 |
|
|
UER Store
|
|
sweet UER decals:
|
|
|
|
Activity
|
|
890 online
Server Time:
2024-04-20 08:56:40
|
|
|
 EvilSpork
  
Location: Miami, FL
Gender: Male
Behold the power of cheese!
 |  |  |  | Motion Sensor
< on 12/30/2003 8:21 AM >
|  | | | Motion sensors are usually used to trigger an alarm, either silent or non-silent. They detect the motion of a human or other object via either infared light or microwave signal.
PIR (Passive IR) sensors
These are the most common motion sensors. They're cheap, effective, and not very prone to false triggering. You will find these on everything from motion-sensing floodlights to alarms to energy-saving light switches (the kind that time out and turn off the lights after x number of minutes). Since PIR sensors do not transmit anything (aside from a bit of red LED light when triggered!), they cannot be easily detected. However, they cannot be easily concealed - they require an optical line of sight in order to function. Watch for a little curved whitish plastic lens. For information on how to possibly defeat these sensors, see this thread, and for more information than you ever needed on how they work (as well as an alternate method of neutralization), see this post.
Microwave Detectors
The microwave detector is the sneakiest motion sensor; it can be easily concealed above a ceiling, and still work right through the ceiling tiles or plaster. You will often find these watching the path to a motorized door, used to trigger its opening mechanism. They are, however, also used as sensors on alarm systems.
(begin geek block)
The microwave sensor consists of a tuned cavity, antenna horn, Gunn diode, mixer , and detector circuit. It detects motion, not presence. Therefore, you could place a big metal trash can right in its active area, and it would cease to trigger as soon as you set it down and left it alone for a moment. The Gunn diode is basically a source of microwave noise when DC current is run through it; by placing it in a tuned cavity resonator, one desired frequency can be resonated and radiated from the cavity, whereas others are attenuated. The signal is bounced shot out the antenna horn on the front, bounces off objects in its path, and bounces back. The returned signal is amplified, and fed to a mixer along with the transmit signal. If there is a frequency difference caused by Doppler shift, a beat frequency is produced, and a detector circuit triggers the detector's output (usually a relay).
(end geek block)
Fortunately for us, the microwave detector can be detected! An inexpensive radar detector, rescued from a life of warning of speed traps, can be used to detect the signal transmitted by a microwave detector.
Infared Beams
While not really motion sensors, these will detect you moving past and breaking the beam. These are not passive devices; a transmitter outputs an infared beam using LEDs, modulated at 40 Khz. The infared beam is passed through a collimator (in this case, often, just a focusing lens and a piece of black plastic tubing), and aimed at a reflector or the receiver. The receiver has a similar collimator assembly on it to block stray light and other infared beam sources, and detects the 40 Khz modulated beam, ignoring constant sources of infared light (allowing immunity to sunlight locking the sensor on, basically). These could be hidden inside walls, but they usually are not. You will often find infared beams watching the top of a wall, in case someone should decide to climb over. I've heard of them being installed inside a utility tunnel somewhere, so watch out. No, you cannot detect them with a spray of water like in the movies... sorry. Some IR beam systems integrate the transmitter and receiver into one unit, aimed at a reflector on the opposite wall. If you come across something that resembles a truck or bicycle reflector attached to the wall in a corridor, doorway, or tunnel, you're probably looking at an infared beam detector. The type of infared beam that shines at a reflector may be possible to defeat by holding a piece of highly reflective material (such as a sheet of Scotchlite) between yourself and the transceiver, and walking behind it.
Mmmm... the sweet smell of damp concrete. <3 |
|
|
|
All content and images copyright © 2002-2024 UER.CA and respective creators. Graphical Design by Crossfire.
To contact webmaster, or click to email with problems or other questions about this site:
UER CONTACT
View Terms of Service |
View Privacy Policy |
Server colocation provided by Beanfield
This page was generated for you in 78 milliseconds. Since June 23, 2002, a total of 738584124 pages have been generated.
|
|
