I am designing a hobby project where I need a small device (powered by RPi that is running an app over linux) to be able to detect when it is within, say, 2 feet of another object. My initial thinking was that this is a perfect use case for RFID, but perhaps there are better strategies/techs out there. Also, this being a personal/hobby project, I'm looking for open source software/hardware and COTS components every step of the way.

If I were to go the RFID route, then I would need an RFID interrogator integrated with the RPi device. This could either be a standalone RFID reader that just passes on tag detection events on to the RPi, or perhaps my own homegrown hardware that does the same thing. When the reader detects a tag, it needs to communicate with an app running on the RPi, probably via serial comm.

My questions:

 1. Is RFID the right solution for this use case? If not, what is? Can you give details on how *you* might implement it?
 2. If RFID is the right solution, then what types of RFID tags and readers would be cheap/COTS, and compatible with RPi? Alternatively, what hardware would I need that could serve as a primitive reader, and communicate with RPi?