17 Jul
2001
17 Jul
'01
6:29 p.m.
On Jul 17, 19:17, Tony Duell wrote:
pixels.
Indeed. If you're not careful, the detector that can "see" the spot on a
"dark" screen gets dazzled by a lit pixel, or fooled if you tilt the pen so
ambient light leaks past the tip. I gave it up.
--
Pete Peter Turnbull
Network Manager
University of York
[Lightpen over dark screen]
If that's the article I am thinking of, they made
a mistake and suggested
the device you needed was the plain phototransistor. The next month they
publised a correction that said you needed the phototransistor + schmitt
trigger Sweet-Spot.
Yes, but the schmitt-triggered one has a different problem -- it's too
slow! It's a long time ago, and the articles and my notes are buried near
the bottom of a heap of other stuff, but I seem to remember that the
frequency response of the schmitt device was quite low. I didn't use it in
the end; I built a custom amplifier with the front end in the pen tube.
brightness was
set so that black was *just* not visible, a suitably
Yes, but as you imply, that's not truely a dark screen :-)...
> adjusted lightpen on a short-persistence monitor could detect dark It was,
however, *much* easier with lit pixels.
And the adjustments (monitor brightness, light pen sensitivity/threshold)
are _very_ touchy!