Categories: Electronics

Dissecting a feature-enhanced digital rest room scale

Round a decade in the past, my now-wife bought an Omron HBF-400 rest room scale (which Amazon stories was first offered in June 2011, though she doesn’t bear in mind the place hers initially got here from):

It was fairly nifty, supporting simultaneous knowledge logging for a number of customers and measuring not solely weight but additionally physique mass index (BMI, calculated by combining weight and top and with categorized outcomes additionally factoring in age and gender) and physique fats proportion (extra on that exact characteristic shortly). However a few half-decade in the past, its monochrome LCD show began going wonky (in response to atmospheric moisture publicity? Mebbe. It was within the shower-inclusive rest room, in any case!). And after a number of years’ price of transferring it round again in its field in my workplace, I’ve lastly determined to actualize my longstanding aspiration to tear it down.

Right here’s a (shaky, sorry…really, it was the smartphone transferring, not the dimensions itself!) video clip of what the show now appears like once I energy on the dimensions:

And listed below are a number of nonetheless pictures in varied working modes, revealing that whereas parts of the show are nonetheless practical, many of the important digit segments have gone AWOL:

Potential moisture publicity apart, the most typical causes of such failures are both harm to the show itself (if the dimensions had been to have been dropped, for instance, albeit not a related state of affairs on this specific case), fraying of the ribbon cable that’s normally current to attach the show to the system board, and/or cold-solder failure of one-to-multiple connections between that ribbon cable and the show and/or system board (which is my upfront guess). Alas, I didn’t discover this video on an identical mannequin till after finishing my teardown, however as my disassembly was non-destructive, I should give it a shot:

Listed below are some outer field pictures, to begin. Entrance:

Again:

Keep in mind my earlier point out of the dimensions’s physique fats evaluation characteristic? Right here’s a bit extra element:

Right here’s even extra data from an on-line useful resource I got here throughout within the midst of my analysis:

How do they work?

Physique fats scales are straightforward to make use of. You merely step on the dimensions, and the software measures each your physique weight and your estimated fats proportion.

Such scales work with the assistance of sensors beneath your toes that use bioelectrical impedance. Whenever you step on the dimensions, a small electrical present runs up by means of your leg and throughout your pelvis, measuring the quantity of resistance from physique fats.

Then, the sensors within the scale measure the extent of resistance that the present met because it travels again by means of your different leg.

Relying on the kind of physique fats scale you could have, the knowledge can hyperlink as much as your smartphone or smartwatch, in addition to any health apps you might need.

As a rule of thumb, larger physique resistance means a better fats proportion. This is because of the truth that fats incorporates much less water than muscle, so it’s tougher for a present to journey by means of it

Are they correct?

Normally, physique fats scales can present tough estimates solely. Whereas protected to make use of, there are numerous variables that may have an effect on your outcomes. These embody:

Your gender. Ladies naturally have extra physique fats than males.
The place you retailer fats within the physique.
Being pregnant. These scales aren’t really helpful throughout being pregnant.
Your age. These scales aren’t appropriate for kids.
Your top and stature.
Frequent endurance and resistance coaching.

And right here’s a hyperlink to the Wikipedia entry on “Bioelectrical impedance evaluation” for additional schooling.

So…while you stand on the dimensions’s metallic pads, weak electrical present shoots up one leg and again down the opposite. The dimensions measures the resistance and estimates physique fats proportion based mostly on it. I assume this technique explains, leaping forward briefly, this ominous sticker discovered on the dimensions’s underside!

Again to the field…proper facet:

Left facet:

Prime:

And backside:

Open the outer field, slide out the interior cardboard sarcophagus, and inside you’ll discover a number of items of plastic-sleeved literature (right here’s a PDF of the consumer handbook):

together with our sufferer, beginning with a frontside view accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for measurement comparability functions (the dimensions has dimensions of 12.01” x 11.81” x 2.24” and weighs 4.6 lbs). Significantly word the conductive pads, three on either side (for every foot):

Bottom, together with the aforementioned warning sticker. Word, too, the eight shiny screws, two in every nook, and the “foot” in-between every pair of ‘em. All of those can be additional showcased shortly…

and an upside-down view of the underside facet’s energy swap (the opposite three sides are unmemorable):

Now to get inside. There’s nothing retentive inside the four-AA battery compartment you possible additionally already seen within the earlier bottom overview shot:

Eradicating these eight shiny screws, conversely, proved way more productive:

Detaching the black-and-red wiring harness that usually connects the system PCB to the frontside multi-switch cluster will get the 2 halves a bit additional aside:

And disconnecting the single-wire strands that mate the system PCB to 4 of the six topside conductive pads, two on both facet (curiously, word that the top-most pad on both facet was seemingly not functionally harnessed on this specific design!) will get us the remainder of the way in which to our desired outcome:

Right here’s a closeup of the within of the entrance half of the dimensions. Once more, word {the electrical} “tabs” for the conductive pads within the higher corners on the opposite facet, left unconnected on this case:

And right here’s the within of the again half, which (for possible already-visually-obvious causes) will garner the majority of our remaining consideration:

At this level, an obvious-in-retrospect confession is so as. See the multi-wire cluster working from the system PCB to every of the 4 “foot” assemblies? My first thought was “for grounding functions” however that didn’t make sense, as a result of particularly within the rest room, the ground the dimensions could be sitting on was extremely prone to be insulative in materials, not conductive. Subsequent, having not consulted the instruction handbook and with my long-term reminiscence of the dimensions’s operation having pale, I assumed they may act as energy switches, robotically turning the dimensions on once I stepped on it, as was the case with our newer (albeit not as richly featured) substitute scales:

But when so, why had been there three wires traversing the span between the system PCB and every “foot”? And why was there a seemingly redundant energy swap already on the backside of the dimensions? Inherent in my idea, maybe clearly, was the idea that the conductive pads on the highest of the dimensions did double-duty as stress sensors for measuring weight…ignoring the truth that there was solely a single wire working from every of them to the system PCB…

The one factor left to do, I made a decision, was to take one of many “toes” aside and see what was inside:

That screw was terribly tenacious, however a giant Philips screwdriver, a modicum of muscle and greater than a modicum of colourful language lastly compelled it to budge:

What is that factor contained in the two-metal-plate sandwich?

Onward…

As you possibly can see, I had much less luck with the earlier-mentioned screw’s sibling on the opposite facet of the plate, stripping it in a failed try to take away it:

So, I wasn’t in a position to fully take away no matter this was from the plate to which it was connected:

However extra random-topic on-line analysis on my half ultimately hit pay dust; it’s a load-bearing bar (there’s one in every nook on the scale-system stage, bear in mind), containing two pressure gauge sensors:

Previous-skool analog scales (you recognize, with the needle that moved) had been based mostly on some intelligent gears and levers that transformed stress on the dimensions into compression of a giant spring, then the spring’s compression into rotational movement that would drive a dial. However digital scales don’t actually have any transferring elements. They’re mechanically designed to distribute your weight evenly to a bar or assortment of bars which bend very very just below the stress. These bars are bonded to {an electrical} component that additionally flexes very very barely, altering its electrical resistance. It is a pressure gauge. Inside the dimensions I hacked, you possibly can see the 2 load-bearing bars. Every bar has two pressure gauge sensors bonded to it. Measuring the distinction in pressure between the 2 sensors can let you know how a lot the bar is flexing.

Right here’s extra (to keep away from confusion, word that what the author referred to as a load bar earlier he additionally refers to as a load cell in what follows):

The vertical bars on the left and proper are load cells, the mechanical elements which might be designed to bend barely. Each has a tiny PCB with two pressure gauge sensors. The 2 sensors share one widespread wire, so there are three wires going to every load cell.

Extra on pressure gauges right here. And I’ll share extra views, now of the assorted sides, earlier than transferring on:

Onward once more. Now that we all know that the “toes” aren’t energy switches, right here’s what the actual one appears like after elimination of the beauty plate in entrance of it:

And now for the system PCB “sandwich” at high and heart. Eradicating the 2 screws under the show:

enabled its elimination from the bottom of the four-AA compartment behind it:

Right here’s a closeup of the system PCB bottom:

The biggest IC close to the middle is presumably the system “brains”. Listed below are its markings:

38024B89WV
H8F-400A

TH39123
0833

I can’t appear to definitively ID it; reader solutions are welcomed. However I suspect it’s a member of Hitachi Semiconductors’, now Renesas’ Electronics’, H8 microcontroller household.

See these two screws on both facet, close to the highest? Eradicating these, and unclipping the white plastic hooks in the identical horizontal areas however towards the underside, ought to allow liftoff of the LCD:

Bingo!

Alas, this facet of the PCB is relatively bland:

In closing, let’s return our consideration to the within of the highest of the dimensions. Right here’s a closeup of the again of one of many conductive pads:

And its different (entrance) facet; they sadly appear to be sturdily glued in place, as I couldn’t get any of them to budge and come out for standalone perusal:

And eventually, listed below are some step-by-step disassembly closeups of the multi-switch array at heart:

And with that, I’ll wrap up. As at all times, your ideas on this “weighty” evaluation are welcomed within the feedback! In the meantime, I’ll be firing up my warmth gun to see if I can repair a number of chilly solder joints and get this factor again collectively and dealing once more…

—Brian Dipert is the Editor-in-Chief of the Edge AI and Imaginative and prescient Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the corporate’s on-line publication.

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