It's important to have at least one, if not more, working smoke and carbon monoxide detectors in your home or apartment. However, if they go off while you're not there, how will you know if something's amiss?Smart smoke detectors, such as the Nest Protect, will send you an alert to your smartphone in the event of an emergency. But there are other reasons why this $100 device is our top pick. DesignNest's smoke detector is a bit larger than your standard model, measuring 5. 3 x 5. 3 x 1. 5 inches. It's square, with rounded corners. The center of the device has a circular LED that changes color depending on what's happening; yellow means there's smoke or carbon monoxide, but it hasn't yet reached critical levels; red means you need to get the heck out of there. You also get auditory alerts, which we'll get into later. Also helpful is that the light will briefly turn on at night if it senses motion, to help you see your way in the dark.