Some uses for Motion Detectors
Motion detectors are sometimes referred to as Motion Sensors or PIRs. The PIR stands for Passive Infra Red which describes how the technology works.
I have a 12 Volt Motion Detector on my veranda. It is aimed between my door and the steps leading up to my house. As soon as I open the door to leave the house it turns my veranda light on. When I approach the house at night it also turns on the light. It is great for keeping an eye open for any snakes, etc, that might decide to come and camp on the veranda in the evening.
Some people use PIR Motion sensors to turn a light on outside and in their outdoor toilet. National Parks have used them so that the lights do not get left on all night in the toilet block. This saves on power and attracts less insects into the building.
The same PIR technology is found in most burglar alarm sensors. The main difference is that you can arm/disarm the siren with a keypad. I think even switching on a light outside can help deter many would- be intruders. It creates an element of surprise and can create concerns that cameras or other devices have also been activated. I believe that this is a more successful deterrent than leaving a light on all night.
Because the light only comes on 'when needed' for a maximum of 12 minutes, a DC Motion detector can save you power and allows you to use a brighter light than you could otherwise use on your solar system.