>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 08 September 2005 09:44
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Piezo buzzers
>
>
>> I have tried this quite recently, and the PIC has enough 
>grunt to get 
>> reasonable volume, better if you can use two PIC pins and drive them 
>> in anti-phase to double the voltage swing
>
>Ah yes, you've reminded me of some of the previous thread. I 
>think there was some mention of a vibrated piezo putting 
>voltage back into the PIC pin
>
>I just got this from the archives, by Dave Van Horn
>
>"Driving a piezo from a micro usually follows a pattern.. 
>One lead grounded, one lead to the micro. Too soft. 
>
>Both leads to the micro, driven in opposition. Nice and loud. 
>
>Then the strange behaviour begins. 
>
>After a while, you notice the similarity in the schematic 
>between a charge pump voltage doubler and your piezo element 
>and port pins, and you realize that every time you flip the 
>piezo, you are flirting with SCR Latchup or glitching your micro. 
>

Interesting, I can see the potential issue.  A pair of schottky diodes on each pin would solve this problem.  Driving a Piezo in this manner from any CMOS based IC, such as HC inverters risks the same problem, though latching up your piezo driver is preferable to the micro I guess ;)  

Regards

Mike

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