The smoke detectors for home use are a very important part for your safety. They are usually housed in a plastic enclosure abouy 6" in diameter. Some of them have a dual power source - direct electric, plus battery. Others have only a battery inside the case which must be replaced every year so that the detector works properly.(Remember to replace Battery on the same date yearly). In the modern house you need at least one per room and kitchen area. If the house has an attached garage, you also need a carbon monoxide detector placed inside the house near the entrance from the garage, so that if by mistake the car is left running inside the garage you will be alerted before the people imside the house become unconscious. Make sure that you change the battery of each detector every year for your family and your own protection