Area detector

I'm working on a project involving a principle of zoning detection: an objet A is supposed to be linked to one or several objects in a certain distance limitation.(around 2meters) When an object is out of the delimited zone of operation, object A reacts.

The question is the following: What could I use to make this system? (considering the fact that I already tried with an RFID and it's not suited for what i want)

What could I use to make this system?

Sadly this is often asked but I have never seen a satisfactory answer.

You could use GPS to determine the position of each object and radio that back to a central point.

You could use a camera mounted high so it can see and track all the objects.

You could use an active RFID tag or transponder system to get a greater range.

No doubt ultrasonic systems will be mentioned too, but that is even more tricky.

Also I have see suggested a sort of DIY GPS type system with local sensors but that is even more tricky.

In short I don't think there is an easy answer to this one.

I've seen Image recognition as a solution (theoretically).

Object A could be a camera connected to PC with program for image recongition with algorithms and stuff, it's more like a code challenge than dealing with hardware. The algorythms are not easy. "A" would be an artificial watchman.

Thank you all for your help, but I forgot to mention that Object A must be portable and all other objects must be very compact. (Ideally small and flat as a sheet of paper :) )

iBeacon:indoor positioning/GPS/MAP

Compatible devices iOS devices with Bluetooth 4.0 (iPhone 4S and later, iPad (3rd generation) and later, iPad Mini (1st generation) and later, iPod Touch (5th generation). Macintosh computers with OS X Mavericks (10.9) and Bluetooth 4.0 Android 4.3+ (e.g. Samsung Galaxy S3/S4/S4 Mini, Samsung Galaxy Note 2/3, HTC One, Google/LG Nexus 7 2013 version/Nexus 4/Nexus 5, HTC Butterfly, OnePlus One) Windows Phone devices with the Lumia Cyan update or above.

Make an iBeacon in Central roles with Arduino Yun

http://forum.arduino.cc/index.php?topic=288624.msg2019795#msg2019795

Davidkoubi: Thank you all for your help, but I forgot to mention that Object A must be portable and all other objects must be very compact. (Ideally small and flat as a sheet of paper :) )

Have you any idea how annoying it is having vital information withheld or being dripped into the thread. Given this new restriction then you have ruled out any soloution whatsoever.

Grumpy_Mike: Have you any idea how annoying it is having vital information withheld or being dripped into the thread. Given this new restriction then you have ruled out any soloution whatsoever.

I am sorry for this unfortunate error . I have no excuses. I was so concerned about finding a solution that I forgot the secondary features of the idea.

Davidkoubi: I was so concerned about finding a solution that I forgot the secondary primary features of the idea.

:D

Actually, RFID is pretty much the answer, but you need a custom system. It would resemble the things used for "shrinkage" reduction in retail.