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Looking around and in the help section it seems my question is on-topic here. If I'm wrong, please tell me and I'll delete it.

For a teaching activity I was asked to develop a "plate" of about 200x300 mm where children can put their hands on. The plate should recognize the touch contact and highlight the contour area with RGB LEDs. Nice-to-have but not mandatory, is to detect the amount of "pressure" exerted, or said in other words, how close are the body parts to the plate.

For the specific application (I mean, not from a technical point of view, but due to the teaching activity background) we cannot use optical devices.

My first thought was to use a grid of capacitive sensors, with an addressable chain of RGB LEDs in between. This seems very hard to achieve due to the minimum resolution requested: 5 mm. It would mean 5 mm from center-to-center of each capacitive PAD sensor. Even if it is possible, it's not trivial (I have to place, connect and handle over 2400 sensors). Furthermore, with such a tiny and close PADs there is very little room for the LED placement. Anyway, this would be solved putting them on the bottom layer upside-down with just a little hole to allow the light to pass.

Probably it might work, but as said is not trivial and there are so many issues with the layout, routing and firmware handling of all these capacitive sensors. On the other side, I can also detect the "pressure" (aka distance) since most capacitive touch controllers can output debug raw data - not only the on-off status.

Is there another kind of technology I can use? Just as side note, the capacitive solution is quite good since I can put a glass panel over the PCB to be easily cleaned.

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  • \$\begingroup\$ Why wouldn't you just use a capacitive or resistive digitizer as used on LCD panels? \$\endgroup\$ Commented Dec 6, 2024 at 8:33
  • \$\begingroup\$ "I have to place, connect and handle over 2400 sensors" A PCB + EMS will do that for you. \$\endgroup\$ Commented Dec 6, 2024 at 8:34
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    \$\begingroup\$ I think you got yourself quite a challenge, there. My first approach would be a hybrid approach. Capacitive array for range and multitouch and another resistive matrix for touch. However, you may run into limitations, since the resistive grid usually isn't capable of multitouch. Maybe with an advanced controller, you can do sensor fusion magic and only poll the resistive array in areas where the capacitive array senses proximity? \$\endgroup\$ Commented Dec 6, 2024 at 9:33
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    \$\begingroup\$ I haven't tried an array of capacitive sensors, but unless you need multitouch and high update speed, could you multiplex them and just scan though your matrix, enable and sense them one by one? IR LED+detector could perhaps be an option too. \$\endgroup\$ Commented Dec 6, 2024 at 9:36
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    \$\begingroup\$ Think "modular". From your description, the functionality basically involves one sensor and its associated LED, with the only "global" function being some sort of latch or clear signal. You can get small MCUs that can handle 16 independent touch inputs, so make a 4×4 module that processes 16 sensors and 16 LEDs, and lay them out in a 10×15 array behind your glass. \$\endgroup\$ Commented Dec 6, 2024 at 12:36

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Far easier to implement that functionality without sensors would be to use a HF glow discharge! Use plasma-state, not solid-state electronics.

This was used in pseudoscience Kirlian imaging, and might be implemented using a flat-plate electrode, argon fill, and phosphors painted over flat top glass.

The resolution would be better than 100 µm, and it would also be immune to ESD, unlike solid-state devices.

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  • \$\begingroup\$ Exciting idea. Yet I doubt A 200x300 uniform electric field and vapour disposition coated panels are the cheapest solution. Not to speak about the High voltage supply and gas containment. \$\endgroup\$ Commented Dec 9, 2024 at 7:14
  • \$\begingroup\$ @S_G True - but the op doesn't want the various cheapest solutions avaiable. They have proposed a solutuon space that eliminates such solutions due to an (apparent) misconception by persons unknown of their perceived issues. \$\endgroup\$ Commented Dec 9, 2024 at 7:20

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