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.