object proximity alarm

Hey all. I'm working on a project, and I would like to add a proximity alarm feature, ie, there are two halves to the device. If the halves move further than a set distance from each other (say, 5 meters) then an alarm in one or both is triggered (ie the devices need to be kept close to each other)?
Can anyone point me to an existing device or schematic for such a circuit?

can you use a reed switch on one and a magneton the other?

As I explained in the post, I need a 5-10 meter range. Preferably with the ability to tweak/set the distance. Ie the alarm goes off when the distance exceeds 5 meters. The reed/magnet switch would only give me mm.

Cheers

Is this for car following?

As I explained in the post, I need a 5-10 meter range.

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No you said

(ie the devices need to be kept close to each other)

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To do 5-10 meters acturatly will be hard. Maybe sonar if thay always point at each other? May be RF? RFID?
Just what are you making?