Security alarm

Wireless home alarm system control panel

A house alarm

A security alarm is a system designed to detect intrusion, such as unauthorized entry, into a building or other areas such as a home or school. Security alarms used in residential, commercial, industrial, and military properties protect against burglary (theft) or property damage, as well as personal protection against intruders. Security alerts in neighborhoods show a connection with diminished robbery. Car alarms likewise help protect vehicles and their contents. Prisons also use security systems for the control of inmates.

Some alarm systems serve a single purpose of burglary protection; combination systems provide fire and intrusion protection. Intrusion-alarm systems are combined with closed-circuit television surveillance (CCTV) systems to record intruders’ activities and interface to access control systems for electrically locked doors. There are many types of security systems. Homeowners typically have small, self-contained noisemakers. These devices can also be complicated, multirole systems with computer monitoring and control. It may even include a two-way voice which allows communication between the panel and monitoring station.


Exterior alarm bell box

Alarm CPU panel with inputs and outputs

The most basic alarm consists of at least one sensor to detect trespassers and an alerting device to indicate the intrusion. However, a typical premises security alarm employs the following components:

  • Premises control unit (PCU), alarm control panel (ACP), or simply panel: The “brain” of the system; reads sensor inputs, tracks arm/disarm status, and signals intrusions. In a modern device, there are typically one or more computer circuit boards inside a metal enclosure. Many newer networks often use sealed plastic boxes out in the open. Some also have their control units built-in to the keypad or other human-machine interface.
  • Sensors: Devices that detect intrusions. Sensors’ locations are at the perimeter of the protected area, within it, or both. Sensors can detect intruders by different methods. For example, sensors can monitor doors and windows for openings or monitor unoccupied interiors for motions, sound, vibration, or other disturbances
  • Alerting devices: These indicate an alarm condition. Most commonly, these are bells, sirens, also and or flashing lights. Alerting devices serve the dual purposes of warning occupants of intrusion and potentially scaring off burglars. These devices can also warn occupants of a fire or smoke condition
  • Keypads: Small devices, typically wall-mounted, function as the human-machine interface to the system. In addition to buttons, keypads typically feature indicator lights, a small multi-character display, or both.
  • Interconnections between components. Interconnections may consist of direct wiring to the control unit or wireless links with local power supplies.

In addition to the system itself, security alarms often offer a monitoring service. In the event of an alarm, the premises control unit contacts a central monitoring station. Operators at the station see the signal and take appropriate action, such as contacting property owners, notifying police, or dispatching private security forces. Such alerts transmit via dedicated alarm circuits, telephone lines, or the internet in many cases.

Sensor types

Hermetically sealed reed switches

The hermetically sealed reed switch is a common type of two-piece sensor. This switch operates with an electrically conductive reed switch that is either normally open or normally closed when under the influence of a magnetic field in respect to proximity to the second piece, which contains a magnet. When the magnet moves away from the reed switch, the reed switch either closes or opens, again based on whether the design is usually open or closed. This action coupled with an electric current (typically at 12V DC) allows an alarm control panel to detect a fault on that zone or circuit. These sensors are common, are found wired directly to an alarm control panel, or are typically found in wireless door or window contacts as sub-components.

Passive infrared detectors

A passive infrared sensor used to detect motion

The passive infrared (PIR) motion detector is one of the most common sensors found in household and small business environments. It offers affordable and reliable functionality. The term passive refers to the fact that the detector does not generate or radiate energy; it works entirely by detecting the heat energy given off by other objects.

Strictly speaking, PIR sensors do not detect motion; instead, they identify abrupt changes in temperature at a given point. As an intruder walks in front of the sensor, the temperature at that point will rise from room temperature to body temperature, and then back again. This quick change triggers the detection.

PIR sensors designed to be wall- or ceiling-mounted, come in various fields of view, from narrow-point detectors to 360-degree radar. PIRs require a power supply in addition to the detection signaling circuit.

Infrasound detectors

The infrasound detector works by detecting infrasound, or sound waves at frequencies below 20 hertz. Sounds at those frequencies are inaudible to the human ear.[1] Due to its inherent properties, infrasound can travel distances of many hundreds of kilometers. Infrasound signals can result from volcanic eruptions, earthquakes, gravity waves, opening and closing of doors, and forcing windows, to name a few.[2]

The entire infrasound detection system consists of the following components: a speaker (infrasound sensor) as a microphone input, an order-frequency filter, an analog to digital (A/D) converter, and finally, an MCU used to analyze the recorded signal.

Each time a potential intruder tries to enter into a house, she or he tests whether it is closed and locked, uses tools on openings, or/and applies pressure, and therefore he or she creates low-frequency sound vibrations. Before the intruder breaks in, the infrasound detector automatically catches sight of the intruder’s actions

The purpose of such a system is to detect burglars before they enter the house to avoid both theft and vandalism. The sensitivity is dependent on the size of a home and the presence of animals.

Ultrasonic detectors

These active detectors transmit ultrasonic sound waves that are inaudible to humans using frequencies between 15 kHz and 75 kHz. The Doppler shift principle is the underlying method of operation which detects a change in frequency due to object motion. This detection occurs when the object must cause a change in the ultrasonic frequency to the receiver relative to the transmitting frequency.

The ultrasonic detector operates by the transmitter emitting an ultrasonic signal into the area to be protected. Solid objects (such as the surrounding floor, walls, and ceiling) reflect sound waves, which the receiver will detect. Because ultrasonic waves are transmitted through air, hard-surfaced objects tend to reflect most of the ultrasonic energy, while soft surfaces tend to absorb the most energy.

When the surfaces are stationary, the frequency of the waves detected by the receiver will be equal to the transmitted frequency. However, a change in frequency will occur as a result of the Doppler principle, when a person or object is moving towards or away from the detector. Such an event initiates an alarm signal. This technology is not active in many properties because many alarm professionals consider this obsolete.

Microwave detectors

This device emits microwaves from a transmitter and detects any reflected microwaves or reduction in beam intensity using a receiver. The transmitter and receiver are usually combined inside a single housing (monostatic) for indoor applications and separate housings (bistatic) for outdoor applications. To reduce false alarms this type of detector is usually combined with a passive infrared detector, or Dual Tec brand or similar alarm.


  • Advantages: low cost, easy to install, invisible perimeter barrier. It is not affected by fog, rain, snow, sand storms, or wind.
  • Disadvantages: May be affected by the presence of water dripping on the ground. Typically need a sterile clearance area to prevent partial blocking of the detection field.

Compact surveillance radar

Compact surveillance radar emits microwaves from a transmitter and detects any reflected microwaves. They are similar to microwave detectors but can detect the precise location of intruders in areas extending over hundreds of acres. With the capability of measuring the range, angle, velocity, direction, and size of the target, a CSR can pinpoint a precise GPS coordinate of an intruder. This target information is typically displayed on a map, user interface or situational awareness software that defines geographical alert zones or geofences with different types of actions initiated depending on time of day and other factors. CSR is commonly used to protect outside the fence line of critical facilities such as electrical substations, power plants, dams, and bridges.

Photoelectric beams

Photoelectric beam systems detect the presence of an intruder by transmitting invisible infrared light beams across an area, where these beams may be obstructed. To improve the detection surface area, the beams are often employed in stacks of two or more. However, if an intruder is aware of the technology’s presence, it can be avoided. The technology can be an effective long-range detection system, if installed in stacks of three or more where the transmitters and receivers are staggered to create a fence-like barrier. Systems are available for both internal and external applications. To prevent a clandestine attack using a secondary light source being used to hold the detector in a sealed condition whilst an intruder passes through, most systems use and detect a modulated light source.
– Advantages: low cost, easy to install. Require very little sterile clearance area to operate
– Disadvantages: Affected by fog or very high luminosity. Position of the transmitter can be located with some cameras.

Glass-break detection



Window foil is a less sophisticated, mostly outdated detection method that involves gluing a thin strip of conducting foil on the inside of the glass and putting low-power electric current through it. Breaking the glass is for all intents and purposes ensured to tear the foil and break the circuit.

Smoke, heat, and carbon monoxide detectors



Motion sensors


Driveway alarms


Electro-mechanical (shaker) sensors

These electro-mechanical devices are mounted on barriers and are used primarily to detect an attack on the structure itself. The technology relies on an unstable mechanical configuration that forms part of the electrical circuit. When movement or vibration occurs, the unstable portion of the circuit moves and breaks the current flow, which produces an alarm. The medium transmitting the vibration must be correctly selected for the specific sensor as they are best suited to different types of structures and configurations. This technology is gradually being replaced by intelligent digital accelerometer-based systems.

  • Advantages: Low-cost and easily installed on existing fences.
  • Disadvantages: Must be fence-mounted. Being mechanical in nature, the system is unable to analyze differences in the pattern of vibrations (for example, the difference between gusts of wind and a person climbing the fence).

Ferrous metal detectors

Change in the local magnetic field due to the presence of ferrous metals induces a current in the buried sensors (buried cables or discrete sensors) which are analyzed by the system. If the change exceeds a predetermined threshold, an alarm is generated.[3] This type of sensor can be used to detect intruders carrying substantial amounts of metal, such as a firearm, making it ideally suited for anti-poaching applications.[4]

Electrostatic field

Sometimes referred to as E-field, this volumetric sensor uses Electric field proximity sensing and can be installed on buildings, perimeters, fences, and walls. It also has the ability to be installed free-standing on dedicated poles. The system uses an electromagnetic field generator powering one wire, with another sensing wire running parallel to it. The sensing wire is connected to a signal processor that analyses:

  • amplitude change (mass of intruder),
  • rate change (movement of intruder),


The barrier can provide vertical protection from the ground to the height of the mounting posts (typically 4–6 meters of height), depending on the number of sensor wires installed. It is usually configured in zones of about 200 metre lengths.

  • Advantage: High-security (difficult to defeat), high vertical detection field.
  • Disadvantages: Expensive, short zones which mean more electronics (and thus a higher cost).

Microphonic systems


Microphonic systems vary in design (for example, time-domain reflectrometer or piezo-electric) but each is generally based on the detection of an intruder attempting to cut or climb over a fence. Usually the microphonic detection systems are installed as sensor cables attached to rigid chain-wire fences, however some specialized versions of these systems can also be installed as buried systems underground. Depending on the version selected, it can be sensitive to different frequencies or levels of noise or vibration. The system is based on coaxial or electro-magnetic sensor cable with the controller having the ability to differentiate between signals from the cable or chain-wire being cut, an intruder climbing the fence, or bad weather conditions.

The systems are designed to detect and analyze incoming electronic signals received from the sensor cable, and then to generate alarms from signals which exceed pre-set conditions. The systems have adjustable electronics to permit installers to change the sensitivity of the alarm detectors to the suit specific environmental conditions. The tuning of the system is usually accomplished during commissioning of the detection devices.

  • Advantages: Relatively inexpensive compared to other systems and easy to install.
  • Disadvantage: Older systems may have a high rate of false alarms caused by wind and other distances, although newer systems use DSP (digital signal processing) to process the signal and greatly reduce false alarms in some cases.

Taut wire fence systems


  • Disadvantages: very expensive and complicated to install.

Fiber optic cable

A fiber-optic cable can be used to detect intruders by measuring the difference in the amount of light sent through the fiber core. A variety of fiber optic sensing technologies may be used, including Rayleigh scattering or interferometry, If the cable is disturbed, the light will be changed and the intrusion detected. The cable can be attached directly to a chain-wire fence or bonded into a barbed steel tape that is used to protect the tops of walls and fences. This type of barbed tape provides a good physical deterrent as well as giving an immediate alarm if the tape is cut or severely distorted.

  • Advantages: Being cable-based, very similar to the microphonic system and easy to install. Allow to cover large perimeters (few tens of Km).
  • Disadvantage: Typically performs in a manner similar to microphonic-based systems but at a higher cost and complexity due to the use of fiberoptic technology.

Ported coaxial cable

Ported coaxial cable installation.png

This system employs an electro-magnetic field disturbance principle based on two unshielded (or ‘leaky’) coaxial cables buried approximately 30 cm (1 ft.) deep and 1 m (3 ft.) apart.[5] The transmitter emits continuous Radio Frequency (RF) energy along one cable and the energy is received by the other cable. When the change in field strength weakens due to the presence of an object and reaches a pre-set lower threshold, an alarm condition is generated. The system is covert after installation. Care must be taken to ensure the surrounding soil offers good drainage in order to avoid nuisance alarms.

  • Advantage: concealed as a buried form.
  • Disadvantages: can be affected by RF noise, difficult to install.

Security electric fence



  • Disadvantage: potential for unintended shock.

Wired, wireless, and hybrid systems

Installing a wireless magnetic contact breaker on a door




Alarm connection and monitoring





In the UK the most popular solution of this kind is similar in principle to the above but with the primary and back up paths reversed. Utilizing a radio path (GPRS/GSM) as the primary signaling path is not only quicker than PSTN but also allows huge cost savings as unlimited amounts of data can be sent at no extra expense.

Broadband alarm monitoring


Dial-up analogue alarm panels or systems with serial/parallel data ports may be migrated to broadband through the addition of an alarm server device which converts telephone signaling signals or data port traffic to IP messages suitable for broadband transmission. But the direct use of VoIP (POTS port on premises terminal) to transport analogue alarms without an alarm server device is problematic as the audio codecs used throughout the entire network transmission path cannot guarantee a suitable level of reliability or quality of service acceptable for the application.



Radio alarm dual signaling


A dual signaling communication device is attached to a control panel on a security installation and is the component that transmits the alarm to the ARC. It can do this in a number of different ways, via the GPRS radio path, via the GSM radio path or via the telephone line/or IP if that has been chosen. These multiple signaling paths are all present and live at the same time backing each other up to minimize exposure of the property to intruders. Should one fail there is always one form of back up and depending on the manufacturer chosen up to three paths working simultaneously at any one time. Prior to the availability of dual signaling systems, police and keyholders were often called out to the premises because of an alarm signal on the telephone path only to discover that it was a network fault and not a genuine alarm

Dual paths allow distinction between hardware failures and a genuine attack on the alarm. This helps eliminate false alarms and unnecessary responses. Dual signaling has helped considerably with the restoration of Police response as in an instance where a phone line is cut as the dual signaling device can continue to send alarm calls via one of its alternative paths either confirming or denying the alarm from the initial path.

In the UK, CSL DualCom Ltd pioneered dual signaling in 1996. In doing so, the company offered the first credible alternative to existing alarm signaling while setting the current standard for professional dual path security monitoring. Dual signaling is now firmly regarded as the standard format for alarm signaling and is duly specified by all of the leading insurance companies.[6]

Listen-in alarm monitoring


Alarm monitoring services


Alarm response



Some alarm systems use real-time audio and video monitoring technology to verify the legitimacy of an alarm. In some municipalities around the United States, this type of alarm verification allows the property it is protecting to be placed on a “verified response” list, allowing for quicker and safer police responses.


Access control and bypass codes




False and absent alarms


System reliability and user error are the cause of most false alarms, sometimes called “nuisance alarms.” False alarms can be very costly to local governments, local law enforcement, security system users and members of local communities. In 2007, the Department of Justice reported that in just one year, false alarms cost local municipalities and their constituents at least $1.8 billion.[7]


False alarm reduction






Audio and video verification




A second video solution can be incorporated into a standard panel, which sends the central station an alarm. When a signal is received, a trained monitoring professional accesses the on-site digital video recorder (DVR) through an IP link to determine the cause of the activation. For this type of system, the camera input to the DVR reflects the alarm panel’s zones and partitioning, which allows personnel to look for an alarm source in multiple areas.

The United States Department of Justice states that legislation requiring alarm companies to verify the legitimacy of an alarm, before contacting law enforcement (commonly known as “verified response”) is the most effective way to reduce false burglar alarms. The Department of Justice considers audio, video, or an eye-witness account as verification for the legitimacy of a burglar alarm.[7]



Enhanced call verification

Enhanced call verification (ECV) helps reduce false dispatches 25–50% while still protecting citizens, and is mandated in several US jurisdictions, although the alarm industry has successfully opposed it in others.[7] ECV requires central station personnel to attempt to verify the alarm activation by making a minimum of two phone calls to two different responsible party telephone numbers before dispatching law enforcement to the scene.



Independent certification

Some insurance companies and local agencies require that alarm systems be installed to code or be certified by an independent third party. The alarm system is required to have a maintenance check carried out every 6 – 12 months (in the UK, ‘Audible Only’ intruder alarm systems require a routine service visit once every 12 months and monitored intruder alarm systems require a check twice in every 12-month period) to ensure all internal components, sensors and PSUs are functioning correctly. In the past, this would require an alarm service engineer to attend site and carry the checks out. With the use of the Internet or radio path and a compatible IP/radio transmitting device (at the alarmed premises), some checks can now be carried out remotely from the central station.


Further reading