Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: TUESDAY, September 5, 1995                   TAG: 9509070005
SECTION: EXTRA                    PAGE: 3   EDITION: METRO 
SOURCE: DAVID BUTLER
DATELINE:                                 LENGTH: Medium

HIGH-TECH SENSORS HELP TO ELIMINATE FALSE ALARMS

A: During the past decade, technology has dramatically transformed the security industry. The most visible evidence is the intelligent wall station - endowed with easy-to-use menus and on-screen help. Less obvious but no less dramatic have been improvements to intrusion sensors. The result: Today's security systems are more flexible and far more reliable than those produced only a few years ago.

False alarms have long been the bane of the security industry. Nationwide, more than 90 percent of the alarm reports are without merit, giving monitored security a black eye. However, a large percentage of false alarms is caused by a small percentage of systems, prompting many cities to impose stiff fines on chronic offenders.

False alarms can result from improper selection or placement of intrusion sensors (user error is usually to blame). For example, as a house settles, door frames can shift enough to trigger a recessed magnetic contact. Wide-gap versions are now available that will maintain contact up to 3/4-inch. Ultra-wide-gap sensors make it possible to reliably protect overhead garage doors.

It's wise to supplement magnetic contacts with a glass-break detector. If an intruder suspects that a window is wired, he can defeat an alarm simply by breaking out the glass and entering through the unopened window. One unit can cover a room full of windows.

Early glass-break units tended to respond to benign household sounds such as the rattle of keys. The latest models avoid false alarms through the use of sophisticated signal processing techniques. One vendor offers a combination glass-break detector and magnetic contact - an ideal sensor for smaller rooms.

However, it is the motion detector that's most often associated with false alarms.

Passive infrared (PIR) technology, used by nearly all residential motion detectors, senses sudden temperature changes within its range - as when a person walks by. Thus sunlight, radiators, heat registers and even certain radio waves can cause false alarms. As with glass-break detectors, a signal processor can help the motion detector filter out background noise.

Pulse counting is the most common filtering technique. Since the detector's field of vision is divided into distinct "fingers," a person moving through the field generates multiple pulses. Background heat sources do not. Some models avoid radio wave interference by tracking the polarity (direction) of the pulse or by employing multiple sensor elements.

In difficult environments, a dual technology detector may be necessary (PIR plus microwave). Although microwave does a good job of detecting motion, it can't distinguish inanimate from animate objects. For example, vibrations from nearby thunder can trigger a microwave detector. Only a person (or animal) can trigger a dual-tech unit. If you have house pets, you can add a special lens that restricts the detector's vision to the upper portion of the room.

Fortunately, today's state-of-the-art security sensors are nearly always wiring-compatible with older versions. This makes it a relatively simple task to replace a problem unit that's been causing false alarms.

To receive a list of companies that manufacture the latest generation of security sensors, please send $1.50 (please don't send cash) and a self-addressed envelope to David Butler F-528, Department TWN, 14713 Pleasant Hill Road, Charlotte, N.C. 28278-7927. The list includes toll-free phone numbers and a summary of each company's products.

Q: I use motion detectors to track room occupancy. However, if someone sits still for too long, my automation system dutifully turns out the lights and sets back the temperature. Any suggestions?

A: Occupancy detection can be tricky. You could try lengthening the delay before taking action. Or you could install a ceiling-mount "curtain" PIR above the door. Your automation system could then count "incidents" to determine occupancy.