@MKRoxton,
The 7805 is a voltage regulator. Since you will be using a 8,4 volt battery then
the +5 volt fan must have +5 volt. If you use a higher voltage then there is some
chance that the +5 volt fan will be faulty. (Eg. smoke.) L7805CV is OK for your +5 volt
fan. Use a piece of aluminium that have a size of 5 times the size of the regulator as
a heat sink. (Or any other heat sink you may have.) The +5 volt fan has two wires,
one red = plus and one black = minus.
All DC fans have a printed circuit board inside. They also have several (usually four)
spools of copper wire onto some soft iron stators. Inside the fan there is also a rubber
magnet and a metal ring. The printed circuit board (PCB) has some IC and other components.
So, the PCB is designed to get a DC volt INPUT to run the fan. To get the
fan to be a generator you need to open the fan and find the two points where the
coil is connected to the PCB. To open the fan you need to remove the sticker in the
bottom of the fan. Then you need to carefully ply off the small locking ring that
keep the axle secured. Then you can lift the fan off the stator. Now you can have
access to the stator coils and PCB. On most plastic fans it is possible to carefully
lift the PCB and stator coils OFF the fan metal housing. This way you can access
the underside of the PCB and solder two thin wires to the coil points. It may be
necessary to make small cuts in the PCB where the coils is connected to insulate
the coils from the electronic. If you do not do that the electronic may steal away
your power output.
There is no need to use a flywheel, as I see it. The easiest way to do it just to
center the smaller motor fan at the larger generator fan hub with glue. Just make sure
you have each hub centered to avoid vibrations. Then you just fix the stator of the
motor fan to the stator of your generator fan. Balsa wood or plastic will do fine.
These two wires will carry the alternating current when the fan is turned by your motor.
These wires can be connect any way to the diode bridge because it is AC.
The 7805 voltage regulator connects as the attached image.
I have added a Microsoft Word document that describe how you convert a PC DC FAN
to an AC generator. The document can be found here:
http://www.overunity.com/index.php?action=downloads;sa=view;down=292Groundloop.