Beware piezos connected directly to processor pins. Tapping
the piezo surface can cause VERY high voltages (ever seen
the spark from a piezo gas lighting wand?). The voltages
will not be that high but they can destroy cpus. PIC has
some reasonable esd protection but I suspect a piezo could
make enough energy to give it a fright.

Try taking a piezo sounder and connecting it to an
oscilloscope. Tap the surface and observe the output. Note
that the scope probe probably had about 10 pf capacitance.
You'll probably want to be sure this doesn't happen when its
connected to your processor.



-----Original Message-----
From: Dr. Imre Bartfai <root@PROF.PMMF.HU>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Friday, 29 May 1998 19:45
Subject: Re: Piezo speaker with PIC


>On Thu, 28 May 1998, David VanHorn wrote:
>
>> >I want to add sounds to a product--beeps and boops and
the like.
>> >
>> >Can I just hook a piezo speaker to a PIC output pin?
>> >Or should I use a resistor and/or capacitor in series?
>>
>> Yes but.
>>
>> You'll be tempted to use two outputs to make it louder.
This can, with
>> the wrong switching sequence, cause a voltage doubling
effect, and
>> hang your processor with CMOS latchup!
>
>Hi,
>at first I could not follow your arguments. The it became
clearer.
>However, I think, it is true only if the device connected
between A and B
>has enough capacity to hold some coulombs during pin
transition. On the
>other hand:
>the piezo devices drain not too much current, i. e. you can
put a serial
>resistor between it and the PIC. Now, assuming the capacity
is high enough
>to satisfy the condition described above, nothing bad will
happen as the
>PIC has voltage clamping diodes. I guess 2 x 10k will work.
>
>Imre
>
>>
>> Wrong sequence.
>>
>> A  B      Voltage on pins at the moment of transition
>>
>> L  L        0     0
>> L  H        0     5
>> H  H       5     10 (!)  Possible CPU hang.
>> H  L       5      0
>> L  L        0     0
>>
>>
>> Right sequence
>>
>> A B
>>
>> L L     0  0
>> L H     0  5
>> L L     0   0 (This state just has to happen, it dosen't
have to stay there
>> any significant time)
>> H L     5  0
>> L L      0  0 (Same as above)
>>
>>
>