Alarm Detection Techniques III - The PIR Sensor

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Psychlonic
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Alarm Detection Techniques III - The PIR Sensor

Post by Psychlonic » Sun Dec 27, 2009 6:11 am

PIR - Passive InfraRed - describes circuits which use thermal transistors with focused lenses to pick up on temperature changes. Think of it as a camera that only sees the infrared band and has a line of sight dependent on it's lens, which is chosen according to where it is intended to be placed. Instead of translating the light (heat, in this case) into imagery, it charges up a portion of it's circuit and either throws it open or clamps it shut, notifying the control panel that part of it's system is amiss.
It's quickly becoming a lower cost technology and is a standard in any building with anything worth protecting. PIR sensors are also commonly found in outdoor and indoor lighting for both convenience and security purposes. You may have seen these placed in front of walkways so that they turn the lights on when someone is approaching the door, in buildings where simply walking into a room turns the lights on, or alongside of buildings where they cover dark corners and may scare off novices.
Here are some various pictures of PIR sensors:

http://www.reuk.co.uk/shopImages/12v-pir-sensor.jpg
http://www.made-in-china.com/image/2f0j ... H-WHW-.jpg
http://www.reuk.co.uk/OtherImages/Fresn ... sensor.jpg
http://www.made-in-china.com/image/2f0j ... -3005-.jpg
http://img.alibaba.com/photo/51515633/PIR_sensor.jpg
http://diycomponents.com/E-Store/images/TLC-360.jpg

They come in all shapes and sizes. It is definitely worth your while to look further on Google for more pictures.

Notice that the lens may be either flat or convex. They may also be either darkened or semi-transparent allowing you to see the faceted lens inside. They may also have a visible LED. This light can mean several different things. This is fucking important so take notes. The most common function of this LED is to turn on when the sensor has been "violated". If you notice there is no light, and suddenly a light turns on or flashes when you get near that sensor, then you know it's on because of you. This may be your only indication that the alarm has been triggered as many of today's alarms are silent and do not utilize a horn to scare trespassers away.
You may also observe others that flash or stay on constantly. This can either mean that the system is simply armed, it could be an indicator that the sensor is a dummy, or in some cases it can be a sign that the battery for the sensor - in a wireless set up - is running low. If you're running a recon mission, learn what it's doing. If there's a company sticker somewhere, remember that too so you can look it up later and see what the usual LED function is. A lot of companies post their user manuals online so take advantage. Just remember that the LED isn't present on every sensor, and when it is, there's no 100% guarantee you can be sure you know what it's up to, but you can at least have a good idea.

As mentioned, PIR sensors see based upon the lens installed. This can often mean it has a limited viewing range where more is needed. Rather than using another sensor installed right next to it, in cases like this you'll probably run into a dual or even quad-PIR sensor. Don't let the name intimidate you, it just means that the sensor has more than one lens installed so it can cover multiple angles of the same area.
Below are photos of assorted sensors so you may become familiar with them:

http://www.alarmsystemstore.com/v/vspfi ... 600-2T.jpg
http://www.bassburglaralarms.com/images ... b12107.jpg
http://www.ness.com.au/ProductPictures/106-202.jpg


While this inherently makes the system superior to standard PIR, often the angles are just as vulnerable. Sometimes however, the lenses are observing the same angle, making it especially sensitive to heat. Use some logical thinking here. It's common to see dual-PIR sensors in the middle of a hallway rather than at one end, because one lens is able to cover one direction, while the second lens covers the other. It's safe to assume that when you see a dual-PIR sensor like this, both lenses are not covering the same area and therefore while it covers twice the area, it is not technically any more sensitive. This is also somewhat common in corners so that one lens covers one direction, the other covers the perpendicular. On the other hand, if you see a quad-PIR sensor sitting in a corner of an enclosed area, you can bet your ass that getting around it isn't going to be very fun.

As passive infrared sensors are just that, passive, they do not emit anything which can be picked up via electronics or any other detection scheme. This makes detection completely reliant on sight. Look for clues that a PIR sensor may be present. What kind of building are you approaching? Look for the stickers, look for other sensors, look for signs of money invested into the place.
PIR sensors are typically installed to cover the largest area possible, while avoiding things which are sure to set them off. You'll usually see them facing away from windows so that sudden heating or cooling of the glass from outside weather does not set the alarm off. (As you'll learn later, PIR cannot detect heat through glass. However, it can certainly detect the temperature of the glass itself) They are also typically placed as to not face heating systems that may turn on and trigger the alarm, and occasionally they may avoid designated doors although this usually isn't the case as the system will be set on a delay allowing the owner to go ahead and trip the PIR while walking towards the panel to deactivate it before signal.

What really bites most about PIR is that it's not overly complicated technology. An operative can learn all about it easily and learn the evasion and blinding techniques, but their simple effectiveness often makes these methods nearly impossible to use. It's never as easy as it seems.
In addition, PIR sensors are often paired up with a second detection scheme in what are called Dual Sensors. You will find more information on these later on.
Knowledge alone is not power, it is the potential for power. That potential can only be unlocked through applying that knowledge and realizing the skill.

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