local power supplies. In addition to the system itself, security alarms often offer a monitoring service. In the event of an alarm, the premises control unit contacts a central monitoring station. Operators at the station see the signal and take appropriate action, such as contacting property owners, notifying police, or dispatching private security forces. Such alerts transmit via dedicated alarm circuits, telephone lines, or the internet in many cases. Sensor types Hermetically sealed reed switches The hermetically sealed reed switch is a common type of two-piece sensor. This switch operates with an electrically conductive reed switch that is either normally open or normally closed when under the influence of a magnetic field in respect to proximity to the second piece, which contains a magnet. When the magnet moves away from the reed switch, the reed switch either closes or opens, again based on whether the design is usually open or closed. This action coupled with an electric current (typically at 12V DC) allows an alarm control panel to detect a fault on that zone or circuit. These sensors are common, are found wired directly to an alarm control panel, or are typically found in wireless door or window contacts as sub-components. Passive infrared detectors The passive infrared (PIR) motion detector is one of the most common sensors found in household and small business environments. It offers affordable and reliable functionality. The term passive refers to the fact that the detector does not generate or radiate energy; it works entirely by detecting the heat energy given off by other objects. Strictly speaking, PIR sensors do not detect motion; instead, they identify abrupt changes in temperature at a given point. As an intruder walks in front of the sensor, the temperature at that point will rise from room temperature to body temperature, and then back again. This quick change triggers the detection. PIR sensors designed to be wall- or ceiling-mounted, come in various fields of view, from narrow-point detectors to 360-degree radar. PIRs require a power supply in addition to the detection signaling circuit. Infrasound detectors The infrasound detector works by detecting infrasound, or sound waves at frequencies below 20 hertz. Sounds at those frequencies are inaudible to the human ear. Due to its inherent properties, infrasound can travel distances of many hundreds of kilometers. Infrasound signals can result from volcanic eruptions, earthquakes, gravity waves, opening and closing of doors, and forcing