Yellow means there's a malfunction or a low battery, while red means there's a fire or smoke condition. Onelink's alarm uses photoelectric technology, which is generally better at detecting smoldering fires than are alarms that use ionization technology. However, the latter are better at detecting fast burning fires. The Onelink can be connected to HomeKit and Alexa but not Google Assistant, so you can ask both Siri and Amazon's voice assistant for the status of the smoke detector, but not much else. By contrast, if the Nest Protect detects fire or carbon monoxide, you can have it automatically turn on a security camera, open Lutron blinds, turn on Philips Hue lights, turn off your heat and more. Setup should theoretically be easy—just scan a HomeKit code in the Onelink app—but it took a few tries to get it to work. The Onelink smoke and carbon monoxide alarm is good on its own, but the Nest Protect does more for the same price. What to look for in a smart smoke detectorFire and smoke detection sensors. All smoke detectors use one of two types of sensors to detect fires. Ionization type sensors are best at detecting fast burning fires, while photoelectric sensors are generally better at alerting you about smouldering, or slow burning fires. You should have both types of alarms in your house.