Piezo Metal Keypads: How do they work?

[userx2]

Hello,

For a new design, I am looking at using an IP68 rated solid metal front keypad that operates on piezo technology.
The manufacturer does not want to disclose how it works but their keypads have a built in CPU.

The general theory so far is that piezo elements are mounted behind the metal panel and will have pressure applied when the panel deflects ever so slightly as a key is pressed.

I am just wondering how that can even work?

As far as I know, a piezo crystal becomes charged and thus outputs a voltage when pressure is applied. But that charge and voltage deplete rapidly and is measurable only as a little spike during the pressure increase.
Likewise, the voltage goes negative while the pressure is released again.
But any static pressure will not be measurable = 0V.

I personally cannot see any 100% way to reliably use that to detect presses, holds and releases. of buttons.

Perhaps someone else here knows more?

Best regards
X

 

[KlausST]

Hi,

to give useful answers we need useful informations first.

Links to the manufacturer, and/or products.

maybe: What material, thickness, mechanically modified? (like etched microstructures...)

Klaus

 

[D.A.(Tony)Stewart]

Imagine a sub-surface mounted electret mic behind every button area and measures the relative amplitude of noise by touching any surface behind a key pad to detect activity of the loudest compared to an inactive zone. No moving parts and waterproof. Does this seem reliable? Possible? Yes with the right design. But verify!

 

[userx2]

Here is one link:

https://www.keyflex.com.au/vandal-resistant-keypads/

i need a custom shape and solution.
The keypads need force ie, not touch.
So there has to be some deflection in the metal sheet when pressing.

I have done some experiments with piezo buzzers as sensors but the limitations are gigantic as the touch/release sensing easily gets out of sync because of the piezo only responding to changes in pressure.