I have recently purchased an 8-bit gaming console out of nostalgia, of course the light-gun that came with it did not work on modern LCD screens. Now I know that when you squeeze the trigger of the light gun this sends a signal to the console to draw an entirely black screen with just the position of the duck replaced by a white rectangle. Then if the gun is pointing at the duck/rectangle light will hit the photodiode in the gun's barrel and it will register an electrical signal and a subsequent "hit".

I know this does not work with modern TV's because of screen update lag. However I tried shining a flashlight into the gun's barrel and it did not register a hit, either. Then I tried checking if the diode worked, at all, so I used a multimeter to measure the voltage when it was being shined with a flashlight and when not. However, what I got was that the voltage only changed the first second or so when light hits the diode and after that the voltage came down to its usual level, even though the diode was still being shined on.

So far I don't know what I am doing wrong.

EDIT: i appreciate the answers, but it seems my question is unclear:
tl;dr - i know how LCD and CRT works, what i want to know is why the diode does not work when shined constantly with a flashlight

  • \$\begingroup\$ I believe (though I’m happy to be corrected) that the gun requires two things: a completely black screen, followed by a white signal at the correct scan point. By shining a torch into the gun you’re only fulfilling the second of these requirements. \$\endgroup\$ – Robin Whittleton Jan 2 '15 at 20:27
  • \$\begingroup\$ The gun requires two things: a completely black screen and a short white signal light at the correct point in time (WHEN the light shines tell the gun which duck was hit - the ducks are illuminated by white light one at a time). It's hard though not completely impossible to emulate this with a flashlight. You just need to control the flashlight with some sort of electronics that can synchronize with the signal from the console. It's completely impossible doing it by hand unless you're Superman or Flash or Quicksilver. \$\endgroup\$ – slebetman Jan 2 '15 at 20:41
  • \$\begingroup\$ @slebetman SERIOUSLY? The detector actually looks for "no signal" then "signal" and then "no signal" to work?! I expected it to be much more simple, considering it's just a game console. :D I thought that it was more in the lines of "if there is signal when trigger is pressed = hit" \$\endgroup\$ – mathgenius Jan 2 '15 at 22:13
  • \$\begingroup\$ @mathgenius The black screen "signal" it to detect cheating by pointing the gut at a light bulb or a white piece of paper. \$\endgroup\$ – slebetman Jan 3 '15 at 10:01

The gun is AC-coupled, and it is probably looking specifically for the 60-Hz pulsations that would come from an NTSC CRT monitor/TV set. It would be tuned for this in order to do a better job of rejecting unwanted signals from other sources of light, including DC (sun, incandescent bulbs) and 120 Hz (fluorescent bulbs).

Note that the light from an LCD monitor (the backlight) will either be DC (LEDs) or a very high frequency (CCFL).

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  • \$\begingroup\$ waitaminute, what do you mean 60-Hz pulsations? I am not good with electronics but photons are photons, regardless of the device that creates them they have a wavelength (nm) and frequency (hz) and different hz mean different colors of light and while the photodiode is sensitive to a range, i doubt 60hz and 120hz electronics produce photons THAT different in color. \$\endgroup\$ – mathgenius Jan 2 '15 at 20:04
  • \$\begingroup\$ No, it has to do with the fact that a CRT "paints" the pixels one at a time, scanning across one row at a time (at a 15.75 kHz rate), moving from top to bottom (at a 60 Hz rate). Your eye's persistance of vision (POV) makes it seem that the screen is continuously illuminated, but in fact the light is pulsating at 60 Hz. If you take a photograph of a CRT screen at 1/125 second exposure or less, you will easily see this. \$\endgroup\$ – Dave Tweed Jan 2 '15 at 20:09
  • \$\begingroup\$ o-o-oh, i see now, so it has to do with how often light is emitted from the screen? \$\endgroup\$ – mathgenius Jan 2 '15 at 20:18
  • \$\begingroup\$ Not just how often, but when. Shining a flashlight into the gun won't work because it is not synchronized to the display. The console expects a pulse of light to occur at the exact same time as the white block is being displayed (flickering on and off in time with the video signal) - anything else is rejected as being noise or ambient light. \$\endgroup\$ – Bruce Abbott Jan 2 '15 at 20:39
  • \$\begingroup\$ SERIOUSLY? The detector actually looks for "no signal" then "signal" and then "no signal" to work?! I expected it to be much more simple, considering it's just a game console. :D I thought that it was more in the lines of "if there is signal when trigger is pressed = hit" \$\endgroup\$ – mathgenius Jan 2 '15 at 21:12

It will never work with an LCD TV because its screen refreshes in a completely different manner.

A cathode ray tube (CRT) creates a beam of electrons that produce a single dot of light on the screen. This dot is scanned rapidly across the screen from left to right, and also moved down slowly to trace a 'field' of horizontal lines that fills the screen. Your light-gun has a lens which focuses on a small part of the screen, and an optical sensor which generates a pulse as the dot passes by. Then the console compares the timing of this pulse to the position of the white block it is displaying (the console is producing the video so it knows where the beam is at all times).

The image on a CRT only looks solid due to the eye's persistence of vision. In reality it is flickering like crazy as the beam traces a single dot that moves across and down the screen (phosphor decay causes the dot to make a trail of light behind it, but this only lasts for a few lines). An LCD screen works completely differently - it has a matrix of pixels that are turned on and off in parallel, so when a block is displayed it is visible continuously - there is no moving dot to be detected.

CRT Versus LCD

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