This DIY Metal Hunting Robot uses some Parallax processing to hunt down metal objects while avoiding bumping into items in the way.
“To avoid any collision with other objects (such as furniture), the robot uses an ultrasonic range detector (radar) as well as 2 side infrared range detectors. The radar, placed on top of the robot, searches the area infront of the moving robot for obstacles. If one is located within close proximity (30cm), the radar scans the area (rotates with the use of a motor) for any other possible paths which contain no obstacles within range. The 2 side IR range sensors make sure that if something misses the range of the radar, a side collision won’t occur.”