Some glass-breakage detectors are fastened directly to window glass. They sense shock or vibration that occurs when glass breaks. Dampened mercury sensors are one type. The contain two electrical contacts and a globule of mercury-all within a small glass vial filled with oil. When vibrations are detected, the frequency must correspond to the resonant frequency of the glass vial. When this occurs, the mercury dispersed in all directions, opening the normally closed alarm circuit. This oil, sometimes referred to as dampening oil, acts as a filter. It ignores erroneous vibrations caused by people knocking on windows and bumping the sides of buildings, as well as passing trains and trucks.
Inertia sensors are also mounted directly to window glass. These detectors use gold-plated, ball-bearing-balanced mechanisms, which simply disrupt normally closed circuits when vibrations of sufficient magnitude are detected. They are excellent on-glass detectors, However there usually is no way to adjust their sensitivity. Adjustments become necessary whenever ambient low-level vibrations cause false alarms.
Vibration sensors use weighted mechanical contacts that interrupt normally closed alarm circuits when sufficient vibration is detected. They typically are mounted on window frames or walls and should be fine-tuned to the environments in which they are installed.
Dampened mercury, inertia and vibration sensors all are fast-responding devices (typically generating 5-msec. pulses). These types of sensors are too quick for some older alarm control panels, which have an average alarm circuit response time of 500 msec. Pulse stretchers or extenders, used to connect fast-acting devices to older alarm circuits, compensate for this. They convert the fast 5-msec. pulse to a 1-sec pulse. Pulse stretchers derive their operating power from the auxiliary power output of an alarm panel. Almost any number of fast-acting devices can be installed in series with a pulse stretcher’s input. The output usually is connected in series with an alarm circuit and its end-of-line resistor, if one is used. Most newer control panels offer at least one fast-acting loop. Some microprocessor-based alarm panels allow installers to program the specific response time of each alarm circuit to match the type of detection device attached to it.
Vibration, inertia and some mercury devices are well suited for detecting intruders breaking through walls and windows. However, most do not contain electronic discriminator circuits that distinguish between “bumps in the night” and intruders. Consequently, any vibration of sufficient magnitude can cause one of these sensors to trigger an alarm. On-glass electronic sensors, however, are available with either built-in discriminator circuits or sensors that can be connected in series to a centralized processor that contains the discriminator circuits. In either case, these detectors constantly monitor for the ultra-high-frequency vibrations that are typical of glass breakage.
Piezoelectric crystal sensors also can be used as glass-breakage sensors. Each piezoelectric sensor contains a flat crystal element that converts mechanical vibrations into electrical impulses. These impulses then are analyzed by discriminator circuits, either inside each sensor or in a centralized processor. Centralized systems can monitor as many as 50 sensors at a time. Piezoelectric glass-breakage detectors generally are powered and connected to alarm circuits in one or two ways. Probably the most common is the four-wire method. Two wires carry 12 VDC power and two others carry a sensor’s normally closed output. The two-wire method involves a four-conductor cable. The first two wires carry both low-voltage power and alarm signals to each sensor. A second set of wires carries both power and alarm connection to the next detector.
Stand-alone detectors usually have external LED’s that signal when detection has taken place. Some models latch and the detector must be reset before it will operate again. This is advantageous when there are multiple sensors in a single zone, because it allows installers to see, at a glance, which sensor has tripped. These detectors may be reset with either a momentary push-button switch or by feeding a panel’s auxiliary power through the normally closed contacts of an auxiliary relay. The user can manually reset the detectors by entering a personal identification number or striking auxiliary keys on a keypad.
Some sensors employ external LED’s as signal-strength indicators as well as alarm-memory indicators. Other sensors use an LED that changes color to indicated various conditions, such as alarm and fringe-detection. Piezoelectric glass-breakage detectors usually are applied to windows using either self-adhesive pads or liquid adhesive that hardens when it dries. In either case, cleaning the glass before mounting the detector is very important. Some sensors come with both a cleaning solution and an adhesive solution. A central processor for shock sensors counts the number of pulses, or shocks, that occur within a present time period. The number of pulses usually is installer adjustable so the system can be made more or less sensitive. This helps to eliminate false alarms.
Another way to protect window glass is by mounting shock-type sensors on frames near windows. When intruders break glass or shatter wood, vibrations that travel through the structure are detected, converted to electrical impulses, and sent to a centralized processor or internal discriminator circuit for appropriate action.