Digital Home Security System, Do all it yourself (Detectors, Glass break, Panic buttons)

Choosing the right detectors

There are numerous detection devices available and choosing the right one to protect a particular area isn’t always easy. There is a whole range of circumstances to take into account — the size of the room, what’s in it, even how it is constructed, as well as what’s outside or even across the road.

You may need one or more types of detectors in any one area — for example, there are bound to be doors and windows to protect as well as the room itself. There may also be very large areas of glass, such as a patio door, draughts or heating, which may make it difficult to site certain types of detectors, and you may have pets in the house which will mean careful positioning and directing of the detection pattern.

Magnetic contacts

The most basic detection device is a magnetic contact (or reed switch). The reed switch is attached to the fixed section of an opening (i.e. frame of a door or window) and is held in the closed position by a magnet fitted to the moving part. When the magnetic force is removed by the door or window being pushed ajar, the switch opens, cutting off the current (if the alarm is set) and triggering the alarm.

Contacts should be positioned carefully, ideally at the top of the door, with the contact flush mounted in the architrave a few inches in from the catch side of the door. Surface mounted contacts, easier to fit, are often provided with DIY kits. One is sufficient for each opening but it is important they are installed on well fitted and sound frames or you may end up with false alarms.

Detecting shock

Shock sensors come in a variety of forms — inertia, piezo electric and vibration. These are designed specifically to detect force and sounds like splintering wood in the area that is being protected. Once again, it is very important to position these correctly.

Pinpoint the most obvious point of attack, where the intruder may gain access with the least noise; on a window this is likely to be close to the handle where the intruder may try to lever the window open. The sensor should be mounted on the window frame and set at a sensitivity level where it will not trigger the alarm other than when a real attack is detected. You should also check that other sources of vibration, like a nearby road or railway line, will not affect the device.

Independent tests on a correctly fitted vibration sensor showed that it didn’t react when the window was tapped, a door knocked or a tennis ball thrown against an adjacent partition. Even a masonry drill used on an external wall about a metre away did not activate the alarm.

Glass break

The problem with glass break detectors is that not all types will work effectively on double-glazing. There are basically two methods. Self adhesive foil strips or tape carrying a current can be applied to the glass. Breaking the tape triggers the alarm (although even a hairline crack can produce this and cause false alarms).

A single strip should be applied no nearer than 30cm (12″) to the edge of the glass; the glass is held firmly and any nearer may not shear the foil. If a rectangular pattern, rather than a single strip, is used, then it must be applied no less than 50mm (2″) and no more than 10cm (4″) in from the edge. It also has to be varnished to insulate it from moisture.

The other method is based on audio (ultrasonic) technology and enables the detector to recognise the particular frequency of breaking glass. Here it is important not to position a detector close to air vents or doors where the noise of glass breaking outside may be picked up, or placed near a telephone.

Laminated glass may also reduce the effectiveness of the device. However, one system is able to measure the flexing effect of a blow on glass as well as the frequency it generates, so glass breaking outside should not create an alarm. These units can also be used to protect several windows at a time, rather than having to fit a device to each window separately.

Panic buttons

A personal or panic attack (PA) button has to be activated deliberately by a person in the house. Usually it’s by pressing a hand button, although it is possible to obtain adapted units which can be activated with a knee or foot (often used by retailers, banks and similar premises or by the disabled). The unit activates the alarm via a special 24 hour circuit even when the alarm has not been set and can only be turned off and reset with a special key. PA buttons should be sited as discreetly as possible and are often positioned just inside the front door and by the bed.

Pressure mats

These aren’t widely used today but are often included in DIY kits. Available in various sizes, they are suitable for protecting staircases (one may be positioned at the top or bottom of the stairs), immediately inside patio doors or other possible points of forced entry. Pressure mats incorporate an ‘open’ circuit which is completed when the intruder steps on the mat. It is necessary to have the underlay of the carpet cut away to avoid the mat showing — an expensive measure if you have to employ a carpet fitter to do it for you.

You should be careful not to place furniture on them by mistake or to install them on uneven floors or on top of carpet tacks or other sharp objects which could pierce the plastic covered mat. They are prone to wear, which could result in a false alarm, and if placed under a thin carpet may eventually show.

Passive infra-reds

The most popular of the movement detectors is the passive infra-red detector (PIR). PIRs look for a change in energy — the infra-red produced by humans — and trigger an alarm if the source of the energy (body heat) moves across a fan-shaped series of zones. In older PIRs, strong sunlight could trigger the detector but most now incorporate a dual sensor system which has zones on two levels so, if the source of heat is static, it should be ignored. Only if the heat source moves across the PIR‘ s range should an alarm be triggered.

PIRs are supplied in a choice of ranges — long, wide, corridor and curtain — and can cover as little as 3m (10′) up to as much as 40m (about 133′). The long and corridor lenses have fewer zones across a narrow width but can detect movement as far as 30 or 40m away. The curtain type is suitable for protecting doors or windows and particularly large areas of glass. The pattern is a vertical one running parallel with the area to be protected and creates a solid protected zone. These are also useful if the house is open plan with a mezzanine floor for the bedrooms. The pattern would be able to detect anyone jumping off the mezzanine or a break-in through the ceiling from a loft.

Pet alley patterns allow an area at ground level for pets to walk around safely without triggering the alarm. However, this means that an intruder crawling by would not be detected and it also doesn’t prevent pets jumping on to furniture and setting off the alarm. The only way really to avoid this risk is to keep dogs and cats, and any other warm-blooded animals likely to roam, confined to one room where a movement detector is not in use. A room protected by contacts or shock detectors would also be secure from pet false alarms.

Some PIR detector manufacturers now supply masks with a standard scanning unit which enables the coverage pattern to be modified to prevent the PIR from seeing ‘hot spots’. You can adapt them to different circumstances rather than having to buy completely different units.

PIRs are susceptible to moisture — ensure that those used to control exterior lighting, for example, are IP rated. This is a grading system which shows the level of resistance to dirt and moisture; they should be at least IP44 to IP55. Insects, too, can cause havoc if they get into the casing or even walk across the sensor if the optics aren’t sealed. PIRs should also be resistant to radio frequency interference (RFI).

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