Here are the video instructions for the DIY version: http://makezine.com/projects/make-31/fetch-o-matic/

For me, I'd just pay the $100+ and buy the one in the link below. There are other brands and sizes available, but be careful as to how the ball is launched. Some dogs will try to wait where the ball exits the machine and they could be injured if struck in the face.

For safety, any time a dog is playing with something mechanical a person needs to be present plus, it's a good time to interact with your dog.
Whether your dog will fetch and return the balls to the bucket depends on the dog's response to training and the trainers patience in teaching the dog.

Here is a link that might be useful: Tennis ball tosser

The GoDogGo ball tosser mentioned in the above post is $140. Looking at the pictures, I think my dog would chew the gizmo up the first day he go it. The plywood box of the Fetch-O-Matic might keep him from destroying it for a while.

But building the Fetch-O-Matic looks like a fairly lengthy project, more suited for those with good mech skills.

Fetch-O-Matic is a niftier name than GoDogGo.

I wish the build plans for the Fetch-O-Matic said why this particular gearmotor was used. Did the designer already have it available? Was it the cheapest from dealer? Did he find it at the junkyard?

Is there a name of the inventor listed? I would try to find that person and ask your questions.

My guess about whether a dog would play with the ball-thrower alone is that it depends on the dog. My Megan was a lab-husky? mix who needed to wear off energy and who would push her Best Ball all over the back yard with no one there to encourage her. She would play with a machine that was there to play ball with her. Toby, who had a smaller Best Ball, does not want to play with his unless you are there and throw it for him, then chase him a bit. Then he gets tired of it in about 5 minutes. No way would he be interested in a mechanical ball-thrower.

I bought my dog one of the cheaper tennis ball machines (they are all pretty expensive). Have gotten a lot of use out of it and it saves my arm lol.