First look at a Hexbug Spider
I recently bought a Hexbug Spider after seeing one in an Instructable. The thing which impressed me was the unique directional control. The spider changes direction without turning by changing the leg sequencing and uses just 2 motors. It's based on work done by Jamie Mantzel, (although he is not credited by Hexbug) and comes with IR control - rotate left / right, move forward / backwards. Build quality is really good and it comes apart with a single screw for the 'carapace' and three more to get to the circuit board. The next set of screws gets to the heart of the mechanics, but I didn't want to risk disconnecting something so haven't gone further.
Inside there's a double sided PCB with a 14 pin MCU on one side and a dual H-bridge driver on the other. The other device is a combined red LED and IR receiver. It's a simple enough job to remove the MCU to get to the H-bridge inputs directly, then add sensors and patch on an MCU of your choice. When time allows I'll be grafting in a rangefinder, light sensor and Picaxe 14m2 to make the spider autonomous.
All I've done so far is to replace the 3 x button cells (max 4.5V) with a lithium ion battery (3.7V to 4.2V). This works fine and the robot stomps around with no reduction in speed. Probably because there's less voltage drop on load with the beefier LiIon cell.