Choosing and Using Fire and Burglar Alarms

In the right hands, an electronic security system can make a home much safer. If you’re looking for a mindless way to secure your home, then a burglar alarm isn’t it. The alarm is just one link in the security chain. It doesn’t eliminate the need for other important security measures. A burglar alarm will be helpful in homes where a family is security conscious — where they have strong doors, deadbolt locks on entry doors and regularly lock their doors and take other ordinary security measures.

As with many things, there’s a learning curve involved with using a new alarm system. Everyone in the home has to be willing to learn to use it properly and to stick with it if adjustments are needed. It isn’t unusual, in the first couple weeks, for an alarm to unexpectedly go off in the middle of the night or right after you walked out the door. A lot of people get so frustrated by those so-called “false alarms” (which are almost always caused by human error), that they disarm the system and stop using it.. They could have saved money by just posting burglar alarms stickers and signs around their home. The person who can benefit from an alarm system is the one who will find out what’s causing the unexpected alarm, fix it (or get it fixed), and continue to regularly use the system. A simple rule of thumb is that if you’re not using your key every time you leave your home or your car then you’re probably not going to benefit much from a burglar alarm system.

If you’re thinking about installing an alarm system or about having one professionally installed, you can save a lot of time and money and avoid false alarms, by knowing how alarms systems work and how to best choose and install alarm components for your specific needs.

How alarm systems work

A basic system consists of at least 3 components: a control unit; one or more sensors; and one or more annunciators. The control unit, often called a “control box”, “control panel” or “keypad” is the “brains” of the system, and is where you program, arm and disarm the system. A small system may have just one control unit, but many home alarms systems have multiple remote control panels or keypads for convenience. Sensors (or “detectors”) come in a variety of shapes and sizes, and are designed to detect sound, air movement, body heat, and other conditions that indicate an intruder is present. An annunciator lets you know that the alarm has been triggered. Common annunciators are sirens, bells and lights. When a properly placed sensor detects an intruder, it signals the control unit and the control unit then responds as programmed — which usually means to trigger annunciators and other components (which are discussed later in this article).

There are two basic types of burglar (or “intruder”) alarm systems: hardwired and wireless. With a hardwired alarm, you need to run wire from the control panel to each sensor and to the siren or alarm or bell.

Sensors

False alarms have always been the chink in a home security sensor’s armor. They are the bane of the owner’s, neighbor’s and local police department’s existence. Burglar alarm manufacturers have responded to this problem with greatly improved detection devices. Many of today’s detectors are more effective, more reliable and less expensive than ever before. What are commonly called “false alarms” are often simply the result of someone using the wrong sensors or installing them in the wrong place.

Most detectors fall within two categories: perimeter and space. Perimeter devices protect doors, windows and walls. They detect intruders before the intruder enter a house or room. The most common perimeter devices are magnetic switches, audio discriminators and foil tape sensors. Space protection devices cover open areas like whole rooms and hallways. They can detect an intruder who has bypassed a perimeter device.

Perimeter Detectors

Perhaps the cheapest and most basic sensor is the foil tape type, which is usually applied to windows. Like wire, the foil acts as an electrical conductor to make a complete circuit in an alarm system. When the window is broken, the fragile foil also breaks, interrupting the circuit and triggering the alarm. Foil tape costs only pennies per foot, but it breaks easily even when you wash the windows. And they generally aren’t considered attractive on residential windows.

The most popular perimeter protection device is the magnetic switch. It is used to protect doors and windows that open. A magnetic switch is reliable, inexpensive and easy to install. It consists of two small parts: a switch and a magnet, usually both are housed in an identical shaped plastic case. The switch contains two electrical contacts and a metal spring-loaded bar that bridges the contacts while magnetism is applied—allowing a complete (unbroken) electrical circuit in the alarm system. When the magnetic force isn’t present, the bar lifts off one or both contacts, creating an open circuit and triggering the alarm. That’s how magnetic switches work in most burglar alarm systems.

In a typical installation, the magnet is mounted on a door or window and the switch is aligned about 1/2-inch away on the frame. When an intruder pushes the door or window open, the magnet is moved out of alignment and no longer holds the switch’s bar on the contacts. Some magnetic switches are designed to be surface mounted; they’re the least expensive and easiest to install. For a more attractive installation, some people prefer recessed mounted magnetic switches. Recessed mounted models are fitted into drilled holes. When properly installed, recessed mounted magnetic switches are hard to notice, and blend in well with the door or window.

The sound of breaking glass can trigger another kind of sensor, the audio discriminator. It converts the sound into an electrical signal. A lot of professional alarm installers like using audio discriminators. By strategically placing audio discriminators, you can protect several large windows at once. Some models can be mounted on a wall up to 50 feet away from the window. Some have an omni directional pickup pattern; those models are for ceiling mounting.

Although they are tuned to the high-frequency sound glass makes when it breaks, other loud sounds may cause the sensors to trigger an alarm. Some sophisticated models require both the sound of breaking glass and shock vibrations before triggering an alarm. That feature reduces false alarms caused by vibrations or sounds alone.

Space Detectors

Among the more sensitive space protection devices, microwave detectors emit high-frequency radio waves and sense any change in the pattern of reflected waves that may be caused by an intruder. Microwaves can penetrate walls and the coverage can be adjusted to protect irregularly shaped rooms and hallways. A microwave detector is easy to conceal because it can be placed behind solid objects and it isn’t vulnerable to loud noises or air movement when properly adjusted.

Because microwaves penetrate walls, however, microwave detectors can be hard to adjust properly. A passing car, radio transmissions, or even florescent lights may prompt a false alarm. In home alarm systems, microwave detectors are usually combined with another type of sensing technology so that both must sense an alarm condition before an alarm is triggered.

Like microwaves, an ultrasonic sensor transmits high-frequency sound waves. But it’s waves bounce off walls, floors and furniture in the room. The problem with an ultrasonic sensor is that it’s prone to false alarms caused by such noises as a ringing telephone or jangling keys.

Passive infrared, or PIR, detectors are the most common space protection sensors. PIRs sense changes in temperature within a protected area by monitoring infrared radiation, a form of heat energy given off by all living things.

When an intruder enters a protected zone, a PIR detector notices the rapid change in IR radiation. If properly positioned and adjusted, the sensor ignores all ordinary and gradual changes in ambient infrared energy levels caused by sunlight and heating systems.

A typical PIR can monitor an area about 20 feet by 30 feet, or a narrow hallway of about 50 feet, depending on how the unit is positioned. It doesn’t penetrate walls or other solid objects, so the PIR detector is easier to adjust than are the microwave types.

Perhaps the biggest drawback to the PIR is that it can’t “see” an entire room at once. A PIR has specific and limited detection patterns, which are determined by it’s lens. One way to visualize a detection pattern is to imagine five or more long invisible fingers coming from the PIR reaching in various directions. Some may point down; some up; some across. Further imagine that the alarm is triggered when someone touches one of the fingers. At this point you probably realized that there are spaces between the fingers that are being reached by the PIR.

Theoretically, you could deal with the problem by having multiple PIRS throughout a room. But even then you wouldn’t cover every single spot in the room. More importantly, however, rarely is more than one PIR necessary in a room. The solution to the PIR problem is to pick the right type of lens that gives the shape of coverage for the room or area you’re protecting. Unless you want be alerted every time an ant gets into your home, there’s no need to have PIR coverage for every square inch of a room. You only need coverage that a person would have to be noticed by. Just ask yourself, if someone were breaking into this room or hallway or whatever, where would the person have to pass or go to. Remember, because nothing is actually emitting from your PIR, so the intruder can’t see the detection pattern.

Some PIRs have interchangeable lenses and offer a wide variety of detection patterns. One pattern, called the pet alley, is formed a few feet above the floor to allow pets to move about freely without triggering the alarm.

Quad PIRs consist of two dual-element sensors in one housing. Each sensor has separate processing circuitry, so the detector is essentially two PIRs in one. A quad reduces false alarms, because both PIRs must detect an intruder before an alarm is triggered. A mouse, for example, may be detected by one of the PIRs but would be too small to be detected by both simultaneously.

Combine PIR with microwave and you have a dula technology devices, also called “dual-tech”. The dual tech triggers an alarm only when both types of sensors detect an intrusion. Movement outside a wall that would trigger a microwave won’t affect a dual tech because the PIR element wouldn’t have simultaneously sense a change in infrared radiation. Dual techs can be costly, so they’re used primarily for commercial and high end residential applications.

Article from IAHSSP (Intl Assn of Home Safety & Security Professionals) www.iahssp.org

Home assessment and suggestions:

Whether you get a wireless home security system or not, you should be aware of how vulnerable your home is to a break in. Have you already taken some basic precautions to protect your family and belongings? Do you have, and do you regularly test, smoke and carbon monoxide detectors in your home? Are you and all your family careful to lock doors and windows? Here are some tips that can help to keep your family safe.

Have fire extinguishers that are easily accessible in your kitchen and garage. Establish a family escape plan and practice it regularly. Make sure your street number is visible from the street, especially at night. Clear shrubbery away from your windows so intruders won’t have a place to hide, and do not leave windows open at night or when no one is home. Install peepholes in all your external doors and use deadbolt locks. Even if you purchase your home new, install new locks or re-key existing ones.

Lock garages, sheds, and gates to the back yard. Put mowers, ladders, bicycles, rakes, and other yard tools away when you are finished with them. Make sure you have locks on all your windows (including basement windows) and extra locking devices on sliding glass doors. Buy window coverings that fully cover windows to prevent burglars from seeing your family or valuables inside.

When you’re going on vacation, stop deliveries, and have your lawn mowed regularly. Set timers on lights in more than one room, and vary the times. Talk to neighbors you trust, ask them to watch your house, and provide them with a contact number. Make it difficult for anyone to see inside your house.

False Alarms

A burglar alarm can initiate a considerable response by police or private security personnel, who may leave other important duties to race to the scene of the alarm. Therefore, it is important to prevent false alarms when designing and installing alarm systems. Modern burglar alarms make use of several different technologies to reduce false alarms.

Passive infrared detectors can be programmed to ignore the first movement detected, as in when the intruder moves from one detection zone to another, and to sound the alarm only when the movement passes through two or more detection zones within a specified period of time. In this way, an insect landing on the detector’s lens, or a sudden rise in background temperature caused by an activated furnace, is ignored.

Dual Technology Another means of preventing false alarms is the dual-technology motion detector. This is probably the most common type of detector used in more sophisticated burglar alarm systems. A dual-technology detector combines a passive infrared device and a microwave device in one small unit. The passive infrared device sees many detection zones and measures the change in background temperature as a target moves across them. At the same time, the detector projects microwaves and measures the Doppler shift when a target moves through the protected space.

An infrared motion detector will detect movement regardless of whether the target is moving across the field of view or toward the detector. But such a detector is more sensitive to movement across its field of view. Thus, it is more prone to false alarms caused by disturbances such as a mouse or rat moving across its field of view than by movement toward it. Microwave detectors are just the opposite: more sensitive to targets moving toward them than they are to targets moving across their field of view. If a large leaf falls off a plant in a room, a microwave detector is more likely to detect the motion than is an infrared detector. But if there is movement outside a window, a microwave detector might detect it when an infrared detector probably would not.

Dual-technology motion detectors use a circuit that requires both devices to detect motion before an alarm is sounded. A bird landing on an outside windowsill might trip the microwave device but not the infrared device, so no false alarm would be transmitted.

About motion detectors:

A motion detector is a type of electronic security device that senses movement and usually triggers an alarm. Many types of motion detectors can sense motion in total darkness, without an intruder becoming aware that an alarm has been triggered.

Motion detectors are an important part of most burglar alarm systems. They help alert security personnel, especially in situations where no obvious break-in has occurred. For instance, if an intruder steals a key to gain access to a protected site or hides within the site during normal business hours, the intruder’s entrance or presence could go unnoticed. A motion detector will detect the intruder’s movements as soon as he or she walks or otherwise moves within the area protected by the detector.

Motion detectors usually protect indoor areas, where conditions can be more closely controlled. Detectors for use in homes usually detect movement in spaces about 11 m x 11 m (35 ft x 35 ft) in area. Detectors for large warehouses can protect areas with dimensions as large as 24 m x 37 m (80 ft x 120 ft). Buildings with valuable or important assets, such as museums, also use motion detectors to detect break-ins at vulnerable points. Such points include walls, doors, windows, skylights, and even air ducts. Special motion detectors can protect the inside of exhibit cases where items such as diamonds are displayed. Others can be focused to a narrow area of coverage, somewhat like a curtain, that is projected in front of a painting to detect even the slightest touch.

Motion detector systems use a variety of methods to detect movement. Each method has advantages and disadvantages. Infrared detectors sense heat, and can detect an intruder’s radiating body heat. Most infrared detectors are passive, meaning they do not send out signals but merely receive signals, such as changes in temperature. Microwave and ultrasonic detectors are active – that is, they send out waves of energy and receive waves reflected back by objects. Any disturbance in the reflected waves caused by a moving object will trigger an alarm. Video systems optically monitor an area, and movement is registered visually on a monitor. Often, these methods are combined to improve accuracy and reduce chances of a false alarm.

Infrared Passive infrared motion detectors are sensitive to infrared wavelengths of energy, which are invisible to the human eye. Infrared energy can be detected as heat, and this type of detector senses the heat that a human or other animal emits. Passive infrared motion detectors have lenses that allow them to “see” an area as several distinct zones within the range of the detector. These zones spread out vertically and horizontally from the detector lens as a series of fingerlike areas, fanning out from the lens across the protected area and down to the floor. Electronic circuits enable the detector to recognize the normal amount of heat that is usually present in the area. When an intruder enters, the heat from the intruder’s body adds to the amount of heat normally present in the area. As the intruder moves from one zone to another within the detector’s field of view, the detector senses an increase in infrared energy and signals an alarm. Infrared motion detectors are very sensitive to heat changes, but they cannot see through solid objects or even through glass.

Infrared energy is also used in active infrared motion detectors. These detectors project a beam of infrared energy from one unit (the transmitter) to another (the receiver). An intruder who moves through the beam breaks a circuit in the receiver, triggering an alarm. Active infrared motion detectors are a more advanced version of electric-eye motion detectors, which use a photoelectric cell to detect an interruption of a beam of light. Active infrared systems use invisible infrared energy, so an intruder is less likely to discover or avoid the system.

Ultrasonic Some older burglar alarm systems use ultrasound (sound of very high frequency) to detect motion. They are called ultrasonic motion detectors. In such a detector a transmitter sends out sound of a frequency that is too high for the human ear to hear. A receiver picks up the sound waves reflected from the room or area under protection. The motion of someone or something in the space between the receiver and transmitter will cause a change, or shift, in the frequency of the sound. A circuit in the device detects any unusual shift in the frequency. A small shift, such as that produced by an insect or rodent, is ignored. When a larger shift, such as one produced by a moving person, is detected, the device triggers the alarm. Ultrasonic detectors are extremely sensitive, and can sometimes be triggered by loud noises or air gusts from an open vent.

The frequency shift discussed above is also known as the Doppler effect, which results from the behavior of sound waves when they are compressed by a moving object. Ultrasonic motion detectors use the Doppler effect to detect movement. The detector’s circuitry compares the frequency of the sound that is emitted by the transmitter when no motion is present to the frequency that results when motion occurs. When no motion is present, the sound is emitted and bounces back in an even, steady pattern. When motion occurs, the sound waves are disturbed and the circuit detects the shift.

Microwave Microwave motion detectors also use the Doppler effect to detect motion. Instead of emitting sound, they emit electromagnetic energy in the form of microwaves. Unlike microwave ovens, burglar alarm systems use microwaves that will not cook food or harm people. One characteristic of microwave energy is that it will penetrate glass, thin walls, or even people. If microwave motion detectors are not properly installed in a protected space, they may actually detect motion outside the building by penetrating the walls or windows.

Video Video motion detectors sense changes in ambient light, or the light that is normally present in a space. They are less sophisticated than the systems previously discussed, because they require ambient light to function. A video camera views an area, and sends the image to a monitor. The motion detector’s computer reads the pixels, or the tiny picture elements that make up a video image, that are displayed on the monitor. The detector notes whether the pixels are light or dark. When an intruder moves across the field of view, the monitor displays the movement, changing the lightness or darkness of the individual pixels. The detector notices the changes and signals an alarm.

Home Assessment

Whether you get a wireless home security system or not, you should be aware of how vulnerable your home is to a break in. Have you already taken some basic precautions to protect your family and belongings? Do you have, and do you regularly test, smoke and carbon monoxide detectors in your home? Are you and all your family careful to lock doors and windows? Here are some tips that can help to keep your family safe.

Valuable Home Safety Tips

Have fire extinguishers that are easily accessible in your kitchen and garage. Establish a family escape plan and practice it regularly. Make sure your street number is visible from the street, especially at night. Clear shrubbery away from your windows so intruders won’t have a place to hide, and do not leave windows open at night or when no one is home. Install peepholes in all your external doors and use deadbolt locks. Even if you purchase your home new, install new locks or re-key existing ones.

Lock garages, sheds, and gates to the back yard. Put mowers, ladders, bicycles, rakes, and other yard tools away when you are finished with them. Make sure you have locks on all your windows (including basement windows) and extra locking devices on sliding glass doors. Buy window coverings that fully cover windows to prevent burglars from seeing your family or valuables inside.

When you’re going on vacation, stop deliveries, and have your lawn mowed regularly. Set timers on lights in more than one room, and vary the times. Talk to neighbors you trust, ask them to watch your house, and provide them with a contact number. Make it difficult for anyone to see inside your house.

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