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[on:]

Ok, this application might sound a bit odd.  I am trying to figure out what kind of sensor I might use that could be used to sense when a person moves his or her hand close to their face.  I need the sensors to be very small and unobtrusive to the wearer -- and, with that said, the sensors obviously have to be wearable.  I am new to physical computing, so I don't know the right direction to explore. 

The precision of the measurement does not have to be exact -- and, the type of data that I am ultimately looking to collect is simply doing a running count.  For example, my ideal sensor would be bracelets, perhaps with embedded magnets.  Then, another sensor would be connected to the persons collar or a necklace.  So, assume that a person has a chronic nail biting habit.  Each time the persons hand went to her or his mouth, it would log the event.  Of course, this isn't precise measurement because it simply indicates whether the hand reached the general area of the mouth.  And, yes, you would have a lot of false positives in the event of sneezing, scratching, and eating, but all these errors in data collection are easily managed from my point of view.  Again, I am really  just trying to solve the problem of identifying the event when the person's hand goes close to her or his face. 

Any suggestions for sensors to consider?

Thanks in advance!

[Reply #1 on:]

what about a necklace (around the throat) that detects some RFID like thingy, that is attached to the wrists?
http://playground.arduino.cc/Learning/PRFID

according to en:WP they can b read from 10cm to 10m... 
http://en.wikipedia.org/wiki/Rfid

[Reply #2 on:]

Any suggestions for sensors to consider?

Not difficult at all: an lc oscillator will see its frequency shift as a human hand / face is approaching. So your mcu solution would be like this:

1) enable your watchdog timer;
2) use an exposed coil as the main oscillator to the mcu.
3) in the loop, count up a variable (do not initialize this variable) until it is reset by the watchdog timer.

the count will be smaller if a hand / face is approaching the coil.

The actual implementation will be more complex than this but the gist is the same.

The same principle can be used as a metal detector / finder.

[Reply #3 on:]

What dhenery is forgetting is the effect the body will have on the coils. This will swamp the signal you are looking for.
You can get wrist mounting RFID tags that look like a watch, it might work. But I think it is one of those projects that sounds a lot easier that it actually is.