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

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/

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.

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.

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

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

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

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...

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.

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.

sensorwatch.net has been around many years.

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

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.

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.

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.

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

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

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.

> 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.

that was very well documented! great job

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.

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

hacking f-91w been very popular

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

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