Magnetic contact detectors belong to the essential devices of perimeter protection. They are used to protect doors, windows, etc., by triggering alarm when they are open. The S-3 detector is designed for flush-mounting, i.e. its elements are to be put in place by screwing their aluminum enclosures into properly prepared holes in wooden or plastic door frame, window frame, etc. S-3 consists of two watertight parts: the reed switch (magnetic) sensor and the magnet, whose faces should contact each other. Moving one part away from the other breaks the sensor circuit, which is indicated as a violation. Additionally, the detector is provided with a tamper loop.