Martin Nilsson wrote:
>
> I have an application where I need to sense four touch pads with a PIC.
> These pads should react to very light touch, or even just proximity,
> so (mechanical) membrane switches are inadequate.
>
> I'm thinking of the following approaches:
> - Optical: Sensing the proximity of a finger with a photoreflector
> - Inductive/Capacitive: Presence of a finger changes inductance/capacitance
>   in an oscillator, which changes the frequency
>
> I would be interested in hearing if anyone has suggestions and
> experience of this problem. Noise and ESD are factors.
>
> Martin Nilsson                           http://www.sics.se/~mn/
> Swedish Institute of Computer Science    E-mail: mn@sics.se
> Box 1263, S-164 28 Kista                 Fax: +46-8-751-7230
> Sweden                                   Tel: +46-8-752-1574

For photoelectrics, noise immunity is best for 'through-beam' mode,
where finger interrupts a beam rather than reflects it.  Also, you will
need to consider whether modulation of the beam is necessary if ambient
light is strong.  Hamamatsu and Sharp offer 4-pin chips that include
everything needed for modulated photosensor except LED and bypass
capacitor.
--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists