I’ve built a few ESP8266[0] (so not Ardunio but same space) based sensors and have them distributed throughout a building so we can see how changes in one part affect the temperature and humidity in other parts.
They’re very cheap to build (we only use the project boards to prototype, final builds are on PCBs) and they can be run on small batteries when they’re just doing simple data collection.
I used KiCad[0] PCB design software and followed a lot of Youtube tutorials. This[1] one was the best tutorial I found, it seems pretty trivial at first glance but touches on all the parts of the process.
I use OSH Park for the actual PCB production[2], they're pretty cheap and good quality boards but be prepared for a bit of a wait. There's also a good price comparison site[3] for alternate suppliers.
I started on KiCad but found the interface very unintuitive.
I got stuck at the point of making my own footprints for custom devices, which I needed to do because the library doesn't seem to include all that much (or maybe I couldn't find devices I was using because I couldn't work out the UI for searching well enough.
They’re very cheap to build (we only use the project boards to prototype, final builds are on PCBs) and they can be run on small batteries when they’re just doing simple data collection.
[0]https://en.wikipedia.org/wiki/ESP8266