I'd like to create a small device that can tell when it's near another created device or sensor. When it detects the other device, it lights up an LED.
Maybe maximum detection range of a few feet. No need for any unique ID values or complex data exchange, just a simple "Yes I detect one, no I don't" sort of control.
Any suggestions on how this could be done in a low power, portable sort of way? Because its requirements are so simple, something like RFID seems overkill (even if it could be made to work).