The intruder alarm detection devices CIA use

On this page we detail the various devices we install with our intruder alarms, from movement sensors – through magnetic reed contacts and vibration detectors – to break glass detectors, and explain a little about how they all work and good system design practices.

The movement detectors CIA use

Like all the equipment we use in our installations, Christie Intruder Alarms only ever uses the very best quality movement detectors.

This is important because poor quality movement detectors are credited as the second most common cause for a system to false alarm.  (The first is user error.)

A persistently false alarming system will not only annoy your neighbours but alert burglars to the potential of easy pickings, and if your alarm is monitored, you will lose police response after three false alarms in any rolling 12 month period.

CIA’s Research & Development department continually assesses the market to ensure that we remain at the forefront of movement detector technology for the lowest price, to protect you and our reputation as the installer with one of the lowest false alarm activation rates in the area.

Today’s movement detector types

These days there are really only two viable types of movement detector to consider. The passive infra-red sensor (PIR) and dual technology sensor (Dual-tec).

PIRs detect changes in the room’s ambient temperature through infra red sensors.  When temperature changes are detected in several different sections of the infra red at once, the sensor’s electronics determines movement and trips the alarm.  CIA only uses PIRs with mirror optics which accurately divide the beams and increases stability still further.

Dual-tec, as the name suggests, is a combination of two technologies – a PIR, and a microwave sensor which detects motion through the principle of Doppler radar – and both technologies must detect movement before it trips the alarm.

Correct design and installation to avoid false alarms

Although high quality movement detectors minimise false alarms, good system design and mindful choice and placement of the right motion detector for the specific location is also of paramount importance.  There is a danger, otherwise, that even high quality movement detectors will false alarm.

And this is where CIA’s expertise comes in.  Most of our sales team have come from an engineering background and know what to look for.  They know that PIRs can trip because of air convection, from the sun heating windows or from reflections off shiny objects, and that Dual-tec’s microwave sensors react to movement through glass and thin walls and even by water moving in pipes.  So they will ensure that the detectors are located in an optimum position to avoid false alarm potential, while our trained intruder alarm engineers will make modifications during installation – such as masking some mirror optics, perhaps around a heat source like a wood burner – to increase the detectors’ stability.

Magnetic reed switches – door or window “contacts”

A magnetic reed switch, commonly referred to as a “contact”, is a normally–closed switch that completes an electrical circuit of an alarm system, when a magnet (usually fitted in the opening portion of the door, window or hatch) is in contact with the switch (usually fitted in the frame) signifying that it remains closed. When the alarm system is set (active), if the door, window or hatch is opened the alarm system is triggered.

Door contacts are also the most commonly used means of completing the exit sequence and setting a system – programmed as a “final exit” door, or (despite the name final exit) starting the entry sequence when un-setting a system.

If a final exit door is the first detection device to change state (from closed to open), the entry sequence begins and a pre-determined amount of time allows the detected person to reach the control keypad to unset the alarm, and any detection devices fitted along the “exit route” are programmed as such and will not trigger the alarm within this timeframe.

Vibration detectors

These can be fitted on walls, ceilings and door (or window) frames and detect vibrations associated with shocks (drilling would normally cause similar vibrations).

Although piezo electronics have replaced the old mechanical inertia sensors (where a ball sits in a cradle completing a circuit, and shocks cause the ball to jump breaking the circuit for a brief moment) and piezos allow for sensitivity adjustment, vibration detectors are still overly susceptible to false alarms from thunder and lightning, heavy traffic, earthquakes (like the Ramsgate earthquake 22 May 2015) along with many other meteorological or otherwise random events.

Because of this we are reluctant to use these unless the situation categorically demands it.

Breaking glass detectors

Break glass detectors sense the range of acoustic frequencies associated with breaking glass.  These are usually mounted on the ceiling or wall adjacent to glass panes being protected.

Two outdated ways of protecting windows are:

Magnetic foil – a method that involved varnishing a thin strip of conducting foil to the inside of the glass which would reliably trigger the alarm even if the glass was only cracked

Tube-and-wire-frame – whereby an unsophisticated frame of wood and aluminium tubes (resembling bars) with wire stretched inside the tubes tightly enough that if the tubes were bowed the alarm would be triggered.

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