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Placing Touch-Screen Behind a 3mm plastic cover

gophert

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My goal is to put a touch-sensitive display behind a clear 2 or 3mm plastic panel of acrylic (PMMA) or polycarbonate (PC).

Unfortunately, when I tested a sheet of PC over my iPad or phone, I get no response. Therefore, I assume the PC will hinder the performance of my Raspberry Pi display as well - assuming a simple sheet of PC is used as the cover.

My question is, does anyone know of a trick to make the touch feature of a cap touch display work behind a sheet of plastic?

The touch features will always be in the same place on the display (four small buttons). Ideally, I am looking for a Raspberry Pi-compatible display with a nominal screen size of 4.3". Any display is fine if one type or another has adjustable touch sensitivity (without reflashing ROMs on the display).

Any ideas are appreciated.
 

alec_t

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#3
Might work if the covering sheet is of a tranparent poorly conductive polymer material.
 

gophert

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So you want your touch screen to work without been touched - I think the flaw in that idea is obvious.
It works fine "not being touched" with a 0.5mm, 1mm and 1.5mm glass or plastic. I'm just looking for an idea to extended the effect if anyone has an idea... oh, looks like alec_t is just posting an option to test.
 

gophert

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Might work if the covering sheet is of a tranparent poorly conductive polymer material.
Thank you.
Do you have a suggestion of a poorly conductive material (of any type? It doesn't have to be transparent for proving out the idea).

Maybe the plastic bags Digikey uses for semiconductor parts? I'll try later today.
 

alec_t

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#6
I can't suggest any specific type. There has to be some current flow via the layer to your finger, but if the layer is too conductive then the touch position would be ambiguous.
 

gophert

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I can't suggest any specific type. There has to be some current flow via the layer to your finger, but if the layer is too conductive then the touch position would be ambiguous.
I was going to cut a circle of material for each button.
 

JonSea

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#8
It appears that RPi touchscreens are available in either capacitive touch (like your phone) or resistive touch, and from the tiny bit of research I did, it's not always clear which type a display has.

While generally, resistive touch performance isn't as good as capacitive, for what you're trying to do, it might work better. Resistive touch relies on physically pressing on the screen to compress it. I wonder if you laser etched or routed a groove around each botton area, that would give the 'button' enough flexibility to deform a resistive touch screen to register. If this does work, I don't know how durable it would be.


At any rate, make sure any testing you do corresponds to the type of touch screen the display has.


SmartSelect_20190216-052730_Firefox.jpg
 

gophert

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Thanks JonSea - I have tried the original R Pi screen (cap touch) as well and found it is well tuned for an actual touch. There are instructions online for certain displays based on an open-source hardware to increase sensitivity to the point where gesture is good enough - no actual touch is needed for cap-touch (unlike Nigel Goodwin 's assumption above). In fact, the laptop touch pads on Lenovo laptops and many other brands use capacitive sensing (capacitive touch) to sense the gesture above the pad.

Unfortunately, the sensitivity settings for cap touch are in hardware on most displays instead of in software - like some touch-sensitive keypads.

We are waiting for any suggestions on how to possibly extend the cap-touch sensitivity with surface tricks before resorting to, as you have also suggested, laser cutting "accordion" buttons into the plastic sheet and use resistive touch. It is a nice looking kiosk so we want to avoid the accumulation of dirt/dust in the thin laser-cut lines. Our laser-cutting partner told us both sides of the panel must be engraved to localize the pressure so there is no good way to eliminate dust collection - even tapering the edges of the laser-cut spiral becomes expensive and slow unless a big setup cost is done to program path, power and speed along with a bit of a research program to optimize. Cutting clear, (even mostly clear in near-IR) makes it difficult to transfer energy into the plastic sheet.

Thanks for the input.
 
Last edited:
#10
Ideally you need a layer of gel-type adhesive - or possibly a clear polymer as used for setting glazing - under the acrylic/polycarbonate, so there is no gap between the cover sheet and the touchscreen.
eg.
https://www.ultrimaxstore.com/soudal-fix-all-crystal-adhesive-sealant-clear-c-35-p-4185959
https://www.aboutroofing.com/ct1-sealant-290ml-cartridge-for-indoor-outdoor-use.html


Any airgap, no matter how small, will reduce the touch sensitivity.

Whichever material has the higher dielectric constant will be better.

Alternatively, could you use an add-on touch panel designed to retrofit to a monitor and mount that in front of the acrylic guard screen?
 

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