█ Introduction
Transmitter uses ultrasonic as its emitter and receiver to emit and receive constant amplitude ultrasonic in the target area. The reflected ultrasonic is constant amplitude when there’s no moving object in the area.Otherwise The amplitude of ultrasonic wave reflected is unequal, and changing. The receiving circuit tests the changing signals and it will control the circuit to drive the alarm response.

 █The work principle of the ultrasonic anti-theft alarm
the ultrasonic anti-theft alarm can be divided into two types according to its structure and installation methods: One type installs two ultrasonic transducers in a same shell, exploiting the advantages of sound wave Doppler effect. This kind is also called Doppler type. The reflected ultrasonic is constant amplitude when there’s no moving object in the detective area.Otherwise the wave amplitude is unequal, and changing. The emitted ultrasound is oriented and in oval shape.The other type puts the two transducer in different places. That is receiver and emitter splitting type which is also called acoustic detector. Its emitter and receiver are non or half directional transducers. Non directional transducer generates a hemispherical energy field distribution model while half directional transducer generates cone type model.

                                                                                            The working principle of Doppler type
█ Ultrasonic continuous wave signal transmitting circuit draft

                                                                          Ultrasonic continuous wave signal transmitting circuit draft
█Application of Ultrasonic anti-theft alarm :
Ultrasonic detector which can detect the moving objects are widely used in many fields such as detection of automatic switch door,controller, elevator automatic starter,anti-theft alarm detector etc.This detector can judge whether there’s any people or moving object in the detective area. It also has the advantages of large detective area and high reliability.