Cheap Smoke Detector or just cheap?

The attractively priced smoke alarm arrived quickly and the packaging was fine. With the manual one can only fail, but if you try long enough, you can pair the devices doh together, just not as described. However, only work 4 out of 5, on complain I have omitted, the price and the expenses are then out of proportion. But the next time I give rather from something more that gives me then also feel that the parts also work safely in Erstfall