Microwave Presence detector works using Doppler Effect
It detects stationary and moving people just like a PIR, but it can do that also behind doors and thin walls, by taking advantage of the Doppler effect. By Boris Landoni @ open-electronics.org:
The detection of people, animals and hot bodies in general has been done for years using passive infrared radars, also known as PIR, which work by placing a pyroelectric sensor, so a heat sensor, behind a Fresnel lens, which has the ability to focus on just one point the infrared rays coming from the frontally detected heat, emitted by moving objects within a certain angle.
PIR sensors cover a wide array and variety of applications and represent by now a low-cost solution to protect ourselves from home invasion, automatically activate utilizers when moving people are detected etc., although they have the limitation to be able to detect only whatever can be seen: they cannot detect, even at a short distance, people moving behind doors and windows, so if we employ them in a home security system, they will only be activated when the intruder is already inside the room where the sensors are installed.
In order to have a preemptive protection, we can make use of radiofrequency sensors and, to be precise, microwave sensors, because they can detect people behind doors and even walls as long as they are not too thick or made of reinforced concrete or metal, or walls containing metal plaster reinforcement grids (for bladder walls) or cavity for sliding doors such as Scrigno.
Microwave Presence detector works using Doppler Effect – [Link]