We coached over 10 teams combined for First Lego League and Junior FLL and were judges in various competitions including State Finals and had the opportunity to observe many teams and the various ways they use various sensors. One thing that kept coming up again and again was how and where to attach the color sensors to the robot. For a FLL robot, it is a near requisite to use two color sensors. The two color sensors are needed for:

  1. Line/Edge following (one color sensor tracks the edge)
  2. Stopping when the line ends (second color sensor stops at the T junction found at the end of all the lines)
  3. Line Squaring: In many cases, when the robot is coming towards a line, because of mechanical drifts, it is not likely to end up on the exact same place on the line all the time. Thus, the robot needs to calibrate itself with regards to the line. The process of turning and making itself exactly 90 degrees to the line is called Line Squaring. This is an advanced technique and is possible reliably ONLY if you have two color sensors.


The color sensors are best positioned when they are about 3 pennies stacked from the ground and completely level with the ground. The color sensors should be about 1 inch apart i.e. just slightly wider than the thickness of the line on the FLL mats. Additionally, it is important that you position  your color sensors as close to the front and slightly under the body of the robot to avoid interference from external light. Here is something critical about the color sensor: The color sensors GENERATE all the light they need to measure the reflected light intensity or the color. So, in an ideal case, you should shroud them in such a way as to completely douse any external light. Use the Lego angular beams (Design ID: 32526) to create a shield around the sensors.

Build instructions:

PDF instructions ColorSensorAssembly