The ultrasonic detector is a popular device for detecting motion in a free space. it is not as versatile as the passive infra-red type mainly due to its restricted range, usually only about 9 meters max. It can be used in situations and environments which mould be unsuitable for the infra-red type as it is not affected by temperature chances.The human ear can hear sounds of frequencies between 20Hz and 20KHz (20,000Hz). Sound waves do exist above 20KHz but me cannot hear them, these very high sounds are called ultrasounds or ultrasonics. Imagine you are standing in a street and a police car or some other emergency service vehicle is speeding towards you. As it passes the sound of the siren seems to change. The reason tor this is because sound waves travel at a finite speed (700mph approx) regardless of the speed of the sound source. In effect as the source moves towards you the sound waves are compressed and sound higher, on the other hand, as the source moves away the sound waves are expanded and sound lower. This phenomenon is known as the Doppler Effect after the person who first discovered it in 1845.
The wiring of motion detectors is a relatively easy task as you can see below. The diagram shows a unit with the cover removed to reveal a terminal block for the connections to the control panel. These consist of:
1. A Normally Closed Alarm Relay. When the detector is stable (no movement) then this relay is closed. This is connected to the appropriate alarm zone at the panel and hence the alarm loop will be closed. If however the detector senses movement then it will open this relay for approximately 5 seconds after which it will reset itself.
2. A Tamper Switch Output. This is closed until the cover is removed and is connected to the tamper circuit at the control panel.
3. 12VDC Supply Terminals. These detectors require a 12Volt supply from the panel.