How to make a wind up tank know not to go over the edges of surfaces

Hi, Prestontim505 here, with my first ever instructable! It is quite simple. You have to unscrew the body, take out the 'input' wire and screw it back on. Then, you add the sensory techonlogy ( I don't know the real name, but the bit that is squared) then it will detect where surfaces stop and it will not go over!