The PIR motion sensor of type HC-SR501 allows motion detection based on infrared radiation analysis. The sensor detects changes from 2 separate IR windows, and the amplifier transforms this pulse into a signal with a high logical level. Such a signal can easily be used to drive a transistor or logic input of processor. When used with a Lan Controller, connect the detector output to the INPD1-4 logic input. However, it should be noted that a low state forced by the detector means rest, and a high state indicates motion detection.
On the board there are 2 potentiometers described as Sx and Tx.
The sensor works in retriggering mode, i.e. in the case of continuous motion detection the high state at the output is maintained at all times.
A more detailed description of the operation of the PIR sensor is available at this site.