I wanted to make a small project in which the lights of a room can only be turned on when there is someone in the room and it will turn off in absence of any person. I thought of using PIR sensors but from what I read it looks like they sense motion and not the presence (non-moving human) of someone.
The standard cheap multi-segment PIR is a motion sensor. It works by having a sensitive thermal sensor that is shaded in certain directions, so detects an on/off/on signal when something hot moves across its field of vision. The sensor drifts too much for it to be DC coupled.
There are plenty of stories of people working in offices where the lights are controlled by PIR to save money, and if they are the last in the office, they have to stand up and wave every few minutes to keep the lights on.