Russel, maybe you could adapt one of the miniature wireless mice or
trackballs. Use the XY axes mechanically to sense head turning and tilting
(or nod & tilt) and maybe sense blinking reflectively. I see this done
using a pendulum type sensor, encased in a small tube filled with oil for
damping. I have made a reflective blink sensor using a lensless spectacle
frame and a modulated IR LED + SFH type detector. It works well. It senses
the pupil to some extent so it could be used for gaze sensing but this
would require a lot of work imho. Several commercial gaze sensors use
infrared cameras observing the pupil. Look for patents etc imho. if you
want to go with a camera type.

Peter

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