WTB (Build): Low power “proximity to a device” detector

Question

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).

Answer 1

You could use a simple Infra-red LED, and a photo diode. You may have to adjust the LED intensity with a potentiometer to calibrate the range.

This will sense proximity providing there is line-of-sight.