I've updated my online catalog with a movie about the CheapBot line follower. It's a four inch wide PCB with two pairs of IR LEDs (IREDs) and phototransistors. The IREDs constantly emit IR. If the neighboring phototransistor detects a reflection, then the surface below the pair is white. No reflection detection means there's a black strip below that side of the robot and corrective action is necessary.
The phototransistor and its collector resistor behaves like a voltage divider. In the non-conducting state, conventional current flows from the positive supply, throught the resistor, and to the robot's I/O port. This means a positive five volts is detected by the robot controller (AKA a logic high). When the phototransistor is conducting, current from the collector resistor flows through the phototransistor to ground. This leaves no current (or more accurately, very little current) to flow to the robot controller. As a result, the controller see less than 1.4 volts, or a logic low.
I read earlier today that the movie, Creation, which is about Charles Darwin, can't get an American company to distribute it. Amazing, we live in the 21st century, but some throw-backs are going to make it too contraversal to show a movie about Darwin's most important contribution to science.