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The SECO-LARM E-931-S35RRQ is a NIR Reflective Photoelectric Sensor made for outdoors gates, garage doors, etc. In our case for defense or in a SHTF situation this is probably the first line of defense as it will alert you if an intruder is coming fro a further perimeter of your fortress (home)
Some of the features it boasts are Weatherproof (IP66) construction for indoor/outdoor usage; Dark ON operation, audible alert for indoors, and adjustable sensing distance