Circuit explaination in brief:


The circuit is designed to be powered at anything above 5V, the 5V regulator gives the required chips the 5V max they need.
The motors will be driven from the full source voltage, so make sure this isn't going to make your robot too fast!

Firebot (www.tombot.net/beam/linefollower_firebot.html) has GM3 motors driven with a 9V battery, however the circuit differed 
and the motors were only driven by 5V from the voltage regulator.

The line detectors drive the relays on or off dependant upon their status, remember that the left line detector should drive the right motor and vice versa.  
The obstacle detector enables/disables the inputs on the line following chip.  This is made possible by the fact that the chip has two sets of inputs, both with 
their own enables.  The obstacle detector uses the second set of invereters which is grounded (enabled) constantly.