Detector with Ultrasonic

The term refers ultrasonic sound frequencies that are beyond the reach of human hearing, the upper limit of which is 16 kHz in healthy people in their twenties. The frequencies used in the ultrasonic detectors vary among different models, ranging from 23 to 40 kHz. The frequency is generated by an electronic oscillator is fed to one or more speakers. These actions must be very small, in fact, that salvation in a tweeter speaker HiFi because the moving parts are moving very quickly and if their weight must be minimized. This is an advantage with an alarm system ultrasonic detector units that can then be small and unobtrusive. The unit contains a microphone with amplification circuits and transformation.
Ultra-high frequency sound is thus projected into the protected area of the ultrasonic detector. Some of it is received directly by the microphone from the loudspeaker, while some is picked up after being reflected from walls and objects in the room. When there is no movement, both direct and reflected sounds are of the same frequency. Should movement of an object occur, the sound reflected from it will undergo a change of frequency owing to the Doppler Effect. Thus the ultrasonic detector microphone now picks up two frequencies: the original received directly from the loudspeaker, and the frequency shifted reflected from the moving object.

If two different frequencies are mixed, the result is that a third one is appears that the difference between the two. When the two frequencies are close, the third is often termed a beat note. The effect can sometimes be heard when two internal combustion engines are idling at slightly different speeds: the beat note is heard by the ultrasonic detector as a throbbing sound that varies in frequency as the speed of one or other of the engines changes.

The processing circuits in the ultrasonic detector detect the presence of the beat note and actuate the relay output of which is normally closed type. The ultrasonic detector can thus be connected into a normal loop, but it is preferable for space detectors to be connected to a separate zone from the perimeter sensors so that rapid identification of the source alarm can be made. A power supply of 25-50 mA is required from the control box.

Ultrasonic detectors usually have a range up to 30ft, but they have a sensitivity control which can reduce the range down to about 10 ft. The polar pattern is in the form of a narrow lobe and so the device affords little protection at the sides. It should therefore be pointed towards the expected point of entry. As the Doppler Effect is greatest for forward or backward motion, the sensor is more sensitive to these than to side-to-side movements. This is another reason to mount the ultrasonic detector facing the possible point of entry.