I have my old Galaxy S1 sitting in my cellar listening to the beeps from my washing machine. It sends me an email when the wash finishes. (I can't hear the beeps from upstairs.)
* These things are seemingly indestructible. In 10 years, the only problems I had was a deteriorating battery (which I replaced once) and a broken back cover (which I replaced by a 2,99 EUR one from eBay). The screen is still unbroken, fully working, and without scratches
* They are small, handy, and slim (thinner than many modern smartphones)
* Browsing is so slow that I cannot do anything except checking Hackernews and reading the local news. So it is definitely not a procrastination tool for me
* The camera still works great, and makes good pictures
* I don't care about security holes. I have no important passwords stored on the phone, I don't use it to check my mail (for the reason above). The only password stored is the ical service password on my private server which is used to update the calendar app. I couldn't care less if anyone had this password, my calendar is really not interesting.
* Osmand (for OpenStreetMap) and the national route planning app for public transit still work
So basically, I have an extremely reliable pocket machine which can be used as a telephone, to send SMS, to do route planning, to check the news, to check my calendar, to provide a Wifi access point for my laptop and which is also a world wide map. I don't need anything else, so why replace it? During these 10 years, I saw my then-girlfriend and now-wife go through 5 new smartphones, which regularly broke. I used it to make pictures of all our vacations, our engagement, our wedding and our child as a new-born. It has accumulated an amount of experience-patina which is very rare for physical things these days, so I also cling to it out of sheer nostalgia.
Anecdote:
6 months ago I got a OnePlus 7 Pro and installed LineageOS (continuation of CyanogenMod) on it. The 7 Pro has very small screen borders and no notch (iPhone) / punch hole (Samsung); instead a motor makes the selfie camera pop up & retract from the top of the phone. I also never updated LineageOS because it went from an unofficial build to officially supported so updating would require a complete reset.
About two months ago I noticed that the camera would pop up every once in a while for seemingly no reason.
I could only conclude that it was hacked; worse, I would have never noticed on a 'normal' phone without a pop-up camera. Was my phone also recording me the entire time?
I don't know what the vulnerability was - it could have been a remote exploit in Android itself that's also exploitable on your phone, or it could have been from an app that I had installed (the only apps I had with network usage + camera permissions were Firefox, and the latest version of WhatsApp from when I bought the phone (no updates since I don't have Google software on it, I just downloaded the WhatsApp APK when I set it up)).
You've said that you don't have any sensitive data on your phone, but still be careful.
I updated LineageOS and since then the issue has disappeared. I update the OS about every week now to hopefully prevent this from happening again.
That seems like a bit of a leap. Isn't it more likely to have been a software glitch of some sort?
And it hasn't happened again since I reinstalled the OS two months ago (though admittedly it took ~4 months to start happening the first time).
Doesn't seem like a very big leap to me.
(Not saying that's what happened, but I can easily hypothesize a scenario where this only happens after a while, after some specific app is installed or configured.)
I tried several approaches to this problem, with an ESP8266 and a water-flow measurement on the intake, via a microphone/vibration sensor catching the spin-cycle, and finally via measuring electrical consumption (again on the spin-cycle).
Sadly I had to remove the setup, but it was a fun project that took a few months of iteration to get working.
2KHz is the frequency of my machine's beep. Just triggering on overall noise level alone would notify me each time the machine goes into spin, but using the FFT allows it to be more discerning.
(I'll try to publish the code soon.)
Perhaps someone (with more patience than I for Android/Kotlin development) can tweak it so that is can listen for knocking on a door to trigger some sort of alert (email/txt/broadcast an intent).
Triggering an intent would be very helpful as then it could then be acted upon by Tasker or Automagic4Android.