For a step-by-step instructable on how to build this toy, go here: http://www.instructables.com/id/Build-a-Motorized-Uniwheel-Toy-Thing-Mutt/
Build : It is essentially a motorized wheel with two out-riggers with wheels and which support a battery and motor. Since I do not have the expertise, money or motivation for a complex project, I decided to keep it very simple : a wheel, motor, battery and axle. I suppose technically this is just a tricycle but once under power it should theoretically balance only on the primary wheel. As it stands it has no directional control except to overweight one of the outriggers to make it move in that direction or balance the two outriggers equally to make it go straight.
Inexpensive : made this for less than five dollars in stuff.
Important Note: Please realize that when the Mutt is not moving and is at rest, it is only balanced on the primary wheel and one of the outrigger wheels. It can't really "fall over" because the center of gravity is so near the ground that it just naturally sits on two wheels. The motor is hanging really low but not touching the ground. The Mutt in effect holds itself up. It is kind of counter intuitive.
Photo 1 : front side view. Basically this is just a dvd used as the primary wheel and a coathanger used as an axle. For a bearing I inserted a plastic wheel into the center hole of the DVD. Metal strips epoxied to the coat hanger axle serve as the battery holder and motor mount. The metal strips also serve to keep the primary wheel centered on the axle - a primitive sort of bearing retainer. The nut hanging off the axle is just a simple counterweight to keep Mutt going straight; the battery side being a little heavier.
Photo 2 : top view.
Photo 3 : side view motor.
Photo 4 : side view motor. The motor is equivalent to one of the small motors in a dvd player. I got mine from a solar hat fan toy. I just fabricated a motor mount which is much more complex and ugly than it needs to be. It is actually just kind of stuck in there with one bolt to hold it tight. The motor pulley and drive belt also came from the dvd player. I used a slightly larger washer epoxied to the dvd plastic bearing to create the larger axle pulley. There is also a simple on and off slide switch wired in there for the motor. Note that the motor never touches the ground. The Mutt's weight is resting on the dvd and one of the outrigger wheels. You can actually run it with one of the outrigger wheels missing.
Photo 5 : close up of bearing. Lots of slop between the coat hanger axle and the plastic bearing. A real bearing would be sweet.
Photo 6 : front view. Just route the wires up over the top and out of the way.
Results : this was just a kind of proof of concept to see how well it would work. I was surprised how much power the little motor generates yet it is underpowered and needs a more torquey motor . The only way to stop it is catch it or let it crash into something. It tends to wobble a lot because it is unbalanced laterally. There is a lot of friction between the outrigger wheels and the coat hanger. Stripping the insulation off the axle or adding wheel bearings would make this thing cruise really easy.
Though it is entertaining, it would be nice to control the speed and direction.
Todo : I would like to build a larger Mutt with a more powerful motor and use a two channel radio to control the motor and some sort of steering mechanism. Since leaning is the only reasonable way to get the main wheel to turn, I have thought about using a motor or servo to extend/retract a counterweight on one side to make the Mutt turn.
Latest: I have begun building a Mutt 2 and am taking photos as I build it so that I can show a step by step process. I will be using a much larger wheel than a dvd and a more poweful motor along with a two channnel radio that I hacked from a cheap radio controlled helicopter.
Mutt 2: Photo 7 is current status of my Mutt 2 construction. The wheel is a hideous pink exercise wheel and the outriggers are plywood with bolts for axles. My next task is to mount the motor, then battery, and radio receiver. Will be all downhill at that point.
Mutt 2 Update 1: I have built Mutt 2 using the exercise wheel. It uses a single channel radio to control the motor speed so there is still no directional input. Once I have a working chassis that won't fall over I will use it to build a Mutt 2 in which I can achieve directional input through two servos, one on each side, that will drop a wheel to the ground and steer the Mutt in the opposite direction by tilting the Mutt in that direction. Pretty simple but effective I think. I just need to find a 3 channel airplane kit to hack for motor control and two servos.
Mutt 2 Update 2: Have received 4 channel radio, mounted servos for steering and just need to attach wheels to the servos, mount the radio and battery packs and test drive the Mutt 2. School is out so now I have time to finish this little project.
Photo 8: This is the completed but not tested Mutt 2 with 1 channel radio to control speed. It has 7.2 battery pack, radio, motor, axles with counterweight washers and bearing wheels. It has pretty good stability but still no directional ability.
Update: Here is link: http://gscustoms.wordpress.com/2009/09/21/uniwheel-an-introduction/ to an interesting uniwheel robot project. This is a true self-balancing uniwheel. Not sure of the final status of this but it looks awesome.