The Merten smoke detector is a 547 090, seen purely externally, very valent equipment. The retrievable via test alarm volume is more than sufficient. That every 45 seconds very briefly appearing red LED light is not disturbing at all, so that the detector can be used in fine bedrooms. The 3-pack was a great offer, supply includes not only the necessary screws / plugs and a 9V. The attachment is easily accomplished. I can only recommend this smoke detector.