An interior intrusion sensor is a detection device that is used to detect unauthorized access to an interior asset (i.e., a room or other structure) in a secure facility. The interior intrusion sensor itself is just one component of an entire electronic monitoring system (alarm system) that is used to detect and respond to an intruder entering the protected asset. The other components that make up the entire alarm system (alarm communications devices, control panel, and other components) are discussed in detail in the Alarms Product Guide and will not be discussed in this document. This document will focus exclusively on detection devices that detect unauthorized intruders attempting to enter or access a protected room or space.

As stated above, intrusion sensors are the devices that detect unauthorized entry to an asset. Depending on the type of unauthorized entry event they are designed to detect, sensors can be located outside (exterior) or inside (interior or space) of the protected facility or other asset. Exterior intrusion sensors are specifically designed to protect the secure exterior zone and detect an intruder before they gain access into the interior of a protected area. Those types of sensors are often located around the perimeters of facilities, and are designed to withstand the conditions of being located in an outdoor environment. In contrast, interior intrusion sensors are designed to protect the interior space of a facility and detect an intruder who has already entered a room or building and is moving within that space. For example, an interior intrusion sensor might employ an ultrasonic motion detector to detect the motion of an intruder who had bypassed the exterior detection device and gained access inside the facility.

In the design and installation of any intrusion detection system, a level of supervision should be planned for. Supervision monitors the system to determine if someone or something tampers with the system (e.g., cuts power to the system). Supervision works whether the alarm is armed and activated or disarmed and turned off and identifies when the intrusion detection system is damaged or compromised.

A common standard to use when specifying an intrusion detection system is UL-1076. This standard describes sensor technology to be applied or the type of intrusion to be detected; how the intrusion detection system is configured and the associated field wiring to protect all system components from tampering and circumvention. The key term of the standard is "line supervision", which typically identifies an end-of-line-resistor that allows for multiple state monitoring. It can detect cable cutting, cable shorting and other methods used to bypass sensors and ultimately circumvent this detection method. The sensors used should be specified with an internal tamper switch that can activate an alarm when the cover or access panel of an intrusion sensor is removed.

In addition to selecting the appropriate sensor technology when designing an electronic intrusion detection system the monitoring and control system must also be considered. This system helps provide the highest probability of detection with minimum in nuisance alarms. Most monitor and control systems are referred to as Security Management Systems (SMS) that usually incorporate or interface access control, intrusion detection and video management. These SMS's should have the appropriate UL listing for intrusion detection and access control. Although UL listings may not be required, they establish a standard to ensure a high level of quality, supervision and performance.

The remaining portion of this document will focus solely on interior intrusion sensors. Exterior intrusion sensors will be discussed in a separate document.

Interior intrusion sensors are classified according to how the sensor detects intrusion within the protected interior area. The three classes of interior sensor technology include:

• Boundary-penetration sensors;
• Interior motion sensors; and
• Proximity sensors.

Each of these sensor types is discussed separately below.

Boundary-penetration sensors detect intrusions over or through the "boundary" of a protected space. For example, a boundary-penetration sensor can be set up to protect a boundary such as a door or a window. In this type of application, the sensor will detect an intruder that enters the protected space by crossing or penetrating through the door or window "boundary," and will communicate an alarm. It should be noted that these "boundaries" could include not only doors and windows, but also walls, floors, or ceilings. Interior boundary-penetration sensors can work in many different ways - such as by detecting changes in vibrations or pressure on the protected asset, or by sensing changes in some sort of field sent out by the sensor (i.e., detecting changes in acoustical or electrical fields), and include vibration, electromechanical, capacitance, infrasonic, passive sonic, active infrared sensors, and fiber-optic cables. More details on each of these sensor types are provided below.

• Boundary-penetration vibration sensors detect vibrations from the surface of the object to which they are attached.

  EVD-1 Electronic Vibration System, Potter Electric Signal Company

  This includes vibrations caused by chopping, sawing, drilling, ramming or any type of physical intrusion attempt that would penetrate the structure on which the sensor is mounted. For instance, these sensor types are designed to detect the low frequency energy associated with vibrations typically generated during a physical intrusion attempt via the surrounding walls, floors, ceilings, and windows and compare that against the normal "vibration" profile. If the vibrational frequency is outside of the pre-set parameter, the detector will trip. Vibrational frequencies that are characteristic of an intrusion attempt to which the sensor will respond is a pre-determined parameter that is to set up during initial installation. Different types of boundary-penetration vibration sensors include inertial switches, piezoelectric, glass-break, active glass-break, and recent models of fiber-optic intrusion sensors. Information on the design, operation, detection, advantages, and disadvantages of various types of vibration sensors is provided in the table below.



Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Inertial Switches	Composed of a sensing element and a metallic ball mounted on metal contacts. The sensor is mounted on the protected surface, and the metallic ball tends to remain stationary relative to the surface.	If the sensor is vibrated at a frequency characteristic of an intrusion attempt, the inertia of the ball causes the ball to lose contact temporarily with the mount, thereby triggering an alarm. Detects at a vibration frequency that is between 2 and 5 kHz.	Able to detect vibrations at low frequencies. Provides an early warning of forced entry.	Susceptible to false alarms if the sensor is installed on walls or any structure that is exposed to external vibrations.
Piezoelectric	Consists of a piezoelectric sensing element that is housed within a sensor body. The sensing element is mounted directly onto the protected surface and moves relative to the mass of the sensor body. Vibrations create a flexing motion on the piezoelectric element, resulting in a voltage output.	Detection occurs when the protected mass is vibrated at frequencies typically associated with an intrusion attempt, between 5 and 50 kHz.	Not as prone to false alarm conditions when compared against inertial switches.	Lower frequencies may be more susceptible to false alarms.
Glass-Break	Mounts on the glass to be monitored; designed to detect vibrations most commonly associated with breaking glass.	Detects vibration frequencies that exceed 20 kHz.	Able to detect glass break vibrations before glass is actually broken.	Subject to nuisance alarms from improper calibration or installation, as well as sharp impact noises.
Active Glass-Break	Composed of a transmitter that emits a vibration onto the protected glass and a transducer that listens for this signal received in a location elsewhere on the glass.	If the glass is broken, the received signal changes, and an alarm condition is generated.	Nuisance alarm rate is considerably lower when compared to regular glass break sensors.	Detection is after glass is already broken.
Recent Models of Fiber-Optic Intrusion Sensors	Consists of a processing unit that transmits light from one end of a fiber optic cable to a receiver at the other end. Minute movements of the cable (vibrations) cause microbending of the cable, which results in changes to the light transmission.	When changes in light transmission are detected, an alarm condition is triggered.	Parameters can be adjusted to meet users need for reduced sensitivity to nuisance sources.	Filtering nuisance frequencies may reduce sensitivity to intrusion if there are no intrusion induced frequencies beyond the nuisance frequencies.

 

• Boundary-penetration electromechanical sensors detect changes in electrical and magnetic fields generated from a lost mechanical connection.

  Magnetic Reed Switch, Sensaphone

  For instance, magnetic switches detect the break in a magnetic field when a protected door or window is opened or closed, while the continuity or breakwire sensors detect breaks in electrical current resulting from a cut wire. Different types of boundary-penetration electromechanical sensors include magnetic switches, balanced magnetic switches, Hall Effect switches, and continuity or breakwire sensors. Information on the design, operation, detection, advantages, and disadvantages of individual types of boundary-penetration electromechanical sensors is provided in the table below.

 

 

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Magnetic Switches	Consists of a switch and magnetic units. The magnetic reed switch is mounted on the fixed part of the door/window, while the magnet is mounted on the movable part of the door/window. The standard switch is designed to be either normally open or normally closed, depending on the design. When the door/window is in the closed position, the magnetic field generated from the magnet holds the reed switch in the closed (or normally secure) position.	Opening the door/window results in a decrease of the magnetic field, causing the reed switch to move away from the magnet to the open (or alarm) position.	Relatively inexpensive and simple to install.	Simple magnetic switches are more prone to defeat than either the balanced magnetic or Hall Effect switches.
Balanced Magnetic Switches	Consists of a switch unit, a magnetic unit, and an additional bias magnet in the switch assembly to make tampering with the switch (and thereby defeating it) more difficult. Operation is the same as the magnetic switch described previously.	Opening the door/window results in a decrease in the magnetic field, causing the reed switch to separate from the magnet, thereby generating an alarm condition.	The additional bias magnet prevents defeat, thereby providing a higher level of protection.	If the intruder cuts through the door or window, the sensor will not go into alarm.
Hall Effect Switch	A newer type of magnetic switch composed of a magnetic unit and a switch unit lacking any mechanical reeds. Operation is based on the Hall Effect, which is the separation of charge in a current -carrying wire or metallic strip when exposed to an external magnetic field. The magnetic field accelerates charge carriers toward one side of the wire, resulting in a separation of charge. These switches are designed for a specific charge separation. The amount and polarity of the charge separation is proportional to the strength and polarity of the magnetic field.	Any significant changes in magnetic field strength results in changes in charge separation, which cause an alarm condition to be triggered.	Provides increased protection against insider tampering and defeat, and thus offers the highest level of security compared to all magnetic switch types.	Requires A/C power supply to operate.
Continuity or Breakwire Sensors	Consists of small, electrically conductive wires. This type of sensor is generally located in enclosed walls, ceilings, or floors and detects penetration through these materials.	If a wire is cut by an intruder, the electrical current is broken, and an alarm condition is generated.	Because wire must be cut to generate an alarm, rates of false alarms are low.	Defeat may occur by jumping the current.

 

• Boundary-penetration capacitance sensors detect changes in capacitance and are used to protect boundaries that have metal in them (metal walls, etc.). These sensors actively emit and receive signals that detect an intruder attempting to make contact with or penetrate the boundary of the protected structure. Capacitance sensors are generally proximity sensors; however, they can be applied for boundary penetration detection. Information on the design, operation, detection, advantages, and disadvantages of the capacitance sensor is provided in the table below.

 

Boundary-Penetration Sensor	Design and Operation	Detection	Pros	Cons
Capacitance Sensor	Composed of a control unit consisting of a tuned circuit that is located inside an oscillator. The sensor is mounted on the protected metal item and connected to the control unit with a wire, creating a protective electrical circuit between the protected metal structure and the control unit. Operation is based on the capacitance between two metal plates, one of which is the protected metal structure, and the other which is the control unit.	Any change in the protective circuit causes a change in capacitance, resulting in an alarm condition.	Able to detect before the physical destruction of the protected structure occurs.	Depending on the sensitivity settings of the control unit this sensor type may be subject to generating nuisance alarms.

 

• Boundary-penetration infrasonic sensors detect slight pressure changes in the volume in which they are installed. This includes the opening and closing of doors or windows leading into a monitored room with a closed volume. For example, a slight change in pressure (2 Hz) occurs whenever a door leading into a closed room is opened or closed. Information on the design, operation, detection, advantages, and disadvantages of the infrasonic sensor is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Infrasonic Sensor	Operates by sensing pressure changes in the volume in which the sensor is installed.	Changes in pressure in the closed room in which the sensor is installed results in an alarm condition.	Useful in areas where only occasional access is necessary.	Air blowing into the closed space can cause a nuisance alarm.

 

• Boundary-penetration passive sonic sensors use a microphone to detect sounds of a pre-specified amplitude, frequency, and duration (or repetition rate) corresponding to unauthorized penetration into the protected zone. Passive sonic sensors do not emit a signal, but instead receive one, making them responsive to frequencies in only the ultrasonic range. Information on the design, operation, detection, advantages, and disadvantages of the passive sonic sensor is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Passive Sonic Sensor	Design consists of a sensor equipped with a microphone that operates by listening to sounds that are generated in the protected area.	Sounds possessing the amplitude, frequency, and duration corresponding to penetration into the protected area result in an alarm condition.	Useful in confined spaces, such as vaults.	Has a limited effectiveness and is rarely used.

 

• Boundary-penetration active infrared sensors detect changes in received light energy. This type of sensor is often used in narrow areas to fill gaps, often near doors and windows. For instance, this type of sensor would most often be employed on the interior of door entry ways where it can provide optimal coverage. Information on the design, operation, detection, advantages, and disadvantages of the active infrared sensor is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Active Infrared Sensor	Consists of an infrared light transmitter that emits a beam of infrared light across a protected area, and a photodetector that receives that beam at the other end of the protected area.	An intruder passing through the field of detection will interrupt the amount of light collected at the receiver and temporarily cause the signal to fall below the threshold value, thereby creating an alarm condition.	If installed in the appropriate locations (i.e., narrow zones), this sensor type has a high probability of detection.	Dense levels of dust and smoke can scatter the beam and reduce the received light energy to an alarm level.

 

• Boundary-penetration fiber-optic cable sensors detect changes in received light energy. These fiber-optic cable sensors are composed of a span of fiber-optic sensing cable and an alarm processor unit. The alarm processor unit contains a light source, a light receiver, and signal alarm processing electronics (detects and initiates alarm). Each end of the span of fiber-optic cable is connected to the processor unit to form a closed loop. Fiber-optic cable sensors have the advantage of being immune to both radio and electromagnetic frequencies, as well as to changes in temperature and humidity. Different types of fiber-optic cable sensors include fiber-optic continuity and microbend fiber-optic sensors. Information on the design, operation, detection, advantages, and disadvantages of individual types of fiber-optic cable sensors is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Fiber-Optic Continuity Sensor	This closed loop design operates by sending a signal in the form of light energy through the cable where it is received by the processor unit. Any breaks in the cable will result in a change of the received light signal, which causes a loss of signal amplitude at the receiver.	Detects breaks in the signal at the receiver caused by damage to the loop, resulting in an alarm condition.	Because this sensor is designed such that the cable must be broken to trigger an alarm, the result is a decrease in nuisance alarms as compared to the microbend sensor.	Because this sensor type alarms only when the cable is broken, it offers a decreased level of protection when compared to the microbend sensor.
Microbend Fiber-Optic Sensor	The fiber-optic loop design operates by sensing any imposed force or motion of the cable, resulting in microbends of the cable and an alteration in the pattern of light energy received.	Detects changes in received light energy when pressure or movement is exerted on the cable, thus generating an alarm condition.	Microbend sensors provide earlier warning of an attempted intrusion than continuity sensors.	Sensor design will result in an increase in nuisance alarms as compared to the continuity sensor.

Interior boundary penetration sensors are designed to detect an attempted penetration of the boundary of a protected space before an intruder gains access to that protected space. In contrast interior motion sensors detect the motion of an intruder that has already gained unauthorized access to the protected interior space and is moving within it. An interior motion sensor can be set up to monitor and protect interior areas that an intruder will move through if they gain access to the interior. For example, where a sensor is set up in an interior hallway leading from a door, an intruder who successfully penetrated through the door would move through the hallway where he would be detected by the interior motion sensor, and an alarm signal would be generated. Interior motion sensors can operate in a variety of ways – such as by detecting a shift in the received signal (Doppler shift), or by sensing a frequency that is characteristic of an intrusion attempt, and can include microwave, ultrasonic, active sonic, passive infrared, dual-technology, and video motion detectors. More details on each of these sensor types is provided below.

 

• Microwave sensors detect electromagnetic frequency shifts caused by unauthorized motion in a protected zone. The microwave transmitter for this sensor sends out a known frequency. If an intruder moves through the protected space, a higher or lower frequency is returned to the receiver, indicating that the intruder is moving towards or away from the sensor. Information on the design, operation, detection, advantages, and disadvantages of microwave sensors is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Microwave Sensor	The design consists of a single antenna that both transmits and receives energy (monostatic) in the electromagnetic spectrum at frequencies of 10 GHz.	An intruder moving within the protected space will interfere with the signal, causing a "Doppler shift" in the signal (returned to the receiver at a higher or lower frequency), thereby indicating an intrusion and triggering an alarm condition.	Immune to air turbulence (gusts of wind), temperature and humidity changes.	Sensor is susceptible to detection pattern drift and may detect motion outside the protected area, resulting in nuisance alarms.

 

• Ultrasonic sensors detect acoustic frequency shifts resulting from the motion of an intruder in a secure area. As with the microwave detector, intruders within the protected area will interfere with this signal, causing a shift in the signal received back at the receiver. Information on the design, operation, detection, advantages, and disadvantages of the ultrasonic sensor is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Ultrasonic Sensor	Designed to emit ultrasonic sound energy at frequencies between 19 and 40 kHz into a monitored area and operate by detecting frequency shifts in the reflected energy pattern.	An intruder passing through the field of detection disturbs the acoustic wave pattern causing it to be reflected back more quickly, thus increasing the pitch and signaling an alarm condition.	Detection is unaffected by heat or changes in the thermal environment. Ultrasonic energy is easily contained in an interior environment and not susceptible to false alarm from activity exterior to the protected area.	False alarms may be triggered by strong air movements from heating/air conditioning systems, drafts from opening doors, hissing from pipes, and telephone ringing.

 

• Active sonic sensors detect intrusion by acoustic frequency shifts generated by an intruders motion in a protected space. As with the microwave detector, intruders within the protected area will interfere with this signal, causing a shift in the signal received back at the receiver. Frequencies used for these sensors are within the hearing range of the human ear and are typically considered quite unpleasant to the listener. Due to this, active sonic sensors are rarely found in environments where there is considerable human activity. Information on the design, operation, detection, advantages, and disadvantages of the active sonic sensor is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Active Sonic Sensor	Designed to emit acoustic sound energy at frequencies between 500 and 1,000 Hz into the protected zone. This sensor type operates by monitoring for frequency shifts in the reflected acoustic energy.	An intruder's motion within the monitored area results in acoustic frequency shifts, which triggers an alarm.	Unaffected by changes in the thermal environment.	Excessive background noise (e.g., airplanes, trains, thunderstorms) may cause a false alarm condition.

 

• Passive infrared (PIR) sensors detect rapid changes in temperature within a protected zone by monitoring infrared energy (energy in the form of heat). The PIR sensor establishes a

  Passive Infrared Motion Detector, Sensaphone

  baseline temperature in the monitored zone by detecting the infrared energy given off by objects in the stable background. An intruder entering into this protected area creates a change in the background temperature, which signals an alarm. For example, the sensor would respond to changes in the energy of the monitored environment resulting from an intruder's presence, which is approximately equal to the heat given off by a 50-watt light bulb. These detection devices typically respond to infrared energy in the wavelength band between 8 and 14 nanometers (nm). Information on the design, operation, detection, advantages, and disadvantages of the PIR sensor is provided in the table below.

 

 

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Passive Infrared (PIR) Sensor	Designed to detect infrared energy generated by sources giving off heat below that of visible light and operate by sensing the contrast between the "hot" image and the "cooler" background.	Detection occurs when an emitting heat source crosses two adjacent boundaries or crosses the same boundary twice within a specified time.	Mounting these sensors on the ceilings or high on the walls provides a 360-degree detection pattern.	As the environment approaches the same temperature as the intruder (80 –100 degrees) the detector becomes less sensitive and, thus less reliable.

 

• Dual-technology motion sensors detect intrusion when both technologies

  RCR/REX, GE Interlogix Sensors and Detectors

  sense motion simultaneously. Generally, dual-technology motion detectors combine either an active ultrasonic or microwave sensor with a passive infrared sensor. These alarms are combined in an AND-gate logic configuration, which requires that both the active and passive sensor types generate coincident alarms to produce a valid alarm. Information on the design, operation, detection, advantages, and disadvantages of the dual-technology sensor is provided in the table below.

 

 

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Dual-Technology Sensor	The sensor design consists of both sensing elements (ultrasonic/microwave and PIR) combined into a single casing. The coverage area for both sensors is similar in shape, resulting in a uniform zone of detection.	Detection occurs when both technologies sense an intruder moving within the protected space.	Sensor design will reduce the rate of false alarms due to both technology types needing to detect an intrusion before an alarm signal is initiated.	Probability of detection is reduced because both sensors must have a positive detection before initiating an alarm.

 

• Video motion detection (VMD) sensors detect intrusion by comparing the current scene with a pre-recorded stable scene of the monitored area. There are two types of VMDs: analog and digital. Analog is the older of the two technologies and operates by monitoring the camera signal to detect changes in brightness that are higher or lower than the known reference. These changes in brightness (i.e., shadows from shifting light) occur when there is movement in the camera's field of view. When a change is observed an alarm is generated, and the section where detection has occurred is highlighted on the closed circuit television (CCTV) monitor. Digital VMDs operate by converting the camera signal into a digital format, which allows digital processors to be used for video signal processing. The video scene is divided into zones, with each zone being monitored separately for changes in brightness or contrast, logical movement across adjacent zones, speed of motion across zone areas, size of objects within zones, and global changes across most or all of the zones. Information on the design, operation, detection, advantages, and disadvantages of VMD sensors is provided in the table below.

 

Boundary-Penetration Vibration Sensors	Design and Operation	Detection	Pros	Cons
Video Motion Detection (VMD) Sensor	Sensor design consists of a CCTV camera, signsal processor, and recorder. The VMD sensor operates by processing the video signal from a CCTV camera and comparing the most recent image against the stable image.	Detection occurs when the sensor detects changes in the monitored zone by comparing the current scene with a previously recorded stable scene.	Analog VMDs are less expensive than digital VMDs. Digital VMDs reduce nuisance alarms.	Both digital and analog VMDs can be susceptible to false alarms from flickering lights or from rodents, birds, and insects on or moving near the camera lens.

Unlike boundary-penetration and interior motion sensors, which are designed to detect the intrusion through and motion within the protected area, respectively, proximity sensors detect an intruder that has made contact with, or is in close proximity to a protected asset that is within the interior protected space. For example, a proximity sensor can be set up to protect a room or area within an interior space containing vital facility assets (SCADA systems, computer servers, safes, etc.). In an application of this type the proximity sensor will detect an intruder that is coming close to or entering into the protected room or area, and will generate an alarm. Proximity sensors include capacitance proximity and pressure sensors. More details on each of these sensor types is provided below.

• Capacitance proximity sensors are located on a protected metal item and detect an intruder approaching or making contact with the protected metal item. As stated previously, capacitance sensors are typically used as proximity sensors when protecting an object constructed of or containing metal. An electrical capacitor is an electronic component consisting of two conductor plates separated by a dielectric medium (air). A change in dielectric medium (electrical charge) results in a change in the capacitance between the two plates. The ground plate is located under or near the protected metal item. The protected metal item rests on and is protected by insulating blocks leaving only air between the protected metal item and the ground. When the sensor is connected to the protected metal item it generally takes less than a couple minutes for the sensor to calibrate (balance) itself. For instance, if an intruder comes into contact with the protected metal item their own electrical conductivity will change the dielectric medium, resulting in an alteration in the capacitance between the protected metal item, and the reference ground plate. This change in capacitance between the two conductor plates disturbs the balance and generates an alarm signal. Information on the design, operation, detection, advantages, and disadvantages of the capacitance proximity sensor is provided in the table below.

 

Proximity Sensor	Design and Operation	Detection	Pros	Cons
Capacitance Proximity Sensor	Composed of an electrical capacitor which consists of two conductor plates, one being the protected metal item, and the other an electrical reference ground plate. While in operation an electrostatic field exists between the protected metal item and the reference ground plate.	Detection occurs when an intruder attempts to gain unauthorized access to the protected metal item, disturbing the dielectric medium, which changes the capacitance between the two conductor plates and initiates an alarm condition.	Considered to be a good choice for securing high priority assets.	Changes in humidity and movement of other metal objects close to or away from the protected metal item can be enough to trigger a false alarm.

 

• Proximity pressure sensors detect intrusion by sensing the pressure an intruder exerts on the floor when stepping on it. These proximity pressure sensors are typically composed of pressure mats that contain a string of ribbon switches arranged parallel to one another. Information on the design, operation, detection, advantages, and disadvantages of the proximity pressure sensor is provided in the table below.

 

Proximity Sensor	Design and Operation	Detection	Pros	Cons
Proximity Pressure Sensor	The design typically consists of a series of ribbon switches, which are constructed from two strips of metal separated by an insulating material, and situated parallel to one another. Each of the metal strips is connected to an electric circuit.	Detection occurs when pressure is exerted on the sensor, forcing the metal strips to make electrical contact with one another, thereby generating an alarm condition.	These sensors can be well concealed under carpet, tile, or other floor coverings.	If an intruder knows of the sensors' existence, they can step around or avoid direct contact, thereby defeating the sensor.

Signal transmission methods for sensors are classified as either active or passive. An active sensor consists of a transmitter that emits a signal into the monitored area and a receiver that reflects that signal back. When changes in that emitted signal are detected by the receiver an alarm sequence is initiated. Active sensors can be either monostatic or bistatic. A monostatic unit consists of a transmitter and receiver together in the same housing. In contrast, a bistatic component consists of separate transmitter and receiver units. A passive sensor does not emit a signal from a transmitter; instead, it only receives energy that is within close proximity and compares that received signal against preset parameters. When an intruder invades the protected zone, the passive sensor will detect changes in conditions that differ from preset parameters and an alarm condition will be triggered.

Sensors are designed to be either covert or visible. A covert sensor, such as a proximity sensor, is concealed from sight and, thus, is difficult for a potential intruder to identify. This can further increase the sensor's effectiveness against serious security threats. In contrast to covert sensors, visible sensors are located out in the open and are easily identified by any would-be trespassers, therefore making them a deterrent against unauthorized intrusion.

Intrusion sensors monitor either a specific volume of space or along a discrete line. A volumetric sensor monitors an entire volume or a wide section of a protected zone. Line detection sensors monitor points of entry or very narrow areas in a protected zone and detect intruders only if they violate a protected point of entry or narrow zone.

Interior intrusion alarm sensors are designed to detect an intruder who has already gained access to a facility through a door, wall, or window. Costs for interior intrusion sensors can vary greatly depending on the size of the facility. As previously discussed, different sensors have different signal coverage size and distance limitations, and users will need to purchase enough sensors to ensure that they cover all of the area they wish to monitor.