Line-following robots were one of the first automatic guided robots. They can follow a marked line on a contrasting background, usually a black line on a white surface or a white line on a black surface. Typically, the line follower robot operates on a closed loop feedback algorithm in which the controller uses line sensor feedback to correct the routing of the robot. The sensors are usually LED / LDR, LED / Photo-diode or LED / Phototransistor pairs and the controller is an electronic circuit that executes the desired feedback algorithm. Gear motors are used to drive the robotic wheels.