With this circuit you can detect the presence of an object from some distance away, depending on the sensitivity and the size of the object. Very handy for detecting whether someone is on the other side of a door without first having to open the door!
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You can, for example, use this circuit to detect people, but you can also use it to detect the presence or absence of the car in the garage. It doesn’t matter whether the detected objects moves or not, it only matters whether an object is within a certain distance of the sensor.