Proximity Control

Hello, I'm not sure whether there is another subject relating to this already, so question is. Is it possible to make a proximity control with Arduino. Not motion sensors,but more of an object you carry close to another that makes it turn on. In the case of my project, a light. I'm trying to make a Blue Lantern Power Battery and Ring that when you bring to ring within a certain distance the battery lights up.

You'd use an Ultra Sonic Sensor

I was reading a bit about RFID stuff. I was wondering if it might be possible to put a small RFID in the ring and a reader in the Lantern.

RFID would work, but you'd have to carefully figure out how to get the distance right by manipulating transmit power from the reader as the antenna on he ring would be powered by it.

Ultrasonic would be simple but would turn on indiscriminate of the ring or not.