There's no way to calibrate an MPR121 for the kind of results you want.
Fundamentally, the sensor measures the time it takes to charge the parasitic capacitance of the touch pad using a known amount of current. Bringing a finger near the touchpad changes the pad's capacitance since water can hold about 80x more capacitive energy than air.
The .setThresholds() function has no effect on the operation of the sensor. It just defines the amount of change in the sensed values the MPR121 defines as a 'touch' and how much change after that counts as a 'release'. The readings themselves don't change.
In addition, the MPR121 compensates for the effects of things like pad size and humidity (both of which have an effect on the readings) by tracking the average charging time and automatically adjusting the default 'normal' timing for each pad based on that information.
ghenk wrote: ↑Sun Jan 08, 2023 6:25 am
if not I think i have to alter the copper pcb board plates that i have designed and am using. They are large and are not covered by a solder mask.
Adding solder mask will have little effect on the parasitic capacitance of the pads.
ghenk wrote: ↑Sun Jan 08, 2023 6:25 am
the added ground trace that surounds the touch plate is also touched when i touch a single node and is connected to arduino gnd seems to work a lot better than without.
Having a GND ring around the touch pad does help by changing the geometry of the system.
A capacitor is any pair of conductive plates with insulation between them. For a single pad, we can calculate the effect of an infinitely large pad an infinite distance away and still get a meaningful value. In that sense, every object in the universe shares a capacitive connection to that infinitely-distant theoretical plate. The stored energy fills a volume of space that starts perpendicular to the surface of the pad, then becomes more spherical as the distance from the pad increases.
Putting a GND ring around a pad changes the geometry of the system by giving the capacitor an explicit second plate. The energy stored by that capacitor starts perpendicular to the pad, and ends perpendicular to the trace around the pad. That changes the field from a sphere to a torus whose center is in the middle of the pad and whose outer edge is at the edge of the GND ring. For the purposes of touch sensing, that's reasonably similar to the capacitor storing energy in a disc above the pad and GND ring.
As you've seen, the way a GND ring concentrates energy makes the pads more responsive. Having exposed conductive surfaces for both the pad and the GND ring will cause some minor problems though: human skin has a resistance between a few kilohms and a couple of megohms depending on the moisture at the surface of the skin, so a finger touching both the pad and the ring will conduct current from one to the other. It won't be much current, but for the small amount of charge stored in the parasitic capacitance it may as well be a short circuit.
Putting soldermask over both the pad and the GND ring will have a negligible effect on their capacitive behavior, but will eliminate conduction between them when a pad is touched.