Unique MOD for my Casio FW91 watch

Introduction

Explanation of the MOD concept

The mod is a simple way to add additional functionalities and smart-watch like feeling to an old watch. I think the aim is to think of this as a hackable watch that you can do and design it to be whatever fits best in your lifestyle.

Additionally, as there are hundreds of colors and compatible designs this could be easily added to your everyday wear with close to no issues.

Original Casio F-91W Features

The Casio F-91W watch has several functions:

Stopwatch: A 1/100-second stopwatch with a measuring capacity of 59'59.99'' (whatever that means, it is a stopwatch and quite accurate). It can measure elapsed time, split time, and 1st-2nd place times.

Alarm: A daily alarm that lasts 20 seconds, and an hourly time signal.

Calendar: An auto-calendar that sets February to 28 days. It doesn't account for leap years.

Light: A green LED backlight on the left side of the display.

Accuracy: The watch is accurate to within ±30 seconds per month.

Timekeeping: The watch displays the hour, minute, second, PM, date, and day in 12/24-hour format

My personal F-91W is further modified and has the following features as well:

Amber LCD

New bright white LED

Scannable NFC tag (New NXP NTAG213 type with 137 bytes of storage, located behind the LCD). That it is not a tap-to-pay chip.

Sensor-Watch Third-Party Circuit Board Modification

Description and specs of the new circuit board

The best way to describe this board to me is pebble board meets the 2020’s. It might not be as feature rich as the pebble watch out of the box but with some creativity and elbow grease or maybe brain grease?? you can turn it into something cool and useful.

ARM Cortex M0+ microcontroller running at up to 32 MHz

256 KB of on-chip Flash, with up to 8 KB EEPROM emulation area

32 KB of RAM with full retention in low-power standby mode

32.768 kHz crystal for real-time clock functionality with alarm support

Red & green PWM’able LED backlight

Temperature sensor

On-board USB Micro B connector

Reset button with double-tap UF2 bootloader

Controller for ten digit segment LCD, plus five indicator segments

Edge-plated contacts for three interrupt-capable buttons

Connection pad for piezo buzzer (requires light soldering)

Open Source

The A1 and A4 test points offer some additional functionality:

Two interrupt-capable digital inputs, with internal pull-up or pull-down resistors

Two digital outputs with PWM capabilities

Two analog inputs

One UART TX/RX pair

One external wake input that can wake from the ultra-low-power BACKUP mode

Installation process

Tools required

Phillips head screw driver.
Tweezers
USB Micro-b

Step-by-step assembly and disassembly guide

Remove the back with the screw-driver
Remove the rubber seal
Remove the board and battery from the case
Replace the original board with he new Sensor Watch board

Connecting the board to your computer

Something that I might have overlooked is that the USB Micro B connection is through that protrude teeth on the board… It took me longer than I would like to admit to figure that out haha

Benefits of the Mod

New Functions Added

The Clock face allows Sensor Watch to function like, y'know, a watch, displaying the time and date.

The World Clock face allows you add a time display for any number of time zones around the world.

The Sunrise/Sunset face displays the next sunrise and sunset times for your location.

The Timer Face allows you to time events up to 40 days long with one-second resolution.

The Moon Phase face displays the current phase of the moon, and lets you play the month forward to see moon phases in the future.

These are sort of the standard function that comes pre-installed on the circuit board but I am using an alternate firmware image that have functionality for the temperature sensor [link]:

The Backpacker firmware adds a digital thermometer and a temperature logger, which lets you track overnight low temperatures outside your tent.

Temperature sensor; It is really slow to get an accurate reading but I rather have a temperature reading than not and it is quite useful to see it change during the day.

The Stargazer firmware adds an astronomy watch face, capable of calculating right ascension, declination, altitude and azimuth for the sun, the moon and all the planets.

The Athlete firmware adds a countdown timer and exercise counter, as well as an old-school pulsometer complication scaled for 30 beats.

Potential Drawbacks

You lose the buzzer unless you are willing to de-solder the buzzer from the original board and solder it to the new board. It is not hard to do but I really did not care about the buzzer so I can personally live without it but that it is still a drawback.

Power draw, the power draw of this board is higher as the CR2016 battery is rated to last for about a year in comparison to about 10 years with the original board.

Conclusion

I think this is a no brainer MOD if you are interested in a hackable and open source watch. This is not necessarily for everyone but those that are interested will know right away if they will get this board or not.

If you were a Pebble Watch owner or always wanted one I think this watch would appear a lot to you, IYKYK haha.