I found a few old phones from my family. II cleaned them, installed LineageOS and rooted most of them. On one I installed postmarketOS, one is still stock Android and one is lets say bricked (after installing lineagesos it stay on boot logo for ever, before that I installed lineageos and nethunter on it). One one disk encryption doesnt work for some reason.
Phones (all samsung galaxy):
S4
A5
A5 (bricked)
A5 (postmarketOS)
J3
J4+ (no encryption)
A31 (stock)
What can I do with them? Something like Monero node or Tor relay, but I'm already running that on old pc. For something that needs speed I have rpis (like a website). Camera security system? Tracking device?
Is it possible to run (and autorun) cli apps and/or services that can access interent, bluetooth, gps, sensors, camera, files, etc. just like on linux? I'm a programmer and I don't like making normal android apps for a simple project.
For anything internet related: Keep in mind that you likely are not getting any security updates anymore, even on a custom ROM.
Also keep in mind that each accu you have in your home rises the risk of fire slightly. Obviously it always depends on the type and age of each accu.
Now some ideas
Someone on Lemmy wrote about using a phone as Home Assistant box, but I don't remember if it was usable enough (e.g. possible to access usb/sensors/etc)
You might use them as sensors for Home Assistant, however the accu might not last that long, and/or having them connected to power might not be great for the accu
Run a media center on it (e.g. Kodi) and cast it to your TV
Connect some speakers and use them as Spotify boxes in different parts of the house (as it is specifically easy switching devices with Spotify. Maybe there is software for general WiFi speakers)
Use one to create a smart mirror that displays current information like weather etc
Nice, I do something similar for my main phone, with the charger at my work place :)
You could improve the battery life by chaning the automation to charge when below 20% and stop charging at 80%. That way the battery is always between these percentages which apparently is best for it's health.
Also deliberately using a slow charger for that setup is also a good idea.
My main concern of using a phone as a server is the battery. It'll get spicy after a year of constantly being plugged in, and the phone usually doesn't work without a battery attached.
If your old phone has a removable battery, you can sometimes replace them with an appropriate capacitor.
That will eliminate spicy pillows from the mix but also eliminate battery backup, so if your power goes out you will need to manually turn them back on.
The cameras are usually nice enough for using as a dash cam if you drive. I use one of mine for a media device around the house, kitchen aid etc.
I also use an old one for keeping my current one free of boatware, extra apps, and ensuring privacy on my carry phone (by not having anything google or tacking/data harvesting if I can help it).
Outside of that you could look into using them as potential network devices, DNS ad blockers, etc. I haven't looked recently but using one as a casting device receiver for TV would be awesome over a Chromecast or the like. But not sure how those projects are going
To run Linux CLI apps, install termux from f-droid and a few of its add-ons. It's nice, you can run sshd on the phone and ssh into it from your computer over USB.
Note that a lot of those older phones are no longer usable as phones, at least in most of the US, because of the 2g and 3g shutdowns and stickiness about LTE/4G. But yeah, they are little tiny computers with cameras and sensors and a screen, etc.
Slap DroidcamX on them and use them as webcams or as surveillance cameras. Droidcam is pretty flexible, lets you connect from phone to server or from server to phone or via USB.
Another idea is to use them as MiFi access points on trips abroad. You buy a local SIM card and share the wifi connection as a hotspot or via USB. You can also install Tailscale and use your home server as an exit point for extra security or to access Netflix in your home country etc.
Buy a little controller and install older games to make them into mini gaming consoles. Bonus if you can find a controller that lets the phone sit in the center (I think they make adjustable ones so it should fit any phone).