Infrared distance sensors

Learn

What you’ll learn

• Three kinds of IR distance sensor and when to use each
• Strengths and weaknesses compared with ultrasonic

IR obstacle modules (digital)

An IR LED and receiver side by side. If enough IR reflects back, the output switches. A screw sets the trigger distance (usually a few cm up to about 30 cm). Cheap and fast, but only says “something is near”, not how far.

Sharp IR sensors (analog)

Sensors like the GP2Y0A21 measure distance by triangulation: the angle at which reflected light lands on a detector. They give an analog voltage for roughly 10–80 cm. The output is not linear and reads wrongly when closer than the minimum range, so use the datasheet’s curve or a lookup table.

Time-of-flight (ToF) laser sensors

Sensors like the VL53L0X time how long a laser pulse takes to return, much like ultrasonic but with light. They talk over I²C, give millimetres directly, have a narrow beam and work to about 1–2 m depending on the model.

IR versus ultrasonic

	Ultrasonic	IR / ToF
Beam	Wide cone	Narrow
Soft materials	Poor	Fine
Black or shiny surfaces	Fine	Can be poor
Bright sunlight	Fine	Can be affected
Speed	~20 readings/s	Faster

Many robots use both: ultrasonic forwards for a wide view, IR or ToF on the sides for wall following.

Examples

Example: wall following with a side sensor

Keep 15 cm from the wall on the right:

function loop() {
  const d = distanceRight();
  if (d < 12) motors(40, 60);        // too close: steer left
  else if (d > 18) motors(60, 40);   // too far: steer right
  else motors(60, 60);               // just right
}

Example: reading a digital IR module

void setup() { pinMode(7, INPUT); Serial.begin(9600); }
void loop() {
  // most modules pull the output LOW when they see an obstacle
  Serial.println(digitalRead(7) == LOW ? "obstacle" : "clear");
  delay(100);
}

Practice

Try it

distanceLeft() and distanceRight() act like side-facing IR sensors.

Wall following

▶ Run↺ Reset

Your code (JavaScript)// Keep about 20 cm from the wall on the right function loop() { const d = distanceRight(); if (distance() < 15) motors(-40, 40); // wall ahead: turn left else if (d < 15) motors(40, 60); // too close else if (d > 30) motors(60, 40); // too far else motors(60, 60); }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

Tune the gapEasy

Change the target distance from the wall and watch how the robot’s path changes.

Reach the goalMedium

Improve the wall follower until it reaches the green goal. When the wall on the right disappears (a gap), turn right gently, e.g. motors(60, 40), so the robot doesn’t clip the wall’s end.

Choose the sensorHard

A robot must detect a black chair leg 30 cm away in sunlight, and a curtain 1 m away. Which sensors would you use? Ultrasonic for the curtain (IR handles soft material fine but ultrasonic has range and ignores sunlight), and ultrasonic or a ToF sensor for the chair leg. A simple IR module may miss black objects in sunlight.

Quick check

1. Which IR sensor only tells you "something is near"?

VL53L0XSharp GP2Y0A21IR obstacle moduleNone

2. A Sharp IR sensor’s output is…

Perfectly linearNot linearDigitalIn millimetres

3. Which sensor is better for a curtain?

Ultrasonic onlyIR or ToFNeitherA bump switch only

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