How Does a Motion Sensor Work?

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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