Although I managed to create a just about working Tiny Mobile Robot in 24 hours I did not quite achieve my initial aim of getting to fit inside some read shiny spray can tops that I have been collecting. Having looked at the design to see if it was ever possible to get it to fit I decided to have a go at redesigning the motor assembly part to see if it could be made smaller. Obviously it could, as it is nearly always possible to improve on something that already exists. I identified that I could make the servo motor band holding the two motors together smaller and reduced it from 10 mm wide to 6 mm wide. By cutting off the mounting tags on the servo motors I was also able to reduce the wheel size from 24 mm to 20 mm. This might not seem very much but the combined effect is to reduce the motor unit diameter from 55 mm to 48 mm which is just enough so that it fits into the spray can cap.
I used TinkerCAD to design some new small wheels that have a grove for a rubber band to make a non-slip wheel. I found some suitable elastic bands in my LEGO in a packet I had never opened so fortunately they were still rubbery. I had to use glue (first time ever) to fix the servo horn (also cutdown in size) to the new wheels as the self-tapping screws I had originally used were too large.
Unfortunately I did not have time to reduce the size of the electronics so that now takes up too much room in the spray can lid and I cannot yet get the motor unit to fit nicely and there just isn't any room for the battery. Too many wires and too large a PCB. I did try using a 3.7V LiPo battery which just fits into the spray can lid but it just didn't have the voltage to run the Nano and the servo motors, it will need a boost converter so I am going to look at that next.
However, when everything except the batteries was stuffed into the top, it just about worked.
Future steps will be to reduce the electronics and the batteries in size.