How do we detect intruders? With intruder detectors of course…

Jan 15, 2021

Opportunists, prowlers, smashers, or professionals. There are many types of intruders, but they all have one thing in common and that is that they most definitely should not be gaining access to your business, and they need to be detected immediately.

So how do you detect an intruder? Simple, you install intruder detectors (as part of an alarm system). Here’s our guide on the most common types of intruder detectors that you should consider when installing an alarm system to protect your business.

Motion Detectors

Easily the most popular and widely used form of intruder detector available, a motion sensor does exactly what you expect it to – it senses motion! Motion sensors can be deployed in almost any environment and can cover reasonably large areas and at relatively low cost.

But (and this is a big but) they must be carefully used as the wrong type of motion sensor installed in the wrong type of environment can quickly become a frustrating cause of nuisance false alarms. Or worst still, they can become completely ineffective at detecting intruders!

Most motion sensors use passive infrared (PIR) technology to measure infrared light radiating from objects in its field of view and detects changes in the amount of infrared radiation impinging on it. To put this is layman terms, they detect heat, so if an intruder passes a motion senor, the sensor detects a change in the reflected infrared heat and triggers an alarm. Oh, if only it was that simple…

You see, it’s not only passing intruders that reflect infrared: almost anything that moves does, plus some things that don’t move such as heaters or aircon units or the sun and car headlights in windows. There is an absolute plethora of things that can cause PIR motion sensors to false alarm, that’s why the use of them needs to always be carefully considered.

To start with, when it comes to motion detectors you get what you pay for: low-cost sensors use inferior technology and have an increased risk of false alarms. The more costly sensors use superior mirror-optic lenses and are much more stable in harsh environments.

Also, careful consideration needs to be taken to the location of the sensor. Normally, motion sensors should be installed to face into a building, as opposed to facing out towards windows – a big source of infrared. And because the P in PIR stands for passive, the sensors don’t emit anything into the surrounding environment; instead, they view the environment in a beam pattern. And if the sensor is not installed correctly an intruder can effectively walk in between these ‘beams’ and avoid detection.

There’s a wide range of different types of sensor technologies that can be deployed to increase accuracy and reduce false alarms, such as the addition of microwave (no, not the kitchen appliance, but a doppler radar (don’t ask!). Or technologies such as anti-mask, anti-creep, anti-tamper, anti-pet, etc. But the principles remain the same: the right motion sensor installed in the right environment is the starting point for a reliable and robust intruder alarm system.

Door Sensors

Invented in 1922, the humble door sensor is the oldest form of intruder detection. Consisting of an electrical reed switch housed in a glass envelope and fitted to the frame of a door with a magnet installed onto the door itself, so when the door is opened the magnetic field is removed from the reed switch, breaking the electrical circuit, and activating the alarm. Simple!

It’s this simplicity that makes reed switch door sensors so immune to false alarms and the cheapest form of intruder detector available. And with a wide range of types available to suit swing or sliding doors, roller doors, display cabinets or windows, surface mounted or concealed, they’re also pretty flexible.

But they’re not without their downsides: for starters, intruders are notorious for not using conventional methods of entry; they don’t always let themselves in through the front door. So, if you’re relying on door sensors as your only form of detection you could be leaving yourself exposed. And the simplicity of a reed switch is also its greatest vulnerability, allowing them to be compromised and bypassed by any intruder with even the most rudimentary understanding of electromagnetic circuits (part of the syllabus at the school of hard knocks, which most career criminals attend).

That said, we use them to great effect as a secondary form of detection to verify an alarm event: by installing a motion sensor on the other side of a door, if an intruder forces entry the door sensor triggers first, followed by the motion sensor. It’s these two events that we refer to as a verified alarm activation and allows the alarm monitoring station to provide a more escalated and appropriate response compared to if just one sensor was activated.

Door sensors can also used to great effect to monitor doors even when the building is occupied: for example, fire exits that must remain unlocked for safety, but need to be monitored for security. Or for ensuring access control doors are closed properly after they have been used and have not been propped open.

So, while they may be unsophisticated and use technology that is nearly a century old, the modest door sensor should not be overlooked when considering protecting your building with an intruder alarm system.

Beam Sensors

Ok, we’re getting to the juicier stuff now. Beam sensors, or more technically referred to as photo-electric (PE) beam sensors are a bit like what you’ve seen in the movies, minus the red laser like beam or any abseiling from the ceiling a la Tom Cruise in Mission Impossible.

It’s a point-to-point sensor that transmits a light beam (of sorts) from one end and is then received at the other end. If the beam is blocked, the sensor activates, and the alarm is triggered

One of the greatest advantages of beam sensors is that they can cover very large areas; commonly up to 200 metres outdoors or 400 metres when installed indoors in a more stable environment, and with a relatively low false alarm rate too. This makes them ideal for monitoring perimeters of buildings or boundary and fence lines.

Again though, as with all types of intruder detectors they do have their drawbacks, especially when used outdoors. Wildlife, moving vegetation and weather events can all be causes of false alarms. And their simple point-to-point nature means they can be bypassed by intruders on foot (not so with intruders in vehicles).

However, if you’re looking to detect intruders in large open spaces and at relatively low-cost, beam sensors are a great option. Also, having been a mainstay in Hollywood, we think they happen to be pretty cool too!

Glass Break Detectors

We’ve already established that intruders tend not to use conventional methods to gain entry to a building, such as entering through the front door, and a more preferred route tends to involve a brick and a window. This is where glass break detectors come into their own: detecting the sounds and vibrations of shattering glass and triggering an alarm.

But as with door sensors, they’re not infallible and should only be used alongside a secondary form of detection such as a motion sensor. For example, an intruder forcing a window open instead of smashing the glass may not be enough to trigger an alarm. Or curtains and drapes pulled across a window may be enough to muffle the sounds of breaking glass, rendering the sensor ineffective.

However, for that extra layer of protection, glass break detectors are a great method of detecting intruders forcing their way into a building and have a very low false alarm rate.

This is by no means a comprehensive list of the different types of intruder detectors that are available, it’s just a starting point when considering installing an alarm systems. There’s a huge range of sensors available for almost any application and if you’re looking for advice on how to protect your business, just give us a call on 1300 685 504.

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