Intro: Techno-geek Roulette (or Who Makes the Coffee?)
Step 1: Bits and PCs
You will need the power supply unit (PSU), CDROM drive and a blanking plate from an old computer - The older the better. Also, a blank writable CDROM, a selection of small tools, a glue gun and a way of decorating the CD. If you're after accuracy, a protractor too.
The blanking plate is the 'L' shaped bit of metal you take out when you put a plug-in card in the PC. If you're like me, you've probably got dozens lying around until you actually come to need one - then you can't find a single one.
The CD drive must be a read-only type. Some drives work better than others for this. If you have a couple of old drives available, power them up in turn with a blank writeable CD in and see which one spins for longest before stopping. This will be the best drive.
The best type of blank CD to use is the type with a matte white topside which you can print onto with an inkjet printer. If you don't have printing capability, you could use permanent ink marker pens or enamel paint.
Step 2: Preparing the Power Supply
It's best to do this first, as the PSU will help us later. The simpler the power supply the better as more recent ones are harder to get working out of a system. The type which has a 20 way large connector is ideal.
Unplug all the power connections inside the computer and remove the power supply unit. You will usually find three screws on the back panel of the PC holding it. Once out, blow hard into the ventilation holes to remove the dust of ages.
Now you're into the PC, you can take out the CDROM drive (usually a couple of screws into the side) and a blanking plate from the back panel.
If the large connector from the PSU has a green wire in the position shown in the photo, connect a wire link to it, and to one of the black wires next to it. This is the lead which would go to the power switch and needs to be grounded to turn the supply on.
Step 3: Hacking the CDROM Drive
There are many, many different models of CDROM drive, but they are all put together in roughly the same way. Your drive may not be exactly like this, but it won't be too different.
What we want to do is to remove the cover and tray of the CD drive so we can see what's going on inside the drive. There is a low power laser in there, so don't look directly down on the assembly when it's running. Once we've got the disk on the laser light will be blocked.
With the PSU connected, turn on the supply and eject the tray. Turn off and disconnect.
Turn the drive over and remove the four screws underneath. Ease the bottom and the wrap-round top cover off the drive. Keep the flat bottom panel - we'll be using it again later.
Remove the tray. It may come out if you bend the middle upwards, or you may need to cut a plastic clip or two with wire cutters. Once you're done, it should look like the fourth photo.
Step 4: The Disk of Destiny
This is where you can unleash your creative talents. Mine are minimal, so I used the CDROM printing software which came with my Canon printer and a bit of clipart.
The important thing here is dividing the disk into the right number of segments. You can either eyeball it like I did, or use a protractor to get the exact angle for the lines. Divide the number of names into 360 to get the angle (e.g. 5 people is 360/5 = 72 degrees).
We are using a blank writeable CD so that the drive is aware that there is a disk in there, but can't make sense of it. It spins and spins, and eventually gives up, leaving the disk in a random position.
Note : Canon Pixma printers sold in the USA have the CDROM printing function disabled and do not include a tray, but you can enable the CD printing with the help of THIS instructable, and get a tray from eBay, or make one like THIS . (I'm in the UK, so mine was already enabled ;¬)
Step 5: Putting It All Together
Use cyanoacrylate glue (superglue) to attach the decorated CD to the drive spindle. We want the disk to sit flat, and hot-glue is too bulky for this. If the spindle isn't in the right position for the disk to spin freely, you may need to find the arm or plate which moves it. Don't force anything. Once you find the right part it should move easily and lock in place. The fourth photo shows where mine was. You may need to power up the drive and press the open / close button to get it to the right state.
Once the disk is attached, this button is going to be a liability, so carefully 'disable' it with a pair of wire cutters (and the power turned off).
Bend the blanking plate so that one end sits over the top of the drive, and the other just overlaps the disk. Cut it to length at the back, and draw on the arrow with a marker pen. Hot-glue the pointer in place (third photo).
Using more hot glue, stick the flat plate from the case to the back of the drive. This will give you a flat surface at the back. Now glue this firmly to the PSU so that that the pointer is pointing down (connector at the top) and the whole assembly sits solidly on the table.
You could cut off all the wires from the PSU except the ones to the drive. If you do this, you will have to cut and strip the green wire and then twist it together with a black wire. This should then be insulated. Make sure the other wires are cleanly cut with no stray whiskers.
Or you could drape and glue the wiring looms and connectors to the drive itself to make things look more interesting, like I did. Use the hot-glue gun for this.
Step 6: Who Makes the Coffee?
If the disk stops precisely on a dividing line, coffee duty will be decided by a round of 'rock paper scissors' between the two people indicated.
The selected person must now go and make the coffee, or, in the case of those unfortunate enough to have one, persuade the drinks machine to produce a beverage which is almost, but not quite, entirely unlike coffee.
As the Kaptin says below, this isn't limited to coffee making. With different disks you could replace throwing a dice, play roulette, make vital life decisions etc. Your imagination is the limit!
(Some drive motors seem to have a couple of dozen angles they prefer to stop at. If you're thinking of using more than ten or so sectors, don't use one of these as it will skew the randomness. You can feel it as a lumpyness when you turn the disk by hand.)