Silent Sentries, Loud Alarms: A Network of Protection

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A hard-wired alarm system. In the house that is shown below, each element of the security system is individually wired to a control box in the basement. The exterior doors have contact sensors, and windows accessible from the ground or the deck atop the garage have both contact sensors and glassbreak detectors. Inside the house, passive infrared detectors check for the telltale heat signature of an intruder. Smoke detectors are located at key points throughout the house: the upstairs hall, the bedroom, the workshop, and the basement stair well. Both the kitchen and the bed room are equipped with keypads, and the bedroom contains a panic button for triggering the alarm in case of an emergency.

A central security system offers far better protection for an entire house than stand-alone alarms. Consisting of an interconnected network of sensors and sirens, such a system is more versatile and easier to maintain than a collection of separate alarms because you can control everything from a single point.

There are two basic types of central systems: hard-wired, in which each sensor is connected to a main control box with wires, and wire less, with battery-powered sensors that communicate with the control box by radio signal. Additionally, some systems combine hard-wired and wireless technologies. Equipment for each type can be found at any alarm-equipment distributor.

Hard-Wired Systems: In this set up, the control box plugs into a standard wall outlet through a transformer, which steps the standard 120-volt household service down to between 16 and 20 volts. By sending a small current through the wires, the control box continuously monitors each sensor and sounds the alarm if any of them is tripped. To provide backup in case of a power failure, a battery must also be connected to the system.

Most systems are activated and deactivated through wall-mounted keypads, one typically located at the main entrance to the house and another in the master bedroom. The keypads allow you to turn the system on when you leave the house, off when you return, and back on again when you go to bed.

Wireless Systems: In homes where fishing wires is unusually difficult, the solution is to install a wireless system, in which typically only the keypads are wired to the control box. Wire less sensors, although much easier to install than hard-wired components, are more expensive, and you must periodically replace the batteries that run them.

The Sensors: Both hard-wired and wireless systems offer a variety of sensors to protect every part of a home. The first line of defense are the contact sensors at each door and window. When the system is armed, these sensors trigger the alarm whenever a door or window is opened. and to alert the system when an intruder breaks a window pane, special glass-break detectors set off the alarm at the sound of shattering glass.

The second line of defense consists of interior sensors, which detect the presence of an intruder who somehow manages to get into the house undetected. The most common of these is the passive infrared sensor, which responds to heat that is emitted by the human body. Other interior sensors discern motion either as a disturbance in a field of low-energy radiation (microwave) or as an interruption of a light beam (photoelectric). Still others combine passive infrared and microwave technologies to create a heat-and-motion sensor.

Finally, smoke detectors are recommended for all central systems. Types of devices are described and placement suggestions are given in a separate article.

Alarm Options: For added peace of mind, you can contract with a local security company to monitor your alarm system. Through an automatic telephone dialer in the control box, the company receives a signal when the alarm goes off. Things to look for in a security company appear in a separate article in this guide.

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