I'm trying to make a fairly large touch panel (no display). Or if that's any easier: a set of touch ribbons.

The Wikipedia page on capacitive sensing and other online resources seem to pretty much boil down to "measure the capacitance created by your finger at the corners of a conductor".

There are also a bunch of breakouts and tutorials on Adafruit, Sparkfun, and similar websites. But they are all binary buttons, they don't sense location. These companies also sell LCD panels with touch screens, but nothing without the display.

The Teensy has touch pins that can be trivially read with touchRead, with similar on/off examples. But owning a Teensy, it seemed easy enough to experiment with sensing position.

I connected about a meter of wire between two Teensy touch pins, and tried to subtract the values I got. That's what the simplified model would led you to believe: take a conductor, measure capacitance at the ends and compare. However, this result was pretty useless, and no clear difference could be found.

So then I thought, alright, maybe it needs to have more resistance? Supposedly there is some R and C involved, and the R of a copper wire might just be too small. Next experiment was to connect a 10k resistor between the pins. Touching either end of the resistor was kind of discernable, but still not super reliable. I got some nichrome wire to play with, but I'm not hopeful.

So this begs the question: how does position sensing with capacitive really work. What kind of material is needed? How is capacitance actually sensed?

  • \$\begingroup\$ um, en.wikipedia.org/wiki/Touchscreen#Capacitive is pretty explicit on how capacitive touch screens work: you "simply" build a "grid" of conductors, between which a finger changes the capacitance. \$\endgroup\$ – Marcus Müller Dec 11 '18 at 18:13
  • \$\begingroup\$ individually, these capacitances are measured just like in the breakout boards: charge the capacitor and then slowly discharge it and see how fast the voltage drops, or use the capacitor in a resonant circuit oscillator and watch the frequency, or speed of envelope decay, if the oscillator is considered heavily dampened. \$\endgroup\$ – Marcus Müller Dec 11 '18 at 18:15
  • \$\begingroup\$ Can you offer a spec for position for XYZ direction & tolerances ?? Contact or non-contact? Hysteresis gap or pressure area. \$\endgroup\$ – Sunnyskyguy EE75 Dec 11 '18 at 18:47
  • \$\begingroup\$ @MarcusMüller Alright, if I manage to make a fine enough grid and a chip with enough pins, that would work. I guess I was referring to what your link calls Surface capacitance. \$\endgroup\$ – Pepijn Dec 13 '18 at 9:30
  • \$\begingroup\$ @TonyEErocketscientist I want to make sort of a guitar neck, so either one elongated plane of roughly 10x50cm or a few separate strips so you only sense one dimension. Don't really know/mind about contact/non-contact, hysteresis gap/pressure area, whatever works and is easy to make. \$\endgroup\$ – Pepijn Dec 13 '18 at 9:33

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