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I want to build a circuit to detect when people bring their hands close to their mouth. This could be used to warn chronic nail biters of impending nail biting for example :)

The idea I have is that a person can wear a metallic ring on their finger (iron/gold?) and a necklace with a small sensor near the neck. A heavy pendant counters the sensors weight and keeps it on top near the neck. Now the circuit's job is to detect proximity of the metallic ring within a 3 inch radius and I am looking for ideas.

I couldn't find anything like this on adafruit et al. Also there is another question that asks about short range metal detector but the solution doesn't fit my constraints.

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  • \$\begingroup\$ Have you considered using NFC instead? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 26 '16 at 1:39
  • \$\begingroup\$ Great comment, actually NFC never crossed my mind. AFAICT for NFC both the ring and the necklace would have to be active/powered devices. Is that correct? Is there a version of NFC in which one of the parts can be passive/unpowered? I found TecTile though which seems to be unpowered \$\endgroup\$ – Pushpendre Jun 26 '16 at 1:46
  • \$\begingroup\$ @Pushpendre, take a look at RFID tags - they do not need power. \$\endgroup\$ – Catsunami Jun 26 '16 at 1:48
  • \$\begingroup\$ NFC is a specialization of RFID. Both sides only need power if you need long range, otherwise it is completely feasible to let one NFC device power the other. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 26 '16 at 1:50
  • \$\begingroup\$ Thanks for pointing me in the right direction. I found further info at adafruit.com/category/55 \$\endgroup\$ – Pushpendre Jun 26 '16 at 1:59
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RFID is ideal for that. There are very small glass encapsulated tags which are typically for injecting into pets. Transceivers are pretty cheap too but I'm not sure you'd want to be installing one in someone's mouth.

A pendant would probably be a bit small for the necessary coil but you could loop the coil around the subject's neck, I guess.

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