I sincerely hope no one has waited all this time to plan their alarm system. If you have I can only scratch my head...
We left off with the promise of some discussion of sensor types as they relate to planning an alarm system. Here we go...
But first, it is important to determine whether the system will be monitored remotely by a central alarm station or just a local alarm with a siren. Monitored systems provide the opportunity to summon help and come with a fee for the service. Local systems may be cheaper but with the limitation that the only help will need to be within the sound of the alarm and choose to respond. Insurance companies will often require the system to be monitored as well
Alarm systems use sensors to detect activity and there are a very wide variety of sensors to choose from. They can be classified a number of different ways. There are sensors to detect movement, changes in temperature, the presence of water (flooding), capacitance (changes to an electrical field), light and so on. The typical consumer system will normally be a combination of balanced magnetic switches, some sort of volumetric sensors, possibly glassbreak sensors, and maybe panic buttons. Here's a real quick translation:
A balanced magnet switch is commonly referred to as a "door contact" and consist of a magnet next to the sensor to complete the electrical circuit. When the door opens the magnet moves and the sensor detects the break in the circuit. The balanced aspect of the magnet makes it more difficult to defeat the sensor as shown in one of the Beverly Hills Cop movies. These sensors are useful on doors and windows or any objects that may be moved.
Volumetric sensors monitor an area (volume) and include such examples as acoustic (microphones), microwave (radar), passive infrared (ambient heat). There are dual-technology sensors that use a combination of these capabilities to either increase the likelihood of detection (increases false positives) or to increase to the certainty of a valid detection (increases false negatives). The difference between the two choices is nothing more than how the logic within the sensor is configured. This can be referred to as an "And" versus an "Or" that either requires one or both of the technologies to detect the activity. Glassbreak sensors are a form of volumetric sensor that listens for the sound of breaking glass. These are nice because they do not need an intruder to actually intrude before activating. They can also react to other loud sounds as well.
Panic buttons are personal alarm devices that simply allow a user to manually activate the alarm system. The central monitoring station sees each sensor type differently and therefore can place a greater emphasis on a panic button.
What makes the system valuable is the selection and arrangement of these sensors. Poorly selected technologies will generate excess false positives (nuisance or false alarms) and reduce the effectiveness of the response over time. You may have heard the story of the boy who cried wolf? Well alarm systems that cry intruder too often stop being believed too.
One important point to the selection and arrangement of the sensors is considering the asset(s) the system is meant to protect. Is it an expensive collection of something in one spot, random stuff scattered about, or just the piece of mind that no one is waiting inside when you open the door enter?
For a much greater (in every way) discussion of sensors try The Design and Evaluation of Physical Protection Systems by Mary Lynn Garcia.
Next... we'll take a look at arranging the sensors - tricks, tips, and pitfalls.
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