Hacking a Casio F-91W digital watch (2023)

[fellerts]

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]

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.

[timerol]

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 λ-lambda (λ/2, λ/4, λ/8, λ/16, etc.) are opportunely chosen.

This sentence is confused enough to be incorrect. λ/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). λ/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 5λ/8 antenna, but none that I'm aware of for λ/8 or λ/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]

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).

[ComputerGuru]

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]

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.

[ajot]

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...

[layer8]

The article was published in June 2023, which makes LLM use less likely for that kind of article.

[netsharc]

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...

[blargthorwars]

You’re absolutely correct! I was totally hooked by the intro—the kind that promises Forbidden Wisdom™—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.

[mschuster91]

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]

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?

[jqpabc123]

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]

Nothing was unnecessary since the goal was having fun & learning.

[Guestmodinfo]

I want to put a better backlight in my A158W. Internally F-91W and A158W are same.

[Latitude7973]

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.

[scrapheap]

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.

[0xbeefcab]

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

[forinti]

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.

[throwaway81523]

sensorwatch.net has been around many years.

[mrweasel]

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.

[askl]

> 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.

[xdavidhu]

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.

[jqpabc123]

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

[jonhohle]

Projects like ollee Watch seem to be able to stash it behind the front crystal or underneath the LCD.

[throwaway293892]

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...

[ekjhgkejhgk]

Curious, I've had one of those watches for years and never got caught.

OR perhaps your meme is out of date.

[camillomiller]

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]

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.

[t1234s]

F-91W backlight fix: https://www.etsy.com/listing/1448973768/back-light-spreader-...

[deadbabe]

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]

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.

[simonmorley]

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.

[conroydave]

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.

[NetMageSCW]

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.

[jansper39]

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.

[netsharc]

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.

[achayala]

Well documented!. I hope you don't lose your watch hehe

[puzzlingcaptcha]

Is NanoVNA still the best entry-level tool in 2026?

[bytesandbits]

that was very well documented! great job

[amelius]

Nice work but a pity they will have to redo it once their payment card expires.

[tguvot]

hacking f-91w been very popular

https://en.wikipedia.org/wiki/Casio_F-91W#Usage_in_terrorism

[anthk]

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.