Taking apart a rechargeable electric toothbrush

Early on in a wireless charging piece published back in March 2019, I shared two photos of one of my rechargeable electric toothbrushes (I’m using “rechargeable” here to differentiate them from also-electric toothbrushes that instead run on standard user-replaceable batteries):

along with the following prose:

Close-range wireless transmission is commonly used for recharging the batteries built into smart watches, mobile phones, tablet computers, electric toothbrushes, and other consumer electronics devices, for example, along with pacemakers and other products surgically implanted into the human body. If you own an electric toothbrush, you may not realize it but you’re benefitting from wireless charging (and yes, I really do need to clean mine).

And yes, I still cringe with regret, every time I look at those photos, that I didn’t clean the toothbrush first …(which begs the question of why I’m proactively sharing them again…)

I had two of them (strictly speaking, still have, but I’ve retired them), one for the upstairs bathroom and the other downstairs, purchased back in April 2013 (yes, I found the archived Walmart purchase invoice email to confirm my recollection). They were described at Walmart’s website as being the “Oral-B Professional Healthy Clean Precision 1000 Power Toothbrush with Bonus Travel Case” and cost $35.97 each (presumably on sale at the time). And for around 10 years, they reliably chugged along. But in the last few months, they’ve exhibited no-longer-ignorable evidence of pending embedded-battery demise:

Sometimes they won’t start up at all unless I remember to manually rotate the bristle tip beforehand, instead immediately exhibiting a red-blinking-light low battery indication on initial power button press.

When I can get them to run, the bristles rotate more slowly than before, and I need to rush my tooth cleaning (I really hope my dentist isn’t reading this) to ensure the battery doesn’t conk out mid-brushing, and
They’re realistically no longer travel candidates, despite their “bonus travel cases,” unless I’m also able to pack the charging dock and find an AC outlet to plug it into at my destination.

Here are some overview shots of one of them (now cleaned), first mated to the charger:

Now by itself, as-usual accompanied by a United States penny (0.75 inches/19.05 mm in diameter) for size-comparison purposes:

Note the model number stamped on the underside: 3756. Hold that thought.

Here are a few closeups of the embossed markings on the sides and back:

and the top-end tip and toward-the-bottom power button and LEDs:

And here’s the charger:

What did I replace them with? Two Braun Oral-B model 3756s, purchased “refurbished” (hold that thought, too) on eBay, for $24.95 each complete with chargers. Here’s what one of them looks like (the replacement-unit chargers are identical to the one I’ve already shown you):

You’ve probably noticed, as I did in the eBay posting’s photos, that they look different than their predecessors. There’s no aqua-colored trim around the front and top; instead, there’s a red (orange?) “swoosh” toward the top, around the sides and back. Initially, I thought the “swoosh was just for looks…until I used one of them for the first time, and it brightly lit up. Turns out I ended up with feature-set upgrades!

Both toothbrush generations briefly “stutter” every 30 seconds to tell you it’s time to switch to another area of your mouth…again, I hope my dentist isn’t reading this, because rarely do I encounter even one stutter during my entire often-rushed brushing routine. But the newer units, again named identically to their predecessors (very confusing), offer both high-speed (first button press) and lower-speed (quick second button press) operating modes, with turn-off achieved by either doing yet another quick button press or via a button long-press from initial high-speed mode (I’m not going to admit how long it took me to figure that out—in my defense, they didn’t come with instructions). And if the toothbrush senses that you’re applying too much pressure to your teeth while brushing them, the red light blinds you until you chill out.

Next, let’s revisit that earlier “refurbished” comment. Turns out there’s a “cottage industry” of folks who take rechargeable electric toothbrushes like mine with defunct batteries, tear them apart and replace the power cells with fresh ones, then put them back together and resell them. To wit, before going any further I want to give full credit where due to Rob (and colleagues? Or is he a one-man shop? Dunno…) from Ionic Industries, whose detailed online disassembly, battery replacement (both same-size and larger) and reassembly guides, not to mention the accompanying video:

were invaluable. Granted, Rob didn’t just do this out of the goodness of his heart; he sells replacement batteries (not only for toothbrushes, by the way) both from the Ionic Industries website and on eBay. Still, the extensive information provided free of charge was priceless in enabling me to get my toothbrush apart (yes, dear readers, we’ll be tearing down a Braun Oral-B Model 3756 today) as smoothly and non-destructively as possible. Because, of course, someday when I have spare time (hah!) I hope to do my own battery swaps on both, a task which, you’ll soon see, isn’t as simple as it might seem at first glance (and, arguably, should be).

Before diving in, I hope you’ll indulge me one more brief topic detour. Admittedly, I’m not a mechanical engineer, so what I’m about to describe may be elementary to some. But I find the operation of these things to be mystically fascinating. You start out with a motor inside the toothbrush, which you’ll shortly see for yourself, which presumably has a normal 360° rotating shaft. That gets converted, via a mechanism which for lack of better words I’ll call a “transfer case”, into rapid side-by-side movement at the toothbrush tip, videos of which I shot using my smartphone. Here’s normal “full” speed (as “full” as the depleted batteries will allow):

and here it is slowed down by the smartphone during capture:

Now stick on the toothbrush tip a brush head, which via another integrated “transfer case” converts that side-by-side movement back into full rotation, albeit 90° from its original orientation. For these videos, I used my new toothbrush in “high-speed” mode to give you an accurate sense of what these things are capable of. Here it is operating normally:

And again, now with the phone slowing down the video during image capture:

You’ll note in those last two videos that I intentionally kept my grimy brush head (which, to my dentist’s inevitable scorn, were he to find out, I likely change less often than I should) out of the frame. Here’s what a new one looks like, accompanied by the color-differentiating ring that enables multiple people to use the same toothbrush (swapping out brush heads as-needed):

Clearly, this is a case of the “razor and blades” business model, where the manufacturer may end up going “underwater” on the initial toothbrush sale in order to hook you on subsequent repeat sales of highly profitable brush heads. And admittedly, you can easily drop several hundred dollars on a higher-end toothbrush like the one that iFixit tore down a couple of years ago (whose insides, you’ll soon see, aren’t that different than mine). Still, keep in mind as you keep reading that mine only cost me $35.97 each…after Walmart’s retailer markup. What’s that say about the bill-of-materials cost and/or the manufacturer’s tolerance to take an upfront loss?

Without further ado, let’s dive in. Ionic Industries’ website suggested that I begin with the tip at the top, specifically removing the plastic ring around the shaft, and that a preparatory soak for five minutes or so in hot water might help (a brilliant means of loosening adhesive, I must say):

After a scalding bath, the ring came right off, prompted by the application of the tip of a small flat-head screwdriver acting as a chisel:

Back in the bath for five more minutes, this time bottom-end first:

Now seat the toothbrush on the charger dock:

Gently push backwards, hopefully not breaking off the retaining tab in the process:

And we get our first glimpse of the inductive charging coil, among other things:

The spring in the base, I assume, holds the insides in place despite the high-RPM gyration vibrations they endure whenever the toothbrush is in use:

See, the retention tab survived intact!

With the base removed, the insides can now exit out the bottom, starting from the tip-top:

Here are four overview perspectives of the inside assembly, oriented the same as the toothbrush enclosure alongside it, which formerly encased it. Front:

Left side:

Back:

And right side:

Let’s now return to the front view. As keen-eyed readers may have already noticed, there’s a “switch plate” (for lack of a better term) covering the top half of the assemblage. Its purpose is to translate user presses of the previously mentioned power button on the case to activations of a switch further down on the PCB:

As you can see, the battery’s still connected:

The power button, by the way, is implemented simply; it’s just a portion of the toothbrush enclosure comprised of flexible rubberized (vs rigid elsewhere) plastic:

With the switch plate still in place, albeit lifted out of the way, we can still see quite a lot. Here’s the motor, viewed from the front:

And here’s an overview of the PCB:

There are four solder points on the right side. They are for the following (from top to bottom):

One motor connection
One of the two “tabs” for the battery underneath the PCB
The pressure warning switch (which doesn’t exist in this particular toothbrush variant but as previously mentioned is implemented in its successor; obviously the same PCB services multiple end product proliferations and generations), and
The other motor connection

To the left of the solder points is, obviously, the previously noted switch. To its left is a faintly marked TSM4424 20V N-channel MOSFET (manufacturer unknown). On either side and below it are two more unused-in-this-case solder points, for the pressure warning light. The other, larger notable IC on the board, further to the left is the system’s “brains”, a TI MSP430G2332 16 MHz microcontroller containing 4 Kbytes of flash memory, 256 bytes (yes, bytes) of SRAM, a 10-bit ADC, SPI and I2C interfaces and an integrated timer (remember the earlier mentioned every-30-second “stutter” feature?). Further to the left is the solder point for the other battery “tab” connection. And at far left are two final solder points, for the inductor coil connections.

About that inductor coil…here’s a closer look:

After unclipping its assembly at two points, you can see one of the battery “tabs” more clearly:

And since the entire inside assembly is now rotated 180°, let’s see what else we can see. First, the other end of the battery. As the Ionic Industries prose explains in fuller detail, the assembly was designed to enclose two different length batteries. Mine happens to be the shorter 42 mm one, for which Braun also included a “tab” extension so that it’d still mate up with the previously noted solder point on the PCB. You can see the extension here:

And next to the battery, unsurprisingly, is the motor (we saw the other side of it earlier), along with the “transfer case” assembly that converts its shaft’s full rotations into the side-to-side oscillations I showed you earlier:

The replacement same-length NiMH battery from Ionic Industries’ eBay storefront (for the one marked 234NV in my toothbrush), by the way, has the following specifications:

Diameter: 14 mm
Voltage: 1.2 V
Capacity: 1100 mAh
Chemistry: NiMH
Brand: FDK
Made in Japan
Tagged at both ends for soldering

Finally, let’s get that switch plate outta there. It unclips from either side pretty easily:

leaving the bulk of the assembly a bit more exposed than it was previously (I didn’t bother shooting another photo of the backside, as it was already unencumbered before):

Yes, dear readers, “finally” is right; as previously mentioned, I want to maximize the probability of being able to get this toothbrush back to fully functional condition at some point in the future, so I’m going to terminate the disassembly at this point. If I ever do tackle battery swaps in the future, I’ll report back (whether successful or not) via a future blog post. That said, I’ll leave you with two more images of the PCB underside, taken from either side, to reassure you that you’re not missing anything notable (at least here) aside from traces and solder points:

Until then, check out the additional images on Ionic Industries’ website to satiate your appetite.

Instead, I’m going to devote my remaining energy and wordcount to an attempted teardown of the charging dock. Here it is again, from various closer-up perspectives:

A hot water bath worked so well the previous two times that I thought I’d try it again:

Yeah…not so much…

Long-duration exposure to a high-intensity heat gun proved more fruitful, albeit marginally so:

But after plenty of effort (and thankfully no loss of blood), I finally got inside, wherein I beheld an interior chock-full of impenetrable white paste, completely obscuring all the circuitry, etc.:

In closing, I’ll share what I hope will leave you with a chuckle. In Googling around for ideas of how (if at all) to proceed, I came across another (quite good) related teardown video:

The above video embed window starts partway through, at the most appropriate point for this discussion (but please do watch the entire thing!). As you can see, he eventually gave up and sawed the charger dock vertically into multiple pieces with a hacksaw! And thanks to “Robert’s Smorgasbord” I’ve also learned a new term: potted!

In striving to keep Aalyia’s wrath in check, I’m going to close now before I cross through 2,500 words. Reader thoughts are as-always welcomed in the comments!

—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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