If you have a Roomba, why not buy a broom and put the Roomba to better use.
“The other day I was marveling at the efficient operation of my roomba when it occurred to me that it would be really nice if roomba would get me a beer. While gestating the idea my first thought was to have roomba follow a line to the fridge and back. And a short while after that I realized that roomba already had 4 downward facing optical sensors AKA the cliff sensors. I decided to hack them up and make them sense light/dark lines instead of cliff/no cliff. I haven’t actually made the roomba get me a beer yet, I’m thinking of using a PIC to interface the roomba serially, and a 4066 switch IC to change between normal and beer retrival modes. Anyway I’ve shown here how to hack up your sensors as a first step. Enjoy!”