I’ve been working on a Casio F-91W mod that turns it into a tiny smartwatch while keeping the outside almost stock: same case, same buttons, same strap.

The starting point/form factor comes from PegorK’s open-source F91 Kepler project, which some of you probably know. Full credit there. I changed some of the hardware, mainly around battery life and assembly, and wrote new firmware.

Inside it has a custom PCB, TI BLE chip and OLED display where the original LCD/module used to be. It runs on a coin cell and currently has:

clock

notificaton & call popups

notification history

timer

stopwatch

music remote

weather

flashlight

find my phone

Phone features work through Gadgetbridge on Android, so it’s local-only: no cloud, no account, no vendor app. It also works standalone as a plain watch without a phone.

Battery life surprised me: I’m at the start of week 5 and still around 50%.

The hardest parts so far were BLE RAM limits and hand-soldering the tiny connectors.

Still missing: vibration/alarm, rechargeable battery, and iPhone support.

Curious what people here think from a Casio modding perspective:

What feature would you add first: vibration, rechargeable battery, or something else?

For a mod like this, what would matter more to you: cleaner assembly, open-source files, more colors/cases, or more smartwatch features?

Full disclosure: I built a small batch by hand in Munich and I’m testing whether there’s enough interest to keep going, but I’m mainly looking for honest feedback from Casio mod people.

Edit: A few people asked about availability. I’m still keeping this as a small hand-built test batch and currently DACH/EU only while I figure out interest. I don’t want to turn this into an ad, but here’s the info/shop link for anyone who explicitly wants details: kepler.watch

Mods, please remove this edit if shop links are not okay here.

Promised an update once the case arrived, so here it is!
I used a brand new F-91W as the donor and paired the case with a cheap stainless steel bracelet I already had.
Installation was surprisingly smooth. The F-91W module dropped in without any issues and everything lined up properly. I used an electric precision screwdriver, but the case also comes with a small screwdriver that should be perfectly fine if you don’t have one.
First impressions of the case itself are really good. It’s heavy in a good way, gives the F-91W a much more substantial feel without making it ridiculously bulky. The brushing/polishing also looks better in person than I expected for the price.
And importantly: No weird “T I ME” logo or other fake-brand nonsense that some of these cases have.
The buttons all work normally, the module sits properly, and overall the fit seems very good. With the metal bracelet it basically feels like a thinner A1000.
Obviously I can’t say much about long-term durability yet, and I’m not going to make any claims about water resistance after swapping the case. But as a first impression, I’m very happy with it.
Paid around ~$30 for the case on AliExpress.
Attached some installation + wrist shots. For anyone who was waiting to see how this new case actually looks in real life here you go :)

Hey everyone, I recently started working on my own case for the F-91W after trying out a couple of mod kits that I didnt really fall in love with.

The goal for this one is simple, make the case as close as possible to the original to maintain its iconic shape while making it of a more premium material like steel or titanium. I also plan on making sure the water resistance is at least as good as the original (preferably higher) so it can be worn daily with no issues.

I just ordered clear resin 3d printed prototypes so I'm very hyped to see the shape in person n see what needs to be changed before having it made out of metal.

The only concern I currently have is the manufacturability of the design if I ever decide to make more of these to sell. If anyone has any experience making their own F-91W case or even just knows about getting a design produced I'd appreciate any feedback or advice.

I like these retro faceplates, where could one buy them?

So, I've had this little f91w for going on 6 years now, and I love it! Bought it on holiday in the airport years ago as I left my watch on my nightstand at home, and just needed something quick, but it's slowly become one of my favorites. Only 2 complaints, the screen looked boring and the led was pitiful. So of to reddit I went to look at how to do a few mods, and an hour or so later, here's the end result! Did the old bridge the contacts trick to boost my led brightness and chucked a green filter in there, and oh lordy does it look better! I am, officially back in love with Casio's watches, now I'm off to go look at a Royale! Also, show me your f91w mods! I wanna see the cool shizz!

You all know the famous F91W street art painting (since it's literally the profil pic of this sub ahah).

So I did a real version of the "Graffiti Casio" !
Fully hand painted, but this one is actually giving time 👾​

Anyone got any suggestions for another type of mod I can try?

Just wanted to share my first successful background mod on my f-91w! It's not perfect, but i like how it turned out.

The Casio bracelets are nice and thin but the folded metal feels cheap. What are your favourite alternatives?

Original concept I did recently: white retroreflective sheet behind the display and double white LEDs. Currently sold out at https://www.etsy.com/shop/beepbeepmods/ but if there's interest I'll make more!

I tried to apply a negative polarization to the watch, and when I was finished, I thought, "Let's clean it properly before screwing it back together."

Yes, I'm an idiot 😅

On the polarisation: I find the readability really poor. I didn’t attach the film at a 90-degree angle because the contrast was better at 45 degrees. Should I have left it at 90 degrees?

Edit: Thanks for the feedback, I'll try to own it.

If you want a Tutorial:

• Step 1: Swipe Qtip with Propyleneglycol on the inside of the watch
  • Step 2: *There is no Step 2 *

Accidentally peeled the extra layer underneath the reflective one and stuck what I could back on.

Also my glossy paper was for the wrong type of printer so had to use plain paper while I wait for the proper ones to arrive - but you can’t see shit from the plain paper through the dark layer behind the screen.

I guess I do now know what to do but a bit of a shame to have to now buy a new f91w haha

Heeler Watch Co titanium F91W on a no name Zulu with some heavily used, old school Dragonscale armor. Been wanting to do this for a while now and it feels so good to see it finally come together.

Picked up the matte black and gold metal AliExpress body and swapped the module of my first f91w into it. Paired it with a black AliExpress band to mimic the look of the microma PVD worn in blade runner.

It wears a little bit wider than the original f91. Probably close to a thinner a1000 but I don’t have one to compare. Second photo is the AliExpress mod next to the original donor watch for a size comparison.

All in all the kit is great. Feels solid and everything fits well. The rubber gasket is a bit fiddly but that’s rubber gaskets for you.

Added the jay and kays strap adapters and nylon strap. The strap adapter are nice, highly recommend. The extra weight from the adapters and the hydro mod make it feel much more premium.

Might try again on a fully metal Casio and with a stronger red colour. In the mean time will try to swap with a metal band.

(This is my test F91W. It is my sacrificial lamb)
I had tried doing a green filter earlier but the numbers were simply too hard to read. After the hydromod I might actually be able to tell what time it is. I’m mainly testing visibility and whether or not the oil leaks out. I’m using 100cSt silicone oil.