THE effectiveness of home burglar alarms is still open to debate. Whether an alarm system works to deter break-ins largely depends on the system itself. It seems that an alarm system with lights and sirens may not be as effective as one that is connected to a law enforcement agency.

Burglars themselves give alarms mixed reviews. Most say they may enter a house with an alarm, but they will retreat if they trip it. Some systems only have internal alarms, that is sirens and lights that can only be heard and seen inside the house. This may be enough to initially scare a burglar out of the house. Once outside he will discover that no one in the neighbor hood can hear or see the alarm and he may return to disable the alarm and resume the heist.

Systems with external alarms are a little better, provided you live in an area with vigilant neighbors who will summon the police if they hear the alarm. These systems should have an automatic cutoff circuit. This will prevent the device from sounding indefinitely if you are away for an extended period. Without the cutoff, the alarm will continue to sound and most likely the neighbors will summon the police to silence it. The police will then use any means available to disable the system to restore peace to the area. An ideal cutoff circuit not only silences the alarm, it also resets the system to normal operating mode.

The best alarm systems are connected to a law enforcement agency. The most elaborate and effective systems are connected to the central office -- either through a radio transmitter or private telephone line -- of a private guard service. When the alarm trips an officer at the office immediately telephones your residence. When you answer, you must give a prearranged code phrase to verify a false alarm. If you give the wrong code, or do not answer the phone he immediately dispatches his guards and notifies the local police.

This type of alarm system is reliable and very sophisticated; it must be installed by technicians employed by the guard service. This will ensure that the system is wired correctly and that it will not trigger false alarms. The main drawback to this type of system is that it is very expensive. In addition to the installation and equipment charge, there is also a monthly service charge. The average homeowner usually cannot afford this type of alarm unless he can write it off as a business expense.

Similar to this is an alarm system that is connected to a local police station. When the alarm is tripped, a transmitter -- there is a small service charge for the rental of this device -- sends a signal to a display panel in the local police station. This system is only effective if the local police are efficient. Small rural police stations may have a display panel that is too small to accept many residential alarm signals. In addition, many police departments are understaffed and cannot respond to every alarm in a timely manner. And keep in mind that most communities charge homeowners if the police have to respond to a false alarm.

Burglary alarm systems can be divided into two broad categories: active and passive. Active systems protect an area with some sort of motion detector. In most units, beams of heat-sensing infrared rays or inaudible ultrasonic sound waves are projected across a room. These waves bounce off the walls and are picked up by the unit. As long as the objects within the room remain stationary, the reflected beam remains the same as the broadcast beam. However, a moving object crossing through the room path will deflect the beam and trigger the alarm.

These systems are called modular systems because they are packaged as a single unit. They are easy to install. Usually, it is only necessary to hang the motion detector and plug the control module into a nearby outlet. There is no need for elaborate wiring to entry points. In most cases the module will only work for one room, so it necessary to buy a number of units to protect an entire house.

If modular systems are easy to install, they are also easy to disable and remove, and this is one of the drawbacks to this type of unit. There are cases reported where burglars entered a house, disabled the alarm and stole it along with the contents of the house.

Another drawback to active systems is that they are easily tripped and often give a false alarms. This is because the motion detector can be triggered by any moving object, a person, a pet or even window drapes swaying in a breeze. Some beams will penetrate window glass so the alarm may be triggered by an outside object, like a bush moved by the wind.

In addition, once you turn the unit on, you cannot enter a protected area without setting off the alarm. So if you activate the alarm when you go to bed at night, you must remember to deactivate it when you get up in morning. You must also confine pets to rooms that are not protected by the system or else they will trigger the alarm.

The other type of alarm system is called a passive system. With this type of alarm, contacts are installed on all entry points, doors and windows, throughout the house. These are connected by wires to a central control panel. When the system is turned on the, alarm can only be set off by disturbing the contacts, that is by opening a door or window. Alarms can either be internal or external signals.