A security alarm is a system designed to detect intrusion – unauthorized entry – into a building or area. Security alarms are used in residential, commercial, industrial, and military properties for protection against burglary(theft) or property damage, as well as personal protection against intruders. Car alarms likewise protect vehicles and their contents. Prisons also use security systems for control of inmates.
Some alarm systems serve a single purpose of burglary protection; combination systems provide both fire and intrusion protection. Intrusion alarm systems may also be combined with closed-circuit television surveillance systems to automatically record the activities of intruders, and may interface to access control systems for electrically locked doors. Systems range from small, self-contained noisemakers, to complicated, multi-area systems with computer monitoring and control.
Passive infrared detectors
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 its own energy; it works entirely by detecting the heat energy given off by other objects.
Strictly speaking, PIR sensors do not detect motion; rather, they detect 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 tobody temperature, and then back again. This quick change triggers the detection.
PIR sensors may be designed to be wall or ceiling mounted, and come in various fields of view, from narrow "point" detectors to 360 degree fields. PIRs require a power supply in addition to the detection signalling circuit.
Using frequencies between 15 kHz and 75 kHz, these active detectors transmit ultrasonic sound waves that are inaudible to humans. The Doppler shift principle is the underlying method of operation, in which a change in frequency is detected due to object motion. This is caused 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. The sound waves are reflected by solid objects (such as the surrounding floor, walls and ceiling) and then detected by the receiver. Because ultrasonic waves are transmitted through air, then hard-surfaced objects tend to reflect most of the ultrasonic energy, while soft surfaces tend to absorb 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 considered obsolete by many alarm professionals, and is not actively installed.
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 "Dualtec" alarm.
Microwave detectors respond to a Doppler shift in the frequency of the reflected energy, by a phase shift, or by a sudden reduction of the level of received energy. Any of these effects may indicate motion of an intruder.
Photoelectric beam systems detect the presence of an intruder by transmitting visible or 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.
Glass break detection
The glass break detector may be used for internal perimeter building protection. Glass break acoustic detectors are mounted in close proximity to the glass panes and listen for sound frequencies associated with glass breaking.
Seismic glass break detectors, generally referred to as "shock sensors" are different in that they are installed on the glass pane. When glass breaks it produces specific shock frequencies which travel through the glass and often through the window frame and the surrounding walls and ceiling. Typically, the most intense frequencies generated are between 3 and 5 kHz, depending on the type of glass and the presence of a plastic interlayer. Seismic glass break detectors “feel” these shock frequencies and in turn generate an alarm condition.
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 electrical current through it. Breaking the glass is practically guaranteed to tear the foil and break the circuit.
Smoke, heat, and carbon monoxide detectors
Most systems may also be equipped with smoke, heat, and/or carbon monoxide detectors. These are also known as 24 hour zones (which are on at all times). Smoke detectors and heat detectors protect from the risk of fire and carbon monoxide detectors protect from the risk of carbon monoxide. Although an intruder alarm panel may also have these detectors connected, it may not meet all the local fire code requirements of a fire alarm system.
Traditional smoke detectors are technically ionization smoke detectors which "create an electric current between two metal plates, which sound an alarm when disrupted by smoke entering the chamber. Ionization smoke alarms can quickly detect the small amounts of smoke produced by fast flaming fires, such as cooking fires or fires fueled by paper or flammable liquids." A newer, and perhaps safer type is a Photoelectric smoke detector. It contains a light source in a light-sensitive electric sensor, which is positioned at a 90-degree angles to the sensor. "Normally, light from the light source shoots straight across and misses the sensor. When smoke enters the chamber, it scatters the light, which then hits the sensor and triggers the alarm. Photoelectric smoke detectors typically respond faster to a fire in its early, smoldering stage – before the source of the fire bursts into flames." 
Vibration (shaker) or inertia sensors
Strain-sensor cable installed on a chain-link/barbed-wire fence
These 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 technology of the devices varies and can be sensitive to different levels of vibration. 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.
A rather new and unproven type of sensor uses piezo-electric components rather than mechanical circuits, which can be tuned to be extremely sensitive to vibration.
- pros: Very reliable sensors, low false alarm rate and middle place in the price range.
- cons: Must be fence mounted. The rather high price deters many customers, but its effectiveness offsets its high price. Piezo-electric sensors are a new technology with an unproven record as opposed to the mechanical sensor which in some cases has a field record in excess of 20 years.
Passive magnetic field detection
This buried security system is based on the Magnetic Anomaly Detection principle of operation. The system uses an electromagnetic field generator powered by two wires running in parallel. Both wires run along the perimeter and are usually installed about 5"/12 cm apart on top of a wall or about 12"/30 cm below ground. The wires are connected to a signal processor which analyzes any change in the magnetic field.
This kind of buried security system sensor cable could be embedded in the top of almost any kind of wall to provide a regular wall detection ability, or can be buried in the ground. They provide a very low false alarm rate, and have a very high chance of detecting real burglars. However, they cannot be installed near high voltage lines, or radar transmitters.
This proximity system can be installed on building 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. Both wires run along the perimeter and are usually installed about 800 millimetres apart. The sensing wire is connected to a signal processor that analyses:
- amplitude change (mass of intruder),
- rate change (movement of intruder),
- preset disturbance time (time the intruder is in the pattern).
These items define the characteristics of an intruder and when all three are detected simultaneously, an alarm signal is generated.
The barrier can provide protection from the ground to about 4 meters of altitude. It is usually configured in zones of about 200 metre lengths depending on the number of sensor wires installed.
- advantage: concealed as a buried form.
- disadvantage: expensive, short zones which mean more electronics (more money), high rate of false alarms as it cannot distinguish a cat from a human. In reality it does not work that well, as extreme weather causes false alarms.
The operation of a microwave barrier is very simple. This type of device produces an electromagnetic beam using high frequency waves that pass from the transmitter to the receiver, creating an invisible but sensitive wall of protection. When the receiver detects a difference of condition within the beam (and hence a possible intrusion), the system begins a detailed analysis of the situation. If the system considers the signal a real intrusion, it provides an alarm signal that can be treated in analog or digital form.
- advantage: low cost, easy to install, invisible perimeter barrier, unknown perimeter limits to the intruder.
- disadvantage: extremely sensitive to weather as rain, snow and fog for example would cause the sensors to stop working, need sterile perimeter line because trees, bushes or anything that blocks the beam would cause false alarm or lack of detection.
Microphonic based systems vary in design but each is generally based on the detection of an intruder attempting to cut or climb over a chain-wire 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 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 analyse incoming electronic signals received from the sensor cable, and then to generate alarms from signals which exceed preset 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.
- advantage: very cheap, very simple configuration, easy to install.
- disadvantage: some systems have a high rate of false alarms because some of these sensors might be too sensitive. Although systems using DSP (Digital Signal Processing) will largely eliminate false alarms on some cases.