This is a remote controlled sphere, not unlike the sphero and several DIY variants. It served as a great opportunity to test out inertial sensors & RF control, as well as being a birthday present for my nephew.

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Sphere mechanism

The sphere mechanism uses a gear motor and a servo. This seems to be the most popular method in DIY implementations. The alternative is the "car in a sphere" arrangement as envisioned by (among many, I'm sure) xkcd, and implemented by Sphero.

Electronics

The receiver electronics are based on STM32L151 processor and LSM303DLHC magnetometer/accelerometer sensor. To control the motor, there is a simple H-bridge. There is also a quadrature encoder composed of IR led and two phototransistors, to detect the rotation speed of the shell.