Toothbrush Speed Controller Secrets Revealed

Typically, when we want to build something with a DC motor, we might grab a bunch of AAs, or a single lithium cell at the very least. Electric toothbrushes often run on more humble power sources, like a single NiMH battery. They’re designed to get useful motion out of just 1.2V, and [Marian Hryntsiv] has taken a look at what makes them tick.

The article focuses on an electric toothbrush built around the Low Voltage GreenPAK™ SLG47513 chip. It’s designed to work at voltages from just 1 to 1.65 V. To make the most of the limited power available, the toothbrush stays in sleep mode most of the time when it’s not working in oral health.

[Marian] steps through the various parts of the circuit, and also explains the unique functionality baked into the brush. Of particular interest are the timer routines that guide the user through brushing each section of the mouth in turn, before a notification that tells them that 2 minutes of brushing time has elapsed. There’s also a useful explanation of the inductive charging method used.

Electric toothbrushes may be mundane home items today, but they’re an example of a product that has largely already been optimized to the nth degree. Until laser-based plaque removal or enamel regeneration technology gets off the ground, this is as good as it gets. We can dream, though!


21 thoughts on “Toothbrush Speed Controller Secrets Revealed

  1. A few years ago I had to buy a new toothbrush because the batteries of my old one were worn out and I opened the old one (from braun) and it had an MSP430 in it. There are also plenty of teardowns of all kind of models of tooth brushes on youtube.

    On a sidenote, I can sort of understand why a toothbrush costs EUR30. Some decent quality Li-Ions. motor, PCB with some chicken fodder and a water resistant housing. Some of those things cost over EUR100 and I’m flabbergasted of how to justify that. Anyone care to comment?

    1. I think the higher cost ones are usually a bit more compact, which may affect the production cost, but mainly it’s a markup cost for better/newer features and heads.

      As for why they can justify selling them for 100 – easy! On cost alone, if it saves you one trip to the dentist it’s paid for itself.

    2. At EUR100, the actual manufacturing cost should be at about 30 to 35 EUR, then you have to add shipping (ocean freight), warehousing and local distribution costs leaving maybe 30 EUR for retail gross profit. Then after their operating costs, retail is maybe making 5 EUR or maybe not for net profit.

      Neither the manufacturer in China, the marketer (the brand owner) or the retailer are getting rich selling one of these toothbrushes or any other consumer electronics to you. Economy of scale and a willingness to sacrifice a strong markup for volume driven profit are how they make any money selling them. But at thousands sold per month, they all make money.

      1. Current average net profit margin for consumer electronics is 3.5%

        However, for up-market and luxury brands there can be a lot of air in the prices. Apple’s net profit margin is currently sitting at 24.49% with a 43.18% gross margin.

        But if you want to go wild, go look at clothing and fashion. If production costs exceed 10% it’s considered too expensive: all the costs are in marketing and advertising, and a huge percentage of the products end up dumped and unsold because the market is in total saturation and the only way to sell anything is to spam cheap s*** everywhere – burn what can’t be sold.

    3. There’s MASSIVE markup involved. The Oral-B Pro 5000/6000 (bluetooth, pretty lights, pressure sensor, lithium ion battery, some adjustable settings) sells for $130-$150 CAD everywhere I’ve seen it, but then Costco will sell you a **2-pack** of them for $115. With extra brush heads.

      (Before you go “blah blah silly extra features IOT” the bluetooth is legitimately useful to me, as it lets me keep a better record of if/when I’m actually brushing my damn teeth and reconfiguring one of the modes has legitimately led to better cleaner teeth.)

      1. Indeed. I guess the EUR30 variant costs about EUR5 to EUR10 for the parts, the rest is manufacturing, transportation and markup.

        The EUR130 variant costs maybe EUR2 more to manufacture, but consumers who are willing to pay EUR1000 for a phone don’t blink at the cost of a toothbrush.

      2. I disagree and I’ve been involved with Manufacturing and Retail for over 40 years.

        That massive markup does exist when comparing material cost or in the case of retail, wholesale cost to sales price. But material cost is just one part of cost. Once you add labor, rent, equipment, utilities, insurance and a long list of other costs to that product that massive markup reduces down to single digit.

        And I’ll give you just one real example. Retail space here in the US is graded into A, B or C categories, with A rated space costing the most per square foot but should also return the most cash-flow per square foot. And if you buy an item from one of those store fronts, that business pays more for rent than he pays for inventory! This is why store front rent costs less per square foot in small towns in the USA then store front rent in a major mall. And I bet it’s the same in the EU. But that small town store won’t have near the traffic or near the sales.

    1. Among the FET’s there are both “enhancement” and “depletion” variants. The depletion variants can even conduct current though their Source-Drain channel without a gate voltage, and these need a negative voltage to close the channel. It’s just a parameter during production.

      It is also quite common for electronics to work on such low voltages. Even the “big” PC processors (both Intel and AMD) mostly work on around 1V internally. Even for small microcontrollers it is common to have an internal voltage regulator and work at around 1V, but often you have to look carefully at the datasheet for this. (STM32 for example has this) Sometimes you just find some reverences to an internal voltage regulator in the datasheet.

      But related to tooth brushes…
      I guess most work on rechargable Li-Ion, and then the electronics will never see such a low voltage.

  2. I feel like GreenPak chips are some of the best things that the hacker community hasn’t found a use for yet. They have an open bitstream and there’s even a few supported in Yosys. The actual parts inside aren’t terribly fancy (a handful of LUTs, some opamps, and in some cases, h-bridges). The form factor is a little tight, but they sell DIP breakouts as well! It feels like Silego did everything right, but now that they’ve been acquired by Renesas, these parts might languish.

      1. Hi NiHaoMike! Thank you for the interest. Mentioned in the article above, the GreenPAK™ SLG47513 does not require coding to accomplish this project (definitely, it’s an advantage for those who can’t program). It has a lower quiescent current in sleep mode. The toothbrush described above can work from 1 volt thanks to the SLG47513, and practically, there are no microcontrollers on the market that work from such low voltages. That’s why most toothbrushes use ASICs, which leads to their increased price. Please, find out more here

  3. I dismounted an Oral-B toothbrush a few years ago and it had basically an Attiny13a and a nickel-CADMIUM! 1.2v battery ,so the Attiny works at 1.2 for sure , maybe a fuses thing, not enabling any BOD or something, but that teardown made me discover the Attiny13a and reprogram it in Arduino IDE

    1. Not to mention these Oral-B toothbrushes don’t use an inductor, they use the ACTUAL MOTOR in the step-up circuit.

      When it’s turned off and you put it to your ear you can hear it.

  4. You would think a brush head is a brush head, but I know Philips spends millions on researching new brush head layouts. There is also research in the market place looking at LEDs in the brush head for gum health, etc. Don’t forget in the US, the electric toothbrush is considered a medical device, unlike other places where it is considered a cosmetic device. I can’t even begin to explain the amount of paperwork that is required to be done to move a single trace on the PCB. So there are the COGs of the item, and then there are they testing, compliance, regulatory, and research that go into building these things.

  5. Easy life hack, but an electric toothbrush (<<£10) which takes standard AA alkaline batteries, then buy AA sized NiMH cells (<£10 for 4 pack) and the appropriate two-cell charger for them. The cheap toothbrushes meant for AA alkalines run just as well on AA NiMHs, better infact, and the heads are cheap to replace every few months. DC motor expects 3V peak, 2 AA alkaline in series when fully chraged and runs down to about 2V. Works just as well for 2.4V (2 series AA NiMH) and stays at nearly full power for more uses before the NiMH cells go over the sharp end of their discharging curve.

  6. Well it’s a small world… I’m the Renesas FAE that designed the GreenPAK program with the LVPAK with a certain electric toothbrush manufacture with a specific buzz pattern. Props to our apps team for always making great apps notes!

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