Raspberry has a lots of guts when speaking of processing power and available resources, but suck at controlling things. The fact is that if you need to connect external device to it – you need an extension board. Depending on your goal the choice of if vary. Brian decided to give a try in building a Raspberry Pi robot. So first thing he thought was a Digilent Cerebot II board which is powered by Atmega64L. It talks to Raspberry Pi using SPI interface. In RaspPi interface is handled by Python script.
Robot has couple DC motors that needs to be PWM controlled. They are connected to Cerebot II using couple H-bridge daughter boards. And last thing is Wii remote control. For this Raspberry Pi is equipped with Bluetooth USB module that talks to Wiimote. So far everything loos promising, but there is still lots of to do including batteries, wireless console for control, Wii support for camera and laser, servo control for camera and front wheels.
Embedds highly recommends EasyEDA for circuit and PCB design