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.
Hints and Tips
- 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.
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