@Nca78 Down to 3V is fine. I've tried a 2025 coin cell. From the touch pad up, I have led diffuser (1.6mm), gap (0.8mm), 3mm glass
The chip marking reads TTP224N-BSB

@Nca78 It is common TTP224 module.

Please ignore the bottom pad as I'm testing conductive foam on that.
Top 3 pads works with Cs removed

I'm doing something similar but using ESP8266. Saw your article and I had to order a TTP224 touch module to test. I found it is ok. I need to remove Cs to increase sensitivity. Now the button activates when my finger is 5mm away from the pad, even without glass.