Introduction
You power up your PIR sensor, and immediately it starts triggering like crazy. Wait—this is normal. Most PIR sensors require a warm-up period of 30-60 seconds before they operate correctly.
What Happens During Warm-Up?
Inside the sensor, the pyroelectric elements and the signal processing circuitry need to reach thermal and electrical equilibrium. When power is first applied:
- The sensor self-heats slightly.
- The internal amplifiers stabilize.
- The sensor adapts to the ambient infrared background.
During this time, the output may randomly toggle. This is not a defect; it’s the sensor calibrating itself.
How Long Is Warm-Up?
For most common PIR modules (HC-SR501, HC-SR505), warm-up takes about 30-60 seconds. Some high-end sensors may stabilize faster; low-power sensors may take longer. Always consult the datasheet if available.
What If I Can’t Wait?
If your application requires immediate detection after power-up, consider these options:
- Keep the sensor always powered (don’t cycle power).
- Use a sensor with faster warm-up (e.g., some Panasonic models).
- In software, ignore the first 60 seconds of output.
Handling Warm-Up in Code
You can add a startup delay in your Arduino code:
void setup() {
Serial.begin(9600);
pinMode(pirPin, INPUT);
Serial.println("Sensor warming up...");
delay(60000); // wait 60 seconds
Serial.println("Ready");
}
Warm-Up After Prolonged Inactivity
If a sensor has been powered down for a long time, it will need warm-up again. For battery-powered designs that cycle power to save energy, consider whether warm-up time is acceptable.
False Triggers During Warm-Up
Don’t rely on motion detection during warm-up. If your system acts on sensor output, ensure that warm-up false triggers don’t cause unwanted actions (like turning on lights).
Conclusion
Warm-up time is a normal part of PIR sensor operation. Always allow for it in your projects to avoid confusion and false starts.