SimpliSafe Entry Sensor (Pack of 4) - Window and Door Security System With this SimpliSafe Entry Sensor (Pack of 4) - Window and Door Security System ($50), you will know when someone is entering your home or opening a window. You can place it on any door or window, and the sensor magnet will work to keep you protected. Image Source: amazon.com SimpliSafe Entry Sensor (Pack of 4) - Window and Door Security System $50 from amazon.com Buy Now