The principle of life detector is extremely simple, because as long as it is life, there will be a lot of special life information in the body, which will be displayed outside the body through various energy methods, such as sound waves, ultrasonic waves, electric waves, light waves and some special waves that the earth people have not mastered, such as some special waves generated by the brain during activities. These waves will naturally emit completely different energies, and this life detector can judge whether they appear on the screen by detecting these different waves. For example, the location of the "living" can be found by using the principle that ultra-low frequency radio waves (generated by the heart) generate electric fields, because the ultra-low frequency electric fields emitted by the human body can pass through reinforced concrete walls and steel plates. When the instrument encounters the above obstacles, the detection distance will be reduced.
There are many kinds of life detectors, some of which sense the ultra-low frequency radio waves emitted by the human heart, generate an electric field, make the antenna swing and finally point to the target. And infrared thermal imaging. There is also the principle of sound wave/vibration, which is a listening device to hear where there is movement. The most earthy is the optical principle, which is an optical fiber probe. The most advanced is the radar type, which uses ultra-wideband impact radar to continuously illuminate and detect the heartbeat signal in the echo. I have seen special reports in discovery before.