There are plenty of smart smoke detectors on the market that connect to your home's Wi-Fi network to provide smartphone notifications when there's trouble, but they're an expensive upgrade to the dumb smoke detector you've (hopefully) already got watching over your home. The $40 Roost promises pretty much the same functionality as those smart alternatives, but on the cheap, upgrading only the 9-volt battery your smoke or carbon monoxide detector already relies on.
Battery life is rated at an impressive five years, even while the Roost is connected to your home's Wi-Fi network. But that's because the Roost pretty much sits completely dormant until it detects your smoke detector's deafening screech. At that point the Roost springs to life, connecting to your home's Wi-Fi network in order to send notifications to your smartphone.
The Roost is even smart enough to know when its own battery is dying, sending you a notification that it's time to swap it out before the detector starts its own annoying chirp. And in the final version of the Roost (that's a prototype pictured above) the bottom half that contains the actual battery can be swapped with a $10 replacement—giving you five more years of added protection. [Roost]