# Battery powered, wearable, non line of sight, short range metal detector?

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.

• Have you considered using NFC instead? – Ignacio Vazquez-Abrams Jun 26 '16 at 1:39
• 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 – Pushpendre Jun 26 '16 at 1:46
• @Pushpendre, take a look at RFID tags - they do not need power. – Catsunami Jun 26 '16 at 1:48
• 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. – Ignacio Vazquez-Abrams Jun 26 '16 at 1:50
• Thanks for pointing me in the right direction. I found further info at adafruit.com/category/55 – Pushpendre Jun 26 '16 at 1:59