How to build a max distance alarm

I've been searching for an instructable that does that but couldn't find it. The basic idea is to put a small as possible tag to something(unpowered if possible) and have another small thing that can fit in your pocket. If the tag goes more than, say, 4,5 or 6 meters away your pocket box would warn you. I think its a great idea to protect from thieves but no idea on how could I build a device like that. Thanks

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Saladino (author) 9 years ago
But you are talking about a powered plane that measures the radio input, I don't know if that would work with a small tags. Maybe if we use a button battery and only use it to answer to a signal, the other box would be able to calculate the distance by periodically signaling the device. I don't knwo how to do this though
I think it might, given the size of the actual circuitry on a lot of planes. Like I said, the specifics are beyond my abilities, but having a button cell "ping" the other unit sounds reasonable. Of course, how would it be timed (i.e. if the other unit doesn't send a signal every 30 seconds, how does the other unit know the scheduled ping was missed)?
Saladino (author)  Rishnai9 years ago
Shouldn't it be easier to just be listening(the tag on the object) and answer to the pings from the central unit? That way the central unit could calculate the distance from the tag and warn you. At this point we need powered tag and powered unit, but still no idea on how to do that
Goodhart9 years ago
Whatever "detection" method you use, it would be "always on" when in use, that is, the alarm state must come from the "breaking" of the connection or the signal. Most home alarm systems use this type of circuitry in case someone "cuts the wires", which sounds the alarm.
Rishnai9 years ago
I've considered doing that with my car keys so valets don't go for a joyride. Are you familiar with remote control planes, where if they go out of range of your controller, will slowly power down and come to a landing instead of continuing in a straight line? Having "no radio input" default to "switch open" sounds like the way to do it. I can't really explain how to do that, but I'm sure someone will shortly.