Home Depot has smoke alarms that are 10 yr sealed battery alarms that come in pairs that you can interconnect up to 8 alarms, so if one goes off they all will. They even fit the plate that is now fastened to your ceilings. You just follow the simple directions to interconnect them, do a test to make sure all are working and then unscrew the old and twist in the new ones and you're all set. Just mark each one where you are putting it so when in the future you start taking one down you know where it came from. Simple and work great. If you have a great neighbor put one in their home too, so if something happens someone will know to call for help if needed. Be safe.