On Tue, 12 May 1998, Bruce Cannon wrote:

> Sorry if this has been discussed before, but I'd like to try to emulate the
> technique which Timex uses, wherein the wearer holds their watch up to the
> PC screen, which flashes, transferring data.  Basic visible light comm seems
> hard enough to me; most likely the watch has a blue light PIN diode or some
> such; but I'm REALLY ignorant about video; are there tricky things to
> consider re scan rates, etc.?
>

  I don't know if this has been discussed before, but I know that
lightpens work quite well with visible light, and that the amount of data
to be transferred into the watch is very small, thus it makes little sense
to use a high speed PIN diode and a power hungry amplifier in a
battery-driven device. Minimal optics can focus a window of about 1/2 14"
screen size (1/4 area) well enough from a few feet and the amount of light
is certainly enough to make it feasible with a low power and low cost
amplifier.

  If one displays a pattern such that each signal bit is displayed in a
different scan line, then bits will arrive at between 15 and 65 kHz
depending on the scan rate. A PIC can cope with this bit rate, including
in bit-banging mode (which you would have to use anyway).

  Of course the protocol and encoding would have to be self-synchronizing
and packet-oriented ;)

  It's a good idea anyway.