active ir motion detector

Posted on Feb 6, 2014

Circuit shows the operation of an ultrasonic motion detector. There are two transducers, one emits an ultrasonic wave and the other picks up reflections from the different objects in the area. The reflected waves arrive at the receiver in constant phase if none of the objects in the area are moving. If something moves, the received signal is shifted in phase.

active ir motion detector
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A phase comparator detects the shifted phase and sends a triggering pulse to the alarm. Ultrasonic motion detectors have certain advantages and disadvantages when compared with other types of motion detectors. The main advantages is that they are very sensitive and extremely fast acting. However, the largest problem with this type of motion detector is that it sometimes responds to normal environmental vibration that can be caused by a passing car or a plane overhead. Some types of motion detectors use infrared sensors to avoid this problem, but even these detectors have some problems. In passive infrared motion detectors, a sensor containing an infrared-sensitive phototransistor is placed in the area to be protected. Circuitry within the sensor detects the infrared radiation emitted by the intruder`s body and triggers the alarm. The problem with using this type of detector is that it can be falsely triggered by warm air movement or other disturbances that can alter the infrared radiation levels in an area. In order to prevent this problem, newer systems use tw1o infrared sensors which monitor different zones within a protected area. Logic within system triggers the alarm only when the tw1o zones are activated in sequence, as would occur if a person walked through the protected area. Figure 2 shows the operation of an active infrared motion detector. In the active system each sensor consists of tw1o housings. One housing...

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