A motion sensor alarm broadcasts an acoustic or optic spectrum into the area being monitored, and measures the reflections of the sound or light that bounce back to the sensor. The sensor relates interruptions in the reflected signals to motion in the area. These motion sensor alarms usually operate in the ultrasonic or microwave wavelengths.
A passive motion sensor does not emit any spectrum at all, but simply monitors the existing background spectra. These sensors typically operate in the infrared (IR) wavelength range and are called passive sensors because they do not actively emit any signal of their own. When these sensors detect an IR emission (body heat) they activate the alarm.
Passive infrared motion sensors are often paired with an active microwave sensor. Combining the two technologies, together with careful placement of the sensor, allows a high degree of discrimination against false alarms. The heat signature of a human body is quite distinct from that of a cat or dog, for example, and so the device can be calibrated to ignore pets. Pairing two sensors also allows the precision of a microwave sensor without requiring its constant power usage. The microwave emission part of the detector is shut off until it is activated by the IR sensor, which uses much less power.
These sensors measure the rate of change in the signal they monitor. That means that only changes which occur relatively rapidly will trigger them. It is possible to defeat such detectors by moving very slowly. Most home intruders, however, are not slow. They want to be in and out of a home in as short a time as possible.
Motion detector alarms can be configured to do whatever the designer wants to do with the signal once they are triggered. Some systems are designed to begin video recording activity once they are activated. These are commonly used by game and wildlife management experts to monitor animal activity in a remote area. Systems in a residential setting are usually connected to a loud audible alarm. They are usually also programmed to send a signal to either a home security service or to the local authorities to indicate that a break-in has occurred.
Some systems are designed not to monitor a broad area, but a specific access point. These types of sensors typically pass an infrared beam across a doorway to a detector on the other side of the opening. When an object passes through the doorway and interrupts the transmission of the IR beam, the alarm is triggered. These types of systems do not have a high degree of discrimination and see only a limited use as a motion sensor burglar alarm, because anything which temporarily blocks the signal will trigger the alarm.
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