Line Sensor Bar
The most commonly used method of sensing a line is to use a light sensor, usually made from a Light-dependant resistors(LDR), to determine whether the robot is over a black area (low light level sensed) or a white area (high light level sensed). By using a several LDR's in a line across the robot, all pointing down, the lines position can be detected. It is possible to achieve this with just 2 LDR sensors, one on the right and one on the left. If either one detects the line then it will move towards it. This will maintain the line in between the two LDR's. Some systems utilize 3 or more LDR's to get a more accurate fix on the lines exact position. However, simpler methods can be used to improve the accuracy, rather than simply adding more LDR's.
One of the major problems that a simple line following robot will encounter is a variable light source. As it move across the course, its direction relative to nearby light sources will change, shadows will also be cast by the robot and onlookers, all helping to reduce the accuracy of the LDR's.
By creating a simple light shade from card or paper that is open only on the bottom, one can shield the LDR from external light sources so they do not effect it. A second light shield can then be created around an LED light source so that it illuminates the area under the LDR but not the LDR directly.