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I'm looking for a way to track people in rooms.

iBeacon track Smartphones. But it requires people to have a smartphone.

I'm looking for a reverse beacon ? People wear iBeacon and trigger a kind of BLE "light smartphone"

(There is RFID answer put "it's complicated" and imply antena, etc ... )

Any Ideas ?

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    \$\begingroup\$ With only one exception that I know of, the chips used to make BLE beacons are capable of acting in the "central" role and detecting "peripherals" (other beacons), they just need different firmware to act as detectors rather than receivers. More prototype friendly, there are several writeups on using a raspberry pi or other embedded linux board with a cheap USB BLE dongle to detect beacons. \$\endgroup\$ – Chris Stratton Sep 23 '14 at 14:58
  • \$\begingroup\$ I agree with Chris. The easiest way is to use an Bluetooth LE dongle (bluegiga.com/en-US/products/bluetooth-4.0-modules/…) as the detector with either a PC or a single board computer (RPi, BBB etc.) \$\endgroup\$ – m.Alin Sep 23 '14 at 15:07
  • \$\begingroup\$ Thanks ! is there article on RPi or product providing such feature ? I ask before rewriting the wheel. \$\endgroup\$ – Jean-Philippe Encausse Sep 23 '14 at 16:00
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Yes, it would work just fine in theory.

However, it will stop working well when you have too many people (beacons) all in the same area. Each beacon is an uncoordinated transmitter and its messages can and will collide with others. Messages will also be garbled by interference from other 2.4GHz transmitters (WiFi, other Bluetooth devices, microwave ovens, etc).

Also, at some point you may have a large number of beacons (e.g., 100) and in each interval (e.g., 1s) the receiver will only see some fraction of those (e.g., 50%). While that may seem okay and you might think that you should see them all in a few seconds, it can take a while to see the last few. It becomes a variant of the Coupon Collector's Problem.

The question of where it breaks down is left as an exercise for the reader. Others on SO have reported at least 20 beacons working nearby with no problems, but your mileage may vary.

In regards to using an embedded Linux computer (Raspberry Pi) to scan for beacons, there's this question on SO and this guy's Github project.

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