The purpose of a smoke detector is to provide early warning for responsive occupants faced with a fire emergency, especially while they are sleeping.
Smoke detectors should be placed on each level of the structure.  They should be located in the hallway outside each sleeping area, also located in other rooms of the home.  The detector should be close enough to the sleeping area so when it sounds it can be heard when the door is closed.
The batteries should be tested once a month to be sure that they are still working properly.
Batteries should be changed twice a year when clocks are changed for daylight savings or returned to standard time.