> Do piezo buzzers generate a back-EMF (do I need a diode for 
> protection) ?

What Jinx said. Plus - with the internal driver they are *probably* 
saafe as is, but diodes are cheap.

Driven bare they don't make great back emf by themselves ***BUT*** if 
mechanically excited (tapping on the piezo or possibly even vibration 
of the right magnitude and freuqency) they will generate voltage. A 
good tap on the piezo element can make tens to hundreds of volts. An 
unprotected PIC may well die. I haven't heard of vibration being a 
problem, but I have heard about processors being destroyed when a 
piezo sounder was pressed mechanically.

> Can I drive piezo buzzers straight from PIC pins? No resistor?

What Jinx said, but also, yes, maybe.

If you drive them from two pins in antiphase (01,10,01, ...) you get 
twice the effective AC voltage. Antiphase 5v square waves on two pins 
will give you almost 5v RMS which will work variably well depending on 
the piezo.


        RM

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