A finger is a capacitor
The pad is one plate of a capacitor and your fingertip is the other. The chip measures the capacitance on the pad over and over, and a finger adds a few picofarads. That passes through plastic, glass and wood, so the pad works behind a sealed panel; it does not pass through metal.
Two plates and a gap
A capacitor is two conductors with an insulator between them. The pad is one conductor. A fingertip, which is salty water joined to the rest of you, is the other. Bring it close and the capacitance between the pad and everything around it goes up by a few picofarads.
The chip measures that capacitance many times a second and compares each reading with what it has learnt the pad normally reads. A rise large enough is a touch, and SIGNAL goes HIGH.
Pick a panel and a thickness. The number is a parallel-plate estimate, one fingertip over the pad, and it is illustrative: a real finger is not flat and the field bends round the edges. What it gets right is the direction. Thinner is more; a material with a higher dielectric constant, glass over acrylic, is more; metal is none.
Why the pad works behind a panel
An insulator between the plates does not block the field, it carries it. So the pad can sit behind a sheet of acrylic or a wall of a 3D-printed case with no hole cut in it, which is the main reason to choose touch over a button: nothing to seal, nothing to wear, nothing to clean around.
Tontek's datasheet sets two conditions. The panel must contain no metal and no conductive element, and that includes paint. And a thinner panel is more sensitive. It does not say how thick is too thick, so the only real test is the panel you intend to use.
The capacitor beside the pad
The chip sees the pad, the track, and C5, the 22 pF capacitor from the pad to GND, all at once. A finger's few picofarads are a share of that total, so more fixed capacitance makes the same finger a smaller change. The datasheet allows 0 to 50 pF: none is the keenest, 50 the dullest. This board fits 22 pF. Tontek also asks for a stable type, NP0 or X7R, so the pad's sensitivity does not wander with temperature.
If a finished panel leaves the pad too dull, a smaller C5 is the fix, and a larger one if it fires from too far away.
When it does not work
The panel is too thick for the chip to notice the change, or it has metal in it. Try a thinner piece of the same material, and check for metallic paint or a foil layer. If the panel cannot change, a smaller C5 than the fitted 22 pF makes the pad keener.
Tontek does not publish a maximum for this chip, so there is no honest number to give. The change falls as one over the thickness, and a material with a higher dielectric constant, such as glass, carries more of it than acrylic or wood. Test with the panel you mean to use.
A thin glove is a thin panel, so often yes, with a firmer press to flatten the fingertip against the pad. A thick winter or work glove puts millimetres between you and the pad and may not. Try yours.
A plastic stylus is not you, and it adds almost nothing, so usually not. A metal tool held in the hand usually works, because it carries your own capacitance to the pad. That is the same reason metal in a panel is a problem: it spreads the pad's field onto itself.
What the chip learns at power-up, and why a finger on the pad then spoils it.
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