DIY S02 Window Door Contact Installation Make sure to put the magnet parallel to the sensor on the side with the two LED lights, not the side with the antenna You can orient the sensor sideways or upside down as long as the magnet is on the side of the sensor with the two LED lights On the back is a small slot that allows you to remove the back cover—here you can replace the 23A battery. Batteries should usually last 1 - 2 years. The back plate simply clicks back on. Once you have the sensor attached to the wall with the double backed adhesive, you can use a butter knife or screwdriver to remove the front part of the sensor. The backplate will remain attached to the wall. Contacts that ship now have the Fortress logo on the sensor, as shown in the image below.