Nest Protect (now owned by Google) has been redesigned from the inside out. Its Split-Spectrum Sensor uses two wavelengths of light to detect smoke from both fast-burning and smoldering fires. Carbon monoxide is poisonous but you can’t see it or smell it. That’s why Nest Protect has the most accurate type of carbon monoxide sensor available. Nest Protect also checks its batteries and sensors over 400 times a day. Available in both wired and battery-powered models.