IR line sensors

Learn

What you’ll learn

• How an infrared (IR) line sensor sees a line
• How to set one up and test it
• How two sensors steer a line-following robot

How it works

A line sensor (such as the common TCRT5000 module) has an IR LED and an IR receiver side by side, pointing at the floor.

• White or light floor: reflects lots of IR back.
• Black line: absorbs IR, so little comes back.

The module turns this into a signal. Most have an analog output (AO), a digital output (DO) and a small screw (potentiometer) that sets the digital threshold.

Check your module

Modules differ: many read LOW over white and HIGH over black, some the opposite. Print the readings over both before writing robot code.

Setting up

• Mount it 2–10 mm above the floor (check the module’s datasheet; the TCRT5000 works best close).
• Turn the threshold screw until the on-board LED changes exactly at the line’s edge.
• Use a matte black line (electrical tape) on a light surface.

Two sensors steer

Place two sensors either side of the line:

Left sensor	Right sensor	Meaning	Do
white	white	Line between the sensors	Go straight
black	white	Line drifting left	Turn left
white	black	Line drifting right	Turn right
black	black	Junction or end	Stop or decide

Examples

Example: testing a sensor on an Arduino

void setup() { Serial.begin(9600); pinMode(7, INPUT); }
void loop() {
  Serial.print("digital: ");
  Serial.print(digitalRead(7));
  Serial.print("  analog: ");
  Serial.println(analogRead(A0));
  delay(200);
}

Move it over white paper and black tape and note both readings.

Example: two-sensor logic in the simulator

function loop() {
  if (lineLeft() && lineRight()) motors(0, 0);   // both see black: stop
  else if (lineLeft()) motors(10, 60);           // turn left
  else if (lineRight()) motors(60, 10);          // turn right
  else motors(50, 50);                           // straight
}

Practice

Try it

The virtual robot has two line sensors either side of the black line (they turn red when they see black).

Two-sensor line follower

▶ Run↺ Reset

Your code (JavaScript)Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Faster lapEasy

Raise the straight-line speed. What’s the fastest speed that still completes a lap?

Smoother turnsMedium

The robot zig-zags. Change the turn speeds so it turns less sharply but still holds the line. Instead of motors(10, 60), try motors(30, 60) and motors(60, 30). Gentler turns zig-zag less, but at high speed they may not turn enough on curves.

Count the lapsHard

Print a message each time the robot passes its starting point. Use millis(): a lap takes several seconds. Or count how many times both sensors leave and re-find the straight at the top.

Quick check

1. Why does a black line give a different reading?

It is warmerIt absorbs infrared, so less is reflectedIt is magneticIt reflects more IR

2. Left sensor sees black, right sees white. What should the robot do?

Turn rightTurn leftStopReverse

3. What does the screw on a line sensor module adjust?

The LED colourThe digital thresholdThe voltageThe range in metres

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