rambling robot

i am trying to build a metal detecting robot. i do not know anything about them but i used to do a lot of programming

  so i just took a self propelled mower, took off the blade. i make it go in an ever-decreasing-circle. I paid a local electrical engineering company to interface to my metal detector thru the headset. When the headset goes off, it activates a signal which activates a solenoid to paint a spot on the ground where the metal is.

I can completely cover an acre or 2 in an hour. I use a second detector and go around the edges.