Inductive coupling / "Near Field Communication" / NFC sounds close to ideal=
..
Can be used to power remote too if desired.
They say 4 cm range here but range can be metres if desired.

           http://en.wikipedia.org/wiki/Near_field_communication

>  several of these devices could be in close proximity to each other

Depending on the value of "close", 50 cm is getting a bit far if you
have multiple units but if you can get the two closer together and
reduce available range then you can use N in closish physical
proximity. Dust no problem. Probably no certification issues either
(YMMV). Potentially low cost and simple.

Capacitive coupling also worth thinking about.


 Russell
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