Motion sensors

Over the years there have been many types of "motion" sensors (sometimes also referred to as "space protection"), including microwave and ultrasonic technology, that were used to sense activity in a protected area. By far the most common type of "motion sensor" in use today is the "Passive Infrared" or "PIR" detector.

PIR detectors work by detecting the level of infrared energy in a room (by measuring the surface temperature at various points) and then looking for changes in this level. A person walking through the room will have a different temperature than the wall behind them and will therefore create a measurable change in the overall level of infrared energy.

The PIR detector has a lens which focuses the energy from the room onto a "pyro" element which in turn translates the energy into a small electrical voltage. Changes in the voltage are analyzed to determine whether they have been created by a person moving through the room, or by some other cause. The circuitry in a PIR is extremely sensitive and there are many things which can potentially cause false alarms, such as;

• Rapid changes in temperature. This could be caused by a forced air heater coming on or a gust of wind blowing through an open window.
• Pets or other animals moving through the room. The size of the animal, the amount of movement, and their proximity to the sensor all have an impact on whether an alarm will be generated. Even a mouse can trigger a false alarm if they are quite close to the sensor. There are "pet immune" sensors but even these must be located carefully to avoid false alarms.
• In commercial buildings objects such as decorations or signs hanging from the ceiling sometimes cause false alarms when they move due to air currents. Paper "snowflakes" hanging in blizzard like formations at Christmas time are always good for a few false alarms.
• A beam of light shining directly into the lens of a PIR can also cause a false alarm. Installers must be aware of reflective surfaces and always be very cautious about facing a sensor toward an exterior window. While a PIR will not "see through" a window to detect someone walking by, they can be triggered when sunlight, or headlights from a passing car, shine through the glass and into the sensor.
• A sensor built from low quality materials will be more likely to false alarm. All low quality electronics generate internal "noise" (voltage fluctuations) and can pick up further noise from outside sources such as radio transmitters and AC power lines. This noise can also lead to false alarms in the highly sensitive circuitry of a PIR.

Despite all of these potential problems, manufacturers have developed very reliable and inexpensive motion sensors over the past decade. It is very easy to design a sensor that will trigger at the slightest hint of movement in a room. It is also very easy to design a sensor that will never false alarm by turning the sensitivity down to the point where a flame thrower aimed at the unit would barely register. But the key to designing a truly useful sensor is to find that middle ground between a high probability of intruder detection and a very low probability of false alarms.

There are hundreds of models of passive infrared sensor that an alarm company can choose from. Many of these models are designed for special applications like ceiling mounting, harsh environments, long range detection or pet immunity. The retail price for PIRs (not including installation) can be anything from about $14 to over $100. One of our favourite models is the Visonic SRN-2000, a very high quality sensor, but unfortunately with a price tag around $80 (not including installation) we don't sell them very often. An alarm company must weigh many factors when choosing sensors including detection sensitivity, reliability, aesthetic appeal, availability and of course, cost. The PIRs that we install in most situations are in the $25 to $45 range, the most common choice being the DSC Bravo 3. We do not use less expensive sensors as we have found that they either have very low sensitivity, or are too unreliable.

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