Old NES systems sometimes have this little gun you an hook up to the controller port…you can play games on it, like Duck Hunt, by aiming the gun at the screen & pulling the trigger. There’s also the SuperScope for the SNES, which works the same way, and some other ones…
I’ve seen these things for years, but I still have no idea how they work! How the heck does the TV screen CRT pick up input signals from the gun!?!? That makes no sense! And, furthermore, how the heck does the Nintendo take control of the TV to make it do that!?!?!
The TV cannot pick up signals from the gun. The gun, on the other hand, can pick up signals from the TV.
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peas on earth
You’ve got it backwards. The CRT isn’t picking up the signal from the gun (except indirectly); the gun is picking up the signal from the TV.
The video game unit knows what it’s sending to the display. It therefore knows exactly where the electron beam will be striking at any given time, and by knowing when the gun detects the beam, the console can figure out what part of the screen the gun must have been pointed at.
Ha ha. So the target is actually “shooting” the gun. That aught to take the wind out of some gamers sails. 
Peace,
mangeorge
Work like you don’t need the money…
Love like you’ve never been hurt…
Dance like nobody’s watching! …(Paraphrased)
Im sure I saw Cecil answer this one before, I hope its in the archives.
After far to much Duck Hunt during my formative years, this is the theory I came up with:
- Pull the trigger on the gun
- The NES draws a square 'round the ducks, for about a frame (watch carefully, you’ll see it)
- The gun sends back what color it saw
- If the color was the same as the color around a duck, you got a hit.
I assume these guns aren’t real accurate measuring devices, and only detect one color, rather than a grid of colors. Otherwise, you’d have to focus the gun, and the NES would have to do some computer vision stuff to match what the gun saw against whats on the TV, which is just WAY to much to ask it to do.
It’s the same principle as a “light pen”, which used to be used before mouses, trackballs, etc. They’ve pretty much disappeared because of the physical strain of holding the “pen” up to the screen, but in the 60’s and early 70’s they were the latest thing in user-friendly design.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
So if I found a wall that was the same color as the color that’s supposed to make the duck die, and I shot the gun at it repeatedly, would I be able to cheat? And also, what about black & white TVs? Does it work on those?
Well, the flash you see on the screen was, as i recall, pretty much white. So i suppose it could also work not by color but by timing. Which could also make the guns work on B&W TVs.
So it could flash all the ducks white, and then measure the time from the beginning of the frame to when the gun saw the flash. Then it could figure out where the electron beam was at that time.
So how do those light pens work? I would guess they have to allow you to pick any pixel on the screen, regardless of its color, and without flashing anything (having the screen flash white while I’m dragging my pen across the screen would suck)
Trivia regarding to the Nintendo light guns and color- for the Super Nintendo the gun was called the Super Scope and looked like a big honking bazooka. The major flaw that no one at Nintendo noticed is that the gun did not recognize the color red- after realizing it, games from that point on could not have targets that used the color red. This pretty much doomed the Super Scope (I can only recall 2 games that even used it).
from www.ask.com:
Light guns work in the same way as light pens. On your typical tv set the electron gun (in the
CRT) scans across the screen (phospor) one line at a time. It starts at the upper left corner of the
screen then draws going to the right. When it gets to the right the electron gun turns off goes
down one line and turns on and draws the next line. It does this until it gets to the bottom of the
screen. Where it goes back to the top and starts over again. These are called raster lines.
Well when the electron beam hits the phosphor it glows brightly and slowly dims until it is
struck again by the electron beam. Our eyes don’t really notice this bright and dimming because
they do not refresh that fast. You can see the bright/dim effect if you record the picture on the tv
with a camcorder. Since the TV and the Camcorder refresh at about the same frequency you will
see a bright group of lines that roll up the screen.
Since the Video Chip in the computer has to create the video signal it knows where it is
currently drawing the curent raster line. The light pen when pointed to the screen detects this
bright/dim effect and when the light goes from dim to bright it sends a signal pulse to the video
chip. The video chip sets a latch which feeds 2 numbers, usually X location, Y location, into a
memory location associated with the video card/chip. The computer program then looks at the
numbers in the memory location. And can tell where the light pen is pointed on the screen by the
two numbers.