>Piezo speakers can generate a voltage? :-O

Sure can - they are 'just' a crystal microphone being used in the reverse 
direction.

>So what do I do, stick a diode in series with it?

The 'approved' way is to use a separate transistor that can withstand any 
voltage that it is likely to generate. It is also possible to get higher 
volume from the speaker doing this as well, but that is a separate issue. 

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