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reading some of the capacitive sensing IC datasheets around, they always recommend an isolation overlay over the sensor pad.

Like this:

enter image description here

I'm wondering, why not use the actual PCB as overlay?

like this (sorry for the bad photoshop):

enter image description here

Please note that I'm not just going to determine the "touched/untouched" state, but the actual amount of touched area (by measuring the actual capacitance).

I've done some experiment with MPR121 sensor and some copper tape on a blank PCB material, and it works pretty good, but I'd like to hear some opinions before I'm going to make some actual PCBs.

Would that be as good as using a proper overlay?

Update: I've read that FR4 (the PCB material) should provide almost the same isolation as acrylic and other plastics, so I think it could work

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  • \$\begingroup\$ It looks like the ground hatch shields the sensitive electronics. What you're suggesting might create lots of unintentional capacitive sensors. \$\endgroup\$ – Oskar Skog Aug 1 '17 at 13:05
  • \$\begingroup\$ I think that it's a good idea, however I agree with Oskar, it may create a lot of false pulses. However if you inlude some electronics or logic to filter false pulses I think it may work. \$\endgroup\$ – Dimitri Aug 1 '17 at 13:18
  • \$\begingroup\$ It'd work fine, just look rubbish. When developing the software for similar types devices I've often worked without the official overlay. \$\endgroup\$ – DiBosco Aug 1 '17 at 13:38
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    \$\begingroup\$ @JimmyB a lot of MCU eval boards have cap touch sensor areas that are nothing but solder resist atop of copper plane. Anyway, that's never an option for an actual product, due to the fact that solder resist is not a long-term touch-safe, dirt-proof surface \$\endgroup\$ – Marcus Müller Aug 1 '17 at 16:45
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    \$\begingroup\$ @MarcusMüller That's true of course; in reality, no one would expose the PCB as a user interface :) \$\endgroup\$ – JimmyB Aug 2 '17 at 9:21
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I've done this before, it's a pain to use it in a product. Do it once and never appear again on another product.

To answer your question, yes it can be done. Just that we don't show customers our PCB directly.

http://www.ti.com/lit/an/slaa576a/slaa576a.pdf

enter image description here

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The reason the PCB doesn't make a good overlay is that it is too thick. That greatly decreases the effects of capacitive coupling to a finger.

I have done capacitive buttons directly on a PCB, and they worked very well, but I used the soldermask as the protective layer.

How robust you need this to be depends on how this product is intended to be used. In my case, the capacitive buttons are not used in normal operation. This is a piece of industrial equipment in wall-mounted metal cabinet with a door. When you open the door, the circuit board with the cap-sense buttons is revealed. The buttons together with a 4x20 character display allow some simple setup and diagnostic operations.

I wouldn't use cap-sense buttons directly on a PCB for more regular use, or when then environment is less well controlled.

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