here is the e-mail that explains on of life's greatest mysteries. thanks again mark

hi,

you wrote:
>i have a general question as to how the old nes gun worked. was it some kind of laser, and how does it know where it >shot? just a general

no, of course not. it is basically a light detector.

>or in depth description of the [fascinating] gun [to me] is appreciated. tia

you can find a full description of how the zapper works in u.s. patent #4813682, "video target control and sensing circuit for photosensitive gun".

you can download scans of the pages of this patent at up to 300dpi from http://patent.womplex.ibm.com/
after entering the patent number, click on "view images". click the demagnify button, and modify the scale=x.xx part of the url to get the resolution you want. scale=1.00 for 300dpi, scale=0.50 for 150dpi, etc. you can also change the format=gif to format=tif to download tiff format scans.

briefly, here is how it works.

when you pull the trigger, the nes program detects this. if there are any targets on the screen, the program does this:

blank the screen (so zapper should pick up no light signal); put a white rectangle where one of the targets was (if the zapper is pointing at this target, it should be picking up a signal); check for the hit signal. if set, the target has been hit. if not, keep blanking the screen & then displaying a rectangle over the next target to check. do this until all targets have been tested.

circuitry inside the zapper prevents you from just, say, pointing it at a light bulb to cheat. the zapper has an electronic filter inside. it only registers a hit when the light it receives is pulsing at a certain frequency (15kHz, the tv line rate).

-- mark