I am trying to build an alarm that works in an opposite way to a security alarm in a shop. In shops RF tags on items set off an alarm as you walk through a detector. I wanted to build alarm that sounded as you move out of range of a detector, rather than into range.
The tags on high value items seem to be simple coils of wire encased in a thin plastic sleeve, they don't appear to have a power source. I presume they emit a small signal that is enough to trigger the walk through loop.
Basically my gran keeps leaving her keys on the table and walking out the front door and getting locked out. I figured if she had a tag on her key fob that would trigger an alarm on her wrist or bag when she was more than 2 or 3 metres from her keys might solve the problem and save her a fortune in locksmiths!