Under Home automation, Motion sensing is an important part of taking a decision to whether start or stop any service
HC-SR501 is a motion sensor having two modes which can be adjusted by jumper on sensor
Complete data sheet can be found here
When any motion is detected it’s OUT pin is turns high and stays high for a time duration
This time duration can be adjusted by a knob on the sensor.
Sensitivity of the sensor can be varied by 3m to 7m