Motion Detectors

Motion detectors are one of the key sensors in most home security systems. Since the object of home security is to detect an intrusion and intrusion means the unwanted presence and movement of an intruder, it stands to reason that a motion detector is essential to any well-planned security system. In many descriptions of motion detectors, you will often notice that the detector is either active or passive. This defines the kind of detection the system will give you – or more precisely, how the detector works.

Active Sensors

An active motion detector emits sound, light, or other forms of energy into the zone and waits to detect change by reception or reflection. A photosensor, for example, is an active light sensor. It emits light across a path. Movement across its emission path triggers the sensor because the object creating the movement reflects light back into the sensor. It is this reflection that causes the active detectors to activate alarms, lights, or even (as in the case of stores) open doors.

Some of the more popular forms of active motion detectors are photosensors, radar, and ultrasonic sound sensors.

1) A photosensor, as stated above, releases light onto a certain path and waits for an object to reflect the light back into the sensor.

2) Radar uses microwaves to trigger the system. It sends out the microwave radio energy into its path and waits for an object to reflect the energy back into it.

3) An Ultrasonic sound sensor employs the same theory as the previous two but by the use of sound instead of light or microwaves. The ultrasonic sound sensor waits for an ultrasonic echo reflected off a body entering within its range.

With active sensors, there are really two applications. The first is what we have seen above: activation through reflection. The other is activation through disturbance. Most security and safety sensors base their alarm triggering on the “echo” or reflection mode, but there are some systems that require a disturbance of a constant reflection to prompt the alarm. These latter kinds have a specific pathway in which a mirror or an object acting as a mirror reflects the energy back into the system. This system is triggered when the energy path is disturbed and the energy is not reflected back.

Passive Sensors

The majority of home security motion detectors are passive. Passive sensors work on a different principle. They use infrared light as a means of measuring temperature change instead of waiting for reflection. These sensors are also called PIR (Passive Infrared) or pyroelectric sensors. Because temperature is always changing, PIR sensors are set to detect rapid temperature change. This works in the case of both intrusion and fires.

Fires obviously change temperature rapidly. In a matter of seconds and within a few inches, fires alter temperatures drastically, sometimes as much as hundreds of degrees. PIR heat detectors operate by sensing sudden temperature change – 15 º / second – or by sensing temperature change above a set temperature.

For sensing intrusions, PIR systems have to be significantly more precise. Since infrared energy is heat, it is understandable that humans emit infrared energy. But the infrared wavelengths coming from a human body is very minute – about 9 or 10 micrometers. The PIR motion sensors have to be extremely precise and sensitive to detect human presence. For this reason, PIR motion detectors are set to detect infrared wavelengths of about 8 to 12 micrometers. Because of the sensitivity demanded in the change, the sensor will not trigger an alarm when room or outdoor temperature changes due to slow environmental change, such as day to night, or season to season.

Even more astounding is the ability of some PIR sensors to detect wavelength difference between human and animals – based on size. Since bigger animals and humans emit more heat energy than smaller animals, pet-tolerant PIR sensors are designed to distinguish between humans and smaller animals (up to around 80 lbs). This neat configuration, making the system perfect for rural areas and homes with pets that wander freely, allows pets or small animals to cross the path of the sensor without activating any alarm.

Sensor Range

Most security motion detectors send out a spray of energy with a wide range of coverage. This spray of energy allows the sensor to “see” a large area. The sensor’s wide range creates a greater chance of detection and, depending on the sensor, a better chance of reflection or temperature change sensitivity.

Creating a wide range of sensitivity is made possible through the sensors lenses. The ability to focus or scatter the energy beam is based on the curvature of the lens. This scattering or focusing allows the sensor to detect motion either passively or actively in that area.

Alerting to Emergency

When the active or passive sensor has determined an environmental change, its action from that point on is fairly simple electronics. The sensor connects an electronic circuit in its system that allows the system to sound an alarm or activate lights or perform whatever activity the system is designed to do.

The most effective motion detecting systems, however, do not stop here. For better efficiency in the motion detector, it is best to connect the sensor to a monitored control panel that is programmed to inform you and the local law enforcement of an intrusion or fire department of a fire. With a monitored security system, the motion detector has the ability to sound two alarms: one for you at your home and one for a dispatcher.

You might also be interested in . . .

Security System Evolution