[ Motion Detector ] electronic guidence

While normal wiring and designing a working submarine is within my knowledge (somewhat) I'm still unsure what I could use motion detectors for. After all, the most obvious would be a automatic door setup, another could be a docking sensor system that triggers when a shuttle is attempting to dock with the sub. Also it could be used as an external proximity alarm system that triggers to alert unaware crew that there is something nearby the submarine. (Weapon wise, it could possibly be wired up to a depth charge system so once it's enabled, any large creature that passes under the motion detector would deploy it for accurate bombardment.)

However it is far too advanced for me as to properly setup and construct such things. If there is someone here who has the knowledge and expertise with such skills and has already created such things, could they post as to how they designed or would design a working wired system for these ?