These smoke detectors appear effective in 3 of them because the 4th ==> emits a beep every minute ==> day and night.! What is particularly unpleasant !!! The other 3 are silent - out of danger detection - they only blink every minute which is more reasonable because they produce no other discomfort and let appear the proper functioning of the use of the battery. TIP: It would be helpful if the manufacturer conducts the most comprehensive testing before the sale of these smoke detectors. Without this trouble this was the subject of five stars and recommended. They practice, they can be easily glued to the ceiling with a special paste, they can be assembled and disassembled easily into a clip. Value ===> GOOD.