PIR Motion Sensor Made by Parallax

The motion sensor was designed using a pyroelectric sensor which is essentially a FET with a window in the case opening onto an infrared-sensitive gate.

The circuit is monitoring the changes in the drain-source resistance caused by the changes in the level of IR light with a wavelength corresponding to body heat. In the design, the sensor is placed behind a multifaceted lens that cuts the view of the area into smaller cones of heightened visibility and intervening areas of lessened visibility. A rapid change in the amount of IR from body heat shining on the sensor is caused by a body moving from an area of reduced visibility into an area of increased visibility. The motion detector circuit watches for these rapid changes and when detected, it triggers the alarm.

The Parallax sensor module has a 3-pin connector for ground, positive, and output, along with a 2-position jumper for triggering mode. The module needs a warm-up period of about a minute, during which time it is adapting to ambient conditions and may trigger randomly. The sensor could be angled toward the ground to be triggered only when feet entered an area.

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