Wow. I definetly have to check this step by step guide to build your own Processing-controlled car.
Toy Hacking is one of the best way to learn how to deal with little hardware. Very well detailed.
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Some Features:

via [NootropicDesign]
Tag Technorati: arduino, Rc Cars, Xbee, Processing,

All logic controlling the vehicle is performed in a Processing program running on remote computer. The Arduino program listens for commands from the remote computer.

Bi-directional wireless communication over XBee radios with (theoretical) 1-mile range. I’ve accomplished 1/4 mile range with these radios.

Sensor events are transmitted from the vehicle to the controlling computer. This vehicle has 3 microswitches – two on front bumper and one at the rear.

Original circuitry of vehicle replaced with dual H-Bridge circuit to control drive motor and turn motor. Drive motor is controlled with variable speed.

Power: Vehicle motors powered by 4 AA batteries. Arduino with XBee shield powered by 9V battery mounted at front of vehicle.

Simple communications protocol: 2 byte commands from controller to vehicle, one byte sensor readings from vehicle to controller.