AVR Freaks

You should not have any problem driving a piezo directly from a port pin,
especially considering that most avrs have pretty stout drive capability.
Also, if maximum possible volume is desired, drive both pins of the transducer
such that when one pin is driven hi, the other is driven lo. You can do this
with two port pins, or a port pin plus an inverter, effectively doubling the
voltage drive to the transducer. I've been doing this using CMOS gates for
many years with nary a problem. You shouldn't need to use a transistor
unless you were using an electromagnetic transducer, or perhaps using
the port pin to also drive something else and you wanted to isolate the
piezo's capacitance from the port line.

Tom Pappano
Tulsa, Oklahoma

hehehe, Colin, I deleted my post, after re-reading the original post, he was referring to oscillation mode form the AVR. I was thinking about self oscillating piezo buzzers, in which case that trick would not work. In the case of raw piezo elements, where you must provide an oscillating signal, than the reversing ports trick is indeed a good one.

I do want to add to rfields, unless that nand gate has driver outputs, it is highly unlikely it will have more drive capability than the AVR itself.

[edit] ok I'm leaving this one for posterity. Colins as quick as I am on post/delete :)

Just for some history, I originally said that I didn't see how you would get double the voltage by swapping the polarity of the port pins. Basically you would always be driving the piezo between VCC and GND. (Note I was thinking of a self oscillating piezo here, thus why I deleted it once I realized they were talking about a raw piezo, where you must provide the excitation signal)

Colin, correctly, replied that it wasn't truly double the voltage, but with respect to the piezo from rest it would appear to be. (which he deleted right after seeing that I had deleted mine)

There you go! Maybe they were allowing for an optional self-oscillating squeaker
or weren't quite sure what they were going to use when they did the layout.

Tom Pappano
Tulsa, Oklahoma

That's really interesting. Murata does say to avoid bias, but several of their
example drive circuits do always keep a voltage on the transducer!

FWIW, I have equipment in the field now with piezos driven with the logic
gate bridge configuration for over 15 years now, and they still make noise.
I have not had an occasion to measure the leakage across one, however.
I suppose to be most proper, one should include a blocking cap.

Tom Pappano
Tulsa, Oklahoma