=

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