http://www94.pair.com/jsoper/autofire.html


Build an Autofire Circuit


NOTE: This was grabbed from somewhere on the net. The only thing I'll add is a quick explanation. A Fire signal is normally held high until a button shorts it to ground. If the oscillating 555 signal is connected to the high side, you have an autofire. If you connect it on the other side of a button, you have turbo-fire. --John

 
Portable autofire

This is a solution for those joysticks which don't have a +5V line (red
wire typically, connected to pin 7). You will need a 9-pin female and a
9-pin male connector, an LM 555 timer IC, two resistors (R1=R2=10K), two
capacitors (C=100 nF, C1=0,42 nF), a two-position switch and some wire.
Since you will need also an appropiate casing, I would recommend you to
use a 9-pin null modem adapter (e.g., Radio Shack #26-264), which will
provide you already with the casing, wiring and DB9 connectors. 
  
First, build the following autofire circuit in the smallest possible
area (less than 0,5"x0,5"):
  
                  +5V
                  red                    firing frequency:
                    |
         ,----------+                    f=1,44/[(R1+2*R2)*C]
         |          |
         Z       ,--+                    ( T=0,694*(R1+2*R2)*C )  
      R1 Z    .__|__|__.
         |    |  8  4  |
         +----7        |                 typical values:
         |    |        3----- org
         Z    | LM 555 |                 C1=0,42 nF
      R2 Z  ,-6        5----,            R1=10 K
         |  | |        |    |            R2=10 K
         +--+-2        |    = C1         C=100 nF
         |    |___1____|    |
         |        |         |            (These are the values I selected.)
         `---||---+---------'
             C    |
                 blk
                 GND
  
If you are using the null modem adapter, skip this paragraph and continue
with the next. If you aren't and are building the adapter with separate
parts, then just connect each pin from the 9-pin female to the same pin on
the male connector. Now you've just built what the adapter looks like on
the inside.
  
Now, if you are using the null modem adapter: just open the casing and
accomodate the circuit inside.  Make sure that all pins on the male are
connected to the same pin # on the female (i.e., pin #1 to pin #1, pin #2
to pin #2, etc.) and not cross-connected.
  
Now for both: Connect the +5V line to pin #7 (either on the male or the
female connector, it's the same).  Similarly, connect the GND line to
pin #8.  Cut the wire from pin #6 just in the middle. Now, proceed to make 
the following connections on the switch:
  
                     /
               A    /  B
               o  o  o
         to    |  |  |    to
        male --/  |  \-- fire
                  |
               to female
  
Now, make the appropiate holes in the casing for the switch. Fix the switch
in place and close the casing. Now the portable autofire is ready for use.
  
Just connect the portable autofire between the joystick port on the
computer and the joystick's plug.  With the switch in the A position, you 
will simply bypass the autofire circuit (autofire disconnected); when it is
in the B position, the autofire is connected and the joystick's fire button
is disabled. The advantage of this design is that you can connect the
portable autofire to any joystick you want, without having to make any 
permanent modification to it.
  
Marco Antonio  Checa  Funcke
Botoneros 270
Lima 33
Peru
  
reachable at jtkirk@urp.edu.pe