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A NEW BRAIN FOR THE CASIO F-91W

Drop a programmable ARM board into a classic Casio F-91W and the watch can show your two-factor login codes.

by Joey Castillo (Oddly Specific Objects)

FULL CAD BOM FIRMWARE DOCS

Open-hardwareDisplays

difficulty
●●○○○
time
an evening
license
PCB designs: CC BY-SA 4.0 per the LICENSE.md summary (the PCB section links CC BY 4.0); library code mostly MIT, plus BSD UART code and Microchip ASF code; Second Movement firmware: MIT
repo
repo ACTIVE2,097 stars

WHAT YOU’LL NEED

Videos
1
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COMPAREE VERDICT

Sensor Watch is a replacement board for the Casio F-91W (or A158W). It swaps out the quartz movement for a Microchip SAM L22 (ARM Cortex M0+), which drives the original segment LCD and runs for over a year on a coin cell. You open the watch, swap the board in, reuse the battery clip and close it up. On the Lite board, moving the buzzer contact takes a little soldering; the Pro needs none. The preloaded Movement firmware gives you clock, world clock, sunrise/sunset, moon phase and timer faces. Through the browser-based Sensor Watch Builder or your own build you can add TOTP codes and dozens of community faces. Note which board you are looking at: the video shows the original 2022 board, with a separate temperature-sensor flex board in a nine-pin connector, and that board is no longer sold. What you can buy today is the Lite (39 dollars, temperature sensor on board) or the Pro (59 dollars, RGB LED, IR sensor, nine-pin connector), both as pre-orders. The catch: you need an F-91W to put it in, and pre-orders mean waiting.

GOOD TO KNOW

  • —Sensor-Watch repo: Lite board schematic plus Gerbers (OSO-SWAT-B1-03); the original board has only an Eagle schematic and no Gerbers listed; Pro files TBD in the README.
  • —Current firmware is Second Movement (github.com/joeycastillo/second-movement, MIT), with board targets sensorwatch_red (Lite), sensorwatch_green, sensorwatch_blue and sensorwatch_pro.
  • —No BOM for self-building. Buy the board assembled from Crowd Supply (Lite 39 dollars, Pro 59 dollars, pre-order).
  • —No case CAD needed: the board reuses a Casio F-91W or A158W case, LCD frame and battery clip.
  • —The video shows the original 2022 board with its temperature sensor flex board, running Movement 'standard' firmware.
  • —The PCB licence is CC BY-SA 4.0 per the LICENSE.md summary, but its PCB section links CC BY 4.0; both permit commercial use with credit.

Parts to buy

3 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • Casio F-91W or A158WFind
  • Sensor Watch Lite or Pro board from Crowd SupplyFind
  • 8 dollars US shippingFind

Can I build this?

Printnothing required
BuyA Casio F-91W or A158W, plus a Sensor Watch Lite (39 dollars) or Pro (59 dollars) board from Crowd Supply, 8 dollars US shipping
ToolsSmall screwdriver, tweezers, a USB Micro B cable; a soldering iron for the Lite buzzer contact
SkillsOpening a watch case and handling a small module; light soldering only for the Lite buzzer; optional C and make if you build your own firmware
TimeAn evening: the board swap is quick but fiddly, and firmware customisation takes longer
CostAbout 58 dollars for Lite plus an F-91W, per Crowd Supply's own estimate; the Pro is 59 dollars for the board alone
SafetyKeep CR2016 coin cells away from children (ingestion hazard). Otherwise no real risks.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

Videos

[mod] Casio F-91W Sensor Watch6:34

obso1337 narrates the install of the original 2022 Sensor Watch board with the temperature sensor flex board (not the current Lite or Pro). Published 25 Oct 2022.

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Gallery

Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Get a Casio F-91W (or A158W)(The board goes into the watch's case and LCD frame, and you reuse its battery and battery clip.)
  2. 2.Order a Sensor Watch Lite or Pro board (Lite costs 39 dollars, with the temperature sensor on board and a soldered buzzer contact. Pro costs 59 dollars, with an RGB LED, an IR sensor and no soldering. Both are pre-orders.)
  3. 3.Pick your watch faces (The community Sensor Watch Builder lets you select faces and download a compiled UF2 without installing a toolchain.)
  4. 4.Or build Second Movement yourself (Install gcc-arm-none-eabi, run git submodule update --init --recursive, then make BOARD=sensorwatch_red DISPLAY=classic (Lite). Choose faces in movement_config.h.)
  5. 5.Flash the firmware(Plug the board into USB and double-tap the reset button. When the LED pulses red and the WATCHBOOT drive appears, run make install or copy the UF2 file to the drive.)
  6. 6.Open the F-91W and swap the board(Remove the back and gasket, take out the module, move the battery clip (and on the Lite, solder the buzzer contact if you want sound), then reassemble. The video shows the original board, but the steps are similar.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

KNOWN ISSUES

  • Version mixing: the video shows the original 2022 board (separate temperature sensor board, nine-pin connector), which is no longer sold. The Lite has the sensor on board and no nine-pin connector, and the Pro adds an RGB LED, an IR sensor and a no-solder buzzer.
  • Both current boards are pre-orders; Crowd Supply lists shipping on 29 Dec 2026 (Lite) and 30 Oct 2026 (Pro).
  • You need a Casio F-91W or A158W as the case; the board is not a standalone watch.
  • On the Lite, the buzzer only works if you move a small metal piece from the donor Casio and solder it on.
  • Firmware has moved to Second Movement. Build with make BOARD=sensorwatch_red DISPLAY=classic for the Lite (or sensorwatch_pro), not the old movement/make instructions.
  • The standard TOTP face needs secrets compiled in; use the TOTP-LFS face if you want to load them over USB serial instead.
  • The module is fiddly to get back into the case. obso1337 found it helps to make sure the buttons are fully extended first.

Can I buy the board pre-assembled?

Yes, on Crowd Supply. Sensor Watch Lite costs 39 dollars as a pre-order (shipping listed for 29 Dec 2026). Sensor Watch Pro costs 59 dollars as a pre-order (shipping listed for 30 Oct 2026). The original 2022 board, the one shown in the video, is no longer available.

Do I need to solder anything?

On the Lite board, only for the buzzer. You move a small metal piece from the donor Casio onto the Sensor Watch board. Skip it and everything else works, the watch just will not beep. The Pro board ships with a pre-soldered spring connector, so no soldering is needed.

How long does the battery last?

Crowd Supply says over a year on a single 100 mAh coin cell; the Pro page's comparison table says 1 to 2 years for Lite and Pro. The board runs on the F-91W's CR2016, and you reuse the watch's own battery clip.

Can I change the TOTP secret on the watch?

Not from the buttons. With the standard TOTP face, the secrets are compiled in (edit totp_face.c or a totp_face.secrets.h file), so changing them means rebuilding and reflashing. The alternative TOTP-LFS face reads otpauth:// URIs from a totp_uris.txt file that you write to the watch over a USB serial connection, so no rebuild is needed.

What watch faces are available?

The preloaded Movement firmware includes clock, world clock, sunrise/sunset, moon phase and timer faces. Custom builds add dozens more from the community: TOTP codes, temperature logging, stopwatch, alarms, an astronomy face, a calculator, Wordle, tarot and more. The Sensor Watch Builder website lets you pick faces and download a ready UF2 file.

Can I use this for commercial projects?

According to LICENSE.md, yes, with attribution. The PCB designs are CC BY-SA 4.0 per the summary (share derivative designs under the same licence), though the PCB section further down links CC BY 4.0. Most library code is MIT, and the Second Movement firmware repo is MIT. The bundled Microchip framework code may only be used with Microchip products.

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Discussion1

FROM THE COMPAREE TEAM

Would you go Lite (39 dollars, solder the buzzer) or Pro (59 dollars, RGB backlight, IR sensor, no soldering), and which watch face would you write first?

CompareeTEAM2d ago

Practical notes from our check of the sources: both repos are active (Sensor-Watch last pushed in October 2026, Second Movement in September 2026). The board you can buy today is the Lite or the Pro, not the original board in obso1337's video, so the details differ: the Lite has the temperature sensor built in, and the Pro needs no soldering. Prices on Crowd Supply are 39 and 59 dollars as pre-orders. TOTP secrets are compiled in with the standard face, but the TOTP-LFS face can load them from a file over USB serial. One oddity to know: LICENSE.md summarises the PCB licence as CC BY-SA 4.0, while its PCB section links CC BY 4.0.

Joey Castillo (Oddly Specific Objects)

Joey Castillo runs Oddly Specific Objects in Brooklyn, NY. The original Sensor Watch was crowdfunded on Crowd Supply in February 2022 (2,733 backers). It was followed by Sensor Watch Lite, which puts the temperature sensor on the board and drops the nine-pin connector, and by Sensor Watch Pro, funded in November 2024, which adds an RGB backlight, an infrared light sensor, a louder buzzer and no-solder installation. The video on this page is by obso1337, who installed the original 2022 board in October 2022.

GitHub Web

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