Spherical Treadmills provide a method for head fixed recording from awake animals while minimizing the animals' distress and the risk of torquing away from the headpost and ripping off the drive/headpost. You can also pair the ball with an optical mouse or other method to track the movement of the mouse during the experiment.
To make a spherical treadmill, you need three basic parts:
1: A spherical styrofoam ball
The ball provides the surface for the mouse to run on. We've had good luck with balls from http://shapeinnovation.com, which require a minimum of sanding to achieve sphericity.
2: A hemispherical base, a few mm wider than the ball
You can either make your own base by making a thick epoxy mold of an extra ball or purchase or 3d print a hemisphere of the appropriate size. I've had good luck with hemispheres from http://www.replacementdomes.com.In either case, you'll need to do some sanding of the ball to create the right gap.
To make your own mold from epoxy, cover an extra ball with a thick layer of epoxy (at least an eighth of an inch). It's important that the mold remains rigid, or it will not remain spherical and the shape mismatch will prevent the ball from working. You can then remove the extra ball by chipping away and dissolving with acetone.
To float the ball you'll want several small holes in the base. Depending on your setup, it's probably easiest to create a chamber underneath the bowl connected to your source of air.
3: A source of air, either a fan or compressed air
Compressed air is the way to go if you have a compressor for your building, however, running it from tanks can get expensive quickly. You can also use a model airplane fan to float the ball, though it will be both electrically and audibly noisy. Practically any airplane fan purchased from a hobby site like http://www.hobbyking.com/hobbyking/store/index.asp will be overkill: the cheapest you can find should work. If you're concerned about noise, the general rule of thumb is the lower RPM, the quieter your engine will be, so a larger fan, a fan with more blades, or a fan with a higher blade pitch will move more air at a lower RPM.
To control the fan, it's relatively simple to adapt a speed controller to accept input from an Arduino rather than the RC controller. The speed controller can also be used to power the fan from a DC power supply.
If you use a fan, it's recommended to set the actual fan away from your ball, which will allow you to shield your prep from the noise (moving it out of your faraday cage) and help mitigate noise. I ran a sheet of copper mesh through my tube and covered the part of the tube going through my faraday cage with copper paint: use your own judgement in how much shielding you need.