The Nest Protect, theoretically, falls into the last category, but things are a bit more complicated because it doesn’t have a ionization sensor, but it does use the Split Spectrum Sensor which nullifies the need of the radioactive ionization sensor as you know, it detects smoke produced by flaming fires which burn at a fast rate so, if the ionized air is disrupted by smoke, it triggers the alarm. The photoelectric sensors is used to detect any smouldering fires that uses a light beam which, normally does not hit the sensor, but when smoke scatters the light particles, it will hit the sensor, therefore triggering the alarms – this method is used to detect a fire in its early stages, therefore reacting a lot faster. Now, the Split Spectrum Sensor should detect both the flaming fires and the smouldering ones, by adding a blue LED light at a wavelength of 450nm to look for small particles which are an indicator of a rapid, fast burning fire and keeping the usual photoelectric sensor to detect smouldering fires. To make sure it doesn’t repeat the mistakes of its predecessor annoying false alarms, the new sensor is isolated from any outside light and the encasing is designed to only allow smoke in and nothing else this worked just fine since the second generation of Nest Protect is indeed less prone to false alarms. Besides the Split Spectrum sensor, there’s also an Electrochemical carbon monoxide sensor yes, the Nest Protect is also a Carbon Monoxide Detector which detects the CO concentration level inside the room and, if it’s beyond safe levels, it triggers the alarm CO detectors usually last about 6 7 years, but Nest advertises that the sensor on the Protect can last up to 10 years. Furthermore, there’s also a heat sensor so you’ll get notified when the home gets either too cold or too hot, a humidity sensor so it makes it unlikely that the alarm will be triggered when there’s lots of steam, an ambient light sensor so the detector knows when the lights are shut off and it can activate the Pathlight feature or the Nightly Promise – the LED will be green if everything is fine, otherwise, it will become yellow and an occupancy sensor it is used with the Pathlight feature, so it will detect movement and will shine a light on your path during the night – so you don’t stumble on the furniture. With the new released WiFi systems and home automation hubs, I noticed an increased interest towards simplicity and minimalism in both design and functionality, which meant a simple design and a single LED which showed the status of the system with different colours. Of course, for me it did not make much sense since there is no intuitive way to understand all those flashing or solid colours, so I said that a voice feedback would make much more sense. It seems that Nest Protect actually implemented such a system and, using an omnidirectional microphone, it will speak to you in words besides also using the appropriate colours: there are two types of alerts, the Heads up it’s a calmly spoken warning + yellow LED, when smoke or CO levels are rising but have not yet reached dangerous levels and the Emergency Alerts when smoke or CO are at dangerous levels, the Nest Protect notifies you where is the problem, it turns the LED red and triggers the loud sound alarm – 85dB. Head Up and Emergency alerts can be silenced by pressing the circular button. Note: This device is not intended for people with hearing problems it lacks a flashing strobe of light and the 85db alarm may not be enough in such cases.