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Light gun functionality

florinanghel

Jun 14, 2010
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I was looking at the circuitry of an old light gun and trying to understand how it works. Although I theoretically understood (in the limits of what a total beginner can understand, of course), I still don't get it how an LED can detect light. Can you please give me an explanation in a way that a beginner can understand? Thank you!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Perhaps you can share the schematic with us? Or at least a picture so we can have the vaguest notion of what you're looking at.

In general though, any semiconductor junction can detect light. That includes LEDs. It also includes most transistors -- which partially explains why they're normally inside opaque epoxy or metal containers.
 

florinanghel

Jun 14, 2010
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I attached a picture containing the front/back side of the circuit plus the stuff that couldn't be seen entirely. The red and blue loose wires were connected to a switch triggered when shooting the gun.

I know the light detection thing may not be easy (at least for a beginner like me), but I'm only asking for some basic info, so that I'll know what to further research.

Thank you very much!

P.S. Where it says PCB, it may be PC8, I can't really figure it out.
P.P.S. Of course it's PCB, silly me. I just found out what it means. :)
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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It looks like it's using the LED as a photodiode. A capacitor in series with the LED eliminates any constant illumination (say from ambient light) and only allows an AC signal to reach the transistor.

The light you're pointing it to probably flashes on and off far too fast to see (it could be a spot on a TV screen)

The circuit looks like it requires some biasing (or at least a collector load) which mat be supplied at the other end of the cable..

I suspect the 3 terminal device is a bipolar transistor.
 

florinanghel

Jun 14, 2010
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I've got lots of stuff to learn before I can understand what the heck you just said. :)

From research, your explanations and some previous knowledge though, I can understand the idea behind it. The LED is hit by light and produces a small amount of electricity, the transistor amplifies it and sends it to the console (at the other end of the cable, as you said), where it's being interpreted. Correct me if I'm wrong. But if I'm right, it's mostly your "fault". So thanks, I owe you one.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Yes, that is a pretty good explanation. If you replace "electricity" with "current" then you're almost all the way there :)
 
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