Weird piezo driver

My Fluke 77, will beep when the battery is low but not low enough to trigger the LCD legend, presumably drift over time

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.

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

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.

--sp

On 16 May 2016, Clifford Heath wrote

Yes, ?quit? beeping.

Thanks.

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.