Even though it's a pretty dated system, I still play my Sega Dreamcast a fair bit. It's really too bad it was such a short-lived system, but even still, the Dreamcast has a high proportion of excellent quality games. Unfortunately, aging systems sometimes need repairs. The fan in my machine started to sound pretty bad with its high pitched whine.
Random project man to the rescue!
I went to the electronics store and picked up the only smallish DC fan I could find that had an appropriate speed for the Dreamcast. I thought it would be a simple matter of cramming the fan in and tying it to a power source. I was really surprised to find that with the stock fan disconnected, the Dreamcast powers up, then shuts down within a few seconds. I realized that it could not be overheating or anything, so there must be a fan speed sensor that shuts down the machine when the fan is not spinning. Sure enough, jam a small screwdriver between the blades of your Dreamcast's fan, and it will shut down!
The fan I had bought at the store did not have a third wire for the Dreamcast's fan speed sensor to work, so I decided to see if I could rig up something with spare parts I had lying around. In summary, I was able to put together a very simple 555 timer based pulse generator that tricks the Dreamcast's fan sensor. This will allow you to use any fan you want. I'm actually using a small 12V fan instead of the default 5V fan the Dreamcast comes with. Of course,it's probably a lot easier to just find a fan that comes with the third sensor wire, but it's less fun.
I am certainly not an electronics expert, but I can use a soldering iron and copy schematics from smart people on the Internet! To make the pulse generator circuit, I followed the schematics at this site:
For capacitor C, I used a 0.1 µF capacitor. R1 is a 4.75K ohm resistor. For R2 I used 60K ohms (actually, two 120K ohm resistors in parallel). I omitted the 0.01 µF resistor because I didn't have one (this leaves pin 5 not connected to anything). The circuit has worked fine for me like this. According to the calculator at the link above, the circuit outputs a square wave at a rate of about 12 cycles per second. That's 720 cycles per minute, which is reasonably close to the RPM of a small fan (I think they are usually 1000-2000 RPM). I arrived at these values based on three factors: trying to get about 1000 Hz; using only parts I had; and of course, a little trial and error (the first R2 values I tried did not work). The Dreamcast is super easy to disassemble. There are numerous sites on the 'net that describe its internals. See this page for an example with nice pictures:
The Dreamcast has a 3 pin connector on the controller port circuit board for the fan. I connected Vcc of my timer circuit to the red pin(5V) on the Dreamcast connector. The circuit ground is connected to the blue pin. The third orange pin is connected to pin 5 on the 555 IC.
With this circuit connected, you can play your Dreamcast without any fan attached -- until it overheats. You can see in this picture my messy circuit (remember, I made it from stuff I had lying around). Also notice the wires attached to the Dreamcast power circuit board on the left. These pins supply 12V DC to my replacement fan, and are conveniently labeled on the circuit board.
Attaching my new fan to the Dreamcast case was a little tricky. It's a lot bigger than the original, so I carved out a bunch of the plastic from the inside of the top half of the case. Then I put two-sided tape on the edges of the fan and stuck it between two plastic "blades" that are present on the case (it just fits!). Unfortunately I don't have pictures of the original fan before this modification.
If you try this project, post a comment, or send me a message. I'm curious to see how other people fare with this. Good luck, and remember; your mileage may vary!