Copyright 2009 Jonathan Oxer Copyright 2009 Hugh Blemings
| +---------------------------------------------------------------------+ | This project is featured in the book "Practical Arduino" by | Jonathan Oxer and Hugh Blemings (Apress, 2009). More information | about the book and this project is available at: | ||
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| www.practicalarduino.com/projects/security-sensors |
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Security system sensors such as motion detectors, reed switches, pressure mats, glass-break detectors, infrared beams, and conductive film, can be very handy for all sorts of things including home automation systems, interactive art installations, and even sometimes even security systems!
Most security sensors provide a “normally closed” (or N.C.) output: that is, when they have not been tripped their output is closed-circuit, and when it has been tripped it goes open-circuit. This is the exact opposite behavior of something like a simple push button switch, which is normally open-circuit and then goes closed-circuit when you press it.
To guard against malicious tampering with the cabling, a security system needs to detect far more than a simple open or closed circuit. It needs to be able to detect if the wire to a sensor has been cut, or a wire short-circuited, or the sensor has been tripped. It also needs to detect if the sensor is being tampered with, even when the alarm system itself is in a disarmed state. Well designed security systems treat all parts of the system as untrusted and can detect tampering in nearly any cable or sensor at any time, whether it is currently armed or disarmed.