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I'd like to monitor a state of rooms in my apartment and turn off heating/cooling/lights when nobody is there.

I see there is a bunch of PIR sensors on the market, however I don't understand why some of them are labeled as "motion sensors" and some as "presence sensors", and whether there a real difference between them.

The experience with motion sensors usually installed on the stairs outside of apartments clearly tells me that motion sensors are not good for the task.

So, are there sensors which can reliably detect presence/absence of people in a room?

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    PIR sensors are invariably motion. Actual "presence" thermal sensors would be expensive and require complex pattern recognition to differentiate a person from a colder or warmer environment which may have hot spots. Feb 18, 2018 at 17:14

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This Adafruit article, PIR Motion Sensor, provides a description of a PIR sensor and the basic mechanics of how a PIR sensor works. A synopsis of the how a PIR sensor is designed.

PIRs are basically made of a pyroelectric sensor (which you can see below as the round metal can with a rectangular crystal in the center), which can detect levels of infrared radiation. Everything emits some low level radiation, and the hotter something is, the more radiation is emitted. The sensor in a motion detector is actually split in two halves. The reason for that is that we are looking to detect motion (change) not average IR levels. The two halves are wired up so that they cancel each other out. If one half sees more or less IR radiation than the other, the output will swing high or low.

Later in the article which is explaining in more detail about the design of the PIR sensor.

The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here is not really doing much and so we see that the two slots can 'see' out past some distance (basically the sensitivity of the sensor). When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected.

So obviously a PIR sensor will not fit your requirements since a PIR sensor depends on motion of an infrared (IR) source that is moving across the field of view of the sensor.

However there are IR cameras, FLIR (Forward Looking InfraRed) cameras is the standard name, which can be used with the appropriate image processing software to "see" IR emitting objects.

This SparkFun article, FLIR Lepton Hookup Guide, describes using a FLIR camera along with the SimpleCV open source framework to build a computer vision device which in this case would be "seeing" in the infrared spectrum.

Here is a PDF of a slide package, Computer Vision Using SimpleCV and the Raspberry Pi, which provides a nice overview of using the SimpleCV framework to do camera image processing.

Here is a wiki with a detailed article, How to install FLIR Lepton Thermal Camera and applications on Raspberry Pi, which may be helpful.

There is also the Omron D6T series of thermal sensors which may provide what you need.

Omron's D6T Series MEMS Thermal Sensors are a super-sensitive infrared temperature sensor that makes full use of Omron's proprietary MEMS sensing technology. Unlike typical pyroelectric human presence sensors that rely on motion detection, the D6T thermal sensor is able to detect the presence of stationary humans by detecting body heat, and can therefore be used to automatically switch off unnecessary lighting, air conditioning, etc. when people are not present.

You may also find this Automated Elephant Detection project helpful.

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