Point Sensors

Point sensors are used to protect specific objects within a facility. These sensors (sometimes referred to as proximity sensors) detect an intruder who touches, lifts, or comes close to an object. Several different types of point sensors are available, including capacitance sensors, pressure mats, and pressure switches. Other types of sensors also can be used to protect objects.

Capacitance sensors detect an intruder approaching or touching a metal object (including a wire) by sensing a change in capacitance between the object (or wire) and the ground (another wire). A capacitor consists of two metal plates separated by a dielectric medium that does not conduct electricity. A change in the dielectric medium or the electrical charge results in a change in capacitance. In practice, the metal object to be protected forms one plate of the capacitor and the ground plane surrounding the object forms the second plate. The sensor’s processor measures the capacitance between the metal object and the ground plane. An approaching intruder alters the dielectric value, changing the capacitance. If the change satisfies the alarm criteria, an alarm is generated.

Pressure mats generate an alarm when pressure is applied to any part of the mat's surface (for instance, when someone steps on the mat). Pressure mats can be used to detect an intruder who approaches a protected object. They also can be placed by doors or windows to detect entry. Because pressure mats are easy to bridge, they should be well concealed. For instance they could be placed under a carpet.

Mechanically activated contact switches or single-ribbon switches can be used as pressure switches. Objects that require protection can be placed on top of the switch. When the object is moved, the switch actuates, generating an alarm. The switch must be well concealed. The interface between the switch and the protected object should be designed so that an adversary cannot slide a thin piece of material under the object, overriding the switch while the object is removed.

Duress alarms are also called “panic switches.” Duress alarms may be fixed or portable. Operations and security personnel use them to signal a life-threatening emergency. Because of the nature of the alarm, the duress devices themselves should never make a noise or show that they have been activated. Normally, these alarms are manually operated.

Fixed duress alarms are mechanical switches permanently mounted in an inconspicuous location, such as under a counter or desk. They can be simple push-button switches activated by the touch of a finger or hand- or foot-operated switches attached to the floor. They can be integrated into telephone systems so that pushing one button on a multibutton phone can discretely signal security responders through the telephone system.

Portable duress alarms are wireless units consisting of a transmitter and a receiver. The transmitter is portable and small enough to be carried conveniently. The receiver is mounted in a fixed location within the facility. Either ultrasonic or radiofrequency energy can be used to communicate. When activated, the transmitter generates an alarm that is detected (within a given range) by the receiver. The receiver then generates an alarm signal in the alarm monitoring system.