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Finding the Right Sensors & Detectors for Your Alarm System

What are Sensors?

If the control panel is the brain and the alarm is the voice of the security system, the sensors would be the sensory organs.  They are what sense and detect disturbances throughout the house, either by distinguishing motion (motion detectors), frequency (shock detectors), inappropriate sounds (sonar, aka acoustic sensors), pressure, air quality (smoke detectors), water and moisture,  and even electro-chemical levels (carbon monoxide detectors).

Types of Sensors

Sensors are made from a variety of technologies, but the most basic (and least expensive) is the magnetic sensor, which detects changes of state.  One magnetic sensor consists of two parts that should be installed adjacent to one another.  When one of the two halves moves apart from the other, the closed circuit becomes open and the control panel is alerted, thereby “triggering the alarm.” Magnetic sensors can be placed on any window or door; they can even be installed on a window that is open detecting any change (further opening).  They don’t, however, detect forced entry, such as breaking the window.  For that, you would use glass-break sensors.

Glass-break sensors come in two varieties: shock sensors, which detect a change in frequency, similar to some motion detectors and audio sensors, which detect sound.  They are available in both hard-wired and wireless versions.  The shock sensors can be mounted directly on windows.  Most glass-break audio sensors can detect sound up to 35 ft. in each direction (although they do not hear around corners), so if there are multiple windows within a 35 ft. area, only one detector is needed.  They can be mounted on walls or ceilings. It’s quite common to combine magnetic and glass-break detectors in one room.

Pressure mats can also be placed at strategic locations.  They exhibit sensors that are activated when stepped upon.

Types of Motion Detectors

Motion detectors are the most common type of sensors and at least one motion detector is usually included with all security systems.  They are typically placed on the interior of the house at key access points, like in front of doors and/ or windows.

On the exterior of your house, motion detectors could be used near the perimeter of the property or at vulnerable areas like a shed, swimming pool or garage. There are several different technologies available and some are better for certain areas of the house than others.  Ultrasonic sensors send out waves and then measure the returning echo. If motion is directed towards or away from the sensor, the measurements will vary, and the alarm will be triggered.  These are perhaps the most susceptible to error, since fluctuations such as wind or even moisture can influence the wave’s echo.  They also might not be able to pick up lateral motion. Ultrasonic technology is considered obsolete by many security professionals and is usually not recommended for accurate, fool-proof motion detection.

An upgrade to the ultrasonic sensor would be the microwave sensor, which discerns radio waves.  Microwave sensors are not influenced by air; their downfall is that they are very powerful and often pick up motion in areas that they are not intended to protect. This, too, can lead to false alarms.  Microwave motion detectors are used primarily in conjunct with automatic technologies, such as automatic doors.

Photo-Electric sensors are the most transparent type of motion detector; they are depicted in movies as the high-tech security used to protect valuable art objects. The technology itself is quite simple; it consists of a beam of light that, when blocked from reaching its source, triggers the alarm.  These motion detectors are less prone to error and can be quite valuable when intruders are unaware of their placement.

The most advanced sensor uses passive infrared technology, which reacts to significant changes in the level of heat within a room; it can be set to account for the motion of pets, insects or even for one person vs. two. 

Environmental Sensors

Believe it or not, more property loss and damage occurs from environmental factors, like extreme changes in temperature and water leaks than from break-ins, at least according to insurance claims.  While these events are not always predictable, as in the case of a natural disaster, like a flood,  they often are preventable if detected early enough.  Environmental sensors can be added-on to any security system: they come in analog and digital (wireless) varieties and are usually purchased separately.  In addition to extreme heat and freezing conditions, they can detect heavy winds, moisture, radon and carbon monoxide, resulting in complete home protection.