HC-SR501 PIR Motion Sensor

Working Principle

The HC-SR501 is a passive infrared (PIR) motion detector based on a pyroelectric sensor element. All warm objects emit infrared radiation (~10 µm wavelength). The sensor contains two pyroelectric elements wired in opposite polarity. When a warm body moves across the sensor's field, it sequentially heats one element then the other, producing a differential voltage pulse that signals motion.

Analogy: Picture two thermometers side by side behind a slotted screen. A person walking past heats one thermometer then the other. The difference between them is your motion signal.

Electrical Characteristics

Parameter	Value
Supply Voltage	4.5 V – 20 V (on-board 3.3 V regulator)
Output	Digital HIGH (3.3 V) on motion
Detection Range	Up to 7 m (adjustable via potentiometer)
Detection Angle	<120° cone (Fresnel lens pattern)
Time Delay	0.3 s – 5 min (adjustable via potentiometer)
Trigger Mode	Single trigger (L) / Retriggerable (H)
Warm-up Time	~60 seconds after power-on
Current	<50 µA (quiescent)

Interfacing with an MCU

The HC-SR501 has just three pins: VCC, OUT, and GND. Output goes HIGH (3.3 V) when motion is detected. Two potentiometers adjust sensitivity (range) and output hold time. A jumper selects single-trigger (L) or retriggerable (H) mode.

Calibration

The HC-SR501 requires no software calibration, but hardware tuning is essential:

• Sensitivity pot (CW): Increases detection range from ~3 m to ~7 m.
• Time-delay pot (CW): Increases output hold time from ~0.3 s to ~5 min.
• Warm-up period: After power-on, the sensor needs ~60 seconds to stabilise — ignore outputs during this window.
• Fresnel lens swap: Replace the dome lens with a flat Fresnel for narrower detection patterns.

Code Example

/*
 * HC-SR501 PIR Motion Sensor — Arduino
 * Wiring: OUT → Pin 2, VCC → 5V, GND → GND
 * Allow 60s warm-up after power-on
 */
#define PIR_PIN 2
#define LED_PIN 13

volatile bool motionDetected = false;

void motionISR() {
    motionDetected = true;
}

void setup() {
    Serial.begin(9600);
    pinMode(PIR_PIN, INPUT);
    pinMode(LED_PIN, OUTPUT);
    attachInterrupt(digitalPinToInterrupt(PIR_PIN), motionISR, RISING);
    Serial.println("Warming up PIR sensor (60s)...");
    delay(60000);  /* Wait for sensor stabilisation */
    Serial.println("HC-SR501 Ready");
}

void loop() {
    if (motionDetected) {
        motionDetected = false;
        Serial.println("** MOTION DETECTED **");
        digitalWrite(LED_PIN, HIGH);
        delay(2000);
        digitalWrite(LED_PIN, LOW);
    }
}

Real-World Applications

Limitations

• Motion only: Detects moving warm bodies, not stationary people — a seated desk worker may not retrigger.
• False triggers: Pets, sunlight changes, and convection heat can trigger false alarms.
• Line of sight: IR radiation does not penetrate glass, acrylic, or walls — place the sensor with a clear path.
• Dead zone: A 2.5-second blind window after each trigger (in single-trigger mode) can miss rapid events.