Could you please try design a gearbox? I need to make it ran at low speeds, but in my shop do not have much motors to choice.

I have motor shield for arduino and ultrasonic sensor. (somewhere)..

I used the example code from the ardunio library to drive the motors. I haven't got around to finallizing this design. I've been working on other stuff. I recommend getting a motor controller from pollolu with a set of gear motors.

And code for the car? I was started something like this few years ago. My son will have birthday next month, so maybe i will print it and build it for him.

The internal gears are split in two. It makes assembly easier. It still takes a little effort but the resulting gear spins much more freely.

I was kinda hoping that the geared bearing "wheels" were actually driven by that sun gear. Still a great use of the printable bearing gear. Looks like you printed the different colors and assembled the planetary gears. Was it hard to get the sun gear in there? I've tried a few times to print the emmett gear but my PLA settings tend to make a solid immovable chunk. Printing in pieces would allow for a nicer moving print.