Science Buddies: "Ask an Expert"

My son completed this project from the science buddies website. Unfortunately ,it is not functioning as intended. The sensors are having a hard time recognizing the white from the black paper. The wheels spin when they see paper or the floor, etc. They stop when they are completely covered or if it is lifted in the air. What they will not do is tell the difference between black paper and white paper, or see a black line on the white paper and stop the wheels turning.

Does anyone have any ideas why this would be happening?

We have reviewed the circuit construction and it does not appear to be a problem with that. Are the sensors ever defective?

Sensors aren't likely to be defective out of the package. If you connected them wrong somehow you might have damaged one. But...

If the wheels turn when the sensors see white paper and don't turn when you lift the robot off the surface, it sounds like things actually are working. You need to make sure the sensors are pretty close to the floor and it's super important that you don't use something shiny black like electrical tape. The prototype in the video is a roll of white paper of the kind you might use for bulletin boards and the black is a Sharpie pen. Reflective/shiny black would probably bounce enough IR back to the sensor to allow the motor to run.

Try moving them up and down, farther and closer to the floor. I found that it can be a little touchy to find just the right distance.

Howard