From my understanding, when a human uses a touch screen their body acts as one "plate" of a capacitor and the screen acts as the other (with protective screen coatings, glass etc. in between). They are connected though a common ground (e.g. earth ground). The touch controller can sense the capacitance at different points on the screen and determine where the touch happened.
I have been experimenting with a touch stylus (like in this question) in two different setups:
- Off-the-shelf touch stylus held by me
- Off-the-shelf touch stylus held by a plastic clamp and connected to earth ground
I have found that the touch screen is much more sensitive to the stylus in Setup 1 compared to Setup 2, however I don't understand why. I understand there are a few differences between the setups
- The human body has capacitance and so the total capacitance that the screen "sees" in Setup 1 is different
- The ground that the circuits (below) see may be different
This answer suggests connecting the stylus to the metal housing of the device to create a good common ground. Unfortunately my device is plastic. I think it is unlikely that any ground connections I have access to are the same as the screen's ground.
How can I improve Setup 2's performance? Would building a human equivalent circuit to add to Setup 2 help?
I simplified and built the "human body" circuit and updated my equivalent circuit with Kevin's suggestions. However, the performance isn't any better.