An energy field is transmitted into an area and monitored for a change in its pattern and frequency, which results in an alarm. Microwave motion detectors also operate on the Doppler frequency-shift principle. The sensitive abilities of these detectors result in many false alarms, which limit use. ![]() These detectors are installed on walls or ceilings or used covertly (i.e., disguised within another object). Passive UM detectors react to sounds (e.g., breaking glass). This detector operates on the Doppler Effect, which is the change in frequency that results from the motion of an intruder. Active UM detectors create a pattern of inaudible sound waves that are transmitted into an area and monitored by a receiver. Ultrasonic motion (UM) detectors focus on sound waves to detect motion. If an offender spots the sensor, he or she may be able to circumvent it.įigure 8.16. An intruder “trips” the alarm (i.e., opens the circuit) when the wire is pulled loose from the switch. Trip wire sensors use a spring-loaded switch attached to a wire stretched across a protected area. Although these sensors are difficult for an offender to spot, they are expensive to install, and an offender can jump the circuit. This type of sensor is applied to grill work, screens, walls, floors, ceilings, doors, and other locations. An interruption in either circuit signals an alarm. Grid wire sensors are made of fine insulated wire attached to protected surfaces in a grid pattern consisting of two circuits, one running vertical, the other horizontal, and each overlapping the other. If an offender detects the mat, he or she can walk around it. The cost is low, and these mats are difficult to detect. These mats are applied as internal traps at doors, windows, and main traffic points, as well as near valuable assets. When pressure is applied, as by a person walking on the mat, both layers meet and complete an electrical contact to signal an alarm. Pressure-sensitive mats contain two layers of metal strips or screen wire separated by sections of foam rubber or other flexible material. Lifting the item interrupts the circuit and signals an alarm. An item is placed on it that depresses the switch, completing the alarm circuit. Mechanical contact switches contain a push button-actuated switch that is recessed into a surface. Switches can be visible or hidden and afford good protection against opening a door however, no security method is ever “foolproof.” An ordinary magnetic switch is similar to the balanced type, except that it is simpler, is less expensive, and provides a lower level of security. An alarm is triggered when the door is opened and the circuit is interrupted. When the door is closed, the magnet holds the switch closed to complete an electrical circuit. A balanced magnetic switch consists of a switch mounted to a door (or window) frame and a magnet mounted to a moveable door or window. In the electronics field, a “switch” is a component that can interrupt an electrical circuit (e.g., as with a light switch). Purpura, in Effective Physical Security (Fifth Edition), 2017 Interior Sensors The very act of placing a magnet next to the switch to fool the switch will be seen by the switch as an alarm. By floating the magnetic switch in its case between two magnets oppositely polarized (one horizontal and the other vertical), any disruption in the magnetic field, such as opening of a door or window, will cause a change in state of the magnetic switch. ![]() One problem with simple magnetic switches is that it may be possible for an intruder to place a magnet next to the switch while opening the door, fooling the system into missing the fact that the door or window has opened.īalanced bias magnetic switches are designed for high-security applications. Switches rated for industrial or outdoor use may be fitted with heavy aluminum enclosures, some even with integral conduit fittings. These cannot be substituted for each other. Switches are made for unique applications, such as concealment within a wood door or a hollow metal door. Switches are available both for surface mount and for concealment within the door and its frame.Įconomical switches are usually surface mounted and enclosed in a plastic case, whereas better switches may be concealed. There is a wide variety of magnetic switch form factors for every need. Some are available in a Form-C contact (your choice of normally open or normally closed, both in one switch). Switches may be either normally open (close on alarm) or normally closed (open on alarm). ![]() A basic magnetic switch comprises two parts- the magnet and a magnetically sensitive switch (usually a reed switch enclosed within a glass envelope). Simple magnetic switches are used to detect the opening of doors and windows. Thomas Norman CPP, PSP, CSC, in Integrated Security Systems Design (Second Edition), 2014 Magnetic Switches Simple Magnetic Switches
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