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The hardest thing about making a smoke alarm.

The hardest thing to do, the thing we spent the most time on, the most research, the most stress, is the thing at the heart of every smoke alarm: sensing smoke.

It’s not App Silence. Not Nightly Promise. Not Heads-Up or Sound Check or the Nest app. The hardest thing to do, the thing we spent the most time on, the most research, the most stress, is the thing at the heart of every smoke alarm: sensing smoke.

It’s not a simple problem. Modern home fires flashover four times faster than 40 years ago. You might only have a few minutes before a fire takes over a room. So smoke sensors need to be highly sensitive, but not so sensitive that they alarm when you’ve just burned some popcorn.

The smoke sensor nestled inside the 2nd generation Nest Protect is called the Split-Spectrum Sensor. It’s the most advanced smoke sensor we’ve ever designed. It has a custom shielding to protect it from dust, bugs and light. And it’s paired with precise and complex algorithms. The Split-Spectrum Sensor is the heart of Nest Protect. And today I want to give it its due.

To really understand how special this sensor is, you need to know a little bit about smoke alarms. Home alarms can have two kinds of smoke sensors: photoelectric and ionization. The Split-Spectrum Sensor is photoelectric, meaning it has a tiny light inside it. When smoke particles hit that light, they trigger an alarm. Ionization alarms work on a similar principle, but instead of light they have two bits of radioactive material that ionize the air between them. When smoke particles enter that air, it sets off the alarm.

We ruled out making an ionization alarm early on. The radiation in them isn’t considered dangerous, but we don’t like that they use radioactive particles – what a weird thing to put in your home, when you think about it. France and the Netherlands have even banned them. But more importantly, ionization sensors are notorious for false alarms. They’re more likely to go off when nothing’s wrong – when you're taking a shower or are just lighting a candle. They’re good at catching fast-flaming fires, but they’re great at driving you insane.

Photoelectric sensors are less prone to nuisance alarms and are generally better than ionization at catching smoldering fires that put out bigger particles of smoke. But they’re slightly slower to react to the tiny smoke particles that rise from quickly-flaming fires.

So our mission for the 2nd gen Nest Protect was clear: Build a smoke alarm that would quickly spot smoldering fires and flaming fires, while minimizing false alarms.

We looked beyond traditional home alarm technology. Commercial smoke alarms, like those made for office buildings and factories, used a different kind of design. While many home builders reach for the cheapest alarm on the shelf, business owners often opt for the most advanced and expensive technology to protect their investments. And we thought there should be equally advanced technology in homes.

Here’s how the Split-Spectrum Sensor works:

Unlike most photoelectric alarms that only use one wavelength of light to look for smoke, the Split-Spectrum Sensor uses two. Red light to see large particles of light, blue to look for small. Custom shielding around the sensor lets smoke in but keeps any other light out. It seems so simple, but it’s incredibly difficult to get right.

That little blue light coupled with Nest’s exclusive algorithms give Nest Protect more data to understand what’s going on, so it can make better decisions. Those decisions help make Nest Protect special. Every smoke alarm can sense smoke. But ours can think. And talk.

So while it’s highly sensitive to both slow and fast fires, Nest Protect can also make helpful choices, like when to give you a gentle Heads-Up or when to kick into high gear with a loud alarm. With Heads-Up, Nest Protect can tell you when smoke is just starting to build, and do it calmly, with no panic – so it’s not a nuisance. It’s actually helpful. Heads-Up helps give you time to open a window or turn down the heat or snatch the candle off the floor or the cookies out of the oven. It won’t make you want to rip the smoke alarm off your ceiling.

Nest Protect also lowers false alarms with Steam Check. The algorithms in Nest Protect take the data from the Split-Spectrum Sensor and combine it with information coming from the humidity sensor built into Nest Protect. Then they figure out if the particles inside the smoke sensor are actually smoke, or just steam rising from your shower.

The smoke sensor is the heart of any smoke alarm. The algorithms are the brains. And in the 2nd generation Nest Protect smoke and carbon monoxide alarm, the combination of the two is changing what a smoke alarm can be.