[HN Gopher] Hacking a Casio F-91W digital watch (2023)
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Hacking a Casio F-91W digital watch (2023)
Author : jollyjerry
Score : 208 points
Date : 2026-01-04 16:05 UTC (5 days ago)
(HTM) web link (medium.com)
(TXT) w3m dump (medium.com)
| ComputerGuru wrote:
| I was captivated by the introduction but I only made it halfway
| through before I gave up. The random bolding and emphasizing of
| almost every other word really put me off. Also, even though I
| can see that it's clearly had a ton of human effort put into it,
| I got a lot of AI vibes from the writing.
| eresonance wrote:
| Yeah I agree, I'm wondering if the author's primary language
| isn't English and they leaned heavily into AI to get this
| article together. Either way I found the subject cool but the
| writing hard to go through.
| FridayoLeary wrote:
| lots of spelling mistakes which is odd for such a
| meticulously detailed article. I'm going for the theory he's
| not an english speaker. Op is wrong, it's one of the most
| interesting things i've seen here in a while and truly in the
| spirit of hn.
| ale42 wrote:
| Well, everything in the article (names & places) seems to
| point out the author is Italian...
| blargthorwars wrote:
| You're absolutely correct! I was totally hooked by the intro--
| the kind that promises Forbidden Wisdom(tm)--and then I made it
| about halfway before my attention quietly left the chat. The
| random bolding of almost every other word felt like getting
| yelled at by typography. I wasn't reading anymore; I was
| flinching. And sure, I can tell there's a ton of human effort
| in there, but the overall vibe? Very "AI but with a human
| supervisor." Polished, eager, slightly over-explained, and
| aggressively formatted. I'm sure it's useful. I just couldn't
| finish.
| ThePowerOfFuet wrote:
| It's leaking
| netsharc wrote:
| Yeah, it's fucking goddamned excessive.
|
| Changing the CSS rule .jb { font-weight: 700; } to .jb { font-
| weight: 500; } got rid of all the bolded parts for me...
| ajot wrote:
| You can read it in the Medium frontend scribe.rip, to avoid
| some Medium shenanigans. I do think bolding stays the same
| though, it seems to have lots of bolding. Weird style.
|
| https://scribe.rip/infosec-watchtower/how-i-hacked-casio-f-9...
| ComputerGuru wrote:
| No, it doesn't. This isn't a Medium gripe (which sucks but
| isn't to blame for once).
|
| Edit: this comment made sense before the post I replied to
| was edited to clarify that there is no material difference.
| layer8 wrote:
| The article was published in June 2023, which makes LLM use
| less likely for that kind of article.
| throwaway81523 wrote:
| sensorwatch.net has been around many years.
| mrweasel wrote:
| That still won't do contactless payments, at least I don't
| think so.
|
| It's a neat idea, to just move the chip. The final result isn't
| pretty, it would have been way cooler if the whole thing had
| fitted inside the watch, but there's not a lot of room in an
| F-91W.
|
| Maybe someone with more experience with antenna designs could
| find a way to use the backplate as part of the antenna.
| tguvot wrote:
| hacking f-91w been very popular
|
| https://en.wikipedia.org/wiki/Casio_F-91W#Usage_in_terrorism
| anthk wrote:
| That's because the F91W was like Game Boy of watches. Or the
| Tetris under a 'Brick Game' branded cheap console. Every dad
| owned one. Reliable, cheap and damn long lasting.
| Findecanor wrote:
| The world's most sold digital watch. Widely available since
| 1989. There are even multiple clones out there for a fraction
| of the price.
| Guestmodinfo wrote:
| I want to put a better backlight in my A158W. Internally F-91W
| and A158W are same.
| scrapheap wrote:
| You can get light spreader kits for the F91-W/A158W that
| replace the bit of plastic that sits behind the LCD for one
| that spreads the light from the LED far more evenly than the
| stock one.
|
| I recently fitted one on my F91-W and it certainly makes a
| difference, but it's not going to make the light brighter like
| some of the other LED mods people have done.
| Latitude7973 wrote:
| n-o-d-e (blogger) has published many mods for this watch, with
| videos included. They have great production quality, and the
| mods are easy enough to pull off. https://n-o-
| d-e.net/casio.html
|
| When I did this mod, I soldered the LED on its side, so that it
| shone directly onto the LCD. This made the watch face much
| brighter.
|
| On reflection, the similar variant of the watch with an EL
| display is vastly superior. The A168 is similar to the A158 you
| mentioned but with a better light.
| fellerts wrote:
| Cool project. Tip: acetone will readily dissolve the plastic
| parts of the card leaving the antenna(s) and chip intact. I
| believe a few other people have made drop-in PCBs with NFC
| antennas. Here's one: https://n-o-d-e.net/datarunner.html
|
| Also, no F-91W thread would be complete without a mention of the
| sensorwatch project: https://www.sensorwatch.net/
| dmaa wrote:
| The dissolution in acetone is nicely demonstrated here
| https://youtu.be/NF4VJJKTjy8?t=835 The guy does it in order to
| be able to pay with a prosthetic eye.
| alejoar wrote:
| This is hilarious!
| afandian wrote:
| I would like to encourage everyone to set some time aside,
| get comfy, and watch Bobby Fingers. His videos are
| extraordinary, special, things.
|
| I think making a giant Jeff Bezos boat would appeal most to
| this crowd, but I'd suggest watching them in order.
| piltdownman wrote:
| For anybody wondering wth is with a guy sculpting dioramas
| about obscure celebrity misadventures - he was formerly half
| of the Hip-Hop Comedy Duo, Sketch Artists, and behemoth Irish
| podcasters 'The Rubberbandits'. Probably one of the greatest
| neo-dadaists and proponents of Aestheticism in Europe today.
|
| https://www.unitedartsclubdublin.com/post/bobby-fingers-
| the-...
|
| Some might even know them without realising it - their
| incredibly provocative Richard D. James-esque 'Dad's Best
| Friend', a sonic assault about isolation and toxic
| masculinity, was featured prominently in the Trainspotting
| Sequel, "T2"
|
| https://youtu.be/iYgPznBrjiA
| NetMageSCW wrote:
| I've been doing Apple Pay without issue with the phone since it
| started, and doing it without authentication since the Apple
| Watch came out. The (very) few issues wouldn't be fixed by
| putting the system in a different case. Seems like a lot of work
| for nothing.
| netsharc wrote:
| I watch a YouTube series of 2 guys who restored a classic Mini
| car (putting the running gear of another car into it, adding
| bells and whistles and more bells and whistles). They've been
| going at it 12 years, when they could've just bought a modern
| car. Seems like a lot of work for nothing.
| jansper39 wrote:
| This is Hacker News is it not? I'm sure if this guy wanted to
| buy an Apple watch, he could have just done that.
| camillomiller wrote:
| Another good effort and well intentioned piece ruined by soulless
| AI writing. If English is not your primary language, it's ok.
| Rather write with mistakes and quirks: it would still feel more
| human and readable than a draft flattened and weakened by using
| chatGPT
| tomaytotomato wrote:
| I think going by the timestamp of the article and checking the
| article on a couple of AI detectors, that this article has a
| low probability of being written by GPT.
|
| More than likely just translation anomalies from Italian to
| English.
| mschuster91 wrote:
| The problem with this approach is, every once in a while - I
| think the bank sets the limit - the card will not accept any NFC
| transactions _at all_ until a physical confirm-with-pin
| transaction is made.
|
| That frankenstein'd chip... no way it can ever be assembled back
| into a "holder" card without risking everything breaking.
| taneliv wrote:
| Yes, at least according to the information I have, the card
| issuer sets the limit. It's not clear to me if the chip
| actually has to be physically slotted in for reading it,
| though. Can the confirm-with-pin step be done without contact,
| using NFC feature?
| rubslopes wrote:
| For me, it'll randomly ask for the pin code as OP reported,
| but card insertion is never required.
| mschuster91 wrote:
| Again I guess it depends on the bank. I have to go through
| the full insert physically so that the contactless-count gets
| reset. And of course, it _always_ happens when buying a pack
| of smokes or a drink at a vending machine that does not have
| a PIN pad -.-
| amelius wrote:
| Nice work but a pity they will have to redo it once their payment
| card expires.
| bytesandbits wrote:
| that was very well documented! great job
| forinti wrote:
| These F-91W projects inspired me to buy one, but I found it to be
| so small (and I don't have a particulary big wrist). So I gave it
| to my 12 year old and bought a W-217H, which is almost identical
| but larger.
| xdavidhu wrote:
| I wish there were a way to put the chip & antenna inside (or
| behind?) the watch casing.
|
| While this is really impressive, putting something in front of
| the faceplate breaks the great F-91W look and feel for me.
| jonhohle wrote:
| Projects like ollee Watch seem to be able to stash it behind
| the front crystal or underneath the LCD.
| xdavidhu wrote:
| Ollee does that by replacing the original PCB. In this
| project the movement is kept original.
|
| I recall the blog n-o-d-e did successfully put an NFC chip
| inside while also retaining the default PCB, so there is at
| least some free space in the case...
|
| https://n-o-d-e.net/casio.html (0:56)
| jqpabc123 wrote:
| Or just forget the watch and casing and build an add-on to the
| strap to hold the NFC chip. This should work with any watch
| strap that isn't metal.
|
| Similar project I did to carry a microSD memory card.
|
| https://www.thingiverse.com/thing:6784665
| simonmorley wrote:
| I enjoyed reading this, a lot. Really not really sure why
| everyone's more bothered by the use of ai to help write it. But
| you can't please everyone.
| t1234s wrote:
| F-91W backlight fix:
| https://www.etsy.com/listing/1448973768/back-light-spreader-...
| deadbabe wrote:
| I don't know why Casio doesn't just do something like this
| themselves, I'd buy it. Don't need a super smart watch but do
| like contactless payment with a wrist.
| jqpabc123 wrote:
| Apparently, Casio fails to recognize that nowadays, most people
| don't really _need_ a watch for timekeeping because they carry
| a smartphone that does time perfectly well.
|
| I wear a cheap $5 Chinese made watch only because I use it to
| carry a backup of all my crucial data with me at all times,
| even in the shower. If something goes bump in the night, my
| data is always with me so all I need to worry about is my own
| personal safety.
|
| https://www.thingiverse.com/thing:6784665
|
| Casio could easily and cheaply add this to their watches and
| instantly make them much more desirable in my opinion.
| deadbabe wrote:
| Is your data encrypted at rest? Someone could kill you and
| steal it very easily.
| speedgoose wrote:
| If the threat model includes murder, they can get my
| encrypted data anyway.
|
| See XKCD #538: https://xkcd.com/538/
| jqpabc123 wrote:
| The watch itself is certainly nothing anyone would kill me
| for <g>.
|
| But yes, the entire microSD card is encrypted using
| BitLocker in case it happens to be lost or otherwise falls
| into the wrong hands. My will contains the decrypt
| password.
|
| If you think about it, my data is as safe as I am --- if
| not safer. The authorities aren't going to spend any effort
| to recover a memory card --- but they will to recover a
| body with a memory card attached..
|
| Since this is always with me, it serves as the off-site
| copy in my 3-2-1 backup plan.
|
| https://www.backblaze.com/blog/the-3-2-1-backup-strategy/
| askl wrote:
| > I thought it would be nice not to have to take out my
| credit/debit card from the wallet or my mobile phone from the
| pocket to pay, but instead, to bring the watch closer to the PoS
| and just pay with a pinch of modern-day magic .
|
| I don't know, every time I see someone paying with their watch it
| looks super awkward when they have to twist their arm so the
| watch gets close enough to the reader.
|
| Most of the time they are holding their phone in their other hand
| which makes the whole thing extra silly and unnecessary.
|
| I still think this is a pretty cool project.
| timerol wrote:
| This is a really cool hack, and I wish I could pay for things
| with a dumb watch. It's just the right level of useful and silly
| to be up my alley. But the article, as others have mentioned, is
| a little off. The author did not "invent" the guess-and-check
| method for verifying resonance. That's been a staple of radio
| since the beginning, which is why original tuner dials were
| actually variable capacitors
|
| > Therefore, an ideal antenna should consist of a 22.12 metre
| long wire, but by convention fractions of l-lambda (l/2, l/4,
| l/8, l/16, etc.) are opportunely chosen.
|
| This sentence is confused enough to be incorrect. l/2 is
| generally preferred as an antenna length (standard dipole
| configuration) because it will resonate at the appropriate
| frequency with desirable standing wave characteristics (current
| maximum and zero voltage at input, voltage maximum and current
| minimum at ends). l/4 can be used as a half-dipole, but it
| requires a ground plane to resonate properly. There are also
| arguments to be made for a 5l/8 antenna, but none that I'm aware
| of for l/8 or l/16.
|
| In practice for small antennas, physical length and electrical
| length are only tenuously related, so it's a matter of creating a
| circuit that acts like an antenna of the chosen length.
| x-n2o wrote:
| Is there a short but decent RF/antenna crash course? I'm
| fascinated by the topic -- though the prospect of going through
| a textbook and revisiting the physics rabbit hole gives me
| massive anxiety (it always ends up recursing to philosophy).
| conroydave wrote:
| i love this and its something i had always wanted to try. I am
| actually kind of ticket off at chatgpt as i asked it about 6
| months ago how something like this work and it told me it
| physically wasnt possible. maybe i need to improve my prompting.
| puzzlingcaptcha wrote:
| Is NanoVNA still the best entry-level tool in 2026?
| achayala wrote:
| Well documented!. I hope you don't lose your watch hehe
| jqpabc123 wrote:
| Interesting --- but it looks like a lot of unnecessary work to
| me.
|
| Forget modifying the watch itself. Instead, build a _strap_ add-
| on to hold the NFC payment chip and antenna.
|
| Here is a project where I did something similar to carry a
| microSD memory card.
|
| https://www.thingiverse.com/thing:6784665
| mrb wrote:
| Nothing was unnecessary since the goal was having fun &
| learning.
| throwaway293892 wrote:
| Doing so gives you free room and board at Guantanamo Bay. If you
| ever wanted to visit Cuba ...
|
| https://www.theguardian.com/world/2011/apr/25/guantanamo-fil...
| 0xbeefcab wrote:
| Quick correction to the opening paragraph: You actually can use
| tap-to-pay without unlocking your phone (for iOS at least) when
| paying for a public transit fare. Theres a setting called transit
| mode or express or something. Once enabled, you just hold your
| phone up and it works without authenticating
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(page generated 2026-01-09 23:01 UTC)