Security alarm
A security alarm is a system designed to detect intrusions, such as unauthorized entry, into a building or other areas, such as a home or school. Security alarms protect against burglary or property damage, as well as against intruders. Examples include personal systems, neighborhood security alerts, car alarms, and prison alarms.
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 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.
Components
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:- Alarm control panel, or simply panel: The panel 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. Some also have their control units built-into the keypad or other human-machine interface.
- Sensors: In a security alarm, some sensors detect intrusions. Sensors' locations are at the perimeter of the protected area, within it, or both. Sensors can detect intruders by different methods.
- Alerting devices indicate an alarm condition. Most commonly, these are bells, sirens, and/or flashing lights. Alerting devices serve the dual purpose of warning occupants of intrusion and potentially scaring off burglars.
- Keypads: Keypads are devices that 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.
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 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, based on the normally closed or open design. This action, coupled with an electric current, 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
The passive infrared motion detector is one of the most common sensors found in household and small business environments. This sensor does not generate or radiate energy; it works entirely by detecting the heat energy given off by other objects.PIR sensors 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. 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 20Hz. Sounds at those frequencies are inaudible to the human ear. Due to its inherent properties, infrasound can travel distances of many hundreds of kilometers.The entire infrasound detection system consists of the following components: a speaker as a microphone input, an order-frequency filter, an analog-to-digital converter, and an microcomputer to analyze the recorded signal.
If a potential intruder tries to enter into a house, they test whether it is closed and locked, uses tools on openings, and/or applies pressure, creating low-frequency sound vibrations. Before the intruder breaks in, the infrasound detector automatically detects 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 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 as many 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 for indoor applications and separate housings for the protection of outdoor perimeters high-risk sites and critical infrastructures such as fuel storage, petrochemical facilities, military sites, civil and military airports, nuclear facilities and more. To reduce false alarms this type of detector is usually combined with a passive infrared detector or similar alarm. Compared to the monostatic, the bistatic units work over longer distances: typical distances for transmitter-receivers up to 200m for X-band frequencies and up to 500m for K-band frequencies.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. Microwave detectors are low cost, easy to install, have an invisible perimeter barrier, and are not affected by fog, rain, snow, sand storms, or wind. 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.
Functioning
The microwave generator is equipped with an antenna that allows it to concentrate the beam of electromagnetic waves in one preferred location and the beam is intercepted by the receiver, equipped with a similar antenna to the transmitter.The graphical representation of the beam is similar to a cigar, and, when not disturbed, it runs between the transmitter and the receiver and generates a continuous signal. When an individual tries to cross this beam, it produces a disturbance that is caught by the receiver as a variation of amplitude of the received signal.
These barriers are immune to harsh weather, such as fog, heavy rain, snow and sandstorms: none of these atmospheric phenomena affect in any way the behaviour and the reliability of the microwave detection. Furthermore, the working temperature range of this technology goes from -35 °C to +70 °C.