Nest Protect

Nest Protect has eight sensors that allow it to know what’s happening in a room and detect dangerous levels of smoke and carbon monoxide. In addition to detecting potential threats, Nest Protect has three sensors that let you silence alarms with a wave of your hand. They also detect movement to light up your path at night.

Nest Protect uses information from its sensors to detect potential threats in your home:

• A photoelectric smoke sensor uses light to detect smoke particles in the air. 
• A carbon monoxide sensor detects carbon monoxide, an odorless and colorless gas. When combined with heat, carbon monoxide often indicates there is a fire present. 
• A heat sensor allows Nest Protect to detect sudden rises in temperature. 

In addition to protecting you with three safety sensors, Nest Protect includes a humidity sensor, as well as activity sensors that light your path at night and let you silence alarms with a simple wave your arm. 

• Two ultrasonic sensors detect movement. This lets you use Nest Wave to silence alarms. 
•  One occupancy sensor detects when you are under Nest Protect so that you can silence an alarm with Nest Wave. It is also used to sense when you are passing under Nest Protect at night to turn on Pathlight. 

For consumers who have a Nest Learning Thermostat, the occupancy sensor improves the Auto-Away algorithms. Learn more about Auto-Away 

• An ambient light sensor lets Nest Protect know when it’s dark or when the lights are off inside your home. This sensor is used for Nightly Promise and Pathlight. 
• When you turn off your lights at night, Nest Protect will briefly glow green, giving you the Nightly Promise that its sensors and batteries are working correctly.
•  As you pass under Nest Protect in the dark, Pathlight will light your way. (Pathlight will only be available if your Nest Protect is connected to Wi-Fi and associated with your Nest Account. During setup, you'll get a chance to set Pathlight on or off. After setup, Pathlight will continue to work even without Wi-Fi.) 

In addition to sense and alarm, Nest Protect will send a message to your smartphone when things happen. You can also setup your Nest Protect with your smartphone.

(Ref.: https://nest.com/smoke-co-alarm/life-with-nest-protect/)