Arduino sketch to read the status of security sensors connected via EOL (end-of-line) circuitry
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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:                    |
|                                                                     |
|                  |

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.