Dissecting a feature-enhanced digital bathroom scale

Around a decade ago, my now-wife purchased an Omron HBF-400 bathroom scale (which Amazon reports was first sold in June 2011, although she doesn’t remember where hers originally came from):

It was pretty nifty, supporting simultaneous data logging for multiple users and measuring not only weight but also body mass index (BMI, calculated by combining weight and height and with categorized results also factoring in age and gender) and body fat percentage (more on that particular feature shortly). But about a half-decade ago, its monochrome LCD display started going wonky (in response to atmospheric moisture exposure? Mebbe. It was in the shower-inclusive bathroom, after all!). And after a few years’ worth of moving it around back in its box in my office, I’ve finally decided to actualize my longstanding aspiration to tear it down.

Here’s a (shaky, sorry…actually, it was the smartphone moving, not the scale itself!) video clip of what the display now looks like when I power on the scale:

And here are a few still shots in various operating modes, revealing that whereas portions of the display are still functional, most of the main digit segments have gone AWOL:

Potential moisture exposure aside, the most common causes of such failures are either damage to the display itself (if the scale were to have been dropped, for example, albeit not a relevant scenario in this particular case), fraying of the ribbon cable that’s usually present to connect the display to the system board, and/or cold-solder failure of one-to-multiple connections between that ribbon cable and the display and/or system board (which is my upfront guess). Alas, I didn’t find this video on a similar model until after completing my teardown, but as my disassembly was non-destructive, I may still give it a shot:

Here are some outer box shots, to start. Front:

Back:

Remember my earlier mention of the scale’s body fat analysis feature? Here’s a bit more detail:

Here’s even more info from an online resource I came across in the midst of my research:

How do they work?

 Body fat scales are easy to use. You simply step on the scale, and the tool measures both your body weight and your estimated fat percentage.

 Such scales work with the help of sensors underneath your feet that use bioelectrical impedance. When you step on the scale, a small electrical current runs up through your leg and across your pelvis, measuring the amount of resistance from body fat.

 Then, the sensors in the scale measure the level of resistance that the current met as it travels back through your other leg.

 Depending on the type of body fat scale you have, the information can link up to your smartphone or smartwatch, as well as any fitness apps you might have.

 As a rule of thumb, greater body resistance means a higher fat percentage. This is due to the fact that fat contains less water than muscle, so it’s more difficult for a current to travel through it

 Are they accurate?

 In general, body fat scales can provide rough estimates only. While safe to use, there are many variables that can affect your results. These include:

• Your gender. Women naturally have more body fat than men.
• Where you store fat in the body.
• Pregnancy. These scales aren’t recommended during pregnancy.
• Your age. These scales aren’t suitable for children.
• Your height and stature.
• Frequent endurance and resistance training.

And here’s a link to the Wikipedia entry on “Bioelectrical impedance analysis” for further education.

So…when you stand on the scale’s metal pads, weak electrical current shoots up one leg and back down the other. The scale measures the resistance and estimates body fat percentage based on it. I guess this methodology explains, jumping ahead briefly, this ominous sticker found on the scale’s underside!

Back to the box…right side:

Left side:

Top:

And bottom:

Open the outer box, slide out the inner cardboard sarcophagus, and inside you’ll find a few pieces of plastic-sleeved literature (here’s a PDF of the user manual):

along with our victim, starting with a frontside view accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes (the scale has dimensions of 12.01” x 11.81” x 2.24” and weighs 4.6 lbs). Particularly note the conductive pads, three on each side (for each foot):

Backside, including the aforementioned warning sticker. Note, too, the eight shiny screws, two in each corner, and the “foot” in-between each pair of ‘em. All of these will be further showcased shortly…

and an upside-down view of the bottom side’s power switch (the other three sides are unmemorable):

Now to get inside. There’s nothing retentive within the four-AA battery compartment you likely also already noticed in the earlier backside overview shot:

Removing those eight shiny screws, conversely, proved much more productive:

Detaching the black-and-red wiring harness that normally connects the system PCB to the frontside multi-switch cluster gets the two halves a bit further apart:

And disconnecting the single-wire strands that mate the system PCB to four of the six topside conductive pads, two on either side (curiously, note that the top-most pad on either side was seemingly not functionally harnessed in this particular design!) gets us the rest of the way to our desired result:

Here’s a closeup of the inside of the front half of the scale. Again, note the electrical “tabs” for the conductive pads in the upper corners on the other side, left unconnected in this case:

And here’s the inside of the back half, which (for likely already-visually-obvious reasons) will garner the bulk of our remaining attention:

At this point, an obvious-in-retrospect confession is in order. See the multi-wire cluster running from the system PCB to each of the four “foot” assemblies? My first thought was “for grounding purposes” but that didn’t make sense, because especially in the bathroom, the floor the scale would be sitting on was highly likely to be insulative in material, not conductive. Next, having not consulted the instruction manual and with my long-term memory of the scale’s operation having faded, I thought they might act as power switches, automatically turning the scale on when I stepped on it, as was the case with our newer (albeit not as richly featured) replacement scales:

But if so, why were there three wires traversing the span between the system PCB and each “foot”? And why was there a seemingly redundant power switch already at the bottom of the scale? Inherent in my theory, perhaps obviously, was the belief that the conductive pads on the top of the scale did double-duty as pressure sensors for measuring weight…ignoring the fact that there was only a single wire running from each of them to the system PCB…

The only thing left to do, I decided, was to take one of the “feet” apart and see what was inside:

That screw was extraordinarily tenacious, but a big Philips screwdriver, a modicum of muscle and more than a modicum of colorful language finally compelled it to budge:

What is that thing inside the two-metal-plate sandwich?

Onward…

As you can see, I had less luck with the earlier-mentioned screw’s sibling on the other side of the plate, stripping it in a failed attempt to remove it:

So, I wasn’t able to completely remove whatever this was from the plate to which it was attached:

But more random-topic online research on my part eventually hit pay dirt; it’s a load-bearing bar (there’s one in each corner at the scale-system level, remember), containing two strain gauge sensors:

Old-skool analog scales (you know, with the needle that moved) were based on some clever gears and levers that converted pressure on the scale into compression of a big spring, then the spring’s compression into rotational motion that could drive a dial. But digital scales don’t really have any moving parts. They are mechanically designed to distribute your weight evenly to a bar or collection of bars which bend very very slightly under the pressure. Those bars are bonded to an electrical element that also flexes very very slightly, changing its electrical resistance. This is a strain gauge. Inside the scale I hacked, you can see the two load-bearing bars. Each bar has two strain gauge sensors bonded to it. Measuring the difference in strain between the two sensors can tell you how much the bar is flexing.

Here’s more (to avoid confusion, note that what the writer called a load bar earlier he also refers to as a load cell in what follows):

The vertical bars on the left and right are load cells, the mechanical parts that are designed to bend slightly. Each one has a tiny PCB with two strain gauge sensors. The two sensors share one common wire, so there are three wires going to each load cell.

More on strain gauges here. And I’ll share additional views, now of the various sides, before moving on:

Onward again. Now that we know that the “feet” aren’t power switches, here’s what the real one looks like after removal of the cosmetic plate in front of it:

And now for the system PCB “sandwich” at top and center. Removing the two screws below the display:

enabled its removal from the backside of the four-AA compartment behind it:

Here’s a closeup of the system PCB backside:

The largest IC near the center is presumably the system “brains”. Here are its markings:

38024B89WV
H8F-400A

TH39123
0833

I can’t seem to definitively ID it; reader suggestions are welcomed. But I suspect it’s a member of Hitachi Semiconductors’, now Renesas’ Electronics’, H8 microcontroller family.

See those two screws on either side, near the top? Removing those, and unclipping the white plastic hooks in the same horizontal locations but toward the bottom, should enable liftoff of the LCD:

Bingo!

Alas, this side of the PCB is comparatively bland:

In closing, let’s return our attention to the inside of the top of the scale. Here’s a closeup of the back of one of the conductive pads:

And its other (front) side; they unfortunately seem to be sturdily glued in place, as I couldn’t get any of them to budge and pop out for standalone perusal:

And finally, here are some step-by-step disassembly closeups of the multi-switch array at center:

And with that, I’ll wrap up. As always, your thoughts on this “weighty” analysis are welcomed in the comments! Meanwhile, I’ll be firing up my heat gun to see if I can fix a few cold solder joints and get this thing back together and working again…

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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