I know touchpads operate based on capacitance and using your finger to sort of complete a capacitor and track movements, etc. Is it possible to confuse or disrupt a touchpad using some other material(s) or a powered little electric field or some other means?
I have no one to be a nuisance to aside from myself. I just want to see how I can induce interference in a reliable, reproducible way.
I did some quick tests with my touchpad:
So basicaly, metals seem to be recognized as fingers (nb : I haven't tested enough different ones to be sure if all work or only some). The shape also seems to matter (the side of the coin don't work, probably because there isn't enough surface very near the touchpad).
I think other conductive materials might work as well, but I haven't tested
Capacitive touch sensors are obviously sensitive to any conductors present nearby - those conductors form an external capacitor plate and at the same time shield the sensor. If you put a piece of conductive metal foil over a touchpad, it won't work anymore - it'll be shielded from further influence of fingers or any other conductors placed outside the foil. Say, a metallized sticker would do the trick.
I'm highly doubtful that some special material properties of insulators could be used to interfere. The mechanism would have to involve some molecular, electron or nuclear relaxation, and the signal would be so weak that it wouldn't affect the capacitive sensor. Furthermore, such phenomena typically require highly uniform magnetic fields stronger than those normally present in the environment. For the world-building folks, that's a subtle hint :)
So, just placing some magic plastic material over the touchpad wouldn't make it not work, if the material was an insulator. Sufficiently conductive plastic would work, but simply as a conductive shield. Touchpads perform continuous calibration of threshold capacitance, so a large slightly conductive "shield" would only decrease the sensitivity but wouldn't be mis-detected as a touch. It could trigger palm detection and switch the touchpad off if a mouse was plugged in at the same time.
Modern touchscreens and touchpads have very low emissions and conversely they are relatively insusceptible to narrowband asynchronous interference present during normal operation. By asynchronous I mean interference that is not specifically synchronized to the readout pulses/bursts used by the touchpad driver chip.
To corrupt the capacitive readout, you'd need to have a sensitive and fast "electrometer" amplifier to pick up the scan excitation signals, to synchronize your interference with them. Then you could amplify the pulses and send them back through an external amplifier and antenna or an induction coil. The sensor will be sensitive only to signals that are synchronous with its scan pattern. Modern touchpads and touchscreens purposefully spread the pulse periods across a range of times, to decrease narrowband emissions and narrowband susceptibility.
Without synchronization only very strong signals would interfere - signals so strong that it would probably be illegal to emit them, unless your emitter was right next to the touchpad.
Typically a nearby ham radio operator would pick those signals up, and if they interfered with their reception - would get rightly upset and help FCC or an equivalent authority nab the source of interference. There's nothing quite like upsetting a ham - they usually have the time and know-how to devote to zero-in on the illegal emissions.
So, in terms of any kind of remote interference: not only would it be not a nice thing to do to someone other than yourself, but even if you did it to yourself it could well be illegal if the antenna was far enough away from the laptop.
And then you just plug your phone to a poor quality, electrically noisy USB power supply, and the touchscreen is effectively disabled.
Low-effort and effective. Been there, done that.
Connecting the output of an HF radio to the metal case of the laptop, or to its electrical reference plane (e.g. USB connector shield) would probably be just as effective, without being illegal :) I won't be replacing your laptop if you try that and kill it, though.
