Door and window switches

The first sensor we will talk about is the door/window contact switch. This is by far the most common type of sensor used in the alarm industry. There are several variations of this sensor, but they all function in the same way. For example, you can buy a garage door sensor that has a large magnet and is physically large, so that when the wind sways your garage door, the rattling won't set off the alarm.

The preceding diagram shows a standard door or window sensor. Usually the contact position is when the reed switch is energized (the window is closed). If you aren't sure, check with an ohm meter.

The magnet normally holds the wiper of the reed switch against the normally closed contact. When the window or door is opened, the magnet can no longer hold the wiper against the normally closed contact and it opens.

You can think of a sensor as a magnetically operated single pole double throw (SPDT) switch. In fact, for testing purposes, you can use a toggle switch to simulate door and window contacts.

Remember what I said about carpentry?

The preceding photograph is of an actual installation. The magnet (bottom arrow) is installed on the window, while the switch is on the window frame. The top arrow shows a poor installation job! The installer should have drilled a hole in the window frame closer to the switch, so that not much of the wire is exposed. The less exposed wire there is, the harder it is for the bad guys to bypass the switch. The same rule applies to all types of sensor installations.

The PIR â passive infrared sensor The next most common sensor is the passive infrared motion sensor or PIR. It is called a passive infrared sensor because it does not transmit anything. If, for example, it used an IR laser, it would be an active sensor. PIRs come in many shapes, sizes, and price ranges. Many of the more expensive models can be configured so that the family pet does not activate the sensor. A passive infrared sensor measures the ambient heat in the room and then waits for a warmer body to pass across its viewing area. The resulting "blip" in the ambience is what it detects. A Fresnel lens and a parabolic mirror are often used to extend the range of the sensor. Think of it as a flashlight in reverse. Instead of concentrating the light from the bulb into a beam, the ambient IR focuses on the sensor. In the preceding diagram, the mirror would be behind the Fresnel lens. The Fresnel lens acts like a camera lens to give the sensor a wider field of view. This is an extremely simplified explanation. Modern PIRs use digital signal processing to help eliminate false triggering. All you need to worry about is the field of view of the PIR, because that is what you will use when you install your PIRs. Most PIRs have about a 90 or 120 degree field of view. It is also possible to buy a PIR with a 360 degree field of view. This device looks like a dome, and mounts in the center of the room on the ceiling. No matter how fancy the innards, in the end the result is a pair of contacts for your panel to read.

Glass break sensors The next type of sensor that should be of interest to the reader is the glass break detector. This detector replaces the foil tape that you may have seen on the front windows of many stores. This tape was a pain to apply properly and depending on the climate, could dry out and crack. Modern glass break detectors use a microphone, an amplifier, and digital signal processing to detect breaking glass. The sound of breaking glass is picked up by the microphone, amplified, and then filtered and detected by the software of the sensor. Once again, the result is a set of contacts that open and close to indicate an alarm to your panel.