A scale that tells inconsistent-weight tales

When a bathroom scale gives you multiple different weight-measurement results from consecutive usage attempts, is it cheating if you pick the lowest outcome of the lot?

Two years ago (with publication following a few months later), I took apart my wife’s fancy bathroom scale, which measured not only weight but also body mass index and fat percentage:

but whose LCD had gone AWOL and had subsequently been replaced by a simpler successor. Speaking of simple, this time we’ll look at the insides of my first digital bathroom scale, which replaced a traditional mechanical forebear. It’s Innotech’s model ID-767, the black-colored variant to be exact, which I’d bought on sale for $14.99 from Amazon in spring 2018.

Simpler vs. better

Stock images to start:

No, I didn’t keep mine next to the bed:

Hey loser, don’t you want to be a weight “losser” too?

Consistent inconsistency

About those “error-free readings within 0.2 lb” and “accurately weighs up to 400 lb” claims…

There was much to like about the Innotech model 767. It was svelte and light, with long battery life. It responded quickly when I stepped on it. And I liked its looks, too. Accuracy, on the other hand, was not its strong suite. I very well might have had a bad unit. But if I stepped on it, read the display, then stepped off and repeated the procedure, my second result would be consistently inconsistent, varying from the first by several pounds (albeit always down). And I never knew which reading to believe. The saying “you get what you pay for” perhaps applies?

And then it decided to take a spontaneous swan dive off the counter (where I’d placed it while cleaning the bathroom one day) to the tile floor below, resulting in my not liking its looks as much as before:

You’ll have to trust me when I tell you that its measurement inconsistency predated the dent!

So, I decided to retire it; more accurately, replace it (meh):

and turn it into a teardown candidate.

Incriminating reflections

Here are some overview shots to start. I have no idea who that is reflected in the first one…and speaking of weight, I’d also appreciate no snide comments about that poor person’s bulbous soft waistline, please:

The short URL printed on this sticker, as-usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes, is presumably intended to redirect here but no longer works, at least when I tried it:

This switch, when repeatedly pressed, toggles between the “3 weight units” featured in one of the earlier-seen stock photos: pounds, kilograms and rarely-seen stones:

A widely available AAA triple-battery power source (my kitchen scale, conversely, takes CR2032 coin cells, I was reminded the other night when I replaced one of the pair) is a nice touch:

Time to dive inside. Underneath each of the rubber “feet” is, to the “4 weighing sensors” highlight in one of the stock images, a strain gauge load cell. I discussed them in detail back in July 2024 so I’ll spare you the repetitive prose; check out my earlier teardown for all the details.

It’s delightfully wiggly ( and yes, admittedly, I’m easily amused):

But underneath…nope, no screw heads:

So, I redirected my attention to the scale’s sides, a decision which ended up leading to success:

Voilà:

Minimalist construction

Boring part first; here’s the inside of the lower half of the scale:

Next, the good stuff:

The first things you probably noticed were the four load cells in the corners (or maybe you saw the display-plus-PCB, in which case, please stand by; your patience is appreciated). Here they are in clockwise order, starting with the one in the upper left (upper right when the scale is in its normal usage orientation):

Here’s the first one again, being removed:

and now flipped upside down (the strain gauge structure is presumably underneath the glue):

Now for the stuff in the center (see, your patience was quickly rewarded!), the PCB, with this side showing nothing notable save for the weight-unit toggle switch:

and the next-door LCD:

Remove a few screws, and they’re free!

Now flip both 180°:

Dominating the landscape on this side of the PCB is…a blob, unfortunately obscuring the identity of the control chip. Generally speaking, considering the price tag therefore the bill-of-materials cost constraints, this design is impressively sparse in response:

The backside of the display backlight strives to redirect the aggregate glow toward the front:

where it’s further diffused by another peel-away-able layer:

Here’s the LCD itself:

As you may have already noticed, a red/black two-wire pair within the broader wiring harness powers the backlight. What about power (not to mention control) between the PCB and the LCD? That’s handled by an elastomeric strip with multiple embedded conductors, pressing against the PCB’s counterparts, an approach which we’ve seen plenty of times before:

Weighing in

For grins, in closing, I decided to put it back together and see if it still worked. Success!

Booting:

And ready and waiting to deliver additional impermanent results:

That’s all I’ve got for you today! As always, please share your thoughts in the comments.

—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.

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