The home security market offers a huge range of burglar alarm systems; some are suitable for DIY installation, but others require fitting by professional installers. There are two basic types: one sounds an alarm if someone tries to break into your home, while the other detects any intruders inside.

When you buy an alarm system, make sure that the alarm is audible over a fair distance. Anything with a sound level below 95 decibels won't be heard, especially if there is heavy traffic noise. Tell your neighbours you are installing an alarm and make sure you choose one with a bell that stops after about 20 minutes so you don't keep them awake all night.

Before you invest in an alarm system, make a thorough check of the security of the doors, windows and other possible access points to your home. Add lights to dark corners at the front and back door; fit window locks and good-quality door locks and make sure french windows and patio doors are also secure. Take advice from your local Crime Prevention Officer, and join (or even form) a Neighbourhood Watch Scheme.

Break-in Alarms

The basic elements of the 'break-in' type of alarm are the sensors, the control unit, and the alarm bell, siren or buzzer. The alarm bell itself is fitted onto an outside wall - in a very prominent position so it acts as a deterrent to burglars. In areas of high crime, it's best to fit an alarm that is also set off if a burglar tries to remove the cover or cut the wiring circuit.

The control box is the 'brains of the unit: when a signal is received from one of the sensors, it sets off the audible alarm. Sensors can be magnetic window and door switches: movement of the magnet operates a switch, which sends an electrical signal to the control box, which in turn activates the alarm. The power supply can be either mains or battery - and some mains-operated systems have a battery-powered back-up, so that if power is cut off accidentally or deliberately, the battery takes over.

In most cases the alarm system is set with a key or a code number, which is input on a push button panel. If you live in a small flat or have a room in which precious items are located, there are also small, single-box devices available, which will set off an audible alarm by the movement of a door, window or by the sound of a break-in. There are also battery-operated door alarms with delay switches: these give legitimate occupants time to enter and leave a room without triggering it.

Internal Alarms

The simplest DIY form of internal alarm is a string of empty tin cans left in a strategic position. Though not guaranteed to work, they are about as effective as some sensors, such as pressure pads placed under a mat or carpet. Not only do sensor pads get worn with age and become unreliable, but professional intruders will simply avoid walking where they suspect they might be located. A further disadvantage of a sensor-pad system is that the alarm can easily be set off by household pets.

Ultrasonic and infrared systems are far more sophisticated types of internal alarms. Ultrasonic alarms send out sound waves that are bounced back to them: anything interrupting the signal - a door opening, or a person moving in the room - will automatically trigger the alarm. An infrared alarm is triggered by an intruder's body heat, yet this type of alarm system is not affected by slow-warming heating systems.

Panic buttons can also be fitted at bedsides: these are particularly useful for the elderly and infirm. These operate an alarm even if the system is not switched on, so they are useful during the daytime as well.

It can be useful to choose a system that lets you separate your house into 'zones' so that, for example, you can have the alarm switched on for the ground-floor at night, but it won't be triggered by upstairs occupants making trips to the bathroom.

• Professionally installed systems should comply with British Standards BS 4737 (wired systems) and BS 6799 (wireless systems). Alarms suitable for DIY installation should comply with BS 6707.
• Inform the police if you are having an alarm installed and supply them details of two key holders who can give them access in your absence.
• It is possible to have your alarm system monitored by a company who will inform the police as soon as it goes off. Otherwise, they will not respond unless they are alerted by a neighbour.