When an electron beam first hits the front of a TV or computer screen to
stimulate the phosphor it creates a (relatively) bright burst of light for
about 1/1,000,000 th of a second for each pixel (which we don't have time to
'see').  The phosphor, once stimulated, continues to deliver a lower level
of visible light for a short time after the stimulus is removed - this is
what we do see.

The start of each frame and the start of each line information is easily
gotten from nearly every video card available (or is built into games
consoles).  So, all you need to do is detect a bright light at the right
time to know that the gun is pointing to a certain spot on the screen - if
that spot is where the baddy is - then you got him!

A cheap lens, a bit of tube (barrel), a phototransistor and a switch are the
external part, internally the video generator feeds position information to
the game engine, a bit of knowledge of the video format in use, a bit of
maths and hey presto, a light gun!

Bye.

        -----Original Message-----
        From:   Andrew Seddon [SMTP:seddona@HOTMAIL.COM]
        Sent:   Tuesday, 27 June 2000 19:35
        To:     PICLIST@MITVMA.MIT.EDU
        Subject:        [EE]: Light guns

        I`ve been meaning to ask this for a while now as it`s realy getting
on my
        nerves. I saw a review of light guns for computers, the kind that
allow you
        to shoot at your TV and most were selling for under 40. Now how the
hell do
        they do this??

        I thought maybe it had a sensor that detected the color of what it
was
        pointed at and correlated it to the screen but that would be
completely un
        reliable. I can`t possibly see how they could implement inertial
tracking to
        that degree of accuracy for such a low cost product. So I figure it
must be
        something to do with the linescan on the TV, as I had a look on my
        playstation and sure enough the bit that you plug one of the guns
connectors
        into breaks out from the scart lead.

        Anybody shed any light on this!

        Andrew Seddon