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Alarm System Components – Part 3

Motion sensors detect any movement in areas covered by that particular device. Some detectors use microwave sensors to detect movement, others use infrared sensors.

How Microwave Sensors Work:

Microwave sensors bounce microwaves off of an object, then measure the frequency of the returning waves. If the particular object is moving, it will send back microwaves of a differing frequency. Microwave sensors can offer a greater range of coverage than infrared sensors, and are more reliable at greater distances. Microwaves can also penetrate walls, which means that microwave sensors can be placed behind objects. The only negative aspect of these types of sensors is their strong sensitivity, and the fact that they are vulnerable to interference from other electrical devices such as lightning, or radios. Luckily, your installer can adjust the sensitivity of your microwave sensors quite easily to work best in your particular environment.

How Infrared Sensors Work:

The most common type of sensor seen mostly in home installation is the infrared sensor, as it tends to be cheaper and more reliable than microwave sensors. This type of sensor utilizes the infrared light spectrum, which is an invisible light that emanates from any heat source. The sensor measures the heat of any object that passes by it, and compares this heat to a background setting, such as a wall. One of the drawbacks of infrared sensors though, is that they don’t actually cover a full room. It covers a scanned area, kind of like fingers pointing out into the coverage area. The area between the fingers are not covered (except in the example shown, which uses dual pir technology). The plus side though, is that different lenses can be used, depending on the environment the sensor is in. For example, a long hallway can use a lower angle lens (appr 30 degrees) that covers a greater distance.

Some motion sensors use a combination of these features to improve its reliability. These are called “dual technology” sensors, or “dual tech” sensors. These are becoming increasingly popular, as they greatly reduce false alarms by combining the penetration capabilities of microwaves, and the heat sensing properties of infrared. Any motion or heat sensing device can have its sensitivity adjusted by your installer.

Pet Immune Motion Sensors:

Most pet motions use a dual lens feature, which gives the device the ability to distinguish humans from all household pets under 2 1/2 feet and up to 85 lbs (depending on the model and sensitivity adjustments). A model with these specifications will not sound an alarm if one of your pets (small dogs, cats) walks into the coverage area. If you have more than one pet, it would be a good idea to keep each pet separated from each other while the system is armed. Two or three pets in front of the same motion sensor at the same time may confuse the sensor, and cause an alarm.

Your pet immune motion detector should be able to tell the difference between intruder threat and a family pet – consistently. It’s a good idea to have your pet walk in front of the sensor during your installation, so the installer can make any necessary adjustments.

Remember that not all pet motion sensors (or any type of sensor, for that matter) are equally effective. Be sure your alarm company uses equipment with a well known brand name in the industry, such as DSC, Ademco, Paradox, etc. A cheap motion sensor will cause many false alarms and headaches for you, the authorities, and your pets!

2 comments to Alarm System Components – Part 3

  • Pamela

    Thanks for the information. I tend to think motion sensors are too consistent at triggering false alarms and should only be used in an accessory capacity with a full security system. And, motion sensor activations should always be verified with either audio or video technology.

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