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PICList Thread
'[EE]: Light guns'
2000\06\27@054923 by Andrew Seddon

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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

2000\06\27@082648 by M. Adam Davis

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They use the same technology as light pens.  There is a phototransister inside
the gun.  The TV scans the picture, and when the scan line gets to where the
light gun is pointed at, the phototransister turns on.  The controller, knowing
where the scan line is at any given time, can give the position of the light gun
to the processor.

Many VGA chips still made today have a light pen input.  Hook that up to a
phototransister with a lense and you've got a light gun.

-Adam

Andrew Seddon wrote:
{Quote hidden}

2000\06\27@104424 by Matthew Fries

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> Many VGA chips still made today have a light pen input.  Hook that up to a
> phototransister with a lense and you've got a light gun.

I know that almost every MONOCHROME adapter card had a specific header for
a light pen, but where would you connect a pen on a VGA card? Is there a
pin on the feature adapter that would do this?

2000\06\27@122346 by M. Adam Davis

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As I said, the chips generally have the input (and registers to get the info).
You'd have to search for your particular chip to see if it has it, and if so
you'd likely have to wire your own connector up.

The VESA feature connector has enough signal information that you could use a
relatively simple circuit to interface between it and the light pen which would
tell you where the light pen is.

http://www.hwb.acc.umu.se/co_VesaFeature.html

And you could get quite a bit of information from the actual VGA signals being
sent to the monitor (H & V Sync), enough to tell you where the light pen is.

-Adam

Matthew Fries wrote:
>
> > Many VGA chips still made today have a light pen input.  Hook that up to a
> > phototransister with a lense and you've got a light gun.
>
> I know that almost every MONOCHROME adapter card had a specific header for
> a light pen, but where would you connect a pen on a VGA card? Is there a
> pin on the feature adapter that would do this?

2000\06\28@025512 by Gennette, Bruce

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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 removed}

2000\06\28@164246 by Peter L. Peres

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Hi,

the light gun itself contains a simple telescope and a photodiode or
photodarlington. There is nothing special about it except it has a decent
preamplifier that should be reasonably fast.

There are two kinds of lightguns: Those that are assisted by a computer to
a greater degree and those that are assited to a lesser degree ;-)

The lesser degree is the same as the lightpen that you can connect to the
CGA computer display cards. It has a photodetector and preamplifier and a
trigger signal. That's it. The rest is done by the computer, which will
modify the picture on the TV where it thinks the lightpen points to to
determine which character it sees, exactly. This may be done with every
read or once in a while for calibration. The output pulse is latched by
the CGA card in the form of a H+V address counter position and is
available for further processing. These systems typically do a great deal
of flashing and moving image parts at the location touched by the
lightpen/gun when you 'shoot'. The extra effects help the computer to
correlate and confirm the position more accurately.

The higher degree has a small microcomputer that takes video H and V
signals (thus the SCART connection), and processes the input pulse into a
numerical delay expressed from the left, top of the TV frame. It then
transmits this to the 'other' computer which proceeds to do 'bang' and
smartly flash the screen so it gives a confirmation of the location of the
pulse to the user. This system also needs calibration (with user
assistance).

The light amplifier and the optics in the gun need to be pretty good
because it only views 9 to 100 pixels out of serveral million on the
screen and they are not full on at that.

Once upon a time you could roll your own using some kits or other. Range
was under 2 meters.

Peter

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