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Capacitive Touch boards - Sensor insulator?

Eric Brundick
Guru 12956 points


Just played with my Cap Touch boosterpack and I'm intrigued at the prospect of using such interfaces with my projects in the future.

Document slaa363a (http://www.ti.com/lit/an/slaa363a/slaa363a.pdf) does a great job at explaining the mechanics and especially the signal processing behind the device, but one thing I'm curious about is the plastic insulator that goes on top of the PCB.  The Cap Touch board appears to have a ~0.8mm piece of plastic on top, and that document recommends some 3M non-conductive adhesives for fusing the plastic insulator but I'm wondering where do you typically find thin sheets of plastic like that (e.g. for hobby or small-run production)?  Would clear plastic strapping tape work in a pinch?  (Seems like it should, and the thinner the better...)

  • 0
    Guru 227245 points

    Eric Brundick said:
    Would clear plastic strapping tape work in a pinch?  (Seems like it should, and the thinner the better...)

    Indeed, the thinner the better. However, the plastic as well as the adhesive must be non-conductive. Not causing static charge is a plus (so some plastic materials are better than others) as any charge buildup on the plastic surface raises the ground level.
    If you don't mind the look, you can use the normal PCB solder-stop coating, as it is thin and non-conductive. However, it is not really sratch-proof. But it offers a cheap way to produce labeling (simply in copper on the PCB). Another way is to use a thin PCB (available in 0.5mm) just for the pads, and turn the back side to the user. Here again, you may put some copper labeling to the surface. Or 'draw' the outlines of the buttons and the label texts with the coating. Well, that's a bit difficult to do (negative thinking required when fooling the PCB layout software), but possible. And usually cheaper than ordering a printed plastic cover. :)

  • 0 in reply to Jens-Michael Gross
    Guru 12956 points

    Those are some good ideas!  Also good note on the static charge issue... I noticed the ebay supplier I typically use for copper-clad FR4 has single-sided available in 0.5mm and even smaller (0.18mm????) although if I'm not mistaken the FR4 has some markings on it that would look unsightly in all but a scrappy DIY project (which is all I ever build anyway, so I shouldn't be worried :D)

  • 0 in reply to Eric Brundick
    Guru 227245 points

    Eric Brundick said:
    the FR4 has some markings on it that would look unsightly

    The thinner ones are less likely to have markings than the standard 1.6mm stuff. Also, some supplier offer marking-free base material upon request. It might be a bit more expensive.
    0.5mm is the thinnest I've used so far. Its already quite flexible. Thinner ones might be too flexible for the intended purpose.

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