The sensor needs about 30 to 60 seconds to "learn" the infrared signature of the room after powering on. Expect false triggers during this time.
. It was a "passive" observer, waiting silently to catch the infrared signature of a human body. He carefully wired the three pins: to ground, and the pin to his Arduino’s digital pin 2. hw-416-b pir sensor datasheet
It was set to the center, which meant the sensor would stay active for three minutes after one tiny movement. With a small screwdriver, he turned it fully counter-clockwise, reducing the delay to a snappy three seconds. The Sensitivity Pot: The sensor needs about 30 to 60 seconds