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!

  • $\begingroup$ Changing the lock is not an option? I personally despise any door that automatically locks when closed. In my opinion it should only lock with a key. It would be much cheaper and fast and also more efficient (because it is impossible to lock the door without the keys, unless someone else lock her out). $\endgroup$
    – Mandrill
    May 29, 2015 at 22:18

1 Answer 1


Reverse the logic for the RF tag detector and alarm. As you state, RF detectors and alarms activate when the RF tag is within range of the detector. Change the logic of the detector-alarm system to sound the alarm, as the default situation, when the tag is out of range. When the tag is in range the alarm turns off.

It's a similar logic to that of dead man switches for train locomotives.


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