Introduction to Home Security Systems: Alarms and Sensors
Introduction to Home Security Systems: Alarms and Sensors

The field of alarms and sensors is a very developed one. New and improved types of alarms and sensors are always coming into the market and the choice between the different types seems quite difficult. Despite the baffling selection, you should understand that all alarm systems have a very similar basic structure and work in accordance to the same principals. An alarm system is comprised of a main control box to which various sensors are connected. When the sensors identify a break-in, they send out a signal to the control box, which, in turn, sounds the alarm or performs other predefined tasks such as calling the police or a security company, turning on certain lights etc.

This guide is intended for people who are interested in installing their own alarm system as well as those interested in purchasing a system from a professional security company. The home security market today offers a wide range of products; from DIY alarm system for only a few tens of dollars, to encompassing and advanced systems costing several thousands of dollars for it to be installed by professionals. If you have decided to install your own alarm system, the key to choosing a system that will deliver the results you need is in understanding the security needs of your home, and taking into account your home's unique structure. Different sensors are suited to different areas; there are door sensors, window sensors, hallway sensors, closet sensors, drawer sensors and so forth. Even if the security company is the one installing the alarm system in your home, previous knowledge of alarm systems will allow you to fully understand the advice you receive from the security company, as apposed to having to reach these decisions blindly.


Sensors are generally divided into two main groups: sensors intended to protect the home perimeter and sensors to protect the interior of the home. The first line of defense of an alarm system are the sensors guarding the perimeter. These sensors identify the burglar when he first enters the house - through the window or door. The most common sensors of this group are Magnetic Sensors. These sensors are composed of two parts, one of which is installed on the frame head of the window or door and the other on the window or door itself. Skylink WD-318T Wireless Magnetic Sensor

Skylink WD-318T Wireless Magnetic Sensor

When the window or door is closed, the two pieces are adjacent to one another. In the event the window or door has been opened, the magnetic sensors are separated and the alarm is triggered. A flaw of this system is seen, for example, when a burglar breaks the window and therefore does not alert the magnetic sensors.

A solution to this problem is found in special windows containing foil wires. When such a window is broken, the wires are severed, triggering the alarm. But what if a burglar manages to bypass even this security measure? This is what pressure mats are intended for; sensors installed under the carpet at strategic points - near windows and doors or other areas likely to have a burglar walk through them. The sensors are activated when pressure- such as a foot step- is applied to them.

The second line of defense of the alarm system are the sensors guarding the interior of the home. Unlike the perimeter sensors, the internal sensors are not programmed to identify a certain action by a burglar (opening of a window or door), since the burglar's actions inside the home are unpredictable. Therefore, these sensors are programmed to identify movement in general, an action that any burglar must do while in the house. There is a wide range of motion detectors which work in different methods. Each one is suited to a different area of the home and to different environments:

Ultrasonic Sensor

This motion sensor works based on the principle of radar. The sensor sends out ultrasonic sound waves and receives their returned echo from the surrounding area. When there is movement in the area surveyed by the sensor, the pattern of returning waves to the sensor changes and the sensor then activates the alarm. Since sound waves travel by air, a false alarm can occur when positioning the sensor in an area susceptible to drafts or fast temperature changes. Therefore, it is advisable not to position an ultrasonic sensor near air-conditioning or heating vents, or near a window or hatch through which wind tends to come in.

Microwave Sensor

This sensor also works based on the principle of radar, but rather than sending out sound waves, it sends out microwave radio waves. Microwaves are not dependant on air for their movement, and therefore this type of sensor in also suitable to areas of the house where strong winds or continuous air movement are prevalent, such as from an air-conditioning vent. Microwaves have a vast range and power, and might therefore bounce between different surfaces and even exceed their intended boundary. This can cause, for example, the alarm to go off even when movement is occurring around the corner, past the intended boundary. For this reason, much attention should be given to the placement of this type of sensor and if there is a need- a professional should be consulted.

Photo-Electric Sensor

This sensor is comprised of two parts: a focused light source (usually a laser beam) and a light sensor. The two parts are positioned at either end of a hallway or passageway that you want monitored. When a person crosses the beam of light, he blocks that beam from reaching the light sensor. The light sensor will identify the disruption and trigger the alarm.

Passive Infrared Sensor (PIR)

This is an advanced version of a motion sensor. In contrast to the previously mentioned sensors, where energy beams or waves are constantly being sent out and received, the PIR sensor is completely passive. PIR sensors know how to identify people using the infrared radiation those people omit, which stems from body heat. PIR sensors are programmed to react only to drastic changes in levels of infrared radiation omitted in their surveillance area, caused by the movement of a person in the room. This is to prevent false alarms are from sounding when the floor cools at night, for instance. These sensors also know not to react to pets moving around in their surveillance area.

Noise Detector

This type of sensor is most suitable to especially quiet areas, in which noise only has one meaning: the presence of a burglar in the area. These sensors are not prevalent in the private home sector.


All the sensor types mentioned above hook up to an alarm's main control box. The control box is the one to respond to the signals omitted by the sensors throughout the house. The response will not always be immediate, but rather shortly delayed for in order to allow the homeowner enough time to enter the secret code and neutralize the alarm, if he is the one the sensors have picked up. There are many possibilities of reaction. The most common is a sound alarm.

The sound alarm may scare off burglars as well as get the attention of the residents and neighbors, who can, in turn, call the police.

Another option is a telephone dialer, in which several phone numbers can be programmed and will be called once the alarm is sounded. The dialer can call the police directly, and play a pre-recorded message stating the address of the house. The recording will repeat itself over and over, so that the person at the receiving end of the call will still hear the message even if the call has been on hold for a while before being answered. If you are using a monitored alarm system, the dialer will call your security company and forward the information regarding the goings on at the house. The information can be very detailed, and include the point of break-in as well as the position of the burglar at any time (if you have installed a closed circuit television system, the security company can even watch the burglar and follow him in real-time). The security company can send security people to the house, or call the police. The dialer can be based on either a landline or cellular.A cellular dialer can work even if the burglars have disconnected your landline. That said, such a dialer will require subscription to a cellular supplier, just as you would need for usage of any other cellular phone.

Another option is to connect the alarm system to the house's outdoor lights. The alarm system will turn the lights on and off repeatedly, which can assist the police in locating the house more quickly as well as alert the neighbors and passersby of the distressed situation. The alarm system can be upgraded by installing a closed circuit television system (CCTV), which can send images of the burglars in real time to all the televisions or computers in the house, or directly to the security company's control center. Security cameras can also send the transmission via the internet to your office computer or to your cellular phone. The main control box is connected to the house's electricity system, but is also equipped with a battery allowing it to work during times of power shortage or if burglars have disconnected the electrical power supply to the alarm system.