Motion sensors are used on lights for a couple of purposes. They alert you to people or animals walking in a certain area, which can help prevent intruders, and they save power by only coming on when needed, such as when you walk up to the door to insert the key. If you have a porch light installed already, there is an attachment available that will add a motion sensor to the existing light.
Remove the cover to the outdoor light. Depending on the type of light, you might need a screwdriver to remove the cover.
Twist the light bulb counter-clockwise to loosen and remove it. Set it to the side.
Twist the motion sensor attachment into the light socket. Turn clockwise to tighten it down. Turn it so that the sensor screen is facing the front of the light.
Screw the light bulb back into the socket on top of the sensor.
Test the sensor to make sure it works. Place tape on the edges of the sensor if there is a spot you use often, and don't want the light to constantly turn on.