Weird piezo driver

http://imgur.com/Trvs5mk
This is a beeper piezo in a Fluke 117 DMM which quite beeping. Scoped the piezo?s pads and... nothing.
An inverter (from the CD4069) is connected across the piezo. Guess I don?t understand anything about driving piezos.
Is this a standard practice? Cheers.
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On 16/05/2016 07:36, DaveC wrote:

My Fluke 77, will beep when the battery is low but not low enough to trigger the LCD legend, presumably drift over time
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On 16/05/16 16:36, DaveC wrote:

Did you mean that "it quit beeping"? If so, you aren't going to hear anything.

The Fluke should be producing a square wave, say 1KHz, across the piezo. The piezo itself looks like a mid-sized capacitor; you won't get any DC resistance reading through it. They are extremely high resistance devices.
If you touch 9v to either side of the piezo, you should hear it click, and click again when you disconnect. The Fluke does that too, but fast.
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On 16 May 2016, Clifford Heath wrote

Yes, ?quit? beeping.
Thanks.
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On 16.5.16 09:36, DaveC wrote:

You'll get AC drive when you drive one side with a logic signal and the other side with the same signal inverted.
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wrote:



If you put an inverter across the piezo you get double the supply voltage drive - push-pull. Not uncommon. Typically you'd drive each side with two paralleled inverters and use another inverter to invert the input to one pair.
The piezo element looks something like a ceramic capacitor electrically. A series capacitor is often used.
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In article

Piezo drivers have a high impedance so they need high voltages. One side of that piezo element is driven with the signal source and the other side is driven with the inverted signal. That doubles the drive voltage so it's loud enough to hear.
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