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?

  • \$\begingroup\$ Look up the University of Washington and also the topic, generally, of ambient backscatter. No licensing requirements, can get a range of about where you are considering, very simple in concept (if not in the software implementation.) Very low power (almost zero.) \$\endgroup\$ – jonk Jul 3 '18 at 6:14
  • \$\begingroup\$ @jonk Ambient backscatter does look like a very interesting concept but everything I've read puts the range at a max of 2.5 feet. The absolute minimum for my application is 5 feet and I would prefer more like 10+. \$\endgroup\$ – RedHatter Jul 3 '18 at 7:25
  • \$\begingroup\$ I've heard that implementations have reached 10 ft, perhaps a little more. But this is an area I plan to experiment with next year. Not yet. So I don't know more than what I've read, so far. \$\endgroup\$ – jonk Jul 3 '18 at 7:33
  • \$\begingroup\$ @jonk You don't happen to know where you read that? The only project I can seem to find is the original one. Even if it doesn't work for this project I'm very interested in following the technology. \$\endgroup\$ – RedHatter Jul 3 '18 at 7:53
  • \$\begingroup\$ How big would you consider the antennas/coils (at both ends of the link) could be - the bigger the better. If you are talking just a few centimetres then it's unlikely this will ever work over the distance you propose. Directionality is a big problem usually. Certain directions will obtain max coupled energy, other directions will be zero. \$\endgroup\$ – Andy aka Jul 3 '18 at 8:43

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