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I want to provide people at an indoor exhibition of ours with a bracelet which allows me to send a signal to it to flash a led light. At the same time I want to be able to determine the location (within 2-3 meters) of that bracelet (everyone wearing the bracelet is assumed to have given permission for this). The exhibition consist of 1 room (mutliple rooms MAY be needed in the future, not sure if that changes anything in technology requirements). I was thinking of some sort of radio receiver, but what kind of equipment & technology would be required to do such a thing? Also for determining the location of the bracelet, e.g. could triangulation work? (I need the cheapest solution, so probably bracelets without GPS). The battery (if any) in the bracelet would only have to last a couple of hours.

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Your question is very vague. 3 meters might be possible by using signal strength from various RF transmitters scattered about in known locations, but there are a lot of problems with that. –  Olin Lathrop Jan 14 '13 at 14:15
@OlinLathrop I think you downvoted my question but what exactly do you find vague about my question? Just downvoting doesn't help me improve the question :) –  Flo Jan 14 '13 at 14:41
I didn't downvote the question, not yet anyway. –  Olin Lathrop Jan 14 '13 at 15:01
This is a thought-provoking problem statement, and has more information that most real-world client briefings contain... So I'm not sure I understand the justification behind close-votes and down-votes. Are we getting too rooted within our ivory towers? –  Anindo Ghosh Jan 14 '13 at 16:20
Thinking outside the box => Bluetooth enabled mobile phone? I know there is a mode (used for commercial advertisements, but I don't know the name) that doesn't require pairing. –  jippie Jan 16 '13 at 17:20
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2 Answers

up vote 4 down vote accepted

You probably want to check out OpenBeacon, which implements this sort of tracking using an 'Active RFID' tag. A bonus of this is that it already includes the 2-way radio you'll need to instruct tags to blink.

Triangulation is done by positioning a large number of basestations around the venue, and having tags transmit pulses at varying amplitudes. By examining which set of nodes can be reached at each amplitude, the system can narrow down where the user is likely to be. They're a little vague on what sort of accuracy the system can claim, though, or how it deals with interference due to things like obstructing walls.

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+1 for an "It's been done" example! –  Anindo Ghosh Jan 14 '13 at 16:17
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To begin with, the location problem:

GPS would not work in an indoor venue, because a GPS receiver works by reading signals off four or more GPS satellites orbiting the earth, to compute a precise location (and elevation).

Further, even if the venue of deployment were open to the sky, non-military autonomous GPS accuracy is ~ 4 to 10 meters, while more sophisticated Wide Area Augmented GPS / Wide Area Differential GPS enhance this to perhaps 1-2 meters. However, implementing such a system in a button-cell operated, bracelet sized GPS would be a stretch.

There are two alternatives that may be viable, neither being foolproof.

  1. RF triangulation using a minimal RF transceiver in each bracelet, and RF transceivers driving antennas distributed around the room, on the ceiling perhaps.
  2. Inertial location, such as is being researched for mobile phones to support indoor location systems, something like an extension of Google Maps that would work within buildings. This would require a multiple degree-of-freedom location / orientation sensor / IMU. Size, cost and battery life would be severe challenges with this approach.

Multiple manufacturers such as SemTech, Texas Instruments, Fujitsu and Analog Devices produce single-chip RF transceivers for various frequency bands. The bracelet itself can be used as an antenna, via a suitably long wire embedded into it.

Flashing an LED on receipt of a message could either be achieved by incorporating a microcontroller, or more trivially by using the status pins of the transceiver IC and some creative signaling from the base station.

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Ah, thank you, very useful information. The first solution sounds like something that is cheap and realizable. We have a square room of 2500 m2 with a height of 8 meters without any objects interfering with the signal (lamps etc). What kind of RF transceivers/antenna's would be recommended and what amount to cover this space? Any ideas on costs of such a setup? Thanks again! –  Flo Jan 14 '13 at 14:47
@Floran Almost any transceiver would work, as long as the wavelength is small enough to allow a bracelet-circumference antenna. A 2.4 GHz Nordic nRF24L01+ for instance could do the trick, the antenna length required is provided in the datasheet sample layouts. Use flexible PCB for the circuit, and the whole thing can be embedded around the inside of the bracelet. Cost: You'll have to work that out yourself. –  Anindo Ghosh Jan 14 '13 at 16:26
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