I would like to build a proximity tracking system. A a central unit small enough to fit on a key chain or necklace that would be able to detect as soon as a tagged object is farther away than a dozen or so feet.
I was thinking the central unit would consisting of a Arduino Mini and some sort of RFID reader. I would use semi-passive RFID tags since passive tags wouldn't be able to provide the range I need.
I haven't been able to find much information on semi-passive RFID tags. Would I be able to get the range I need with the form factor constraint I have? How can I obtain some semi-passive tags? I wasn't able to find any on digikey or any information on designing them myself.
Will I need to use fully active tags? Or maybe there's a better solution that doesn't use RFID?