It's the most full featured alarm, blaring out a loud siren and telling you where the problem is. It works with a lot of other smart home systems, so for example it can turn on all your Philips Hue lights, making it easier to find your way out. However, if you don't want to spend more than $100 on a smoke detector—you should have several in your home—then check out the Roost Smart Battery. This 9 volt Wi Fi connected battery costs just $35, goes into any smoke detector, and alerts your phone when the alarm goes off. Make sure you check out all of our top picks for smart speakers, lights, locks, and more on our best smart home gadgets page. Like the Nest Protect, the OneLink Safe and Sound detects smoke and carbon monoxide, sends an alert to your smartphone, and tells you the type and location of the fire or condition. But for those times when it's not warning you of fire, you can use it for a great many other things. With Amazon's Alexa and a very competent 10 watt speaker, the Safe and Sound can play music, listen to audiobooks, and access Alexa's thousands of skills. It's also compatible with Apple's HomeKit. However, the Safe and Sound requires a hard wired connection, and we found that the location of these connections doesn't make it the most conducive for using all of Alexa's capabilities. Of the 200 odd reviews on Amazon where it has an average rating of 3.