You’ve probably seen “bang-bang” line followers that just use a photocell to turn the robot one way or the other. [Will’s] uses a hardware PID (proportional integral derivative) controller.

You can throw a Raspberry Pi camera and OpenCV at the problem and approach it through software, or you can buy an off-the-shelf RGB sensor and wire it up to an Arduino.

In a nutshell, the Arduino reads the photocell and temperature sensor values and converts them into a data stream, one line of data per program loop that is fed out over the USB port.