Tesla Key Watch

Using a Casio F-91W to access my Tesla Model 3

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Embedding a J3A040-CL chip and a custom antenna into a modified Casio F-91W for accessing my Tesla Model 3.

Tesla Key Watch


Before getting into the nitty gritty details of this project I want to thank two people that helped me a lot and inspired me:

Without these two this project wouldn't have been possible.


Let's start with the usual: "I love my X but...", in this case the "X" is my Tesla Model 3 and the itch I cannot scratch is that, from time to time, the keyless entry doesn't work as expected. This usually happens in the worst of circumstances (Water is pouring from the sky? Well I won't open for you sorry...). Something hacky was needed....

The main inspiration for this project came from N-O-D-E and his Data Runner watch. After seeing it I decided I wanted to open my car using my watch. Researching the project I came accross the Gauss-Key-Card project and decided to follow Darconeous steps by making my own matched antenna for a J3A040-CL card from Smartcard Focus. I didn't want to use an official Tesla card for this because the phone call to request a duplicate, if things went south, would have been quite awkward. I didn't want to use the PCB antenna from the Data Runner watch either, because the J3A040-CL antenna matching could be way off, compromising usability. So, custom hand-crafted antenna it is.


I'm quite happy with the results. The NFC communication is quite good and usable even in a pinch (more than taking out my phone or the key card from my wallet for sure) and the watch is comfortable to wear almost as it was before.

The fact that the mod is completely hidden is a big plus to me. You wouldn't know the watch act as a key from the look alone, and once worn the chassis modifications can't be seen.


As always there's things that can be improved on:

  • The watch is almost as comfortable to wear as before. "Almost" is the key word. It scratches a little on the side where the layers of the 3d print join near the screws. I'll need to think of a better design or change the medium (a resin cast maybe? transparent would be a lot cool).
  • The buzzer was removed with the backplate, would be nice to find a replacement that can fit in the new case. If anybody knows of a buzzer that can be put in place of the standard piezo please tell me!
  • The NFC range is good but can always be improved on. N-O-D-E's idea of a PCB antenna is great but we need to take matching into account and without a datasheet to at least go in the neighborhood of 13.56MHz for the resonant frequency we're flying blind. I also considered the removal of some metal parts from inside the watch, but I'm waiting to think of a better alternative before doing destructive modifications to the watch.
  • I don't think the watch is water resistant anymore.


Modded F-91W backplate

Standard Tesselated Geometry - 86.02 kB - 09/01/2020 at 17:10



Loop antenna holder for the NanoVNA

Standard Tesselated Geometry - 196.57 kB - 08/30/2020 at 14:44



Backplate antenna guide

Standard Tesselated Geometry - 46.18 kB - 08/30/2020 at 14:44


  • Part 9: Ambition

    Mattia Dal Ben2 days ago 0 comments

    Today I designed the new frontplate, taking into account the new bit I bought which is 2 mm in diameter.

    I also realized that I have a few engraving bits laying around that I could use for making this frontplate sexier. Here's the idea:

    Am I getting overly ambitious? Yes. Will I try this anyway? Hell yes.

    Whish me luck!

  • Part 8: Getting there

    Mattia Dal Ben3 days ago 0 comments

    I was finally able to get my hands on some 2mm thick acrylic and test the production process of the antenna I thought.

    Let's start with the 2mm thick acrylic frontplate.

    This looks AWESOME! (Since I was at it I alsso modded the screen for a darker look). 

    The matte look left from the CNC machining is really cool and I love it. I'm trying to leave it as is.

    Regarding the production process I designed and 3d printed a guide that can be broken up into parts after winding up the antenna.

    This way I was able to buld the antenna on the guide in a much more comfortable way and, once finished tuning the resonant frequency, I was able to break the guide into parts and tranfer the antenna on the frontplate without damaging it.

    I then wanted to try fixing everything up using some UV curable resin. That was a bad idea...

    I lost the matte look on the top half of the frontplate and it now looks like its foggy. Furthermore some resin creeped on the screen ruining its transparency.

    Finally I decided to stop using the resin altoghether and keeping things simple: I will now design the new frontplate without the hole for the chip. It will be just a recess in the acrylic. This way, if things go wrong, I will be able to rescue the chip.

    Last thing to do is decide how to mount the frontplate to the Casio chassis. I have two options:

    • 2mm thick double sided tape (already bought for this purpose).
    • T-7000 glue

    The second seems more fit for the purpose at this point. I'll have to try thing out.

  • Part 7: Changing gear

    Mattia Dal Ben09/13/2020 at 13:35 0 comments

    The UV curable resin front glass was a dead end...

    After trying to obtain a glass-like effect multiple times and failing I abandoned this path. I couldn't get a totally clear look, no matter how I hard I tried. Here's the best looking one so far:

    Then it hit me... the original front plate is made of acrylic. If only I had access to something that can cut acrylic...

    Oh wait, I do! My dear Shapeoko!

    It took me an embarassingly long time to figure out I could CNC this part out of acrylic. To try things out I cut it out of a 3 mm thick sheet I had lying around and the result is amazing!

    It looks AMAZING! Apart from the awkward thickness. Keep in mind that in the pic above it wasn't seated properly so it look thicker than in reality.

    I plan to use a 2 mm thick acrylic sheet for this which shouldn't get so obnoxious. We'll see...

    Now I need to try and build the antenna out of this and see what it looks like. Depending on that I'll figure out if it'll need some paint. I already experimented a bit with nail polish and resin to see if there's any bad reaction with the acrylic and things should be good.

    Final note: Here's what happens if you forget to use the proper mill for acrylic...

  • Part 6: Resin backplate and frontplate progress

    Mattia Dal Ben09/08/2020 at 17:02 0 comments

    While I was waiting for the necessary instruments for the front plate (2mm thick double sided tape and UV torch light) I ordered a resin printed backplate thinking it would solve the little scratching feeling from the FDM printed one (I know I could have gone with the sending route but hey, I wanted to try a resin printer for once :D).

    Not having a resin SLA printer I used an online service to print it. Albeit not being 100% satisfied by the looks, the print actually delivers on the smooth factor. No more scratchy feeling and super comfortable to wear.

    I think that this idea cannot go further then this so I'll focus on the frontplate antenna for now. Speaking of which...

    I was able to build a working antenna using 0.1 mm thick enameled wire... and it was actually easier than using the 0.25 mm thick one! Unfortunately I'm having trouble in building the glass for the screen using the UV curable resin due to the high temperatures it reaches while curing under the sun. To this end I bought a UV torch light from Amazon hoping it will solve the issue...

  • Part 5: Partial success

    Mattia Dal Ben09/04/2020 at 20:15 0 comments

    I was able to go through a first prototype of the new version and I'm pretty happy with the results.

    Aesthetically I love it (still need to dial in the resin curing process), I've lost the stealth factor but this thing is super cool to look at. Even more so when the real chip will be in place. Unfortunately the thing is really hard to build too: I was able to cram this amount of copper wire before running out of vertical space and unfortunately this wasn't enough to get reasonably close to 13.56MHz. Furthermore getting the wire in place required an unreasonable amount of nail polish.

    I'll try to find some more space by thinning the faceplate (I don't want to get too high since it's already 1mm thicker than the original one) and I still have my secret weapon:

    0.1 mm thick enameled wire.

    It'll drive me crazy but could be a solution.

  • Part 4: Front antenna

    Mattia Dal Ben09/02/2020 at 17:37 0 comments

    So... I got an idea for improving the watch mod.

    The original idea by N-O-D-E was to put the antenna behind the frontplate using a PCB antenna, which is great but leave me no room for resonant frequency tuning. What if I can tune the antenna anyway?

    The idea is the following: what if we can build an antenna using the above part as a guide and as the front plate? Showing off the back of the chip (which is awesome) and leaving all the room needed for the tuning?

    But Mattia, you say, how do you protect the watch now that we have a big gaping hole in front of the screen?

    UV curable resin.

    This will cure perfectly clear and fill all the needed gaps, with the added benefit of making the frontplate and the antenna a whole thing.

    But Mattia, you say, how do you glue this new frontplate to the watch?

    2mm wide double-sided tape usually used in smartphone repair (again thanks to N-O-D-E for the idea).

    This new design should solve almost all the shortcomings of the old version:

    • Buzzer gets back in place
    • No scratches from the 3d print
    • Improved range because the antenna is not behind all that metal

    Let's see if it can actually be done...

  • Part 3: Building and matching the antenna

    Mattia Dal Ben08/30/2020 at 13:34 0 comments

    The first step in building the antenna is to wrap the wire around the guide. I used 0.25 mm thick enameled wire for this and then glued it together with some nail polish.

    The nail polish will stick together the wires and make it possible to later remove (with some caution) the antenna from the 3D printed part. You need to be sure to add enough loop of wires for the resonant frequency to be way lower than 13.56 MHz. How do you know when to stop you ask? Experience.I got the hang of it after a few tries. Always add a few loops more than what you think you'll need.

    After the nail polish was dry, I scraped off the first layer of enamel from the antenna ends and soldered the chip on.

    After checking that the resonant frequency was low enough (9 MHz for me at this point) I moved the antenna to the modified F-91W backplate for the fine tuning.

    And now for the hard part: fine tuning the resonant frequency. The PCB and the thin metal casing inside the F-91W will modify the resonant frequency of the antenna A LOT! I then needed to test the resonant frequency while the chassis was mounted on the watch.

    Fun fact: even the screws of the watch modify the resonant frequency! I learned it the hard way.

    The fine tuning goes like this:

    1. Start by a lower frequency than the target one (13.56 MHz)
    2. Remove a loop
    3. Is it lower? Go to step 2. Is it higher? Go to step 4.
    4. If necessary add some copper tape directly to the antenna to lower the resonant frequency a little.

    Here's the final results for my watch

    Apparently a higher (13.7MHz) than normal resonant frequency is what the Tesla Model 3 likes the most.

  • Part 2: 3D printing

    Mattia Dal Ben08/30/2020 at 13:12 0 comments

    First thing I designed was the new F-91W chassis

    Obviously it took me a few tries to make the new chassis fit properly.

    Well... maybe more than "a few".

    This process was made way easier thanks to N-O-D-E sharing his project files.

    Then, to make my life easier, I started printing a few instruments for building the antenna. The first thing I designed was a guide for the wire to be wrapped around.

    Then I needed a loop antenna for my NanoVNA for the antenna matching stage.

  • Part 1: Getting the chip

    Mattia Dal Ben08/30/2020 at 12:56 0 comments

    As suggested by darconeous I got myself two (because you never know) J3A040-CL Java cards from Smartcard Focus. I uploaded the Gauss-Key-Card applet using an ACR122U and the GlobalPlatformPro project and then paired them with my car. This part was surprisingly easy thanks to darconeous and GlobalPlatformPro work.

    Then it was time to extract the chip from the card. To this end I used the usual acetone trick. To save on acetone I cut the chip from the card (usually you can spot the chip through the PVC layers of the card) and put it in an empty tea candle holder. 

    After two hours of acetone bath I could take it out and extract the chip using tweezers and a knife.

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