Looking at the interest my comment generated I decided to create a post here on HN about PJDL, please if you liked up it: https://news.ycombinator.com/item?id=23591953

The Italian accent made me think of Marconi, also your accent isnt heavy at all, just adds a little bit of color.

That is really a good compliment :)

The awesome thing with wireless IoT is that it can be set up without any additional cabling, which can be extremely painful process in most homes.

In case of wireless all you need is ESP32, relay and a power supply. And all of that together will cost less than 10$.

Can you easily (cheaply) use the already existing power cables for PJDL? I know there are network repeaters that use power cables to transfer data, but I have no idea how much it would cost in IoT scenario. Do you have some examples?

Yes many users have squatted old phone wiring, here in italy in most houses it is embedded in walls, and it is present in most rooms. Being unused and obsolete it can be used for home automation with PJON :)

As a software person who doesn't know much about this sort of thing: how does this work electrically? Presumably there's no current flowing as it's an open circuit? Is it akin to a voltmeter using the ground as a point of reference? Why don't other wire-based protocols use the single-wire approach?

Does it have anything in common with 'single-wire earth return', in power-distribution?

The protocol is really simple +5v is 1, 0v is 0. The circuit is just a single conductor and a common ground pin. If you connect a LED between the bus and ground you can visually see packets in form of light pulses.