UART connection between ATSAMD20 and ATtiny4313

It's hard to see how the installed cost could cheaper. Using 16V allows use of much cheaper wire, you don't need to install junction boxes or conduit, the routing requirements for mains voltages don't apply, and you don't need a certified electrician to do the work.

There's a junction box on the outside of the house for light switches?

Those switches are on the outside of the house and one of them is the doorbell button?

I don't want to defend installations that THEY do here where I live. I'm just describing what usually happens.

In the apartment there are already "in-wall" tubes and junction boxes for mains distribution and frames for lights switches. In this case, I think it's worth it to "waste" one module to install a 230Vac buzzer inside and one module to install a 230Vac pushbutton to activate the buzzer outside. During installation of mains circuits you already have

The prices of 230Vac and 12Vac doorbells are similar (see [1] and [2]), with the plus you can avoid an additional power supply/transformer, even if it is cheap.

Sorry for misunderstanding. It isn't for outdoor use, it is usually installed "out of the door" of the apartment, but inside a multi-apartment building. Of course, in isolated building where the doorbell switch is really outdoor, you can't use that.

When the apartment is one of many in a building (condominium), there's usually a public light on the common part of the floor (balcony? landing?) inside the building. And you could want to switch on when you are outside your apartment. Both the light button and doorbell button are 230Vac, so no mixing.

I don't think this[3] is worse than this[4]. Of course, it depends on personal taste.

See above for the misunderstanding.

Yes, I never said there aren't low voltage buzzers, but they aren't frequently used here.

See [1] and [2]. The prices are the same.

Yes, sorry.

I don't want to spend any money for any doorbells ;-) This is what I see, I'm not an expert of mains distribution in buildings and apartments.

I'm only supposing that in a new house, where you already have 230Vac tubes and junction boxes and you need one or more light push-buttons out of the door (not outdoor), you can share everything and possibly share some money, considering that 230Vac and 12Vac buzzers are similar in price.

I was thinking to a typical situation here, an apartment in a condominium, a bigger multi-houses building. Your doorbell pushbutton is inside the building and you often need to switch on a lamp.

Yes, of course. Anyway, most of the time the doorbell is installed by the certified electrician during a renovation or during construction of a new building. In this case, you already have a lot of cables and you can share already present conduits and junction boxes.

Only the frame for push-buttons. Junction boxes are inside the house. On the floor of 2-3 apartments, it could be a junction box for common assets, such as common lights, antennas, elevator and so on.

Outside, not outdoor. Yes, one of this is the pushbutton of the doorbell.

Ah, OK. we're talking about doorbells for single-family, detached houses. There's typically a chime mounted on a wall in a central location and one or more buttons (two is very common) wired to that chime.

You're talking about apartment buildings. Here in the US, apartment buildings don't use the 16V doorbells we've been describing. They usually have buzzer/intercom/remote-door-lock systems that are completely different than a "doorbell".

I haven't seen you post anything that would be used outside. Worse, you have not explained why there would be a gang switch box outside where the doorbell button would be. Others have asked the same thing, and as far as I can tell, you ignore them as well.

You are picking one line item from a single catalog. Price out all the parts required.

I don't know what you are talking about. Sorry. You seem to be addressing doorbells with buttons inside your house. We just don't have much of that in the US.

Yeah, that makes more sense. In the US, older apartments that were made from a single family dwelling, have a number of low voltage buttons on the outside of the main door, or just inside in a foyer. More modern apartments have no doorbells. You just knock on the doors. Works pretty well. We have these things called, "knockers".

Even in such apartments, we just use low voltage doorbells for all the reasons that have been explained to you. Every part of the system is a lower cost than a 240V bell system. It requires no special consideration for the wiring and can be done by the kid next door, rather than under supervision of a licensed electrician. I don't know why you can't understand this simple matter.

Sorry, I don't know what 3 and 4 are.

Apparently where you live, no one goes outside. All front doors are inside. Nice world.

You literally know nothing of the issues of wiring high voltage vs. low voltage. You keep talking about the buttons in a catalog, without understanding how they are used. I can't spend any more time trying to explain it to you. So, you may hold your ideas dear.

Ok, Enjoy.

I don't know how we are fighting for this kind of arguments. I don't want to convince you, I'm not a professinal electrician and for me is good what you write. Keep on with embedded things.