Instructables
This is my first autonomous robot. I built it to explore the interfacing of various sensors to a PICAXE processor. His mission is to search for a magnet (South pole) via his Hall-effect sensors and avoid obstacles via 2 IR reflectance sensors (one looking forward for walls and the other looking downward for cliffs).

The robot's protocol is: go forward until you find a magnet or encounter an obstruction respond to obstructions by stopping, backing up, turning and resuming search. if magnet is found, stop and flash green LED.
 
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Step 1: Gearbox Assembly

Picture of Gearbox Assembly
The Tamiya 89915 Twin-Motor Gearbox comes from Pololu as a kit. You have a choice of 58:1 or 204:1 gear ratios (I chose 204:1 for 61 RPM operation). I find just assembling these gear boxes to be a lot of fun. Then I added the Pololu round laser-cut chassis and caster kit. The nice thing about using this chassis is that I did not need to drill a single hole, it comes with all kinds of holes and slots. A lot of engineering went into even the caster kit, which has 3 height options (I used the tallest one). With all of these pieces you have a differential drive 2-wheeled robot.