Introduction
After a power outage, some PIR sensors fail to resume normal operation. They may stay stuck in one state or not detect motion. This article explains why and how to fix it.
Why It Happens
1. Warm-Up Period Required
PIR sensors need 30-60 seconds to stabilize after power-up. If your system starts using the sensor immediately, it may not detect motion during this period.
2. Microcontroller Boot Sequence
If the sensor is connected to a microcontroller, the microcontroller may boot faster than the sensor stabilizes, missing the first detections.
3. Stuck State
In rare cases, a power glitch can leave the sensor’s internal logic in an undefined state.
4. Battery Backup Issues
If the system has battery backup, the switchover may cause a glitch that affects the sensor.
Solutions
1. Add Power-Up Delay in Code
In your microcontroller code, add a delay of 60 seconds at startup before using the sensor.
void setup() {
pinMode(pirPin, INPUT);
Serial.begin(9600);
Serial.println("Warming up sensor...");
delay(60000);
Serial.println("Ready.");
}
2. Hardware Reset Circuit
Add a power-on reset circuit that holds the sensor in reset for 60 seconds after power-up.
3. Brownout Protection
Ensure your power supply has enough capacitance to ride through brief outages. Add a large capacitor at the sensor’s power input.
4. Use a Sensor with Faster Warm-Up
Some sensors (like Panasonic EKMB) have faster stabilization times (10-30 seconds).
5. Implement a Self-Test
Have the system perform a self-test after boot, checking if the sensor responds to a test stimulus.
Conclusion
Power outage issues are usually due to insufficient warm-up time. A simple software delay is often enough to fix it.