[sdiy] Has anyone seen (or played with smthing like) this ?

[Colin f]

> >Wouldn't it be easy to put together a negative version, where it  
> >sensed lack of ambient light from whichever diodes were covered?
> 
> Or a modulated light source, and synchronous detection?

Possibly, but how would you deal with the larger number of LEDs obscured by
the operator's finger and hand ?
The trick with the LEDs as detectors is that the light they are picking up
is directly reflected into them by the users fingertip - a very small area,
and a consistent level of light to find the detection threshold for.

Cheers,
Colin f