The only extrinsic problem of note that had to be considered at the conceptual design stage arose from the circuits subsystem. The design of the sensors for the device required first that the sensor be in close proximity to the test units, and second that the each test unit be optically isolated from all other units. Together, these requirements led to the design of the second major electromechanical unit in the final product. This unit, the sensor array, had to be held above the level of the tray so that the tray could be inserted, and lowered at the beginning of operation of the machine to isolate the test units and bring the sensors close enough to detect light from the units.