Body Cardio Weighing Scale Teardown

If you weigh yourself by standing on a bathroom scale, not liking the result, then balancing towards one corner to knock a few pounds off the dial, you are stuck in a previous century. Modern bathroom scales have not only moved from the mechanical to the electronic, they also gather body composition measurements and pack significant computing power.

Yet they’re a piece of domestic electronics that sits in our bathroom and rarely comes under scrutiny. How do they work, and what do they contain? The team at November Five tore down a top-of-the-range Withings Body Cardio scale to find out.

After a struggle with double-sided sticky pads, the scale revealed its secrets: a simple yet accomplished device. There are four load cells and the electrodes for the body measurement, and the PCB. On the board is a 120 MHz ARM Cortex M4 microcontroller, a wireless chipset, battery management, and the analogue measurement chipset. This last is particularly interesting, a Texas Instruments AFE4300, a specialised analogue front-end for this application. It’s a chip most of us will never use, but as always an obscure datasheet is worth a read.

The rather pretty fractal antenna.
The rather pretty fractal antenna.

Finally, the wireless antenna is not the normal simple angular trace you’ll be used to from the likes of ESP8266 boards, but an organic squiggle. It’s a fractal antenna, presumably designed to present a carefully calculated bandwidth to the chipset. A nice touch, though one the consumer will never be aware of.

We’ve shown you quite a few bathroom scales over the years. There was this wisecracking Raspberry Pi scale, this scale reverse engineered to gather weight data, and this one laid bare for use as a controller.

20 thoughts on “Body Cardio Weighing Scale Teardown

  1. “A nice touch, though one the consumer will never be aware of.”

    The audience in here maybe isn’t what you’d call ordinary consumers, but I really appreciate these kinds of details (and I often go looking as well, just because…) :)

  2. It’s a pity TFA doesn’t mention how the bioimpedance body composition sensors are arranged or how they work. 20 years ago when i was in the field the operating principles for these body fat devices were based on voodoo, black magic, and wishful thinking: DEXA and MRI were far superior (though have radiation dose and cost concerns, respectively). But it looks like the accuracy and precision of bioimpedance devices have improved a bit. The datasheet links to an app note http://www.ti.com/lit/an/sbaa202/sbaa202.pdf, which in turn links to the wikipedia entry, which does list academic papers that do show reasonable performance of modern devices.

  3. Finally, another fractal antenna has been found!
    The only time I have seen one in the wild has been on an HD antenna, but it may have just been a gimmicky frill added to the tips of the visible trace.

    1. Sharp corners act like a stub and cause reflection.

      Still true about art though as the second iteration of the fractal element doesn’t add anything to the performance of the antenna (at least that I can see).

        1. NEC2 was and maybe still is the standard go-to software for this. It’s been around forever (i.e., before the dawn of the interwebs). When I first started using it, the 80287 coprocessor was highly recommended…

          If you’re not used to dealing with 80-column cards, an easy-to-use and prettyfied Mac port is CocoaNEC (itself already ancient, as you might guess from its name, but still runs in OSX).

          There are lots of front ends to automate & script it too, and your favourite search engine will provide many examples of so-called fractal antennas with it.

          1. Hey Paul,

            After 30 years and well over 2000 scientific publications, pray tell us why “so- called fractal antenna” is an inappropriate nomenclature. It is amusing to do Monty Python ‘nod nod wink wink’ putdowns, but don’t pretend you are sufficiently informed on this matter. Neee!

  4. As inventor of this antenna, consider the following facts: 1) the use is not authorized and based upon this published piece, the patent holder is pursuing; 2) it is not ‘organic’. I have no idea what is meant by that adjective in this context. It is a copper trace on FR4 substrate; 3) the antenna is hardly ornamental. Who the heck would see it?; 4) The use of this antenna provides a smaller size, and robustness against detuning. Detuning is caused by a large ‘organic’ dielectric standing on top of the scale. IOW, ‘people’; 4) NEC2 is a poor predictor of performance for this geometry: review the proximity limitations in MOM approaches. Best wishes from Nathan Cohen. Over and out.

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