Watch out for the rabbit holes

So after digging this old DBX-100 from a drawer about a month ago, I’ve ended up saddled with a new obsession…

To date:

  • Bought new batteries for all the interesting watches in that drawer
  • Performed a heroic repair to a DB-380 with a broken lug; it’s slightly fabulous and a somewhat sought-after vintage Casio people can pay quite a bit for, and has become my new daily… My Swatch Skin Chrono is retired to the drawer
  • Bought a $9 smartwatch from TEMU, with an eye to gutting it
  • Bought an interesting $3 digital watch from TEMU, mostly out of wonder that the pictured item could cost so little (the wonder diminished when the physical item turned up and it became apparent the pictures were computer renders, but it’s still impressive)
  • Bought a very cheap (new) analog dress watch for a few quid from a UK seller on eBay because it was supposed to be really slim, but it turned out the photos were deceptive (it’s pretty decent for the money though, except for the band)
  • Bought a dead example of a slightly later model of Casio calculator watch in order to cannabalise its buttons for the DBX-100 (hit a wall when it turned out the connector ribbon was a different pitch, considering commissioning custom flex PCB now)
  • Bought a new old stock DBX-100 on eBay after I went looking for a way to repair or replace the scratched crystal (it has worse screen bleed and the rubber on the buttons is bad, no idea if the module has been leaked in and destroyed yet… checking the tracking info on that almost daily)
  • Realised there’s something wrong with my lovely Swatch Skin (the Chrono is 6.5mm thick, the plain Skin is 4mm), when it stopped shortly after battery replacement (which it did the last time I replaced the battery, but I just blamed the battery)… bought a new old stock one of those on eBay after the seller offered it for only $32 plus postage, knocking it down from $75
  • Started a thread about quartz watches in a London fixed-gear cycling forum, which is gathering steam and throwing others down rabbit holes, where I saw somebody has had a red-hot go at jamming a scratch-built OLED smartwatch inside a Casio F91-W case, which I found to be ultimate nerd catnip
  • Searching for details of what turned out to be the sadly stalled F91 Kepler led me to the discovery of Oddly Specific Objects’ Sensor Watch, a highly-refined PCB replacement for the F91-W / A158-W, featuring a modern MCU rocking open-source firmware and lasts more than a year on the F91’s CR2016 cell… The original version had a nine-pin connector for various sensors, but the only available version currently has deleted the connector in favour of an onboard temperature sensor, which it uses to make itself uncommonly accurate; bought one of those and an A158 to put it in, have to grab myself one of those nifty light spreaders for it too
  • Along the way, was reminded that the inexpensive and highly intriguing PineTime is a thing; I’ll probably end up getting one of those too

:cat: :pager:
That was fun to read :slight_smile:


It was one of these. This one is double what I paid for mine, but you can find very similar units with slightly different firmware, like this and this.

So I went the hack on it today; shot some terrible quality vid on my phone which I might edit together and chuck up on YouTube. In the meantime, I learnt the best way to crack one of these open is the opposite to what I did - pop the lens off the front and it should all come out pretty easy.

Long story short, here are all the interesting test pads… Should be possible to hack the crap out of these for the right sort of person…


Hi Kimmo
One thing I have learned from all of this.
If you want something systematically destroyed send it to Kimmo.
Just joking Ha Ha. Have fun.
Cheers Bob


Neglected to mention the reason I cut it open was because it refused to charge, after being forgotten for a couple of weeks. After I got it open, the battery tested at 0V… Pretty crappy BMS.

And after reading up a bit, I’m beginning to realise the enormity of my ambition to get some custom firmware on it… Seems like I should be trawling XDA to see if anybody’s sufficiently far along with such a project, and then I’d have to get hold of pretty much the exact model to hack, I gather.

… At least now it’s had a diet… 9.3mm


This is more like it though


Well this is interesting… Reading about those cheap smartbands, I saw reference to fake HR sensors, which I thought was a bit odd - the cheapie I cut open seemed to have gone to some length to fake it. Turns out the one with the orange and white time display is a better grade of cheapie - I bought a couple more to play with, even cheaper at around $5 each, thinking they were basically the same thing, but not quite…

These ones with the purple and yellow time display just pop right open; the screen cover is merely clipped in! The good one is glued, and inside there was a screw holding one end of the PCB, which contacted the touch interface and charging plug with nice little pogo pins, whereas this PCB is just clipped in and uses little sheetmetal spring contacts, just what you’d expect in this price range. The battery is half the size of the other one, and the ‘HR sensor’ is indeed totally fake; just a green LED connected by two wires soldered to the PCB… Here’s the proper sensor I left out of the other one as well:

The charging plug on the nasty one is at the top with contacts facing forwards; the nicer one has them on the bottom with the contacts facing backwards. The firmware is similar but different, with absolutely terrible kerning around 'i’s in the font. The LCD seems like the same unit, but the IC is different; googling “TLSR8232” led me right to this article: Reverse Engineering the M6 Smart Fitness Bracelet |

So maybe this junk could form the basis of a proper homebrew smartwatch…


Wow… what a shady move to ship a fitness / monitoring product with a fake sensor… :sweat_smile: thanks temu

That aside, in the hands of a master hacker what an affordable and flexible platform to start off with! Thanks for the updates :smiley: what’s the next step?

Next step is to email the master hacker who cracked it to see if we can give this project a bit of a push…


The 1990’s Watch Master Hacker launching into a magical girl transformation sequence upon receiving Kimmo’s email.


It’s gonna have to be one inspirational email.

Oh yeah, got my A158 and Sensor Watch PCB yesterday… Ordered on the same day, arrived on the same day, the PCB came from Canada and according to eBay, the watch was supposedly from the other side of town… Makes me wonder if it was just the vendor on the other side of town.


Turns out the market is flooded with counterfeits, and today in Oz you have to pay at least $30 to get a real F91, and at least $60 to get a real A158. So of course my $17 one was a fake, and I spent hours trying to figure out what was wrong before some folks in a watch forum set me straight…

Got a F91 turning up tomorrow, supposedly - a Sunday, thanks to those slave-drivers at Amazon. Apparently the SKMEI A158 copy is in metal even though it only costs $13 (not on Amazon though; AliExpress), so I grabbed one of those too… Interested to see if the F91 module can fit in it.

So I’ve got:


Time left
Set time
Ship’s bell
Tuning (can’t find the documentation; it just has an octave of notes)
Thermometer log

Those last two can be used to fine tune the calibration to the point it qualifies as high accuracy - +/-10s per year.


I thought to myself, you know what this thing needs - clicky blister buttons. So I spent a week looking through the optivisor futzing around with superglue and bicarb and the dremel, trying to get the packaging right…

But when I pressed the right button, sometimes it would go blank and restart… I had a squiz through the loupe, and the wire I had going from the back of the button to where it touches the clip holding everything together was really close to the end of a component, and was shorting out sometimes. Fixed that, but then then the watch was dead :weary:


Ordered another one…


So I spent bloody ages doing the blister button mod again, but as soon as I needed to redo the soldering once the glue/bicarb support was in place (and half ground away again), the solder really didn’t want to take. I got there in the end, but along with the general fiddliness of it, it meant the buttons aren’t 100% - the light button doesn’t click, and it sometimes double-taps when pressed, and the other two don’t always make contact when pressed.

So instead of trying to get the bare elements in place, I’m thinking I need to get hold of some complete units that are more suitable than those I first tried…

Anyone know of some small SMD buttons that have decent-sized solder pads for me to attach some wires to?

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How small do you need the button? Like 1mm? 1 cm?
This too big?


Yeah, those are too big.

Small. Had a dig around, went with these: 3x4x2mm to 2.5mm Momentary Tactile Push Button Switch SMD SMT PCB Mounted 2-Pin | eBay

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