How IR Sensors Work: Infrared Proximity, Line Following & Obstacle Detection

The Physics of Infrared Detection

Infrared light occupies the electromagnetic spectrum just below visible red (700nm to 1mm). The human eye cannot see it, but silicon phototransistors are highly sensitive in the 850–950nm range. An IR LED emitting at 940nm is practically invisible to humans, making IR the ideal choice for discreet sensing applications.

The fundamental principle of reflective IR sensing: the emitter constantly floods the detection zone with IR light. Nearby objects reflect this light back to the receiver. The further an object, the less reflected light reaches the receiver (following the inverse square law). A comparator circuit sets a threshold — above which an obstacle is detected.

Fig 1: Active IR Sensor Circuit — IR emitter LED, phototransistor receiver, LM393 voltage comparator, and threshold tuning potentiometer

Types of IR Sensors

Arduino Line-Following Robot with TCRT5000

The TCRT5000 outputs a digital HIGH when over a white/reflective surface, and LOW when over a black/non-reflective line. Mount two sensors side by side:

// 2-sensor IR line follower
// IR module OUT pin: LOW = white surface, HIGH = black line

const int LEFT_IR = 2;   // Left TCRT5000 digital output
const int RIGHT_IR = 3;  // Right TCRT5000 digital output
const int LEFT_FWD = 5, LEFT_BWD = 6;   // Left motor PWM
const int RIGHT_FWD = 9, RIGHT_BWD = 10; // Right motor PWM

void setup() {
  pinMode(LEFT_IR, INPUT);
  pinMode(RIGHT_IR, INPUT);
  // Motor pins setup...
}

void loop() {
  bool leftLine = digitalRead(LEFT_IR);   // HIGH = on black line
  bool rightLine = digitalRead(RIGHT_IR);
  
  if (!leftLine && !rightLine) {
    // Both on white = go straight
    analogWrite(LEFT_FWD, 180); analogWrite(RIGHT_FWD, 180);
    
  } else if (leftLine && !rightLine) {
    // Left on line = turn left
    analogWrite(LEFT_FWD, 80); analogWrite(RIGHT_FWD, 180);
    
  } else if (!leftLine && rightLine) {
    // Right on line = turn right
    analogWrite(LEFT_FWD, 180); analogWrite(RIGHT_FWD, 80);
    
  } else {
    // Both on line = stop (junction)
    analogWrite(LEFT_FWD, 0); analogWrite(RIGHT_FWD, 0);
  }
}

PIR Motion Sensors: Passive Infrared Explained

PIR sensors detect changes in ambient infrared radiation caused by warm bodies (humans at ~37°C emit significant IR at 9–10µm wavelength). A Fresnel lens focuses this IR onto a pyroelectric sensor element — when a person moves across the field of view, the differential IR causes a voltage spike that triggers the output pin.

The HC-SR501 has two key adjustments:

• Sensitivity potentiometer: Adjusts detection range from 3m to 7m.
• Delay potentiometer: How long the output stays HIGH after detection (5s to 5min).
• Jumper (H/L mode): Retriggerable (H) re-extends the timer on each detection; Single-trigger (L) waits for timeout before re-triggering.

Frequently Asked Questions