Can someone explain my oscillating touch readings?

[boarchuz]

I don't know much about AC, EMI, etc.
I have a simple program on a basic custom ESP32 WROOM breakout board reading a touch pin and printing out the value via an ESP-PROG. When I am not touching the pin, it maintains an expected stable value ~1200. However, when I hold my finger on the pin, there is an uneven 'oscillating' pattern as seen here with T7 (GPIO 27):


https://ibb.co/74nBr51

And here's another, using T9 (GPIO 32). This time in the second half of the chart I was tapping the pin rather than holding steady contact; you can see the value changing back and forth from the expected idle value (~1200) to tracking my mystery wave, as I touch and release:


https://ibb.co/mckfvMy

Sometimes it oscillates so dramatically that it climbs above the idle value (ie. touching the pin might reasonably be interpreted as a release event, and vice versa)!

Those are with my laptop charger attached as well as mains-powered peripherals, monitor, etc. When I disconnect all of these, such that my laptop, the ESP-PROG, and the ESP32 are all powered from the laptop battery (hence floating), I get expected values (with just a tiny oscillation):


https://ibb.co/FX1k0kz

More out of curiosity than anything else, I wonder if anyone could explain this phenomenon?

[username]

It radiated energy from everything around you. Lights, power supplies and so on. You body is an antenna that picks up this noise.
You can think of it if you ever dealt with audio amplifiers and touched the input cable while it was plugged in and head that typical "hum" noise.

[PeterR]

You're alive!
Just don't touch any unprotected silicon devices....
(so aside from 'hum' you may also have a electrostatic potential tom discharge)

[boarchuz]

Turns out it was an incredibly noisy 12v DC adapter. Even plugging it in a few metres away made the touch sensor go wild. Values are very clear and smooth as soon as it is unplugged.

It's a dirt cheap one from eBay, I'll be replacing it one from a reputable brand and checking again.