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Basically, I'm working on a project for a sports team. I want to strap something to each ankle of an athlete (preferrably not much bigger than an oversize wristwatch). These devices should have no connection between them (i.e. allow the athlete to move freely).

If the ankles of the athlete come within approximately 30cm or less of one another, I need an audible sound. The idea is to encourage a "wide stance". I'm not too picky about the accuracy, even an error close to 5-10cm would likely still be alright.

I don't think IR sensors would work because there's no way to keep them "aligned" with each other easily. Would an RF sensor come in this size and allow for this kind of fidelity? I'd basically need something constantly telling difference between two moving points within 1-2 meters of each other, to an accuracy of about within 5cm, and doing so regardless of the orientation of each device (though it's not expected that they will have anything other than possibly the ankles come between them.)

Thank you all for your help/suggestions!

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    \$\begingroup\$ ultrasonic might work. \$\endgroup\$
    – user57037
    Nov 9, 2015 at 20:36
  • \$\begingroup\$ You could consider low frequency coupled coils .It wouldnt be too tricky to lash something on a table and test it . \$\endgroup\$
    – Autistic
    Nov 10, 2015 at 1:21

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Longish magnet on one leg and reed switch or hall effect switch on the other? The hall effect switch would be better as it will provide an analogue output proportional to the magnetic field strength. Feed this into a threshold detector (a comparator with one input voltage determined by potentiometer and the other by the hall effect output) and let the output generate the beep.

I'm assuming that you would only care how close the feet were at their closest point of passing.

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