Let's Make Robots!

Entertain the Dog robot

I have an idea to put an obstacle avoiding robot inside a hamster ball for the purpose of entertaining the dog.  The robot would have to be small enough to fit inside the ball and I'm thinking that when it encounters something it would simply reverse directions.  Would infrared ignore the ball itself and be able to see what is going on outside of the ball?  I'd like to keep this project as simple as possible since my dog has a tendency to destroy any toy given to it.  I would really like to do this project but wouldn't want to spend a great deal of time on it since it would inevitably be broken.  Anyone have any thoughts or experience on this sort of thing?

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It's hard to say whether the plastic would be IR transparent or not without checking out what type it was, but in any case IR detection through thick, curved plastic would be quite difficult.
My suggestion is to use regular photodiodes/phototransistors or light-dependant resistors to monitor the level of ambient light  entering the ball from different directions. If the ball hits an object, the light sensor on that side will typically receive less light, and the bot can switch directions. The suggested light sensors are all easy available and cheap, plus interfacing them with a microcontroller is a piece of cake. You could also do this project with some standard digital/analog logic circuits, but a microcontroller will be more flexible, so it's up to you.

I asked a similar question here and answered it as well. Black plastic which is very transparent to IR light still reflects just enough to mess up a sharp IR ranger. You might have better results with clear plastic, but any scratches that get on the outside will quickly degrade it.

A better idea might be to use piezo buzzers as knock sensors and glue them to the inside of the ball. Once it bumps into something it can change to a new random direction. I love dogs but most of them are dumb as bricks. Keeping one entertained won`t require a very smart robot ;)

I saw this link on the solarbotics twitter feed. http://www.instructables.com/id/Solar-Powered-Miniball-Wannabe/ This is pretty much what I was imagining.  Are they reading my forum posts?
Here is another dog entertainment robot, posted here a while back.