Burglary is one of the most worrying types of crime to both householders and flat dwellers alike. Not only are possessions vulnerable: senseless vandalism and personal attack have become increasingly common.
Burglar alarms help to make your home secure in a number of ways: they warn you of an intruder if you are asleep at night; they alert your neighbours if a burglar strikes while you are away; their presence helps deter would-be thieves; and the sound of the alarm should frighten a burglar out of your home before he has a chance to take anything or do any damage to your property.
Types of burglar alarm
There are several types of alarm available, some of which you can fit yourself. Ultrasonic detectors are alarms which employ frequency sound pulses generated by a tiny speaker. The pulses are bounced off room surfaces and are constantly monitored by a microphone, usually housed in the same unit. Any change in the sound pattern triggers the alarm. Ultrasonic detectors are small, easy to fit, and easy to disguise from the burglar. Some can be disguised as hi-fi speakers, others can be housed in unobtrusive corners. The disadvantages of this system are that they may be triggered by such things as traffic noise or blustery wind, and they cannot really be used while you are still moving around elsewhere in the house.
Microwave detectors work on a similar principle to ultrasonic detectors, except that they use radio waves. This kind of alarm is simple to fit, is easily disguised, and very effective. It is less prone to false alarms than the ultrasonic system but, again, is of little use when the house is occupied.
Microwave alarms are a cheap and effective way to protect a fairly large, isolated property while you are away from home. They detect movement over a wide area and are difficult for the intruder to disable because there is very little vulnerable wiring and the alarm cannot be approached without triggering it.
If you consider a microwave system on the ground of cost and ease of fitting, try to select the ceiling-mounted type, which may, with adjustment, allow you to move about in the upstairs part of the house when you go to bed. However, you will not be able to come downstairs without first turning off the alarm.
At least one type of ultrasonic alarm is available in the form of a dummy hi-fi speaker, which has the further advantage of being battery powered. However, this system is fairly expensive, and lacks the flexibility that can be incorporated in an electronic loop system.
The basis of electronic loop systems is an unbroken wiring circuit, along which is strung a number of different sensors, all fixed at strategic points. There is nothing to stop you moving about the house once the system is activated.
Open circuits are completed when one of the sensors is activated. As soon as current flows, the control box triggers a bell or siren. The bell circuit is electrically latched so that it cannot be switched off except by a master key.
Closed circuit systems are kept alert by a trickle of electric current. When a sensor is activated the circuit is broken and this triggers the alarm in the same way as an open circuit.
Alarm systems of this type are available for do-it-yourself fittings. Most kits contain a control panel which can be used for both open and closed circuits. They have a basic number of sensors of different types, though additional sensors can be attached. Included in the kits are an exterior or interior ringing alarm bell and the necessary wiring to connect up the whole system. Electronic loop systems can be powered by mains or battery, or a combination of the two to guard against mains failure.
A projected beam of invisible light aligned between an emitter and a sensor, forms the system used by infrared detectors. If the infra-red beam is broken, the alarm is triggered. To increase their effectiveness, beams can be laced over an area- using a series of reflectors.
Infra-red systems are often used in high security areas in conjunction with other types of alarm. They have the advantage of being confined to a certain area, enabling free movement elsewhere in the house, but can work to the burglar’s advantage, unless you have a really sophisticated lacing of beams covering a wider area. Such systems are complex to install, and are best left to a professional security firm.
Types of sensor
Sensors are the detection devices fitted along a loop circuit to doors and windows and other possible break-in entry points. When a door is opened or a window smashed the circuit is disrupted and the alarm is activated.
There are several types of sensor on the market:
A microswitch sensor is a tiny electrical switch activated by a plunger arrangement. The plunger is held down by a closed door or window to complete a closed circuit. If that door or window is opened, the plunger is released, breaking the circuit and triggering the alarm.
A magnetic reed contact is a very simple sensor which is featured in very many alarm packages in one form or another. Current is passed through two fine magnetic reeds, sealed in a glass tube. The tube is fitted in a case, in line with the loop circuit, to the frame of a door. A similar case containing a magnet is fitted adjacent to the reed tube, on or in the door itself. The two cases are fitted parallel to each other at the opening edge of the door.usually less than 3-5mm apart. It is particu-
C. larly important that the two halves of this sensor are correctly aligned.
While the door remains closed, the magnet holds the two concealed reeds together to complete a closed circuit. If the door is then opened by more than the release distance—which varies from 10-35mm depending on the type of switch installed’—the reeds spring apart and activate the alarm.
This type of sensor should be fitted either recessed into both door and frame, or on the inside face of the doorway where it cannot be detected. Cylindrical sensors can be fitted simply by drilling holes of the appropriate size in the door and frame. Concealment is important, as an experienced burglar will certainly be able to use a magnet to keep the reeds together as the door is opened.
Metallic foil detectors are available in self-adhesive rolls for attachment to window glass. Current flows down the foil in a closed circuit and if the window is broken, this is disrupted and the alarm sounds. The foil is attached at each end to connector blocks to ensure correct electrical connection. These, too, are available backed with adhesive so they can be attached easily to the glass.
Metallic foil detectors have the advantage of advertising to the would-be intruder that your home is protected. They should be fitted to the inside of the glass, close to any window catches. A potential disadvantage is that metallic foil is rather fragile, which makes the windows more difficult to clean properly.
Vibration contacts are taking over from metallic foil as window sensors because they look neater, are easier to install, and are a better safeguard for modern, large-area windows. They are closed circuit devices contained in self-adhesive cases which can be mounted at suitable points on the glass itself. A setting screw is then adjusted to ignore ordinary low-level vibrations caused by wind and traffic. Heavy vibration breaks the circuit and the alarm sounds.
Some of the kits contain vibration contacts and most other kits offer them as an optional extra. Since wind-dows are the most obvious means of entry for the intruder and because metallic foil can look unattractive, this kind of sensor is well worth fitting.
Sound detectors are basically tiny microphones which are fitted to windows to detect the sound of breaking glass. They are very sensitive, but not normally used in domestic installations because they need additional wiring to amplify the minute amounts of current running through them.
Inertia detectors, sophisticated sensors used in high security systems, also require additional circuitry. They respond to movement in windows, doors, gates, and even walls. But these are unlikely to be used in a domestic context.
Most installations also include one or more pressure mats. These are open-circuit devices, wired up to a separate pair of contacts in the control box. An intruder stepping on the mat completes the circuit and activates the alarm.
Pressure mats must be installed against a firm floor, under both carpet and underfelt. They are usually employed as a second line of defence at the foot of the stairs, inside the front door, in front of the television set, or next to a particularly vulnerable win- dow. You may be able to cut down on wiring by using special connectors.
Panic buttons are open circuit devices linked to their own separate terminals in the control box and, usually, they can be kept constantly activated during the day. If you are attacked by an intruder, you press the button to activate the alarm.
Panic buttons are usually fixed just inside the front door but the bedroom is another likely site for such a device. There is no need to conceal the wiring, save for appearance’s sake.
Loop alarm kits
Most alarm kits come complete with a range of different sensors; they also contain wiring and instructions for linking them to the control box. This contains a number of terminals for connection of open circuits, closed — circuits, panic buttons and—sometimes—smoke detectors.
Into the control box comes electric power to maintain the closed circuits in the alert mode and to power the alarm bell itself.
Some systems rely entirely on battery power. They use very small currents to maintain the circuits, but the batteries drain rapidly once the alarm is sounded. As batteries are becoming extremely expensive, it is worth considering an alarm system which uses either tiny voltages from relatively cheap batteries, rechargeable batteries, or operates from the mains.
Some systems take their mains power from an ordinary plug socket or fixed outlet, others through a transformer , so in each case a little electrical expertise is required to fit them.
The disadvantage of a mains powered system is that it is affected by power cuts. Also, the power and alarm bell cables must both be well concealed to prevent tampering, as the easiest way to immobilize this kind of an alarm system is to disconnect either of these.
The best alarm kits use a combination of mains and battery power so that if the mains fails or is tampered with, the system switches automatically to battery power.
Regular sounding of the alarm will annoy your neighbours and tends to make them ignore the alarm if there is a real emergency. Most control boxes have facilities for testing, such as a small light to show that the circuit is working. The test provides a ready check that you have closed all the doors and windows before you go out or to bed. And for systems that rely on battery power there should be a battery life indicator as well.
All alarm systems have some kind of keyed master control switch which enables you to set the alarm and turn it off. There is usually a time delay to give you a few seconds to enter or leave the house or get away from the alarm-sensitive areas.
Some systems merely have a key switch on the control box. Others can incorporate remote switches and these may be elsewhere in the house, often by the front door. A remote shunt switch can be incorporated into some kinds of door lock, and these are wired into the alarm circuit of a closed circuit loop. Other kinds of switches are wired independently to the control panel and control the power to all the alarm circuits, whether open or closed. Many alarms incorporate both types.
Alarm systems usually make use of small bells which produce a very loud sound. Once the bell is activated it can only be turned off by the master switch key. Closing the door does not stop the alarm, because the bell circuits are electrically latched. Alarm kits may incorporate an interior bell, an exterior bell, or both.
The purpose of interior bells is to frighten the intruder out of your house, so locate these in an inaccesible spot such as at the top of the stair well. Exterior bells, which come in weatherproof cases, are intended to alert neighbours and passers-by. They should be installed high, out of reach, with the wiring to them passing —1 directly through the wall to prevent tampering.
Opinion is divided as to whether a visible exterior alarm is a deterrent or an advertisement to the would-be thief. Some authorities recommend that external bells be fitted at the back of the house. If you believe in the deterrent theory, it is possible to buy dummy alarm cases with your kit.
Most alarm kits come with a number of magnetic reed sensors, perhaps a couple of vibration sensors, and usually one or two pressure mats. They also contain wiring, one or more bells, and a control box to which additional sensors and panic buttons can be fitted. The kit illustrated is suitable for a small house.
Planning and fitting the system
When you plan your alarm system, concentrate the alarm sensors at vulnerable points such as windows and doors, and around any inside region where the burglar is likely to go—such as the hallway. Start by thoroughly surveying your ground floor and draw up a plan, noting the vulnerable areas likely. Mark out a circuit line for the closed circuit sensors which links them up in the most economical way. The actual positioning of the sensors may depend to some extent on your circuit plan.
All doors to the outside should be included in the closed loop, especially those with any areas of glass such as sliding patio doors. The latter can be fitted with a magnetic reed sensor which is shielded from the metal.
Fit sensors to some interior doors as well. Select those which a burglar would have to open to get to your valuables, assuming he had slipped in through a vulnerable window or door.
All ground floor windows are vulnerable but pay special attention to large bay windows, and windows concealed from general view at the back of the house and down alleyways. Fit either metallic tape sensors or vibration sensors.
Look also at your first floor windows. Those that are close to a porch roof or drainpipe are vulnerable and can be included in a closed circuit.
Fit pressure mats at strategic points, such as at the bottom of the stairs or in front of valuable objects. Small pressure mats are available for fitting further up the stairs.
Locate the control box in a position which is out of sight, but not so far from the door that you cannot leave the house having just activated the system. The control box is fixed to the wall, or a mounting panel, using plugs and screws.
Remember that the power cable, and sensor and alarm connections must be concealed to prevent tampering. Open circuit wiring for pressure mats can run under floor coverings.
Where wiring must cross a door, run it under the carpet, over the door frame or through a flexible door cable which most kits offer as an accessory. Power and alarm bell connections should be concealed at ground level: if possible run the wiring beneath floorboarding. It may be advisable to chase other wires in plasterwork for maximum protection.
You do not need to conceal closed circuit wiring because if anyone cuts it the alarm will sound automatically. However, its presence may betray the exact position of flush-mounted sensors in a door frame.
Finally, test the circuit according to the manufacturer’s instructions. Not all kits have facility for testing open circuits without sounding the alarm bell momentarily. The most sophisticated kits have a low volume buzzer which cuts in when testing.
In the UK you can obtain free professional advice about the installation of an alarm system from the Crime Prevention Officer at your local police station. Inform the police if you fit an alarm and give them the names of two keyholders who can switch it off.