We will soon start a renovation of our cabin and I'm about planning a home automation backbone from scratch. My idea is to have an identical (but modular) "thing" in each room that will work as a room node. I have made a basic first idea of design criteria (see attached sketch).
What is this "thing" typically called?
Anyone done/seen anything similar?
Is it a good idea to build it on a Arduino, and if so what board and shields would you suggest me to include in the design?
Not common in todays practice.
Current systems typically use one microcontroller per function, so that means multiple microcontrollers per room. To keep costs under control people typically use Arduino clones.
Search for ESPHome and similar projects.
POE will work ok for a "do everything node". Not so practical when you have lots of devices per room due to the ethernet star wiring configuration.
Take a look at ESP32 based modules. They are good value for the performance and features. There are pretty affordable modules with the Ethernet, like WT32-ETH01 or ESPDUINO-32. And SoC has WiFi + BLE onboard, with mesh capabilities (but it won't be easy to write a firmware). Arduino framework are available for all ESP32 parts.
Sort of; I have a sensor in each room that records temperature and RH and logs it into a database every few minutes. It doesn't control anything as such. The sensors are DIY PCB's based on an ESP8266 and powered from a simple USB charger. Connectivity is through WiFi. The setup has been working dependably for a year or two now. Here's an unused spare that I never put to use:
It's a simple affair made with a generic/off the shelf ESP module, a linear regulator to make the 3.3V supply, a 3-color LED, DHT sensor and a button. Programming is through the pin header which breaks out a UART interface.
There's one slightly 'smarter' device part of this same system in our living room that has an ePaper display that plots energy use/generation, temperature humidity and air pressure; it's ESP32-based and also WiFi connected + USB powered. It gets some of its data from a smart meter, some from its own sensors, some from the central database.
Here's some sample output of one of the monitoring plots I made:
Temperature at the top, RH at the bottom. The different colors are different rooms in the house. The rooms at the top floor have big south-facing windows so they warm up quicker than the rest of the house. The red line is the ground floor living room, which is the most thermally inert.
You demand more functionality for your distributed IoT devices and given your needs, I'd look at ESP32 as a platform. I find WiFi perfectly acceptable in terms of connectivity; you seem to label it as a 'worst case', but I'm not sure what's so bad about it. Ethernet will work, too, but adds some hardware complexity, makes you less flexible in terms of placement and will result in a physically bigger box especially if you use ready-made Ethernet 'Arduino' modules.
In your case I'd probably design a modular setup with a couple of 'feature connectors' that you can plug (solid state) relay modules and user interface elements into. Maybe work with a couple of housing sizes/variants, or maybe even a modular approach with blocks that click into each other. This is feasible mostly if you are willing and able to make your own PCB's. Not sure how far you want to take this.
If you want to stick with official Arduino products and are up to quite a steep learning curve. the Nano Matter boards might be an option. So far, they work pretty well since Silicon Labs have done most of the work writing the libraries. They work with Apple Home and apparently with Google, Alexa, and HomeKit.
Matter/Thread is starting to catch on and there are sensors becoming available from multiple sources.
You may not be willing to invest the time to learn this technology but I believe that it will dominate in the future. Older technology will not quickly become obsolete though so it's still a viable option.
