Clean the surface of the door inside the structure, including the doorjamb and the side of the door, with a pH-neutral cleanser and a sponge, removing any dirt or debris that may interfere with the ADT door sensor’s adhesive. Clean a vertical stripe down the wall extending about 4 inches from the frame of the door on the side opposite the door hinges for placement of the main alarm unit.
Locate a wall stud near the doorframe on the section of cleaned wall. Pass a stud finder over the wall and wait for an indication of the stud’s presence. Depending on stud finder manufacturer, indicators can be either a tone or a light on the unit. Check manufacturer instructions to determine which one your particular model uses. Mark the location of the stud with the carpenter’s pencil.
Place the door sensor’s main unit onto the wall over the stud. Position the unit so that it’s about chest level. Place a carpenter’s level on top of the sensor to make certain that it’s leveled and then place a mark over the top of the sensor with the pencil. Place a mark through each of the mounting holes on the unit as well. Remove the unit.
Drill pilot holes into the wall at the marked mounting holes with the electric drill. Use a drill bit slightly smaller than the drywall screws that you will use to mount the sensor.
Place the main door sensor unit onto the wall with the mounting holes aligned with the pilot holes and then screw the main unit in place using the drywall screws and screwdriver. Make certain you use screws that are long enough to pass through the drywall covering and into the wall stud beneath.
Place a magnetic switch onto the doorjamb facing the side of the door opposite the hinged side. Anywhere along the jamb will do as long as it does not block the lock or interfere with the door's closing. Remove the protective backing from the rear of the sensor that covers the mounting adhesive. Press the sensor onto the jamb, holding it for 30 seconds to give the adhesive time to stick the unit firmly to the jamb.
Push the door almost closed and then use a straightedge to mark the location of the top of the switch onto the inside edge of the door, creating a line on the interior of the door that's level with that of the magnetic switch already installed on the doorjamb.
Place the second magnetic switch onto the inside edge of the door opposite the switch on the doorjamb. Use the marked line to align the top of the switch. Remove the backing from the magnet and mount the switch to the door using the adhesive on the rear of the unit. When you close the door, the two switches should meet one another squarely across the joint between door and doorjamb with no more than a 3/8-inch space between them.
Set the main unit to recognize the switches, following the setting instructions for your particular ADT model provided by the manufacturer.