Passive Infrared Sensors
Passive Infrared Sensors

These sensors are among the most common devices used for detecting motion. Passive-infrared sensors detect the heat emitted by animate forms. Because all living things emit heat, a system of recording measurable changes in a specific area provides a means of detecting unauthorized intrusions.

When the unit registers changes in temperature in its area of detection, it relays the information to a processor which measures the change according to detection parameters. If the change falls outside the parameters, the processor sends a signal to the unit's alarm.

Infrared Sensor Design

These systems' design detects infrared heat emitted by living objects. They do not emit any signal or energy wave like ultrasonic sensors. The design includes a mirror or lens that captures the heat emission and transfers the emission to a solid-state chip on a circuit board which records the changes. Only a part of the chip is affected and as the subject moves, so does the "hot spot" on the chip.

The processor then records this movement and triggers the alarm. Many companies offer a range of lenses to address different situations. For example, homeowners may not want their alarms to sound when it registers a pet moving across the detection area during the middle of the night. A special lens that does not detect floor-level movement addresses this specific scenario.

You must ensure that the pet will not climb into the detection zone. However, because of the regularity of body heat and its propensity to influence measurable temperatures in the surrounding environment, these types of sensors are regarded as among the most reliable of motion detectors. As they are also smaller and cheaper than most other types of sensors, many people find them to be an attractive choice for their home security.

Units consist of two (dual) or four (quad) sensor units. The dual-sensor model projects two contiguous detection zones. This projection allows the sensor to track movement as changes occur in both detection zones. The quad-sensor model also includes a vertical element registering changes in upper and lower sections.

Different sensors may cover different zones to reduce false alarms The detection range for these sensors is not as complete as it is for active-type sensors. Detection areas radiate out from the central point where the unit is located. Thus, gaps appear between the detection areas. For this reason, many people decide to use these sensors in conjunction with other types of detectors such as microwave sensors.

The different sensors record data at different times. Usually a combined system such as these requires all systems to register abnormalities within their detection parameters. Typically, the range for models suitable for home security extend for about thirty feet from the lens. Ideally, this type of sensor should be attached to ceilings where, in theory, it provides a 360-degree coverage area. Nevertheless, sensors placed on walls may operate effectively.

In addition, some lenses allow reversible angles of 110 degrees and narrow curtain coverage.

Avoid False Infrared Motion Sensor Alarms

You should place the sensor where it will not receive inordinate amounts of heat or cold. Therefore, the sensor should not face open windows as sunlight will affect its operation or in an area where wind or air conditioning will blow on it. If part of your coverage area includes spots where it will detect temperature change, you may be able to utilize special tape provided by many companies to address that issue.

Heated pipes represent another potential source of false alarms. You should follow your manufacturer's instructions to mask areas prone to false alarms. Experts recommend several other ways to avoid false alarms.

First, your choice of locating the sensor should depend on its detection range. A sensor which has a range that greatly exceeds the distance that it is tasked to monitor may cause false alarms. You should also seal all openings to prevent infiltration by insects; this also insulates the unit from drafts.

Hanging objects, such as balloons, have also been known to cause false alarms if they move with shifts in air circulation. Some models use white-light filters which automatically exclude direct sources of light. Others use pulse-count circuits to prevent the unit from processing alarm systems unless the unit detects a heat source crosses multiple detection zones.

How Intruders Try to Trick Infrared Sensors

An intruder may avoid the field-detection pattern by adjusting his path so as to bypass the detection zones, however doing so requires prior knowledge of the pattern. It is also conceivable to cloak one's heat signature and avoid detection; however as the sensor records measurable temperature changes, cloaking requires precise adjustment.

An intruder can also deceive the system by walking directly toward the sensor instead of across it. Extremely slow movement may also not trigger the sensor. Finally, using these sensors in conjunction with other types of sensors decreases the probability of false alarms; but, it if the system is programmed to require detection by both systems, the combination also inhibits activation of the system when it may be needed.

In general, these types of sensors provide reliable detection of motion and many people find them a suitable choice for home security. You should remember to install them where they will be most effective and not susceptible to regular changes in temperature.

You should also remember that their coverage areas are not complete and should consider using them in conjunction with another type of sensor.