Basic information on intruder alarm systems

Intruder/burglar alarm system is a set of interconnected devices that is aimed to protect an object, usually a facility, against intruders and to notify the owner or/and the monitoring station/center of any violation of the protected zones. Depending on the requirements and financial possibilities, it can be a simple alarm system suitable for a basic protection of a home/apartment, or a more advanced structure, which in addition to the basic functions will greatly improve the life comforts of the users. The designer of the system, usually the installation company or simply the installer, can greatly help the investor/user (customer) to choose an optimal solution, depending on the characteristics of the facility and the customer needs, also taking into account expansion options for the future. More advanced alarm systems can perform functions like access control (motorized gates, electric strikes in wickets/doors), lighting and heating control, and many other tasks of home/building automation. The interface devices (manipulators) are modern keypads and touch panels, which beyond the aesthetic appearance and intuitive operation have the ability to store maps of the facility on memory cards for easier control of the entire system, as well as (optionally) mobile phones or handhelds and PCs with dedicated applications and Internet connection.

Alarm control unit/panel (switchboard). The main unit is mounted in a dedicated housing/box together with a backup/buffer battery guarantying operation of the system during power outages. The housing is equipped with an anti-tamper switch connected to the dedicated input of the panel. If the system is not put in the service mode, any attempt to open the case ends up triggering an alarm. The control panel monitors the proper operation of all the equipment installed in the system. It collects information from the sensors, keypads, anti-tamper circuits, detects any faults in the wiring, and in the event of an irregularity, switches on optical and acoustic signaling. If the control panel is equipped with a communication module, it can notify the user or a security company about the alarm action. Any interference into the system will not go unnoticed, being stored the memory of the unit. During the selection of an intruder alarm control panel, one should pay particular attention to the number of inputs, outputs (high and low current), and system expandability options (if ever needed in the future).

Keypads, touch panels. These manipulators allow the installer to setup the control panel and program all the functions of the alarm system. They also are the user interface for communication with the alarm system, which provides information on the system status and enables arming and disarming operations. An alarm system usually contains one main (master) keypad and (if needed) zone/partition manipulators. The master device has LCD display and numeric keys, or touch screen. The partition keypads have numeric keypads and a simple optical signaling in the form of LEDs. They are used to arm and disarm the partitions, while the main keypad is responsible for the complete control of the entire alarm. The daily handling of the alarm system is limited to arming and disarming operations with the use of the appropriate codes, and viewing/reseting the alarm history.

Sensors. Sensors/detectors are the most essential elements of any alarm system. The proper selection, deployment and operation of sensors determines the actual usability of the entire system. The popularly employed sensors are PIR and PIR+MW motion detectors and reed switch sensors mounted on the door/window frames. These and other sensors used in alarm systems can be classified as follows:

The principle of operation of passive infra red motion detectors (PIR) is the detection of changes in thermal radiation of external objects in the far-infrared range by a PIR sensor. The electrical output signal signal is processed by a suitable electronics which is able to distinguish the small differences caused by a moving person from other disturbances that could cause false alarms. The risk of false alarms is minimized by means like Fresnel lenses or mirror segments that divide the protected area into multiple segments. The man crossing the segments causes cyclic changes of IR radiation focused by the elements on the PIR sensor, which triggers the alarm.
Micro-wave (MW) sensors are based on the use of the Doppler effect, which is a physical phenomenon involving the change of the wavelength/frequency of the signal received by the observer caused by the movement of the wave source relative to the observer. The transmitter emits waves that are reflected from the various fixed objects inside the room and return to the receiver without a change of the wavelength/frequency. However, in the case of a moving object in the detection area, the signal reflected by the object (a "secondary" source) will show a difference in the wavelength/frequency interpreted as a violation of the protected area.
Dual detectors (PIR + MW) combine the advantages of infrared detectors and microwave detectors. By using two different detection elements, they are more resistant to false alarms. The electronics of the detectors analyzes both signals and triggers an alarm only in the case of simultaneous detection of an intruder.
Magnetic sensors (reed switches) are closed in sufficiently strong magnetic field of the magnets (when the magnet is close to the reed contact) and open when the magnet is moved away. Such sensors are basic elements of perimeter protection systems and their primary task is to detect attempts of opening doors and windows.
Depending on the requirements, alarm systems may incorporate smoke, carbon monoxide, gas, glass break, curtain etc. sensors/detectors that increase their functionality and increase the security of the protected facilities.

Input/output modules (I/O expanders or expansion modules). I/O modules expand the capabilities of alarm control units with additional inputs and/or outputs. The type of the outputs depends on the needs (OC or relay). Additional individual relay modules controlled by OC-type outputs can control high-power AC electrical devices/appliances, eg. with current consumption up to 12 A. In addition to the increase of the number of inputs/outputs, the expanders are often used for wired connections of detectors located in more distant locations (e.g. in a detached garage). As a result, there is no need to lay cables from each detector to the control panel. The communication between the sensors and control panels is performed via the expanders, with the use of the system bus.

Wireless controllers. The controllers enable the expansion of the system with additional wireless devices, e.g. detectors. They are connected to the control panel via the system bus. The programming is done via the control panel, using a keypad or computer running a dedicated software. Battery-operated wireless sensors can work approx. 2 years without battery exchange. The battery life depends on the transmission power and on how many times the transmission takes place. Wireless subsystems are ideal for use in each case where any additional wiring is unacceptable, e.g. due to aesthetic reasons. The installation is very fast and the maintenance is limited to periodical replacement of the batteries in the wireless detectors.

GSM notification modules. If the control panel does not have a built-in GSM module, an external GSM notification/control module will allow the transmission of information via GSM to a monitoring station/center or to phone numbers defined in the module. Usually the versatile modules can be used with control panels from different manufacturers. They communicate with them via dedicated inputs and outputs. Using the CLIP or SMS service, they enable control of some devices via the main unit. Other notification/control modules communicate with the control panels via system bus and are fully integrated with them (full exchange of data between the devices).

Signaling devices. Optical-acoustic signaling devices (sirens, sounders, flashers) can be divided into internal/indoor and external/outdoor groups. The alarm control unit is usually configured in such a way that the optical alarm is active until resetting, while the acoustic signal is generated for a specified period of time. For example, some municipal bylaws limit alarm volume to 91 decibels for up to 4 minutes In this way, even in the case of a false alarm, the acoustic signal does not worry neighbors throughout a long time. The external devices are placed so that they can be well visible by people around the facility. Indoor sirens with very high alarm volume (120 dB) are intended to quickly deter thieves and prevent them from staying for a longer time in their vicinity.

The diagram below includes components that are usually used in security systems for protection of homes, cottages and other medium-sized facilities. The control panel with AC transformer and backup battery is placed in the dedicated installation box. The system is expandable up to 32 inputs/outputs and has GSM notification module, main and zone/partition keypads, expansion module with additional 8 wired inputs (zones) and 8 programmable relay and OC outputs, wireless controller allowing for connection of wireless sensors.