Light Pens ... - test-drb@ccmp.vtda.org

18 Jul 2001

On Jul 18,  0:45, Tony Duell wrote:

...
  > 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
...

 Right... It's generally recomend for BBC lightpens (See, for example,
 the Advanced User Guide page 369). Are you saying none of these designs
 work properly? 
Well, mine ended up having more in it than the one on p.369 (which has just
a Sweet Spot phototransistor mounted in the barrel) - it had a transistor
and bias resistor in there too, along with a CMOS chip to make a
touch-sensitive switch.  I can't remember what the Sweet Spot part number
in the AUG is (schmitt or plain) so I can't tell you if that's the one I
used.  I bought both types, and one other photodiode (or phototransistor?)
...
 from the RS cat. 
I see that page also refers to a fudge factor to be subtracted from the
register contents -- I remember having to do that too.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York