Peter L. Taylor wrote:
>
> -- [ From: Peter L. Taylor * EMC.Ver #2.5.02 ] --
>
> > 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.
> >
> > Paul Mathews wrote:
> >
> > 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.
>
> O.K. but how do you "Fly-Proof" the buttons?
> What I mean is how do you keep a fly or other object from creating an
> erroneous button press?
>
> --
> Peter L. Taylor
> ~~~~~~~~~~~~~~~~~~~~~~~~
> Email: petert@transera.com
> ~~~~~~~~~~~~~~~~~~~~~~~~
> http://www.transera.com
> ~~~~~~~~~~~~~~~~~~~~~~~~
> TransEra Corp.
> 345 East 800 South
> Orem, Utah 84058
> ~~~~~~~~~~~~~~~~~~~~~~~~
> Voice:  (801)224-6550
> Fax:    (801)224-0355
> ~~~~~~~~~~~~~~~~~~~~~~~~

Two methods are often used:
1. Timing: use routine like 'debounce' and require minimum 'keypress'
time
2. Optics: adjust sensitivity and beam spread so that only a huge fly
could block the beam.

--

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