Nest Protect Teardown
Nick Poole 
Clever Enclosure Tricks
Before looking at the electronic bits let's take a moment to admire the very clever engineering that went into this enclosure. I've broken it up into a few features that I admire and that contribute to both the form and functionality of the device.
Ventilation
Packing everything into a fairly standard size for a smoke detector is hard enough when all it does is beep. This full blown computer has to fit in the same space as your old smoke detector and still allow for smoke to get in behind the dense brick of electronic gizmos inside. This was achieved through a combination of PCB and enclosure layout. First of all, in order to save space, the Nest Protect uses a custom made photoelectric smoke detector that's built through a hole in the PCB. This allows the electrical part of the sensor to sit on the "sensor side" of the board while the detection chamber sits on the ceiling side. By building through the board, they save some vertical space as well, which keeps the device slim.
The other thing you'll notice is that the ceiling side of the device is vented along the perimeter and a series of baffles, molded into the back panel, direct air into the detection chamber without allowing the mounting tabs to disrupt the flow. A smoke detector only works, after all, if the smoke can get in.
Invisible Sensors
Despite being loaded with sensors the Nest Protect has somehow avoided looking like some sort of advanced spy drone. In fact, to look at the face of the Protect, you'd be forgiven for wondering how it's supposed to work at all. It sort of looks like an intercom speaker with a page button in the middle, but behind that friendly perforated exterior is a small army of components that need to see through to the outside world.
The way that the PCB fits into the front panel of the Protect is really slick. Just behind the grill there's a plastic baffle that directs input to all of the sensors. Sandwiched between the grill and this baffle there's a fine mesh that keeps dust out of the circuits but allows CO to get into the detector as well as sound to get out from the speaker.
The plastic sensor baffle couples to a pair of ultrasonic transducers presumably to avoid weird echos from inside the enclosure interfering with their operation. There's also a light pipe that focuses the ambient light from outside the unit directly onto the on-board light sensor. This baffle also supports what I call the "Button Spider", a portion of the plastic that floats on thin plastic legs and allows you to push the central button while taking some of the mechanical stress off of the switch itself. Actually, that gets its own paragraph...
Button Spider
While this is technically the same piece of plastic as the baffle, it displays some of its own cleverness. By joining the LED ring to the actual plastic button piece this part contributes significantly to the Protect's HAL9000-esque appearance. The 'Nest' button snaps into place on the front face of the spider, which houses the LED ring electronics behind it. A system of fins are molded in to keep the button from deforming as you push it (although a little bit of give seems to have been left intentionally as it makes the button feel friendlier). These fins also lock the flexPCB in place so that it can't rotate and free itself from the tacky glue that secures it to the spider. Finally, the inside face of the spider has a feature that keeps the switch in place on the backside of the button. Oh yeah, the spider has one more feature, it maintains the proper spacing between the PIR sensor and a part so cool that I gave it a German compound word: Die Objektivtaste.
"Die Objektivtaste"
Or "Lensbutton" in English (also a made-up word), is the button cover that's visible from the front of the unit. This rubberized plastic disc makes the pushbutton switch attractive, displays the brand name of the device, and is the visual focal point of the design. Ironically, besides being a visual focal point, it's also a lens. I know, you're thinking, "That's no lens! I can't see through it!," and you're right, you probably can't. However, the PIR sensor behind it has no such trouble. Removing the lens-button from the device and flipping it over reveals a Fresnel lens configuration molded into the plastic. It's pretty kaleidoscopic, really, to look at your own reflection in it. You'll recognize that configuration and the milky white color from other PIR/lens modules, like this one!
Cool Speaker
I really want to find a source for these speaker modules. It's just a plastic speaker in a little plastic box, and it's great! It sounds much bigger than it is (a function of the plastic enclosure), but that also has a lot to do with the fact that the sound and speech effects are probably tailored to the frequency response of this admittedly somewhat plastic-sounding speaker. Let this be a lesson: 50% of any speaker is the box you put it in!
Custom Photoelectric Detector
As I mentioned before, the photoelectric smoke detector in the Protect appears to be something they designed for this unit and not an off-the-shelf device. It's comprised of two plastic pieces: one of which houses an emitter/detector pair and the other, which acts as the optical chamber. These devices work by projecting a beam of light into a chamber and past a detector. When there's no smoke, the light sails right by, but when there's smoke present, it scatters some portion of light into the detector. This is a part that needs to meet rigorous standards as a safety device, and the temptation would be to buy a monolithic device that they know operates to code and just pop it into their design. Designing their own to save space? Good move.