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4 Sep, 85 tweets, 20 min read
I saw a tweet recently that I wanted to confirm. Sadly I can't find it right now, but it was about digital pregnancy tests.
So, I went out and grabbed a 2-pack for 7 dollars: let's tear it down! Image
This end pulls off, and has the absorbent "PEE HERE" material inside it. ImageImage
The other end has a clear "SQUEEZE HERE" section, so there's probably a button under it.
The logo says these are 733282/As from SPD GmBH, Switzerland.
That's Swiss Precision Diagnostics GmbH. Image
And... there's no screws under here. This thing is glued or clipped together. (spoilers: it's clipped)
Time to get the spudger. Image
So, with it open, we've got a single PCB here.
There's some kind of pill-looking thing to the left, a little LCD screen, and a I can see a battery on the other side. Image
So, flipping it over, we've got a battery on the left, an IC, and a long paper strip. Image
And check it out: this paper strip has a line on it... this is a pregnancy test strip, isn't it? Image
That's the kind used in the non-digital testers. The ones that show lines, right?
What's it doing inside a digital tester?

Oh boy. Image
Well if we look opposite the paper we can see there's three little holes here, and there's something inside. We need to get this plastic shroud off! Image
But first, a battery!
It's a CR1616 3V button cell. Makes sense. That's basically a cheaper smaller version of the more common CR2032, and this thing doesn't need a ton of battery life, it's a one-use device. Image
BTW, it turns out under where the wick goes is two little connectors. This isn't a test, it's just a power switch. When the wick gets wet, it starts conducting electricity, and this device turns on. Image
So inside the plastic, there's the 3 holes and a sort of 45-degree-angle ramp that had some shiny metal on it (that unfortunately fell out before I could take a picture) Image
That sits on top of this.
You know what this is?
This is three LEDs and two photosensors. Image
and it turns out that the LEDs do the edge one, then the center, then it does it in the opposite direction.
It then measures the light hitting the two sensors while it does that.
and why does it do that? so it can detect the lines on the paper!
In other words, the whole point of the digital part, the battery, IC, LEDs, and photodiodes... is to read the lines and tell you "PREGNANT" and "NOT PREGNANT" instead of "||" or "|+"
(that above GIF is what happens when I glitch it into a test mode)
So, what else?
Well, there's that IC...
It's a Holtek HT48C06! Image
That's an 8-bit microcontroller. 64 bytes of RAM, 1024 words of ROM, 13 GPIO pins, running at either 4mhz or 8mhz. Given that they're only doing a 3v battery here and I don't see any voltage conversion circuitry, I think it's running at 4mhz. Image
It's a surprisingly complicated chip. You might think it's very limited because it's only got 64 bytes of ram, but it's actually using a pipelined architecture to operate at 1 instruction per cycle, giving it quite good performance for a 4mhz CPU. Image
This thing is probably faster at number crunching and basic I/O than the CPU used in the original IBM PC, and this one is in something you pee on and throw away.
Sadly, the "C" in the HT48C06 means this is the mask ROM version, so there's no reprogramming this chip to run other programs. We can't run Doom on this pregnancy tester, I'm sorry.
It also doesn't look like you can pull the ROM off the chip, at least without their proprietary online-activation-required programmer (and possibly not even then), so unless someone wants to decap this chip and read out the bits from the silicon, we can't put it in MAME either. Image
Here's the post that I was referencing:
Mine's a different brand and has some minor internal differences, but it looks to be basically the same.
The one I got: Image
The strips inside are probably the kind that show 0-2 lines.
I may have to try printing that on some paper and seeing if that'll make the device tell me I'm pregnant or not pregnant. Image
Currently if I power it on, after doing the above light pattern, it then settles into showing me the "READ THE MANUAL, IDIOT" symbol. Image
WAITING...
it better not make me wait 9 months, because while that would technically determine if I'm pregnant or not, it certainly isn't worth the money. Image
There's also a hole on the grabby end of the tester, but you're not supposed to put anything any here, apparently.
I think it's just here so they can wedge a bit of plastic in here before final distribution, which'd keep the battery from making contact and potentially draining Image
also am I the only one who sees the USS Discovery from 2001? Image
They're both rebooting right now. They take a surprisingly long amount of time to boot, but it's the only way to get them out of the "READ THE MANUAL IDIOT" mode.
Nope, still get a MANUAL sign. I'm just not sure how these indicator strips are supposed to look, so maybe I'm putting marks in the wrong places or maybe they're too dark? I used a sharpie because it's what I had on hand.
but yeah, as for "WHY DOES THIS EXIST" when it adds basically nothing over the manual version that just shows you lines?

It's a scam, basically. Computers are cheap now. People are buying the digital one thinking it's the more accurate fancy model, but it's the same.
in fact, if anything, it's LESS accurate.
The batteries can't die in the manual model with the lines, and if your indicator paper doesn't light up a very bright line, your human eyes are probably more sensitive than the photodiodes in this thing.
And it's way more expensive.
The one time use manual version, as a generic? 88 cents, vs 7$ for 2 digital ones. Image
And that's actually one of the more expensive test strips.
Depending on how many you buy at once, you can get the generic strip versions for as cheap as 20 cents each.
SO YEAH, they're basically making money by misleading customers into thinking the digital model is better at 3.50$/test when it's just the 0.20$/test strips plus a dollar or two of electronics to read it for you
I dunked one in water (PLAIN WATER! I DIDN'T PEE ON IT, I PROMISE) and this is how it looks after about 5 minutes.
A little more transparent, and with a single blue line. Image
BTW, to answer a suggestion no one has made yet:
Yes, I could replace the microcontroller with an alternative one that CAN run doom, but:
1. it has to run on 3v, which is tricky
2. it has no inputs but "pee on it"
3. the LCD can only show "pregnant","not","manual","wait"
I should mod this into a flash drive.
I wanna hand someone some Important Work Files on a 128gb USB Pregnancy Test. Image
it has been [0] days since I stuck a metal tool into a device to trick it into thinking I peed on it Image
Yeah, I can't get it to read anything but MANUAL.
(which I don't have: I already tossed it)
Maybe just using water means it didn't light up enough places on the indicator paper? or maybe I damaged it while opening it... Image
and yes, it's surprisingly hard to read.
You'd think if they were sticking LEDs in the thing, they could have stuck one near the screen and given it a backlight.
That'd at least help you read it.
in any case, the final tl;dr: don't waste your time and money on the digital readers. Just get the cheap analog ones, specifically the kind that are just a box of test strips.
For the price of this device you can get like 25 test strips, and it'll create less e-waste.
uhhhhhhhhhhhhhh Image
Easy to use! Find out if your glass of water is pregnant in only 20 seconds! Image
walmart also sells this box, which has two tests (one analog and one digital) in the same box, and they suggest you confirm with the digital one.
That's kinda insultingly pointless if it's the same test strips in both of them... Image
Anyway, if you want to support me buying stupid things like pregnancy tests just to take them apart, feel free to send me a dollar or two on ko-fi!
ko-fi.com/fooneturing
or set up a monthly donation on patreon. Thanks!
patreon.com/foone
I don't have a full list yet, but you can see a bunch of my previous teardown threads here:
floppy.foone.org/w/Reverse_Engi…
BTW, one thing that should be said about the argument that this is actually easier to read:
You could totally design this in a reusable way to use the cheap test strips. You'd just use them and insert them into the device to have it read them.
that'd give you all the advantages and none of the disadvantages.

It just wouldn't make Walmart as much money.
ok this was definitely not the thread to post and then go to sleep on. Some updates:
Naomi Wu makes the very good point that the accuracy of tests in the lab is way higher than in the field, due to misreading tests:
So while this type of digital test may be exactly as sensitive as the non-digital kind (because they're the same test strips), by having the computer do the reading, you can take out a lot of that user error, as well as eliminating issues like different lighting conditions
So calling it "exactly as sensitive" was a bad way for me to phrase it: it's technically "as sensitive" because it's the same strips... but it implies the whole system is just as accurate, which is not true. It's going to be far easier to misread a analog test than a digital one.
I also didn't really mention the accessibility issues: A digital test is likely to be easier to read if you have limited vision.
This particular one is hard to read because of the design, but would still be easier to read if you're color blind, for example.
I definitely didn't intend to say NO ONE NEEDS THIS, but I think I went overboard in my annoyance and made it sound like I discounted the idea that anyone would find this more useful.
Accessibility is important and I didn't mean to support that anti-accessibility-for-environmental-reasons narrative that gets spread around a lot of places. That's bullshit.
Anyway, one other thing that was pointed out in a reply:
The test strips probably have to go through a length FDA certification process, so it probably made sense to build the device around a standard test strip, as it'd be prohibitively expensive to certify a separate device
And as wasteful as this device seems (I still really don't like the battery being integrated!), there's definitely a purpose for it existing, even if it's just an aid to reading a standard test strip.
Not everyone reading the strip is going to be an adult in a sensible frame of mind, with perfect vision.
You could easily be a panicked teenager in a public restroom, and having some of the uncertainty and possible user error taken out is quite helpful.
But yeah. The way I wrote this came off as misogynist to a lot of people... I'm sorry about that. It definitely wasn't my intent but I can certainly see where I overlooked aspects and phrased things badly.
Oh and one neat thing in the Naomi Wu thread that was interesting:
How the indicator paper works, and why they can't just make it say "YES" when you pee on it:
Another thing I wanted to mention but was too tired: There's actually precedent for environmental damage from pregnancy tests! Back in the 30s through the 60s, the go-to pregnancy test was African clawed frogs.
You could inject urine into them (into their leg, weirdly) and within 24 hours they'd lay eggs if there was hCG in the urine.
This was a cheaper test than earlier tests based on mice or rabbits, because those required killing the animal.
This one didn't, and the frogs could live quite a long while in captivity, allowing them to be used over and over.
The direct hCG-based tests were developed in the 60s, and these replaced the frogs.
But the issue is that these are an african frog species, and they were being used all over europe and the americas. Even if they're supposed to be contained in labs, there's going to be escapes and accidentally contact with native species
and apparently some labs just released their testing frogs into the wild.
This was a bad idea for two reasons:
1. You now have a invasive species introduced into the areas they were being used in tests
2. They spread diseases to the local frogs
One example of the latter is the disease chytridiomycosis, spread by the fungus Batrachochytrium dendrobatidis.
It was only found in africa, and then spread to north america through the pregnancy test frogs.
nature.com/news/pregnancy…
And other than the disease, they're an invasive predator that are now competing with native species. So there's been a bunch of ecological damage from pregnancy-test-frogs.

So this digital test isn't exactly unprecedented!
but yeah. The final summation is probably:
Get this digital version of the test if you need it, it's fine. If you're worried you won't read it correctly, if you have vision problems, if you're just scared... go ahead! Just please recycle the battery.
but if you don't need the accessibility features and you're going to be doing a bunch of tests (like if you're trying to get pregnant), you'll save money and generate less waste by getting the test strips. They're like 15$ for 50 of them.
And you can always get a digital one (or go to a doctor for a professional test) if you need confirmation of an uncertain result.
BTW, one thing I didn't mention because I didn't know about it:
There's a tiktok meme going around that the "pill" I mentioned is a morning-after pill.
Since this got big, lemme just confirm: NO IT IS NOT, PLEASE DON'T EAT IT
not only will it not WORK, it's a moisture-absorbing material to keep the test from being thrown off by too much liquid.
So it's basically the same as those desiccant packets you get in a lot of electronics, the ones that are covered in "DO NOT EAT". Image
when I called it a "pill" I mean "pill-shaped", but that was some bad phrasing given that hoax meme.
Anyway there's an alternate universe where I tear down one of these and find, instead of a test strip, a tiny frog in a little self-contained aquarium, and I think that's the image we should all take away from this.
I'm seeing a lot of people replying to this without reading to the corrections later on, so this is probably not worth explaining (if you're mad at me after 25 posts you're not gonna change your mind when you get to post 57).
I'm sorry about the thread's tone.
It seemed dismissive and flippant to a lot of people, which wasn't intentional. I do a lot of tech teardowns here, but usually I'm doing it to less important stuff, like computer mice or telephones and the like.
The tone I take with them is usually like "let's see what silly stuff they jammed into this thing, and what weird ways they made it work!"
I'm doing it because I find it interesting to see how things are built and how they work.
the thing I didn't consider with this thread is that while doing it that way is fine for something like a keyboard, doing it for a medical device like this, especially one that's sex-specific?
It sounded like I didn't care about the issues related to the device.
so a lot of people got the impression that I was a techbro being like "PFFT, WASTING GOOD SILICON ON SILLY WOMEN'S PROBLEMS! WE SHOULD INSTEAD USE THOSE CHIPS TO PLAY VIDEOGAMES AND CALCULATE PRIME NUMBERS!"
again, not intended that way AT ALL, but at the same time it's not like I'm blameless for how it came across. I should have thought more about how this would come across, especially once it got retweeted out of my particular follower-bubble.
So it's something to keep in mind for the future, when I'm talking about things like this which stray outside of my usual "some weird/old computer thing" teardowns. This one definitely has wider social implications I should have thought more about before tweeting as usual.

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