Arduino - PIR Sensor

Arduino: Interfacing with a PIR Motion Sensor

Passive Infrared (PIR) sensors are essential for detecting motion, making them a cornerstone of security systems, automatic lighting, and interactive projects. They detect changes in infrared radiation emitted by warm bodies, such as humans.

Key Features and Advantages

• Low Power Consumption: Ideal for battery-powered applications.
• Cost-Effective: Inexpensive and widely available.
• Simple Interface: Typically provide a simple HIGH/LOW digital output.
• Durable: Solid-state design means they don’t wear out.

A PIR sensor works by using two pyroelectric sub-sensors. It looks for a change where one half of the sensor detects a different IR level than the other, which indicates movement across its field of view. Note that PIRs detect change in IR, not static presence. If a person stands perfectly still, the sensor will eventually stop detecting them.

Calibration and Adjustment

Most PIR sensor modules have two potentiometers for adjustment:

• Sensitivity: Adjusts the detection range (e.g., from 3 to 7 meters).
• Time Delay: Adjusts how long the output signal remains HIGH after motion is detected (e.g., from a few seconds to several minutes).

Important: When first powered on, most PIR sensors require a warm-up and calibration period of 30-60 seconds. During this time, they may trigger unpredictably. Keep the area clear of motion while it stabilizes.

Components Required

• 1 × Arduino UNO R3 or compatible board
• 1 × PIR Motion Sensor (e.g., HC-SR501)
• 1 × LED (for visual feedback)
• 1 × 220Ω resistor (for the LED)
• Jumper wires

Wiring Diagram

// PIR Sensor Connections
// (Pin labels may vary, check your sensor's datasheet)
PIR VCC Pin   --- Arduino 5V
PIR OUT Pin   --- Arduino Digital Pin 2
PIR GND Pin   --- Arduino GND

// LED Indicator Connections
Arduino D13   --- [220Ω Resistor] --- [LED Anode (+)]
Arduino GND   --- [LED Cathode (-)]

Modernized Arduino Code

This revised code uses a clean, state-based approach to monitor the PIR sensor. It is non-blocking and easy to extend.

/**
 * @file modern_pir_sensor.ino
 * @brief A clear, state-based approach to reading a PIR motion sensor.
 * @author Modernized Tutorial
 */

// Define pins for better readability and maintenance.
constexpr int PIR_PIN = 2;
constexpr int LED_PIN = 13;

// State variable to track motion.
// Using an enum makes the code more self-documenting.
enum MotionState { NO_MOTION, MOTION_DETECTED };
MotionState currentMotionState = NO_MOTION;

void setup() {
  Serial.begin(9600);
  while (!Serial); // Wait for Serial to be ready (for modern boards like Leonardo/Micro)

  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);

  Serial.println("PIR Sensor Warm-up and Calibration...");
  Serial.println("Please keep the area clear of motion for 30 seconds.");
  delay(30000); // Blocking delay is acceptable in setup() for calibration.
  Serial.println("Sensor Ready.");
}

void loop() {
  // Read the current state of the PIR sensor.
  int pirState = digitalRead(PIR_PIN);

  if (pirState == HIGH) {
    // Motion is currently being detected.
    digitalWrite(LED_PIN, HIGH); // Turn on the indicator LED.

    // If we were previously in a NO_MOTION state, this is a new event.
    if (currentMotionState == NO_MOTION) {
      Serial.println("Motion started!");
      currentMotionState = MOTION_DETECTED; // Update state
    }
  } else {
    // No motion is currently being detected.
    digitalWrite(LED_PIN, LOW); // Turn off the indicator LED.

    // If we were previously in a MOTION_DETECTED state, the event has ended.
    if (currentMotionState == MOTION_DETECTED) {
      Serial.println("Motion ended.");
      currentMotionState = NO_MOTION; // Update state
    }
  }

  // loop() can continue to do other tasks here without being blocked.
}

Troubleshooting and Common Issues

• False Triggers: If the sensor triggers without motion, try decreasing the sensitivity. Electrical noise from nearby appliances can also be a cause.
• Not Detecting: Ensure the sensor has had time to calibrate. Check wiring and increase sensitivity. Remember the sensor detects movement across its view, not directly towards or away from it.
• Stays HIGH: The time-delay potentiometer might be set to its maximum. Adjust it to a shorter duration for testing.

Practical Applications & Next Steps

Now that you can detect motion, you can build exciting projects:

• Automated Lighting: Control a relay to switch AC-powered lights.
• Security Alarm: Trigger a buzzer or send a notification via a WiFi module (like an ESP8266 or ESP32).
• Interactive Art: Wake up a project from a low-power sleep mode when a person approaches.