Sensor (detail)

Top of the sensor assembly.
Each LED illuminates the sensor directly opposite it through a slot in the cap. Each sensor is a pair of phototransistors. The difference in light intensity seen by the detectors in each pair indicates where the slot is relative to the line between the LED and the detectors. There are three horizontal and three vertical slots in the cap, allowing the orientation and position of the cap to be calculated in three dimensions.

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