Alarm and Sensor Technologies

Overview of Alarm Technologies

The point of physical security measures is to provide a complete triangle of Deterrence, Detection, and Delay, ultimately leading to Response from the site personnel, police, etc.
Alarm technology exists to augment humans in meeting these goals. The typical alarm system includes overlapping sensors to check for door/open closed status, glass breakage, motion, and more. These sensors are electrically "supervised" by a microcontroller and monitored constantly for changes in state. Multiple sensors may be placed in series to form a "loop."
A proper implementation will include a series resistor somewhere in the middle of each loop, a resistor at the end of each sensor loop, and the ability to check voltage or resistance at the alarm panel. Sensor zones are always wired up as "normally closed" circuits.
This allows up to 4 states to be identified:

• Sensor activated (i.e. door opened or motion detector sensing motion)
• Sensor not activated (i.e. door closed)
• Open Circuit (i.e. wiring fault or cut)
• Shorted Circuit (i.e. deliberate shorting or bypass of a sensor)

Sensors

• Passive Infrared (PIR)

This type of sensor is used to protect inside and outside spaces by watching for changes in radiated IR.

• • Does a good job of detecting any type of motion by warm bodies
  • Inexpensive and widely available
  • Is vulnerable to false-alarms or deliberate attempts to set it off
  • Wikipedia article on PIRs
  • EEVblog teardown of a PIR