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.
Electronic – arduino – Are the PIR sensor modules motion sensors or proximity sensors
arduinoautomationpirsensor
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Especially for a home hobby project I'd probably start with PIR (Passive InfraRed) sensors. They are cheap and very effective at detecting something warm like a human body moving around.
However, PIR sensors will not detect static warm objects like someone sleeping or sitting still on the couch. With enough PIR sensors around the place, you can probably infer where people are motionless by where you know there was movement and in what direction. PIR sensors don't inherently give you direction, but enough of them activated in sequence does. For example three sensors triggered in sequence in a hallway is a strong clue someone is walking down the hall in that direction. If you saw motion of someone entering a room and then motion in the room, but nothing at the doorway, then you can make a good guess the person that entered is still inside but motionless.
This system isn't foolproof, but PIR sensors are cheap and remarkably sensitive, so with enough of them I think you can get to quite a useable level.
One thing to keep in mind is that other warm moving things will trigger PIR sensors too, like pets moving about. If you have a dog, then aiming the sensors so they only see motion a few feet off the floor helps. Cats jump around a lot, but are smaller, so maybe there is a way to not trigger on cats. This system will be a lot easier if you know the only warm moving things are humans though.
You could try a Doppler radar sensor such as the ~10GHz motion sensor modules which are available on the surplus market and from China.
As unlicensed and likely unapproved intentional emitters, they are probably of dubious legality to use, if that bothers you. They're probably safe to use (aside from possible interference with other electronics) but caution is called for- some people get sensitive if they think their DNA might get scrambled or their 'nads nuked.
Best Answer
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.