PIR Motion Sensor (HC-SR501)

1. Introduction

The HC-SR501 is a Passive Infrared (PIR) sensor that detects motion by sensing changes in infrared radiation emitted by warm bodies (humans, animals). It's the most common motion detector used in security systems, automatic lighting, and presence detection.

"Passive" means it doesn't emit any signal—it only receives infrared radiation. This makes it low-power and undetectable.

2. Technical Specifications

How PIR Sensors Work

The sensor contains two pyroelectric elements that detect infrared light. The Fresnel lens focuses IR radiation onto these elements. When a warm body moves across the sensor's field of view:

1. One element sees more IR than the other
2. This creates a positive differential signal
3. As the body moves away, the signal reverses
4. The sensor interprets this change as motion

3. Trigger Modes

The jumper on the HC-SR501 selects between two trigger modes:

Recommended: Use Repeatable mode (H) for most applications—it provides continuous detection.

4. Wiring Guide

Figure 1: HC-SR501 PIR sensor wiring

5. Project: Motion-Activated Alarm

Components

• ESP32 or Arduino
• HC-SR501 PIR Sensor
• LED (optional, for visual feedback)
• Buzzer (optional, for alarm)

Basic Motion Detection Code

const int pirPin = 2;     // PIR sensor output
const int ledPin = 13;    // Built-in LED

void setup() {
    Serial.begin(115200);
    pinMode(pirPin, INPUT);
    pinMode(ledPin, OUTPUT);

    Serial.println("PIR Motion Sensor");
    Serial.println("Warming up (30 seconds)...");
    delay(30000);  // Wait for sensor to stabilize
    Serial.println("Ready!");
}

void loop() {
    int motion = digitalRead(pirPin);

    if (motion == HIGH) {
        digitalWrite(ledPin, HIGH);
        Serial.println("Motion Detected!");
    } else {
        digitalWrite(ledPin, LOW);
    }

    delay(100);
}

Motion Counter with Debounce

const int pirPin = 2;
int motionCount = 0;
bool lastState = LOW;
unsigned long lastMotionTime = 0;

void setup() {
    Serial.begin(115200);
    pinMode(pirPin, INPUT);

    Serial.println("PIR Motion Counter");
    Serial.println("Warming up...");
    delay(30000);
    Serial.println("Ready! Counting motion events.");
}

void loop() {
    bool currentState = digitalRead(pirPin);

    // Detect rising edge (LOW to HIGH transition)
    if (currentState == HIGH && lastState == LOW) {
        unsigned long now = millis();
        
        // Debounce: count only if 2+ seconds since last
        if (now - lastMotionTime > 2000) {
            motionCount++;
            lastMotionTime = now;
            
            Serial.print("Motion #");
            Serial.print(motionCount);
            Serial.println(" detected!");
        }
    }

    lastState = currentState;
    delay(50);
}

6. Applications

• Security systems: Intruder detection and alarms
• Automatic lighting: Hallway, bathroom, and closet lights
• Energy saving: Turn off devices when room is empty
• Occupancy sensing: Count people, detect presence
• Smart home: Trigger automations when someone enters
• Pet doors: Detect approaching animals

7. Troubleshooting

Sensor always reads HIGH

• Wait for 60s warm-up time
• Reduce sensitivity (turn potentiometer)
• Check for heat sources in view (heaters, sunlight)

Sensor not detecting motion

• Increase sensitivity potentiometer
• Check if target is within 7m range
• Ensure subject is moving (stationary = no detection)

False triggers

• Avoid pointing at windows (sunlight changes trigger it)
• Keep away from air vents and fans
• Shield from pets if needed

8. Summary

• Detects motion by sensing infrared radiation changes
• Range: 3-7m, Angle: 120°
• Needs 30-60 second warm-up after power-on
• Digital output: 3.3V HIGH when motion detected
• Two potentiometers: sensitivity and delay time
• Use Repeatable mode (H) for continuous detection