A line follower robot uses light sensors that shine light at the floor and measure how much comes back. White reflects a lot (a high reading); black soaks up light (a low reading). The threshold is the cut-off between them — a good choice is halfway between the black and white readings. With two sensors, one on each side of the line, the robot knows where the line is: if the left sensor reads black, the line is under the left sensor.
Below the threshold is black — “black soaks up light, so a low reading means the sensor is over the line”
Reading below the threshold → black; above → white.
Threshold = (black reading + white reading) ÷ 2.
Two sensors: the one that reads black is over the line; both white → the line is between them.
Use it for: Building or programming a line follower robot, robotics competitions and science lessons on light.
Worked example: 🔦 A robot has a left and a right light sensor. Below 50 means black. The left sensor reads 25 and the right sensor reads 80. Which sensor is over the black line? → the left sensor
Left: 25 is below 50 → black. Right: 80 is above 50 → white.
So the left sensor is over the black line.
Check: With readings 20 and 80, a halfway threshold is (20 + 80) ÷ 2 = 50 ✓
Work it out in your head or on paper, then tap your answer — or type it and press Check. Stuck? Press “Show me how”.
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🏁 Lesson 18 test
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🖨️ Robotics Worksheet: Line Follower Light Sensors
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