Inside the magic of modern sensors

The latest generation of wireless sensors can last for years on a AA battery while running 24x7 — here’s a quick peek inside the case.

Ever since I was a kid, one of my favorite hobbies has been taking different pieces of electronics apart to see what’s inside. In the early days it was mostly for the challenge of opening up a tiny puzzle and trying to put it back together, but in more recent years I’ve found myself appreciating the feats of engineering that make everyday products work. This week we’re introducing a new family of sensors at Samsara, so I thought it’d be fun to offer a quick teardown to share what makes modern sensors such interesting technology.

Industrial design for industrial environments

First a little bit of background: the Samsara EM21 is designed to help our customers understand temperature and humidity in their industrial environments. The sensors themselves are small enough to fit in the palm of your hand, and look something like this:

Unlike most pieces of electronic equipment, these sensors have to survive extreme day-to-day conditions in the world of supply chains, from sub-zero temperatures in cold storage facilities, to pressure washers and steam cleaners, to getting run over by the occasional forklift.

All of this makes for an engineering challenge — how can we build a simple and robust product, at a reasonable price, that can survive for years in an operationally difficult environment?

We found the key was to keep the overall design as simple as possible, and to choose cutting-edge components for the display, CPU and sensors that would survive in harsh environments with very little power.

Here’s what we ended up with on the inside:

Our hardware team is a pretty clever bunch, and they were able to build on their experiences working on products like the Apple Watch (shock and waterproof) and Nest Thermostat (mounting and usability), to build an unique industrial product. The robustness of the product is pretty amazing:

Looking deeper into the world of tiny chips

Perhaps the most amazing part about the Samsara EM21 is the part that hardest to see with the naked eye — the technology within the silicon.

At the very core of the product is a tiny 32MHz CPU that fits on silicon die that’s around 2mm by 2mm, about the size of a single grain of rice. Here are some photos of equivalent products for perspective:

Left: a 1980s IBM PC, which had similar horsepower to the Samsara EM21 in a much larger, more expensive and less water resistant case. Middle: the relative size of a sensor’s CPU/radio module compared with a pencil. Right: the inside of the core under a microscope, which shows the memory, CPU and radio. The silicon chip itself is smaller than a grain of rice.

For those of you who remember the good old days of MS-DOS, the 32MHz CPU at the core of the sensor has a similar amount of computational power to an IBM PC. This brings us to the most amazing part of the sensor — its power efficiency. The sensor can run on a single AA battery for 3 to 5 years while taking readings every 4 seconds, and that’s while storing months of data, with the wireless radio reporting to the cloud and the display on the entire time. By comparison, the IBM PC above would have lasted around 30 seconds on the same amount of energy, and that’s without the display. The 1,000,000x improvement in energy efficiency is pretty amazing!

So how is such a huge improvement possible? Some of the biggest gains have come from CMOS scaling, where the density of transistors on a chip doubles every 18 month due to advanced manufacturing techniques (also known as Moore’s Law). The smaller transistors use less power and can operate at faster speeds, which has led to new devices like tablets and smartphones.

The sensors act like fireflies and save energy by only transmitting a small percentage of the time, on a precisely synchronized schedule. (Image courtesy of NPR’s Science Friday)

However, there have been two other big breakthroughs in the last ten years that have made the modern sensor possible. First, the CPU and other components are now so fast they are active for less than 1% of the time, which means they can spend the other 99% of the time sleeping. Because the sensors have a two way radio, they have to have very precise clocks that “wake up” with micro-second precision (which is hard to do when the temperature changes all the time!). The second is when they are sleeping, they draw very little “leakage current”, which extends the battery life considerably.

It turns out there’s a name for these improvements in energy efficiency — similar to the size and cost improvements of Moore’s Law, there is also a lesser known version called Koomey’s Law that says the chips will be do the same amount of work with half as much energy every 18 months. Combining the latest low-power CPU technology with new LCD display and radio technologies has made it possible to extend the battery lifetime from minutes, to years while still bringing down costs by over 100x.

Put a sensor on it!

Now that we can put a powerful, robust, yet tiny computer on things, and have it last for years on a single AA battery, entirely new possibilities open up for capturing data about the real world: we can track the food safety of every pallet of food, and get a detailed picture of the entire supply chain from farm to table.

Hopefully you found this quick guided tour of the Samsara EM21 interesting— if you spend your days living in the industrial world like we do, and would like to take a closer look, please get in touch with us and try one out for yourself!