Drive mechanism uses two 3.7g servos from ebay. Both are modified for continuous rotation and cases are striped down to minimum. They are quite slow, but also ridiculously strong. I have tested its strength with 200g soldering tin and the robot had no problem dragging it around.
There are two small plastic gears glued to servos to act as wheels. Gears were used because they are easy to come by and they have reasonable grip. On the downside the grip isn't so reasonable on floor and the narrow gears get stuck in some carpets.
On the back there are two contacts. One for charger ground connector, other for sensing if floor is made out of conductor. There are two problem with those two skids. First - they float in air, second - the robot rocks back and forward. First problem could be fixed by using iron whiskers instead of copper ones. Second one was fixed with plasticine. On the top there is horn for charging.
The middle layer is the battery. I use single cell 400mAh lithium battery for whole robot.
- 2x servo €4.2 ($5.5)
- battery €3 ($3.9)
Total: €7.2 ($9.4)