NASA software engineer Al Bencomo of San Jose produced this very impressive example of what can be done by combining an obsolete or second-hand smartphone with open source software and auxiliary hardware. Not only does his robot automatically track and chase a rolling green ball, it avoids obstacles along the way.
Bencomo’s bot consists of the following major components:
Android Nexus S smartphone (from eBay)
1/18 RC Mastadon truck with brushless motor
USB Micro Bluetooth adapter
25C 2S 1500mAh 7.4V LiPO battery
IR sensors (4)
Lynx B Pan and Tilt Kit with 2 Hitec 422 servos
Xbee shield for Arduino (optional)
RN42-XV Bluetooth module to communicate with laptop (optional)
He puts the total cost of the build at less than $400 including the (used) phone. The OpenCV (Open source Computer Vision) library does the heavy lifting on the software side. Extensive, detailed technical info about this approach to ABR (Android Based Robotics) is available here.