NOTE: This ascii schematic was done by Jay Tilton and posted to usenet on April 24, 1997. Jay has controller port pinouts on his own page: Deathskull Labs Archive

 
Can anyone send me a true way to convert a SEGA phaser or an NES Light
>gun to work on my ATARI 7800?

Yup.

The 26/52/7800 FAQ tells how to do it through internal modifications
to the SMS gun.  A few weeks ago, I posted this simple external
adapter that requires no modifications to any hardware.  Only needs
9-pin male and female connectors and a 7404 inverter.

7800 port                         SMS gun
---------                         -------
pin 6-----------------------------pin 7
pin 7 (+5V)-----------------------pin 5
pin 8 (GND)-----------------------pin 8
            +---------------------pin 6                             
            |
pin 1----------+
            |  |
        +5V |  |
        _|__|__|_____________
       |14 13 12 11 10  9  8 |
       |>        7404        |
       |_1__2__3__4__5__6__7_|
                           |
                          GND

RATTY: "After you make the gun, please shoot me because I paid $4 for this cart"