Motion sensors have become a staple among many home security
systems. Most of us are familiar with motion detectors, as they allow us to dry
our hands without pressing a button or enter a store without touching the door.
But how do they work?
There are two types of motion sensors: active and passive. Active
sensors inject energy into the environment to detect a change. When you step
into this field of energy as you approach the grocery store, a sensor notices
that the amount of reflected energy has changed, and uses this signal to open
the door. Active sensors can also work
with sound, as they emit pulses of sound higher than humans can hear and read
the resulting echoes to detect a change in movement.
Passive sensors detect body heat, or infrared energy. When
it senses a change of heat within its field of vision, the sensor signals the
alarm to trigger. To avoid false alarms, passive sensors are sensitive to the temperature
of the human body (which equals infrared energy with a wavelength between 8 and
12 micrometers).
Often, motion sensor alarms will
have more than one type of detection device. This helps to avoid false alarms.
Today, most motion sensor alarm systems are wireless and include other
features, such as pet immunity (ignoring moving figures that are up to a
certain weight).
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