And the real product pictures now:
Short demo of touch functionality (will post later on the bluetooth mesh network functionality):
Sorry for the solder bridge replacing the R20 inrush current limiting resistor..that's just temporary until I get the real component . And yes, the capacitor across the bulb is still needed but it's a very small one of 0.1uF/250Vac and it should fit near the light bulb on the ceiling (only one is needed if there are two light bulbs on the same circuit).
The aluminum foil wrapped around that sponge is to "extend" the capacitive sensor so that it reaches the front plate fake plastic button. The 3d printed clear plastic around it is to hold it in place and also to spread the small leds light.
Everything is custom made except for the front plastics.
@Mishka Thank you very much for that link. Gosh, it sure would have been awesome to have such an ultra low power wake-on radio such as that described there. Unfortunately, I'm still getting no reply to even my second email attempt at contacting the company. Maybe they'll reply later, but for now I'm going to assume they are closed for business during the Caronavirus attack.
Fortunately, Figure 5 in the paper you linked shows an equivalent transistor layout for the voltage detector. It lacks a BOM with part numbers, but I'll nonetheless take a quick run at trying to simulate it in LTSpice--maybe I'll get lucky. If you were in my shoes, exactly which simulated transistors would you be trying?
As for alternatives to the UB20M, the nearest I could find is this:
which, admittedly, isn't as nice because it is an ultra low current detector rather than a low voltage detector. Its quiescent current is higher than the UB20m, but it appears to be still quite low in absolute terms. What the S5470 does have that the UB20M lacks though is that the s5470 is well stocked at Digikey and similar places.
Have you run across any other parts that might fit the UB20M role?