Smoke detectors hang on the wall unnoticed most of the time.
We assume it will respond if it is ever needed to warn of fire. It saves many thousands of lives each year. It is a very important, though somewhat ignored, item consequently, we should take time to understand how it functions. Armed with knowledge we can then, choose the best one for our home. In newly built homes smoke detectors are required to be hard wired 120 volt systems, with battery operated detectors as backups.