How Does an Infrared Alarm Sensor Work?
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Infrared Defined
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Infrared radiation is a type of electromagnetic radiation similar to visible light but with a longer wavelength. Almost everything produces infrared light. The hotter something is, the more infrared it produces. Infrared is visible to some animals, but it can not be seen by humans without special equipment such as night vision goggles
Infrared Detection
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Most infrared cells use a Cadmium Sulfide or CdS cell to detect infrared light. CdS cells are resistors--elements that resist the flow of electricity. CdS cells are special types of resistors called photoresistors. When they are exposed to light, their resistance goes down. The CdS cell in a motion detector is set up to only detect infrared light. The circuitry inside the motion detector monitors the cell's resistance continuously and uses changes in the resistance to detect motion.
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Active Infrared
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In active infrared systems, an infrared laser shoots pulses of infrared light at the CdS cell, which is attached to a detector circuit. As the pulses of light come, the resistance of the CdS cell drops, producing a spike in current. As long as the pulses of infrared light continue to produce pulses of current, the detector keeps the alarm off. If someone steps between the infrared laser and the cell, however, it breaks the beam. The current in the detector circuit then drops, triggering the alarm.
Passive Infrared
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Passive infrared sensors also work by detecting changes in infrared but in a more subtle way. A passive infrared sensor absorbs infrared in a large arc. As the sun rises and sets and the day gets warmer and colder, the infrared signature changes gradually. If a person suddenly walks past, however, the infrared from his body creates a more drastic change in the infrared being picked up by the sensor. The sensor is programmed to ignore gradual changes,but will trigger the alarm if something changes quickly.
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