Hardware Teardown: the Withings GO activity tracker

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Nick

Co-CTO & Founder

19 Aug  ·  8 min read


Here at November Five, we like to know exactly how stuff works. That’s why we’re introducing a new recurring topic: the hardware teardown! Our first “victim”: the Withings GO.

For this first edition of the hardware teardown, we’re dissecting technology and lifestyle brand Withings’ cheapest activity and sleep tracker: the Withings GO.   Since they launched their first connected weighing scale back in 2010 (the WiFi Body Scale), I’ve been a big fan of Withings. They make beautiful products that integrate seamlessly with their online platform and that seem to have a very decent lifespan: that scale I bought in 2010 still works just fine (although I recently upgraded to the new Body Cardio to get some of the new features).

In this post, we’ll be treating their small activity tracker less kindly: we open up the casing, look at all of the main components and what they do, and try to recreate the BOM (Bill Of Material) and COGS (Cost of Goods Sold) to determine the actual cost of this tracker retailing for €69,99.

The first step: unboxing!

When you open the box, there are 4 items in it. First off, the tracker itself, with an e-ink display in a round plastic casing. Then a wristband, to wear the GO as a watch, and a clip to attach it to your pant pocket or belt. Finally, the packaging includes a small plastic coin with which you can open the back of the tracker to replace the battery.

Contents of the box

Next up: Opening the tracker

I used only basic tools to take this tracker apart: a cutting knife, tweezers, and a USB microscope.

First, we removed the battery. This is easy: you can simply open the back of the casing with the included tool or with a regular coin. The included battery turned out to be a Panasonic 3V CR2032 with a capacity of 225mAh. In other words, it could power a device consuming 225mA for one hour. According to the Withings GO product website, the battery can last up to 8 months, so simple math tells us that the tracker consumes only 43.4 microamps. With real life usage, that number will probably turn out a little higher, but even then it’s a very low-power device.  

Removing the battery also exposed a seal ring used to make the enclosure waterproof – necessary, as the Withings GO tracker is advertised as being able to detect when you are swimming.

Battery, lid and seal ring

Removing the seal ring already revealed the back of the PCB and its FCC ID: XNAWAM02.

Back of PCB

A quick look-up tells us a few things already:

  • Withings is, of course, the applicant of the FCC certificate
  • The address is Karaportti 3, Espoo, Finland (Nokia’s headquarters – Nokia recently acquired the originally French Withings for $191 mio)
  • The frequency band used is between 2402.0 and 2480.0MHz (due to the 2.4GHz bluetooth radio)
  • The Dutch company Telefication issued the FFC certificate.

The casing itself consists of three parts: the top, the bottom and a thin cover for the e-ink display.

Separating the parts seemed tricky at first, because they were sealed together, but by chipping some plastic off the side with my cutting knife I managed to create a small opening. After that, I could easily cut open the casing around the seam.

Casing

Going in: the circuit board

The PCB was manufactured by a Chinese company called PLOTECH.

PCB under

The back of the PCB immediately showed a bunch of testing points, which are labelled on the silkscreen layer with TPXX. This indicates they have a (semi)automated testing setup, which is always a good thing.

The PCB itself is very thin (0.6 mm) and on the bottom you can see (besides the testing points) 4 connectors for the e-ink display and the connectors for the battery. Being slightly overenthousiast after prying off the casing, I accidentally cut one of those display connector ribbons with my cutting knife… whoops.

The connectors Withings used are called “Front Flip FFC/FPC Connectors” (FFC stands for Flat Flexible Cable) and are very easy to open with a pair of tweezers.

Between the e-ink display and the PCB, a support frame was installed to keep the display in place and to avoid contact between the display and the PCB components. We’ll talk about the display later in this post, but we’ll go into some of the other elements first (in a more or less clockwise order).

Support

First off, the push button. On the top of the PCB, in the middle, there is an SMD push button, which you can actually press with the thin and flexible display. It’s used to set up the tracker when you unbox it and to switch from “tracker” view to “watch” view while using it.

Push button

To its right is the ultra low-power 3-axis accelerometer, the ADXL362 from Analog Devices.

Accelerometer

Next to the accelerometer there is another chip: a push button reboot controller, the XC6190 to be exact. (Previously, I assumed it was a load switch, but one of our readers, Nick Harvey at Torex Semiconductors, corrected this. Thanks Nick!) The XC6190 is an ultra low-current, push-button reset timer. The XC6190 uses a long timing setup delay to provide the intended system reset, and avoid resets from short push-button closures or key presses. This reset configuration also allows for differentiation between software interrupts and hard system resets.

reboot controller

Bluetooth and display

The bluetooth radio circuit consists of a few main components, which you can see on the image below:

  • BLUE: bluetooth radio chip
  • YELLOW: two crystal oscillators
  • PINK: a chip antenna
  • ORANGE: a balun. 

bluetooth circuit

The Bluetooth radio chip Withings selected is the nRF51822 from Nordic Semiconductors. It’s a 2.4GHz ultra low-power bluetooth chip built around a 32-bit ARM® Cortex™ M0 CPU. It’s accompanied by an RF ceramic chip antenna from Johanson Technology.

Below the bluetooth chip, there are two crystal oscillators , labeled “SF547” on the small one and “e160 610a” on the larger one. Unfortunately I could not determine the manufacturer or the frequency just by looking at their packaging.

Crystal oscillators

So I dug a little deeper into the documentation of the nRF51822 and found a mention of two clocks on the datasheet: ”The system depends on, and generates, two different clocks: a high frequency clock (HFCLK) and a low frequency clock (LFCLK).
These clocks are only available when the system is in ON mode. The HFCLK is fixed to 16 MHz and the LFCLK is fixed to 32.768 kHz.”

With that, we can reasonably assume that the two crystal oscillators are used as frequency-determining devices to generate the clock waveforms, and that they have a frequency of 32.768kHz (left) and 16MHz (right) accordingly.

Completing the Bluetooth setup is the balun, BAL-NRF02D3 from STMicroelectronics, which is used to match the impedances. This balun is actually optimized for the nRF51822. You can read this excellent blog post from Nordics Semiconductors to learn why a balun is used in this circuit.

balun

Next up, we move to the big black “blob” (glob-top) on the top of the PCB, which I assumed to be the display driver.

Glob top

The blob itself is actually a protective cover for the chip and the wire-bonds. This method is called “chip-on-board” (COB). Black epoxy resin is used for this process, because the chips can be sensitive to light – it’s the same material used for the packaging of ICs. We found a very nice article here detailing how these types of chips are made.

E-ink display

Last but not least: the e-ink display itself! The display is only 0.45mm thick – in other words, extremely thin! The display does contain a serial number “SCD72E00-160418-1”, but unfortunately I could not locate any documentation online. We can assume Withings will have partnered with a company to make custom e-ink displays for this device.

The cool thing about e-ink displays, though, is that they keep displaying their last state even if they are not connected to a battery. The ultimate proof that these displays are ultra low-power!

Our estimations: Bill of Material and COGS

According to this video from David Jones (EEVBlog), the Steve Irwin of electronics, the rule of thumb is that a hardware device on the market is priced 2.5 times the amount of the COGS. Taking that and the tracker’s retail price of €69.99 into account, the COGS of the tracker should be around €28. This includes hardware, casing, tooling, shipping, labor (solder and assembly), …

Let’s see if that is a correct assumption.

Finding all the exact parts on Digikey or Farnell wasn’t easy, so the actual cost of each part can be a little bit off. And off course, Withings probably has its supply chain all figured out, meaning they can get those parts a lot cheaper than I can find them online, but it will still give you a pretty good idea. What is not included: development costs, advertising, warehousing and management/supervising at the factory.

BOM

According my calculations the total cost of 1 unit is €28.46, which times 2.46 gives the retail price of €69.99. That means we’re quite close to the real number, taking into account their supply chain discounts and the exclusion of warehousing and factory supervising.

Wrap up

And that’s it for our first teardown!

Roundup

To round it up: the Withings GO tracker has a fairly simple design, a bluetooth chip with integrated CPU, an accelerometer and an e-ink display with driver – and that’s basically it. We might do a teardown of a similar activity tracker in the next weeks to compare the insides… Stay tuned!


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Nick

Co-CTO & Founder

19 Aug  ·  8 min read