I'd like to setup a system to detect when I'm home or away, but my phone can not be the source of the trigger/detection.
There's a couple of reasons. I'm not too keen on the idea of an app running in the background, using resources just for this purpose. For various reasons I turn on/off my phone wifi, so I would get very inconsistent results if connecting to my home network was the trigger.
What I'm thinking is an RFID or bluetooth tag in my wallet or key ring. I always leave the house with both.
Yes, understood. If you had said iPhone, I would have offered a suggestion that would not have an effect on your phone battery. Since you said Android, I would also suggest ESP32 is likely your best bet.
How accurate do you want this to be. Do you want to be in your house/on your property or are you looking for accuracy within a few hundred metres. If it's the latter, then Macrodroid has a useful function which detects which phone mast(s) you're connected too. You could use a cloud endpoint to trigger a routine when you connect/disconnect to your local phone mast. Also, as @sidjohn1 suggested, the smarthings arrival sensors are very good but they eat batteries so I'd recommend hacking them with larger batteries and then to leave them in your car. Finally, I appreciate you don't want to use WiFi, however I've found this option to be rock solid for the last 2 years
Is your goal to make sure somebody at home or not or to detect only arrival/departure conditions?
In a past I was using BLE Beacons with DIY ESPresence + Home Assistant.
This worked very well and could be used for detecting all presence/arrival conditions.
But unfortunately BLE Beacons are not secure at all. I am using arrival detection for unlocking
and opening the apartment door. The security for this application is very important.
My presence detection is based on multiple Motion Sensors plus RM rules. For the arrival
detection I switched to Active RFID fobs. here is a link to my Active RFID project:
Been using Samsung's built in Modes and Routines app to manage my presence when I connect/disconnect from my home WiFi from my phone. If you don't have a Samsung, you could look at something like MacroDroid to do the same.
The way I have mine setup is:
When I connect to my home WiFi, it use a webhook to turn on my virtual presence switch in my C8 which triggers my outdoor lighting routine. I have another step to display a pop up button on my phone to run another webhoook to a virtual button which runs my unlock door routines.
When I disconnect from my WiFi, I set my virtual presence switch to off and display a pop up on my phone to re-enable presence in case I'm doing something that disconnects me from my WiFi.
I have the same setup mirrored on my wife's phone for her virtual presence switch. On my c8 I have a rule machines rules to auto engage HSM when both phones are away for more than 5 minutes. I had a second rule to lock the door only when just me or the wife leaves but find that I have auto-lock already on the door so it was redundant.
I keep an ST sensor with AA battery modification under the seat of my motorcycle. It has endured modest off-road activity and western PA roads (which are worse) without issue.
The one in the glovebox of my wife's car somehow had a battery pop off the contact after 2.5 years. Not sure what kind of stunt driving she must have been doing.