What Is a Passive Alarm?

Passive Motion Detecting Alarm Sensors

• Passive motion-detecting alarms sense movement in the protected area. Pets and other animals can inadvertently trigger the alarm although some manufacturers have developed thresholds in their motion-detector software to eliminate the problem.

Passive Infrared Alarms

• Infrared alarms sense changes in the temperature of the protected area. Infrared sensors "see" images based on thermal radiation. A human body, for example, is warmer than the surrounding materials and could be detected by a passive infrared alarm.

Infrared Detection of People

• According to the website faraonline.org, the human body produces 7 to 14 microns of infrared energy. Infrared sensors are programmed to trigger when a heat signature in that range is detected.

After the Passive Alarm is Triggered

• If the passive alarm sensor detects a person's heat signature or motion in the protected area, it returns a signal to the alarm's central unit. Depending on the system, alarms may sound or an alert may be signaled to law-enforcement personnel.

Preventing False Alarms

• According to the Scottsdale, Arizona, website, understanding how the alarm sensors work is important to preventing false alarms. False alarms are irritating to the property owner and can pose a cost in time and manpower to law-enforcement agencies.