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Sensor Watch: Home

Not sure where to start? If you bought a Sensor Watch board on Crowd Supply and need to assemble it, follow the instructions in the video below. But first, read these two notes:

Did you also get a Sensor Board?

If you have a sensor board (such as the Temperature Sensor included with most early orders), you must install it first, before you begin the steps in the video. Your sensor board will come packaged with instructions describing how to install it in the Sensor Watch 9-pin connector.

Do you want to install alternate firmware?

Sensor Watch ships with a build of Movement that includes a Clock and World Clock, as well as a Sunrise/Sunset and Moon Phase complication (backer boards also include a Temperature watch face). If you wish to install an alternate firmware, you should download it and install it before installing your Sensor Watch board in the watch case, as installing it in the watch case renders the USB port inaccessible. Instructions for flashing new firmware can be found here.

Need to buy a watch?

Casio’s F-91W uses ‘Module 593’, which was first produced in the 1980s. Whether a watch uses this module is indicated on the back of the case in a rectangular box. We expect that any watch using this module can act as a donor. However, as many of these Casio watches are counterfeited, take care to buy from a reputable seller.

Watches using this module include:

Model Works Counterfeits Current variants
F-84W F-84W (black)
F-91W ⚠️ F-91W (black), F-91WG (black, gold face), F-91WM (coloured case), F-91WC (coloured), F-91WS (translucent)
A158W ⚠️ A158WA (silver, black face), A158WEA (silver, gold face), A158WETG (gold)
A159W ⚠️ A159WA (silver, black face), A159WGEA (gold, black face), A159WAD (silver, black cut glass face), A159WGED (gold, black cut glass face)
A163W A163WA (silver, black face)
A164W A164WA (silver, black face)
A171W A171WE (silver, black face), A171WEG (gold), A171WEGG (black), A171WEMG (gold)
W-31 W-31 (vintage, stainless steel case & strap)
W-78 W-78 (vintage, larger and rounder with a bit of a 90s look)

The ‘Works’ column indicates that someone has successfully transplanted the sensor watch module into that watch. The ‘Counterfeits’ column means that someone has seen probable counterfeits for sale, so extra caution is warranted.

Watches that will not work as they use different movements, although the button layout is the same: A168W, A700W, LA680W, B650W. In general, if the watch is a 3 button digital Casio that has a nice ‘illuminator’ backlight rather than a side light, it’s NOT a 593 and will not work.

1 - Sensor Watch Firmware

While you can build any number of bare-metal applications for Sensor Watch, when we refer to the Sensor Watch firmware, we’re really talking about Movement, the community firmware for Sensor Watch. You can read more about Movement here; for now, the important thing to know is that it manages a series of watch faces that you advance through using the mode button. It’s a very similar idiom to the classic Casio F-91W, which advances from Clock -> Alarm -> Stopwatch -> Time Set, and then wraps around to Clock.

Movement offers many more watch faces — some of which, like World Clock, you can include more than once! Still: you’d be pressing the Mode button for hours if we included them all, so instead, we only build a subset of the available faces into any given firmware. This means that you can download an alternate firmware that includes the kind of functionality you want, or build custom firmware that includes just the watch faces you desire.

1.1 - Flashing Firmware onto Sensor Watch

The small spade at the top of Sensor Watch is designed to plug directly into a USB Micro B cable. This allows you to flash new firmware directly onto the device via the built-in UF2 bootloader.

To accomplish this, disassemble your watch completely, and plug the Sensor Watch board into a USB Micro B cable plugged into your computer. Double tap the tiny Reset button on the back of the board; the LED should begin to pulse red (or blue, for Special Edition boards). You should see a disk drive called “WATCHBOOT” appear on your computer.

Drag a UF2 file with the firmware you want to use onto the WATCHBOOT drive. You can find some prebuilt firmware images right here on the Sensor Watch website, or you can build your own firmware with just the watch faces you want to wear.

After you drag the file over to WATCHBOOT, the LED should pulse intensely for a few seconds, and then turn off. This signals that the firmware has uploaded successfully; you can now reassemble the watch and wear the new firmware on your wrist.

NOTE: As a failsafe (in case the watch inadvertently enters bootloader mode while being worn), the bootloader is programmed to exit after 60 seconds of inactivity. This means that once you enter bootloader mode (red LED pulsing), you have one minute to copy over your firmware, or the watch will exit bootloader mode, and you will have to double-tap reset again.

1.2 - Prebuilt Alternative Firmware for Sensor Watch

This section contains the stock firmware for Sensor Watch, as well as several prebuilt firmware images featuring an alternative set of watch faces. To flash one of these firmware images to your Sensor Watch, follow the instructions here.

Prebuilt Firmwares

Note that each firmware comes in two flavors, depending on the color of your circuit board. This is because blue boards have a red/blue LED and green boards have a red/green LED, and the pins are swapped. No damage will result from loading the wrong firmware onto your board, but the LED colors will be wrong.

Also note that all of these have the “Preferences” and “Time Set” screens at the end; for brevity they aren’t included in this listing, but you will always have access to them.

  • Movement Standard Edition: Simple Clock, World Clock, Sunrise/Sunset, Moon Phase, Stopwatch
  • Movement Backer Edition: Simple Clock, World Clock, Sunrise/Sunset, Moon Phase, Temperature
  • Focus: Simple Clock, Tomato Timer, Stopwatch, Countdown
  • The Athlete: Simple Clock, Stopwatch, Countdown, Exercise Counter, Pulsometer
  • The Backpacker: Simple Clock, Sunrise/Sunset, Moon Phase, Temperature, Temperature Log, Blinky Light
  • The Stargazer: Simple Clock, Astronomy, Sunrise/Sunset, Moon Phase
  • Deep Space Now: Simple Clock, Mars Time, World Clock x3
  • alt.time: Simple Clock, Beat Time, Day One

Movement Standard

This is the standard firmware for Movement. It’s packed, but not jam-packed:

  • Simple Clock - A basic clock with date for timekeeping.
  • World Clock - You can configure this with a custom two-letter title and any time zone.
  • Sunrise/Sunset - Local sunrise and sunset times.
  • Moon Phase - Tonight’s moon phase. (press Alarm to see days in the future)
  • Stopwatch (by Wesley Ellis) - A simple stopwatch with one-second resolution.

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

Movement: Backer Edition (with temperature display)

Required Sensor Board: Temperature Sensor

The first Sensor Watch boards that shipped were with orders that automatically included a temperature sensor board. As such, the set of watch faces that shipped with those boards included a temperature display.

Post-campaign, the temperature sensor board was optional, so we replaced the temperature display with a stopwatch. If you bought a temperature sensor board and wish to have a temperature display, use this firmware; it’s identical to the standard firmware, but replaces the stopwatch with a temperature display.

  • Simple Clock - A basic clock with date for timekeeping.
  • World Clock - You can configure this with a custom two-letter title and any time zone.
  • Sunrise/Sunset - Local sunrise and sunset times.
  • Moon Phase - Tonight’s moon phase. (press Alarm to see days in the future)
  • Temperature - As measured from the temperature sensor board. (Alarm toggles C or F)

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator


Required Mod: Tomato and countdown timer requires soldering the buzzer connector.

For folks who want to use the watch for productivity.

  • Simple Clock - A basic clock with date for timekeeping.
  • Tomato Timer (by Wesley Ellis) - A productivity timer varietal.
  • Stopwatch (by Wesley Ellis) - A simple stopwatch with one-second resolution.
  • Countdown (by Wesley Ellis) - A simple countdown timer for eggs, tea or tasks.

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

The Athlete

Required Mod: Countdown timer requires soldering the buzzer connector.

For folks who want to use the watch for fitness.

  • Simple Clock - A basic clock with date for timekeeping.
  • Stopwatch (by Wesley Ellis) - It counts up.
  • Countdown (by Wesley Ellis) - It counts down.
  • Exercise Counter (by Shogo Okamoto) - Designed for tracking a number of exercises by pressing the Alarm button.
  • Pulsometer - Hold the Alarm button while touching a vein on your neck, and count 30 beats. Release for your heart rate.

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

The Backpacker

Required Sensor Board: Temperature Sensor

This watch face is focused on helping you when out in the backcountry.

  • Simple Clock - A basic clock with date for timekeeping.
  • Sunrise/Sunset - Local sunrise and sunset times. Don’t get caught in the dark!
  • Moon Phase - Tonight’s moon phase. For when the sun fails you.
  • Temperature - As measured from the temperature sensor board. (Alarm button toggles C or F)
  • Temperature Log - A 36-hour log. Toss the watch outside your tent to measure overnight lows.
  • Blinky Light - This will chew through your battery, but could be useful in an emergency.

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

The Stargazer

For folks who think space is the place!

  • Simple Clock - A basic clock with date for timekeeping.
  • Astronomy - Calculates altitude, azimuth, right ascension and declination for all planets
  • Sunrise/Sunset - Local sunrise and sunset times.
  • Moon Phase - Today’s moon phase. (press Alarm to see days in the future)

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

Deep Space Now

For folks who work in deep space. (no, really!)

  • Simple Clock - A basic clock with date for timekeeping.
  • Mars Time - Displays Mars Standard Time, or local mean solar time and mission sol for each active mission on Mars.
  • World Clock - One slot for the ground station in Goldstone (California, USA)
  • World Clock - One slot for the ground station in Madrid (Spain)
  • World Clock - One slot for the ground station in Canberra (Australia)
  • Day One - Counts the number of days from a given date. Try plugging in the start of a mission (i.e. August 20, 1977) to get mission day.

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator


For fans of alternative timekeeping systems.

  • Simple Clock - A basic clock with date for timekeeping.
  • Beat Time (by Wesley Ellis) - AKA Swatch Internet Time, a decimal time standard centered on Switzerland.
  • Day One - Counts the number of days from a given date. Try plugging in your birthday!

I hope to expand this face in the future. (International Fixed Calendar, anyone?)

Download it now: for Green Boards and Blue Boards

Try it out: Sensor Watch Emulator

2.1 - Calibration with Nanosec

You can include the Finetune and Nanosec faces in your Sensor Watch firmware to achieve exceptionally high accuracy within 10 seconds per year.

Accurate timekeeping with a quartz crystal

The Sensor Watch measures time using a quartz crystal that oscillates at a frequency of 32,768 kHz. In practice, the frequency of every crystal deviates a little from the nominal value. Because the difference is very small, we don’t express it as a percentage (“parts per hundred”), but as ppm (“parts per million”). For example, a deviation of +1 ppm means the frequency is 32,768.03278 kHz. This would make the watch gain about 31.5 seconds per year, because there are about 31.5 million seconds in a year.

In addition to the static offset of individual crystals, their frequency also varies dynamically with temperature. Crystals are manufactured in a way that their frequency peaks at 25 °C and decreases at both higher and lower temperatures.

For accurate timekeeping, you need to correct for your crystal’s static frequency offset, and your watch needs to dynamically compensate for the variation due to temperature changes. These are the things that the Nanosec and Finetune watch faces let you do.

Calibrating the Sensor Watch

For really good results you need a watch with a temperature sensor, but you can also get a significant improvement without one. Without a temperature sensor, select correction profile P1 instead of the default P3 (details in the last section).

The firmware has to include these faces:

  • A regular clock face like Simple Clock (simple_clock_face), because you want a way to see the time;
  • Time Set (set_time_face), because you need a way to set the date and time;
  • Finetune (finetune_face), so you can set the time precisely and adjust the static frequency correction;
  • Nanosec (nanosec_face), because it contains the behind-the-scenes magic that dynamically compensates for temperature-driven variation.

To calibrate your watch, follow these steps:

  1. Flash a firmware that contains the necessary faces onto the watch, assemble it, and set the time via the Time Set face.

  2. Open in a browser to display the accurate time. Go to the Finetune face and adjust the offset until the seconds on the watch precisely match the seconds on the screen. Apply the change without updating the frequency correction value with a long ALARM press.

    See the Finetune section below for the details of doing this.

  3. Wait a few days, then fine-tune your watch again using and Finetune, saving your changes with a long LIGHT press. Your watch now knows how many hours have passed since the last time it was accurate, and how much it has drifted since then. From these two values Finetune can calculate an update to the static frequency correction value, beyond adjusting the current time itself.

  4. Your watch is now a lot more accurate. Wait a few weeks until it has drifted a second or two, then repeat Step 3.

  5. Wait a few months, then repeat Step 3. At this stage your Sensor Watch will be accurate to within 10 seconds per year.

Some useful details:

  • You need a special site like for this type of calibration. The normal time on most websites (and on most devices like laptops or smartphones) is not accurate enough.
  • The Finetune face lets you adjust the time in increments of 25 msec. That is below most people’s threshold of perception. You need to wait for the watch to build up a noticeable amount of drift before re-calibration is practical.
  • The static frequency correction value is preserved when the watch resets. You don’t need to calibrate from scratch after replacing the battery or re-flashing the firmware.
  • The 10 seconds per year accuracy puts your watch right up there with the most expensive high-end quartz timekeepers.


When you reach Finetune by repeatedly pressing MODE to cycle through the installed watch faces, it will first look a little like this:

The four 0s mean that you haven’t input any time adjustment yet. The number on the right keeps updating to show the seconds part of the current time.

Finetune has two secondary faces. You need a long MODE press to move to the first of them; after that, a short press is enough.

  • FT means Finetune, and it’s the main screen where you can enter the desired time adjustment.
  • DT stands for delta time and it shows the number of hours since you last adjusted the time with the Finetune face. At first you will see a small value like this because your watch hasn’t been on for very long yet. The small numbers are fractional hours. E.g., the image above shows 0.05 hours, which is 3 minutes.
  • F stands for frequency, and it shows the amount by which your current adjustment will update the static frequency correction value. However, unless at least 6 hours have passed, Finetune will not mess with the frequency correction, which is why at first you will see “6Hr” here.
  • Back on the main screen, you can use LIGHT and ALARM to change the adjustment. A long LIGHT press increases the offset by 250 msec; a short LIGHT press adds 25 msec.
  • The first and third digits of the display have some limitations, which is why the “7” in 0275 looks a bit odd.
  • You need to go towards a positive value as shown above if your watch is ahead of the true time.
  • You can verify the adjustment’s effect by holding your watch next to the time on an accurate clock, like Your adjustment offsets the constantly ticking seconds value.
  • A long ALARM press decreases the value by 250 msec. A short ALARM press decreases it by 25 msec.
  • The odd shape in the top right is meant to be a double minus sign, indicating that this is a negative adjustment.
  • You need a negative adjustment if your watch is behind the true time.

Now let’s see what a real calibration looks like!

  • On the FT screen I have input an offset of 1025. My watch was just a bit over a second fast.
  • The DT screen tells me that 693.72 hours, or a little over four weeks, have passed since the last time I adjusted the time with Finetune.
  • The F screen tells me that if I apply this adjustment, the static time correction value will increase by 0.4104 ppm. (This will make my watch about 12.9 seconds slower per year.)
  • A long LIGHT press applies the adjustment to the time and updates the static time correction value. This is what I did above. Sensor Watch will be more accurate going forward!
  • A long ALARM press applies the adjustment to the time but does not update the static frequency correction. This is what you need to do the very first time you use Finetune, when you initially set the exact time.
  • After either a long LIGHT or long ALARM press on the F screen, Sensor Watch returns to the default face that shows the current time.

What if you have come to the Finetune face only to experiment but do not want to change the time, let alone mess with Sensor Watch’s calibration? Easy: on the FT screen, change the value 0000 again. If you now press MODE a couple of times, Sensor Watch will move on from Finetune. Or if you just wait for a minute without pressing anything, it will return to the Simple Time screen.


The Nanosec face has two functions. The first of the two is invisible but absolutely crucial. The face contains code that Sensor Watch executes regularly in the background. This code dynamically adjusts the time in minuscule increments to compensate for the effects of the current temperature on the crystal’s frequency.

The second, visible function is a collection of screens where you can review and change various parameters that affect Nanosec’s internal calculations. You essentially won’t need to change any of these settings, but they are fun to review.

In each of the screens below, you can use LIGHT to increase the value and ALARM to decrease it. Where it makes sense, a long press means a larger change.

  • FC: The static frequency correction value.
  • T0: The crystal’s center temperature, in °C. Nominally this is 25 °C for all crystals, but small variations are possible. A true center temperature improves the precision of dynamic frequency adjustments. However, in order to determine this value, a number of fine measurements are needed at controlled temperatures, which is beyond the reach of most Sensor Watch users.
  • 2C: The quadratic coefficient in the formula that Nanosec uses to approximate the quartz crystal’s temperature characteristics.
  • 3C: The cubic coefficient in the formula that Nanosec uses to approximate the quartz crystal’s temperature characteristics.
  • PR: The correction profile (formula) used to model the crystal’s temperature characteristics. You will most likely want to stick with P3, but see description of profiles below.
  • CD: The cadence of time correction, i.e., the length of the interval (in minutes) at which Nanosec makes a minuscule adjustment to the time.
  • AA: Aging compensation (ppm/year). The static frequency of crystals changes with age, but the exact value can only be determined empirically. If you notice that you keep adding 0.5 ppm each year to the static frequency correction to stay accurate, you can instead enter that value here.

Nanosec correction profiles

This is what the various correction profiles do.

P0: Static hardware correction with 1ppm resolution. This has the least additional power draw because the correction is done entirely in the hardware.

P1: High-resolution (0.01ppm) static correction implemented in software. This is the profile you’d want to choose if your watch has no temperature sensor.

P2: High-resolution static correction, plus dynamic frequency correction for temperature changes. The dynamic correction uses the quadratic formula from the crystal’s datasheet.

P3: Like P2, both static and dynamic correction, but using an empirically measured cubic formula.

P4: Custom settings for a specific crystal, using values that you can edit in the source code. You need to determine these values yourself; your crystal is unlikely to match what you find in the source code.

2.2 - Clocks

The core function of a watch is telling time. All of the watch faces in this category tell time in one form or another.

Simple Clock

The Simple Clock watch face echoes the classic time and date display of the stock F-91W. It displays the day of the week and day of the month on the top line, along with the current time on the bottom line. Pressing the Light button illuminates the LED so that you can read the display in the dark. Holding the Light button keeps the LED on while the button is held.

If you soldered the buzzer connector to your Sensor Watch board, you may also toggle the Hourly Chime feature by pressing and holding the Alarm button. When you release the Alarm button, the Signal indicator will turn on, indicating that the hourly chime is enabled.

The Simple Clock face also incorporates a low battery warning: this watch face will display the LAP indicator when it detects that the battery voltage is low. This does not mean that power failure is imminent, but it does mean that your battery has only about 5% of its original capacity remaining and you should start thinking about a replacement. The battery is a CR2016 coin cell.

World Clock

The World Clock watch face looks similar to the Simple Clock watch face, but you’ll notice that at first launch the day of week indicators are blank. That’s because this watch face does not display the day of the week. Instead, you may customize these letters to display the name of a time zone of your choosing.

To customize this watch face, press and hold the Alarm button. The first letter in the top row will begin flashing. Press the Alarm button repeatedly to advance through the available letters in the first slot, then press the Light button to move to the second letter. Finally, press Light again to move to the time zone setting, and press Alarm to cycle through the available time zones. Press Light one last time to return to the world clock display.

Note that the second slot cannot display all letters or numbers. Also note that at this time, time zones do not automatically update for daylight saving time; you will need to manually adjust this field each spring and fall.

Beat Time

The Beat Time face displays the current Swatch Internet Time, or .beat time. This is a decimal time system that divides the day into 1000 beats.

The three large digits in the bottom row indicate the current beat, and the two smaller digits (normally the seconds in Simple Clock) indicate the fractional beat; so for example you can read “67214” as “beat 672.14”.

Mars Time

This watch face is dedicated to Martian timekeeping. It has several modes, and can display either a time or a date. Pressing the Alarm button cycles through different time zones on Mars:

  • MC - Mars Coordinated Time, the time at Airy-0 Crater on the Martian prime meridian
  • ZH - Local mean solar time for the Zhurong rover
  • PE - LMST for the Perseverance rover
  • IN - LMST for the Insight lander
  • CU - LMST for the Curiosity rover

Press the Light button to toggle between displaying time and date:

  • MC S - the Mars Sol Date, Martian days since December 29, 1873
  • ZH Sol - Mission sol for the Zhurong rover
  • PE Sol - Mission sol for the Perseverance rover
  • IN S - Mission sol for the InSight lander
  • CU S - Mission sol for the Curiosity rover

Note that where the mission sol is below 1000, this watch face displays the word “Sol” on the bottom line. When the mission sol is over 1000, the word “Sol” will not fit and so it displays a stylized letter S at the top right.

2.3 - Complications

In horology, a complication is an additional set of gears in a mechanical watch movenent that enables a secondary function, such as a sunrise/sunset dial. The watch faces in this category do just that.


This watch face displays local sunrise and sunset times. During the day, it displays today’s sunset; at night, it displays tomorrow’s sunrise.

When you first see this watch face, it will display “No Loc”, or No Location. This is because your latitude and longitude are required to calculate sunrise and sunset. When on this screen, you can set your location in a similar way to the World Clock screen.

Press and hold Alarm to enter location setting mode. The top line will read “LA” (Latitude), and the bottom line “+ 0000”. The large digits are the whole number part of the latitude, and the smaller digits (in the seconds place) are the fractional part. Enter your latitude and longitude (“LO”) by pressing the Alarm button to change the sign or advance the digits, and the Light button to move to the next character; for example, a latitude of 40.73° N would be “+ 4073”, and a longitude of 73.94° W would be “–07394”.

Once you have set your latitude and longitude, the Sunrise/Sunset face will display the next sunrise or sunset on the bottom row, and the day of that sunrise or sunset at the top right.

A short press on the Alarm button will advance to the following sunrise or sunset: for example, on Monday afternoon, it will display Monday evening’s sunset, but a short press on the Alarm button will display Tuesday morning’s sunrise.

If you made a mistake while entering your location, or if you simply wish to change your location, you can re-enter location setting mode with another long press on the Alarm button.

Moon Phase

The Moon Phase face is similar to the Sunrise/Sunset face: it displays the current phase of the moon, along with the day of the month and a graphical representation of the moon on the top row.

This graphical representation is a bit abstract. The segments that turn on represent the shape of the moon, waxing from the bottom right and waning at the top left. A small crescent at the bottom right will grow into a larger crescent, then add lines in the center for a quarter and half moon. All segments are on during a full moon. Then gradually the segments at the bottom right will turn off, until all that remains is a small waning crescent at the top left.

All segments turn off during a new moon.

On this screen you may press the Alarm button repeatedly to move forward in time: the day of the month at the top right will advance by one day for each button press, and both the text and the graphical representation will display the moon phase for that day. Try pressing the Alarm button 27 times now, just to visualize what the moon will look like over the next month.


The Stopwatch face provides basic stopwatch functionality: you can start and stop the stopwatch with the alarm button. Pressing the light button when the timer is stopped resets it. This face does not count sub-seconds.

Stock Stopwatch

The Stock Stopwatch face implements the original F-91W stopwatch functionality, including counting hundredths of seconds and lap timing.

  • Use the alarm button to start and stop the stopwatch.
  • Press the light button while the stopwatch is running to view the lap time. The stopwatch continues running in the background, indicated by a blinking colon.
  • Press the light button again to switch back to the running stopwatch.
  • Press the light button when the timekeeping is stopped to reset the stopwatch.

There are two improvements compared to the original F-91W:

  1. When the stopwatch reaches 59:59, the counter does not simply jump back to zero but keeps track of hours in the upper right-hand corner (up to 24 hours).
  2. Long-press the light button to toggle the LED behavior. It either turns on with each button press or remains off.




The timer watch face is an advanced countdown face, providing the functionality of starting a countdown by choosing one out of nine programmable timer presets. A timer/countdown can be 23 hours, 59 minutes, and 59 seconds max. A timer can also be set to auto-repeat, which is indicated by the lap indicator.

How to use in NORMAL mode:

  • Short-pressing the alarm button cycles through all preset timer lengths. Find the current timer slot number in the upper right-hand corner.
  • Long-pressing the alarm button starts the timer.
  • Long-pressing the light button initiates settings mode.

How to use in SETTINGS mode:

  • There are up to nine slots for storing a timer setting. The current slot is indicated by the number in the upper right-hand corner.
  • Short-pressing the light button cycles through the settings values of each timer slot in the following order:
    hours -> minutes -> seconds -> timer repeat
  • Short-pressing the alarm button alters the current settings value.
  • Long-pressing the light button returns to normal mode.




The Interval face provides 9 customizable interval timers, which can be used for hit training and/or time management techniques.

  • To cycle through the 9 interval timers, press the alarm button (short press). For each timer slot, the relevant details for each timer phase are shown in a “carousel” (see below).

  • To start an interval timer, press and hold the alarm button.

  • To pause a running timer, press the alarm button (short press).

  • To completely abort a running timer, press and hold the alarm button.

  • Press and hold the light button to enter settings mode for the current interval timer. Short pressing the light button cycles through the settings of each timer.

  • Each interval timer has 1 to 4 phases of customizable length like so:
    (1) prepare/warm up --> (2) work --> (3) break --> (4) cool down
    When setting up or running a timer, each of these phases is indicated by the letters “PR” (prepare), “WO” (work), “BR” (break), or “CD” (cool down).

  • Each of these phases is optional, and you can set the corresponding minutes and seconds to zero. If you want to use the timer, at least one phase needs to be set to a non-zero value.

  • You can define the number of rounds either only for the work phase and/or for the combination of work + break phase. Let’s say you want an interval timer that counts 3 rounds of 30 seconds work, followed by 20 seconds rest, like so:
    work 30s –> work 30s –> work 30s –> break 20s
    You can do this by setting 30s for the “WO"rk phase and setting a 3 in the lower right-hand corner of the work page. The “LAP” indicator lights up at this position, to explain that we are setting laps here. After that, set the “BR"eak phase to 20s and leave the rest as it is.

  • If you want to set up a certain number of “full rounds”, consisting of work phase(s) plus breaks, you can do so at the “BR"eak page. The number in the lower right-hand corner determines the number of full rounds to be counted. A “-” means that there is no limit, and the timer keeps alternating between work and break phases.

  • This watch face comes with several pre-defined interval timers suitable for HIIT training (timer slots 1 to 4) as well as doing work according to the Pomodoro technique (timer slots 5 to 6). Feel free to adjust the timer slots to your own needs (or completely wipe them).


The Alarm face implements 16 customizable alarms on the sensor watch.


  • In normal mode, the alarm button cycles through all 16 alarms.
  • Pressing the alarm button long in normal mode toggles the corresponding alarm on or off.
    (Whereas pressing the alarm button extra long jumps back to the first alarm.)
  • Pressing the light button enters settings mode and cycles through the settings of each alarm.
    (Long pressing the light button enters settings mode without illuminating the led.)
  • In settings mode an alarm slot is selected by pressing the alarm button when the slot number in the upper right corner is blinking.
  • For each alarm slot, you can select the day. These are the day modes:
    • ED = the alarm rings every day
    • 1t = the alarm fires only one time and is erased afterwards
    • MF = the alarm fires Mondays to Fridays
    • WN = the alarm fires on weekends (Sa/Su)
    • MO to SU = the alarm fires only on the given day of week
  • You can fast cycle through hours or minutes by holding the alarm button.
  • You can select the tone in which the alarm is played. (Three pitch levels available.)
  • You can select how many “beep rounds” are played for each alarm. 1 to 9 rounds, plus extra long (‘L’) and extra short (‘o’) alarms.
  • The simple watch face indicates if any alarm is set within the next 24h by showing the signal indicator.


The Pulsometer is an implementation of a sort of a classic mechanical watch complication. A classic pulsometer complication involves a chronograph with a scale calibrated for counting a certain number of heartbeats (often 30). You start it and begin counting heartbeats, and stop it after counting the specified number of beats. Once stopped, the needle will point to your heart rate.

The pulsometer on Sensor Watch flashes its instructions at launch: “Hold Alarm + count 30 beats.” Using the hand on the side where you wear your watch, touch your carotid artery (in your neck) and feel for your pulse. Once you find it, use your other hand to press and hold the Alarm button, and count your heartbeats. When you reach 30 beats, release the Alarm button. The display will show a number such as “60 bpm”; this is your heart rate in beats per minute.

Two notes:

  1. For the first few seconds of a measurement, the display will read “Hi”. This indicates that it’s too early for the measured value to be a valid heart rate. Once the measurement is below 240 bpm, the display will update.
  2. If you hold the button down for more than 45 seconds, the display will read “Lo”. If it took this long for you to count 30 heartbeats, this indicates that your heart rate is below 40 beats per minute.

Tomato Productivity Timer


TOTP Generator

This watchface generates time based one time passwords (two factor auth codes) allowing you to sign in securely to many popular websites (e.g. Google, GitHub). Time-based one-time password (TOTP) is a computer algorithm that generates a one-time password (OTP) that uses the current time as a source of uniqueness.

Press the Alarm button to cycle between your configured websites / TOTP secrets.

The watchface supports multiple websites / TOTP secrets, which need to be extracted from TOTP QR codes and added to the source code for the watchface as follows:

  1. Obtain a TOTP secret or QR ode from the website you want to generate codes for.
  2. If you have just the QR code, Stefan Sundin’s web site will allow you to extract the secret - it will be an alphanumeric string around 32 characters long, which is the TOTP secret encoded in Base32.
  3. To add the secret to the watchface code, you need to convert it to hexadecimal bytes. This page will allow you to do that conversion. Note you’ll have to enter your TOTP secret in uppercase. Shell tools can be used to convert secret to bytes:
  1. Finally, you’ll need to take the Hexadecimal bytes and add them to the TOTP watchface source code and recompile movement:

Edit totp_face.c

You may want to remove the demo keys. Assuming you want to add a key to the end of the list:

static const uint8_t num_keys = 2;

Add one to the number on this line.

static uint8_t keys[] = {
   // Add the hex bytes for your key

Add the hexadecimal bytes from step 3 to the end of this array, comma separated and each one preceeded by 0x. Don’t forget to add a comma after the previous final byte.

static const uint8_t key_sizes[] = {

Add the size of your secret (the number of hex bytes you just added) to the end of this array.

static const uint32_t timesteps[] = {

Add another 30 entry to the end of this array.

static const char labels[][2] = {

Add a label for your secret… E.g. if it’s for your Google account you might want to add { 'g', 'o' } as a friendly label.

That’s it - enjoy the convenience of TOTP codes on your wrist!

Day One

This watch face displays the number of days since a given date. It was originally designed to display the number of days you’ve been alive, but technically it can count up from any date in the 20th century or the 21st century, so far.

Long press on the Alarm button to enter customization mode. The text “YR” will appear, and will allow you to set the year starting from 1959. Press Alarm repeatedly to advance the year. If your birthday is before 1959, advance beyond the current year and it will wrap around to 1900. Once you have set the year, press Light to set the month (“MO”) and day (“DA”), advancing the value by pressing Alarm repeatedly.

Note that at this time, the Day One face does not display the sleep indicator in sleep mode, which may make the watch appear to be unresponsive in sleep mode. You can still press the Alarm button to wake the watch. This UI quirk will be addressed in a future update.

Time Left

The Time Left Face helps you visualize how far you have progressed in a certain time span. Similar to the Day One Face, you can set your starting date. Additionally, you can also set your target or destination date. You can then use the face to display your progress in various ways.


  • Long-pressing of the light button starts the settings mode:

    • First, you set the beginning date (indicated by a ‘b’ in the upper right corner).
    • Begin by setting the year (indicated by the letter ‘YR’). Use the alarm button to cycle through the values. Short-pressing the light button brings you to the next settings page.
    • Set the values in this order:
      1. beginning date (indicated by a ‘b’): year - month - day
      2. destination date (indicated by a ’d’): year - month - day
    • After cycling through all settings pages, the face returns to display mode.
  • In display mode, use the alarm button (short press) to cycle through these four types of display:

    1. number of days left (‘DL’) until the destination date is reached.
    2. remaining days expressed as a percentage of the total time span. The value is shown with two decimals, using the colon as a decimal point.
    3. number of days passed (‘DA’) since the beginning date. (This is the same value the Day One Face would give you.)
    4. number of days passed expressed as a percentage of the total time span.

What is this for?

You can use this watch face to be reminded of any kind of progress between a set start and end date. The brave among us can use it as a kind of memento mori visualization. Set your date of birth and look up the average life expectancy of your age cohort based on publicly available mortality tables. Then, set the statistically expected day of death as the target date, and you will be able to see how much of your time has passed and how much is still to come.

Blinky Light

The blinky light watch face was designed as a tutorial for making a watch face in Movement, but it actually might be useful to have a blinking light in a pinch.

The screen displays the name of the watch face (”BL”), as well as an S at the top right for slow blink or an F for fast blink. The bottom line selects the color: green, red or yellow. You can change the speed of the blinking light by pressing the Alarm button, and change the color with the Light button. A long press on the Alarm button starts the blinking light, and another long press stops it.

Note that this will chew through your battery! The green LED uses about 450µA at full brightness, which is 45 times the normal power consumption of the watch. The red LED is an order of magnitude less efficient (4500 µA), and the yellow setting lights both LEDs, which chews through nearly 5 milliamperes. This means that one hour of yellow blinking is likely to eat up between 2 and 3 percent of the battery’s usable life! Still, if you need to signal your location to someone in a dark forest, this watch face could come in handy.

Just try to use the green LED as much as you can.


The Astronomy watch face is among the most complex watch faces in the Movement collection. It allows you to calculate the locations of celestial bodies in the sky, as well as distance in astronomical units (or, in the case of the Moon, distance in kilometers).

When you arrive at the Astronomy watch face, you’ll see its name (“Astro”) and an animation of two objects orbiting each other. You will also see “SO” (for Sol) flashing in the top left. The flashing letters indicate the currently selected celestial body. Short press Alarm to advance through the available celestial bodies:

  • SO - Sol, the sun
  • ME - Mercury
  • VE - Venus
  • LU - Luna, the Earth’s moon
  • MA - Mars
  • JU - Jupiter
  • SA - Saturn
  • UR - Uranus
  • NE - Neptune

Once you’ve selected the celestial body whose parameters you wish to calculate, long press the Alarm button and release it. The letter “C” will flash while the calculation is performed.

When the calculation is complete, the screen will display the altitude (“aL”) of the celestial body. You can cycle through the available parameters with repeated short presses on the Alarm button:

  • aL - Altitude (in degrees), the elevation over the horizon. If negative, it is below the horizon.
  • aZ - Azimuth (in degrees), the cardinal direction relative to true north.
  • rA - Right Ascension (in hours/minutes/seconds)
  • dE - Declination (in degrees/minutes/seconds)
  • di - Distance (the digits in the top right will display either aU for astronomical units, or K for kilometers)

Long press on the Alarm button to select another celestial body.


The Orrery watch face is similar to the Astronomy watch face in that it calculates properties of the planets, but instead of calculating their positions in the sky, this watch face calculates their absolute locations in the solar system. This is only useful if you want to plot the planets on graph paper, but hey, you never know!

The controls are identical to the Astronomy watch face: while the title screen (“Orrery”) is displayed, you can advance through the available planets with repeated short presses on the Alarm button. The available planets:

  • ME - Mercury
  • VE - Venus
  • EA - Earth
  • LU - Luna, the Earth’s moon
  • MA - Mars
  • JU - Jupiter
  • SA - Saturn
  • UR - Uranus
  • NE - Neptune

Note that the sun is not available in this menu, as the sun is always at (0,0,0) in this calculation.

Long press on the Alarm button to calculate the planet’s location, and after a flashing “C” (for Calculating), you will be presented with the planet’s X coordinate in astronomical units. Short press Alarm to cycle through the X, Y and Z coordinates, and then long press Alarm to return to planet selection.

The large numbers represent the whole number part, and the two smaller numbers (in the seconds place) represent the decimal portion. So if you see “SA X 736” and “SA Y -662”, you can read that as an X coordinate of 7.36 AU and a Y coordinate of -6.62 AU. You can literally draw a dot at (0, 0) to represent the sun, and a dot at (7.36, -6.62) to represent Saturn. (the Z coordinates tend to be pretty close to zero, as the planets largely orbit on a single plane, the ecliptic)


The Invaders watch face is an authentic remake of the “famous” number invaders game, found on the Casio calculator wristwatches CA-85 or CA-851.

How to play:

  • Press the alarm button to start the game. “Invaders” (just digits, tbh) will start coming in from the right-hand side.
  • Press the light button to “aim”. The digit on the top of the display cycles from 0 to 9.
  • If your aiming digit is identical to one of the invaders, press the alarm button to “shoot”. The corresponding invader will disappear.
  • If the invaders reach beneath the very first position, you loose one defense line. When all three defense lines are gone, the game is over.
  • Also: If you shoot more than 29 times per wave, you lose the game.
  • There are 16 invaders per wave. There is a short break between waves.
  • Long pressing the light button toggles sound on or off (not while playing).

The “n” invaders are ufos!

Whenever the sum of all invaders shot is divisible by 10 the next invader will be an ufo, represented by the n-symbol. Shooting a ufo gets you extra points. Example: shoot 2, 5, 3 –> ufo next

As for points: the earlier you shoot an invader, the more points you get.

To anyone interested in the original game manual, see here:

RPN Calculator

The calculator uses RPN notation. So instead of using infix operators, the operators always follow the parameters.

Example: To calculate 4 - 3 one would enter 4 3 -

Using this notation also does not require any braces, so instead of 2 * (3 - 2) one would enter 2 3 2 - *.

The parameters are put on to a stack, which currently has a size of 4. So if one enters 2 3 2 - * the stack would contain 2 3 2 0. The subtraction then uses the first two values from the stack and pushes the result back on the stack, which leaves 1 2 0 0. The multiplication then uses the next two values from the stack and pushes the end result: 2 0 0 0.

Normal mode

In normal mode the top of the stack is displayed (initially zero), which usually represents the result of the calculation.

  • ALARM enters number mode
  • LIGHT enters operator mode
  • MODE switches to the next watch face

Number mode

Number mode pushes a new parameter on the stack. The first four digits are whole numbers, the last two (smaller) digits are decimals.

  • LIGHT cycles through digits
  • ALARM increases the selected digit
  • MODE pushes the parameter on the stack (and goes back to normal mode)

Operation mode

Operation mode executes operations on the stack. Parameters are taken from the stack and the result is pushed back. If there aren’t enough parameters on the stack, the calculator will go into error mode.

The display shows the current selected operation.

  • LIGHT cycles through the available operations
  • ALARM executes the selected operation (and goes back to normal mode, which will display the result)

Currently implemented operations:

  • add (2 params)
  • sub (2 params)
  • mul (2 params)
  • div (2 params)
  • pow (2 params)
  • sqrt (1 param)
  • pi (0 params, will just push pi onto the stack)

Error mode

An error has happened (currently only caused by too few parameters on the stack).

  • ALARM goes back to normal mode
  • MODE switches to the next watch face


To calculate the following equation:

2 * (3 - 2)

which would be the following in RPN notation:

2 3 2 - *

the following button presses are needed:

  • ALARM enter number mode
  • ALARM 1
  • ALARM 2
  • MODE push number, back to normal mode
  • ALARM enter number mode
  • ALARM 1
  • ALARM 2
  • ALARM 3
  • MODE push number, back to normal mode
  • ALARM enter number mode
  • ALARM 1
  • ALARM 2
  • MODE push number, back to normal mode
  • LIGHT enter operation mode
  • LIGHT change operation to sub
  • ALARM execute sub, back to normal mode
  • LIGHT enter operation mode
  • LIGHT change operation to mul
  • ALARM execute mul, back to normal mode

2.4 - Demo Faces

These watch faces demonstrate functionality of the watch, and aren’t generally watch faces you’d build into a working firmware.

Character Set

This watch face displays all of the characters in the Sensor Watch character set. You can advance from one character to the next with a short press of the Alarm button.

This watch face may be useful to watch face developers, in that it can help them to understand which characters will work in different positions.


This watch was designed for the Crowd Supply marketing team, so they could photograph the various functions of Sensor Watch. The Alarm button advances through static screens that simulate different watch faces.

This watch face may only be useful to you if you need to photograph Sensor Watch, i.e. for a blog post.

2.5 - Sensors

Sensor Watch is designed to display data from sensors. The watch faces in this category read sensor data and present it in a useful way.


This watch face is very simple and has no controls to speak of. It displays the battery voltage as measured by the SAM L22’s ADC.

Note that the Simple Clock watch face includes a low battery warning, so you don’t technically need to this watch face unless you want to track the battery level.

Temperature Display

This watch face is designed to work with either the Temperature + GPIO sensor board or the Temperature + Light sensor board. It reads the current temperature from the thermistor voltage divider on the sensor board, and displays the current temperature in degrees Celsius.

When the watch is on your wrist, your body heat interferes with an ambient temperature reading, but if you set it on a bedside table, strap it to your bike handlebars or place it outside of your tent while camping, this watch face can act as a digital thermometer for displaying ambient conditions.

The temperature sensor watch face automatically samples the temperature once every five seconds, and it illuminates the Signal indicator just before taking a reading.

Pressing the Alarm button toggles the unit display from Celsius to Fahrenheit. Technically this sets the global “Metric / Imperial” flag, so any other watch face that displays localizable units will display them in the system selected here.

Temperature Log

This watch face automatically logs the temperature once an hour, and maintains a 36-hour log of readings. This watch face is admittedly rather complex, and bears some explanation.

The main display shows the letters “TL” in the top left, indicating the name of the watch face. At the top right, it displays the index of the reading; 0 represents the most recent reading taken, 1 represents one hour earlier, etc. The bottom line in this mode displays the logged temperature.

A short press of the “Alarm” button advances to the next oldest reading; you will see the number at the top right advance from 0 to 1 to 2, all the way to 35, the oldest reading available.

A short press of the “Light” button will briefly display the timestamp of the reading. The letters at the top left will display the word “At”, and the main line will display the timestamp of the currently displayed data point. The number in the top right will display the day of the month for the given data point; for example, you can read “At 22 3:00 PM” as ”At 3:00 PM on the 22nd”.

If you need to illuminate the LED to read the data point, long press the Light button and release it.

Temperature Testing

This watch face is similar to the Temperature watch face, but it updates the temperature several times per second. You likely don’t need this watch face, but it is useful for testing the temperature sensor boards.

LIS2DW Accelerometer Data Logger

This is an experimental watch face for logging data on the “Sensor Watch Motion Express” board. I will add more documentation for this watch face once this sensor board is more widely available.

2.6 - Settings

The watch faces in this section relate to watch configuration.


The Preferences watch face allows you to configure various options on your Sensor Watch. Like all other screens, you advance the field you’re setting with the Light button, and advance its value with the Alarm button. The Preferences watch face labels each setting with a two-letter code on the top row; the following list describes each setting and their options:

CL - Clock mode. This setting allows you to select a 12-or 24-hour clock display. All watch faces that support displaying the time will respect this setting; for example, both Simple Clock, World Clock and Sunrise/Sunset will display the time in 24 hour format if the 24 hour clock is selected here.

BT - Button tone. This setting is only relevant if you installed the buzzer connector, and it toggles the beep when changing modes. If Y, the buzzer will sound a tone when Mode is pressed. Change to N to make the Mode button silent.

TO - Timeout. Sets the time until screens that time out (like Settings and Time Set) snap back to the first screen. 60 seconds is a good default for the stock firmware, but if you choose a custom firmware with faces that you’d like to keep on screen for longer, you can set that here.

LE - Low Energy mode. Sets the time until the watch enters its low energy sleep mode. Options range from 1 hour to 7 days, or Never. The more often Sensor Watch goes to sleep, the longer its battery will last — but you will lose the seconds indicator while it is asleep. This setting allows you to make a tradeoff between the device’s responsiveness and its longevity.

LT - Light. This setting has three screens:

  • The first lets you choose how long the LED should stay lit when the Light button is pressed. Options are 1 second, 3 seconds and 5 seconds, or “No LED” to disable the LED entirely.
  • The second screen, titled “blu”, sets the intensity of the blue LED. Values range from 0 (off) to 15 (full intensity)
  • The third screen, “red”, sets the intensity of the red LED, again from 0 to 15. On the last two screens, the LED remains on so that you can see the effect of mixing the two LED colors. On the Special Edition boards, you’ll have red, blue and a variety of shades of pink and purple to experiment with!

Time Set

The Time Set watch face allows you to set the time on Sensor Watch. Use the Light button to advance through the field you are setting, and the Alarm button to change the value in that field. The fields are, in order: Hour, Minute, Second, Year, Month, Day and Time Zone.

For features like World Clock and Sunrise/Sunset to work correctly, you must set the time to your local time, and the time zone to your local time zone. This allows Sensor Watch to correctly offset the time. This also means that when daylight savings time starts or ends, you must update both the time and the time zone on this screen.

3 - Movement: The Community Firmware for Sensor Watch

Movement is an application that manages the display of different screens of content on the watch. These screens are called watch faces. Watch faces can be passive displays of information like a clock or a calendar, or they can be fully interactive user interfaces like the Preferences face, which allows the user to customize Movement’s behavior.

Movement handles the instantiation of your watch face and manages transitions between screens. It also provides a low-power sleep mode, triggered after a period of inactivity, to preserve the watch battery.

Several faces are provided that offer baseline functionality like a clock, a settings screen and an interface for setting the time. You can change and reorder the watch faces that Movement displays by editing movement_config.h, and you can write your own watch face using the guidance in this chapter.

3.1 - Building Sensor Watch Firmware

Install the cross-compiling toolchain on Linux

  • Debian/Ubuntu: apt install gcc-arm-none-eabi libnewlib-arm-none-eabi

  • Archlinux: pacman -S arm-none-eabi-gcc arm-none-eabi-newlib

Other Linux:

  • Download the GNU ARM Embedded Toolchain for 64-bit Linux
  • Extract it: tar xjf gcc-arm-none-eabi-10.3-2021.10-x86_64-linux.tar.bz2
  • Add it to your PATH: export PATH="$PATH:/path/to/extracted/gcc-arm-none-eabi-10.3-2021.10/bin"

Note: if you want, you can add the above PATH export to your shell’s startup script to make it persist between shell invocations

Install the cross-compiling toolchain on MacOS

Note: if you want, you can add the above PATH export to your shell’s startup script to make it persist between shell invocations

Compile firmware

  • From within the repo:
  • cd movement/make
  • make

The built firmware will be at build/watch.uf2. You can now flash this firmware to your watch.

“I just want to pick my own set of watchfaces”

The list of included watchfaces can be found in the watch_faces array in movement/movement_config.h. Simply add, remove, and/or rearrange faces in this list to your liking and re-compile/re-flash your firmware.

You can also use the unofficial online firmware builder to build both the watch firmware and the emulator.

3.2 - Building Firmware for the Sensor Watch Emulator


3.3 - Designing a New Watch Face for Movement

In this document, we’re going to describe designing a single watch face for Movement, the community firmware for Sensor Watch. It’s a simple watch face that simply blinks an LED, and provides some options for blink speed and color.

We’ll start by generating a header and an implementation for our blink watch face. First, open a terminal and navigate to the movement/template directory: cd movement/template/.

You can create a watch face in any of the major categories of watch face that Movement offers:

  • clock faces tend to display the time, whether in a standard time system like a world clock or an alternative system like decimal time. The standard clock, Mars time and beat time all fit in this category.
  • complication faces tend to display some non-time information, but don’t generally require any data other than user input or the time. Sunrise, TOTP generation and dice rolling all fit into this category.
  • sensor faces are a lot like complication faces, but tend to display information from sensors attached to the watch. Temperature display, accelerometer display and battery voltage fit in this category.
  • settings faces are watch faces that involve configuring the watch.
  • demo faces are faces that involve demonstrating or testing functionality.

Once you know the category of watch face you’re building — blink is pretty clearly a complication — create the watch face files with the command python3 complication blink in a terminal window. That generates two files in the watch-faces/complication folder, called blink_face.h and blink_face.c. It also adds the watch face to both Movement’s Makefile and its include files.

Take a look at blink_face.h. Watch faces in Movement are just plain old C, and we’ll implement our blink watch face in the four functions that script has generated for us:

  • blink_face_setup is called when the watch boots, and when waking from deep sleep.
  • blink_face_activate is called just before our watch face comes on screen.
  • blink_face_loop is called every time the watch ticks, which is normally once a second, but watch faces can request something faster.
  • And blink_face_resign is called just before our watch face goes off screen.

The script also generates a structure to hold the state of our watch face. We’ll want to add some stuff: a boolean to keep track of whether the LED should be actively blinking — an on/off switch, if you will — as well as the speed and the color. It looks something like this:

typedef struct {
    bool active;
    bool fast;
    uint8_t color;
} blink_state_t;

Next, we’ll look to our implementation file, blink_face.c, and implement those four functions. Our setup function is simple: when the watch boots, Movement will call this function with a spot for us to stash a pointer to our watch face state. We’ll just allocate some memory for that purpose, and zero it out:

void blink_face_setup(movement_settings_t *settings, uint8_t watch_face_index, void ** context_ptr) {
    if (*context_ptr == NULL) {
        *context_ptr = malloc(sizeof(blink_state_t));
        memset(*context_ptr, 0, sizeof(blink_state_t));

Next, we’ll implement our activate function. When the wearer activates our watch face, we’ll want to set up any initial state that makes sense when arriving at the watch face. The temperature log watch face, for example, moves to the most recently logged entry. For our blink face, we make sure to set the active property of our state to false so the light doesn’t start blinking right away. (Note that we get a void pointer here, which we cast to our blink_state_t type. This “context” pointer may look confusing, but it’s just Movement giving us back the same pointer that we malloc‘ed in setup above.)

void blink_face_activate(movement_settings_t *settings, void *context) {
    blink_state_t *state = (blink_state_t *)context;
    state->active = false;

Before we declare our loop function, I’m going to add a little helper function to update the LCD. This just takes the state of our watch face, and translates it to letters on the LCD. This function formats a ten-character string that looks something like this: “BL F Green”. The wearer can read that as “Blink face, fast blink, green LED”.

static void _blink_face_update_lcd(blink_state_t *state) {
    char buf[11];
    const char colors[][7] = {" red  ", " Green", " Yello"};
    sprintf(buf, "BL %c%s", state->fast ? 'F' : 'S', colors[state->color]);
    watch_display_string(buf, 0);

The watch_display_string function then displays it in the ten positions available on the watch:

Rendering: an L-shaped flex PCB labeled “Temperature+GPIO Sensor Board”

Almost there! Next, we write our loop. This is where the action happens!

bool blink_face_loop(movement_event_t event, movement_settings_t *settings, void *context) {
    blink_state_t *state = (blink_state_t *)context;

    switch (event.event_type) {
        // TODO: handle events!

    return true;

The core of this function is just a big switch statement that handles events. Movement abstracts away all of the button and timer interrupts, and instead gives our watch simple events that correspond to things like button presses or ticks. All we have to do is implement the cases. For example, we get an event when the watch face is first activated. We can use that to update the LCD for the first time:


Next, let’s respond to a button press. When the wearer presses the Mode button, we’ll want to move to the next watch face:


Simple enough! Now we need a way to change the color. We’ll assign that function to the Light button: when the wearer presses that button, assuming the LED isn’t already blinking, we’ll move to the next color and update the display:

    if (!state->active) {
        state->color = (state->color + 1) % 3;

We’ll also need a way to start and stop the blinking. We’ll use the Alarm button for that. When the wearer presses that button, we want to do one of two things. If the blinking isn’t active, we set it to active, clear the display, and request a frequency to match the selected speed. If on the other hand the blinking is active, we’ll stop it and go back to displaying our state.

    if (!state->active) {
        state->active = true;
        movement_request_tick_frequency(state->fast ? 8 : 2);            
    } else {
        state->active = false;

Of course, we also need to give the wearer a way to select the speed. But we’re out of buttons! Not to worry, there’s another event for that. Movement can detect a “long press” of a button, which is when the wearer holds the button for more than a half second. We’ll make a long press on the Alarm button change the speed (again, assuming the LED isn’t blinking):

    if (!state->active) {
        state->fast = !state->fast;

Finally, we need to blink the light! We’ll use the tick event for that. Movement issues this event every time the clock ticks. Normally that’s once a second, but watch faces can request a faster tick, like we did above when setting the blinking state to active. We’ll use this tick function to toggle the LED off on even numbered ticks, and on odd numbered ticks:

    if (state->active) {
        if (event.subsecond % 2 == 0) watch_set_led_off();
        else if (state->color == 0) watch_set_led_red();
        else if (state->color == 1) watch_set_led_green();
        else watch_set_led_yellow();

That’s it for our loop! There’s only one function left, and it’s a short one. Our resign function is responsible for any last-minute cleanup before relinquishing control to the next watch face. In our case, it’s possible the wearer might press “Mode” while the LED was on, so we need to make sure to turn it off here, just in case:

void blink_face_resign(movement_settings_t *settings, void *context) {

With that, we’ve implemented a whole watch face in just a few dozen lines of code! All that’s left is opening up movement_config.h and adding it to the list of selected watch faces:

const watch_face_t watch_faces[] = {
    blink_face, // that's us!

4 - Sensor Watch Accessory Boards (aka Sensor Boards)

Rendering: an L-shaped flex PCB labeled “Sensor Watch Environment”

You may have noticed that there are no sensors on this board. That is by design: rather than pick sensors for you, the goal is to add a tiny flexible PCB with the sensors YOU want, and interface them over the nine-pin connector. The connector provides the following options for power and connectivity:

  • 3V power (3.08 V nominal; can drop to ~2.7V near end of life)
  • An I²C interface with built-in pull-up resistors
  • Five general purpose IO pins, which can be configured as:
    • Five analog inputs
    • Five interrupt-capable digital inputs, with internal pull-up or pull-down resistors
    • Five digital outputs
    • SPI controller (with one spare analog / GPIO pin leftover)
    • One UART TX/RX pair (with three GPIO leftover)
    • Up to four PWM pins on two independent TC instances
    • Two external wake inputs that can wake from the ultra-low-power BACKUP mode
Pin Digital Interrupt Analog I2C SPI UART PWM Ext. Wake
TX or RX
TC2[0] RTC/IN[1]
TX or RX
TC3[0] RTC/IN[0]

These tiny “sensor boards” have a set outline, and the available area for your electronics is quite small (5.7 × 5.7 × 1 mm). Still, this is plenty of room for an environmental sensor, MEMS accelerometer or magnetometer and a couple of decoupling capacitors. Note that you will likely be limited to QFN and LGA type parts; SOICs are too large, and even SSOP packages are generally too thick. You can find reference designs for several sensor boards in the PCB/Sensor Boards directory in the Sensor Watch repository.

4.1 - Sensor Watch Accessory Board: Temperature + GPIO

Rendering: an L-shaped flex PCB labeled “Temperature+GPIO Sensor Board”

This sensor board shipped (or will ship) to all Sensor Watch Crowd Supply backers. It is also available for separate purchase at the Sensor Watch page on Crowd Supply.


The Temperature and GPIO board includes two passive components on the front, a very precise 10 kΩ resistor and a 10 kΩ thermistor, that together form a voltage divider. While the resistance of the 10 kΩ resistor stays very close to 10 kΩ at all temperatures, the thermistor’s resistance goes down as the temperature rises, and goes up as the temperature falls. This changes the voltage that’s measured at the middle of the voltage divider, and with some clever math, we can turn that voltage into a very precise temperature reading.

While the watch library’s thermistor driver is preconfigured with all required parameters for this calculation, we’ll repeat them here for completeness:

  • The resistor has a value of 10 kilo-ohms with a tolerance of 0.1%
  • The thermistor has a nominal value of 10 kilo-ohms at 25° C, and a B coefficient of 3380K


In addition, this sensor board breaks out test points on the underside for six nets: power and ground, the I2C pins SDA and SCL, as well as two GPIO pins, A1 and A4. It will be fiddly, but you can solder fine enamel wires to these test points to connect other kinds of gadgets to your Sensor Watch, whether to test out possibilities for future sensor boards or to bodge in a device with additional functions you want.

Note that if you solder to these test pads, you must cover the five test points on the Sensor Watch board with tape! It’s possible for the solder bumps you create on the underside of the sensor board to touch the main Sensor Watch board, so you must electrically isolate them with tape.

The A1 and A4 pins are very versatile. You can use these two pins as:

  • General purpose digital IO to drive a low current device like a sensor or LED
  • Analog inputs to read an analog value from a circuit like a phototransistor
  • PWM output (they are both on the same TC3 peripheral)
  • Input to the TC3 counter peripheral for frequency measurement
  • UART transmit (A4) and receive (A1)
  • External interrupt inputs

In addition, you can use pin A4 as an external wake interrupt, which can wake the watch from the ultra-low-power BACKUP mode.

5 - Watch Interface Guidelines (WIG)

This document outlines best practices for developing watch faces for Movement, the community firmware for the Sensor Watch. While this is a rather specific scope, many of these concepts will be relevant to writing a bare-metal Sensor Watch app as well. After reading this document, you should have a sense for the hardware capabilities and limitations of Sensor Watch, the kinds of applications that are well-suited to the hardware, and the interaction models that tend to make for a rich and usable watch face.

Before we get to details, there are two major points that underly every piece of guidance in this document:

Sensor Watch is a watch

You’ll wear Sensor Watch on your wrist, and you’ll interact with it to answer questions about your day and the world around you. “When does the sun set this evening?” “How late is it in Berlin right now?” “What’s the high tide today?” These are the kinds of questions that Sensor Watch can excel at answering quickly and clearly.

Rendering: two screens of a tide prediction app, showing the high tide of 7.7 feet at 7:30 AM

Example: a tide prediction app, showing the time of the high tide and its predicted height.

Even when you add sensors like a thermometer, light sensor or accelerometer, you should think about how your watch face can answer these kinds of simple questions. “How cold was it last night?” “What shutter speed should I use in this lighting?” “Was I active enough today?”

Keeping these prompts simple keeps the focus on the things that Sensor Watch can do well, because:

Sensor Watch is not a smart watch

While Sensor Watch can have sensors added to it to measure things like temperature, light or acceleration, it’s not generally designed to synchronize these measurements to another device like a smart phone; in fact, by default, it has no way of syncing data to or from a device. This makes Sensor Watch less well suited to answering complex questions that require external data or a longer data trail; questions like “Will it rain tomorrow” require data the watch does not have, and questions like “How active was I last month?“ may be too complex to answer on a small watch with only a few buttons.

There are three key hardware differences between Sensor Watch and a smart watch:

  • Whereas a smart watch has a high-resolution screen capable of displaying arbitrary graphics, Sensor Watch has a segment LCD with a fixed layout.
  • Whereas a smart watch generally has a touch screen, Sensor Watch has just three buttons for input: the Light, Mode and Alarm (Wake) buttons.
  • Whereas a smart watch has powerful processor and a battery that can be recharged daily, Sensor Watch has a much simpler processor, and a battery that must be discarded and replaced.

These differences have implications for how a watch face displays content, how the wearer will interact with a watch face, and how the watch face goes about running its logic:

  • You will need to think carefully about how to format the information you want to display and make it work within the constraints of the segment LCD layout.
  • You will need to make the most of the limited button inputs, especially since all of these buttons have default functions like changing the mode or illuminating the LED.
  • You must take care to ensure that your watch face does not consume too much battery power. In addition, your watch face must be able to work gracefully with Movement’s low energy mode.

Road Map

This document is divided into five sections. First, we’ll talk about the “big picture” stuff: some of the concepts you should keep in mind when designing watch faces for Sensor Watch, some of what it does well, and some of what it doesn’t. Next, we’ll have three sections dedicated to each of those three key areas of functionality: the display, the buttons, and the low-power microcontroller. Finally, we’ll bring all of these concepts together to make a very simple watch face that says hello.

5.1 - The Sensor Watch Display

Before we talk about the Sensor Watch display, let’s go through some key terminology to clarify our discussion:

  • Segment - A single addressable element on the liquid crystal display
  • Indicator - A segment like “PM”, “24H” or the bell icon that indicates status
  • Digit - A grouping of segments that can display a number, character or glyph
  • Position - The placement of a digit on the display; for example, the first digit at the left of the top row is Position 0

We also tend to label the segments of each digit clockwise from the top segment: the top segment is segment A, top right is B, bottom right is C, all the way to the center digit which is segment G:

TODO: A diagram of a single digit with its segments labeled

In this image of a clock, you can see where the 8 has all segments (A-G) energized. You can display a 1 by energizing only segments B and C, and a 2 by energizing segments A, B, D, E and G.

The Sensor Watch has five indicators, ten digits, and one segment dedicated to the colon in the time display. We choose to number the digits as follows:

  • Positions 0-1 - The digits in the top middle, usually used to display the current mode or the weekday. We can call these the “Weekday” digits.
  • Positions 2-3 - The digits at the top right, usually used to display the day of the month. The “Day” digits.
  • Positions 4-9 - The digits on the bottom row, usually used to display the time; the “Clock” digits.
    • Positions 4-5 are normally the hours display.
    • Positions 6-7 are normally the minutes display.
    • Positions 8-9 are normally the seconds display, and are slightly smaller than the other digits on the bottom row.

TODO: A diagram of the F‑91W LCD with positions grouped and labeled

As alluded to in the “Big Picture” section, this document is not going to be prescriptive about how you use the digits in these positions; there is no “must” about how you display information on your watch face. Still, there are some broad guidelines on where these digits can be most useful:

  • The “Weekday” digits are particularly useful for displaying alphanumeric information.
  • The “Day” digits are best for displaying a single number from 0-39, such as a day, an index or a countdown.
  • The “Clock” digits are well suited for the main informational display of your watch face.

You are going to need flexibility to adapt the information you want to display to the Sensor Watch LCD. This section is merely going to walk through some useful patterns, some limitations, and some best practices.

The Weekday Digits

The digits in these first two positions, positions 0 and 1, have some extra segments that make them useful for displaying letters. The stock F‑91W uses these positions to display the day of the week (SU, MO, TU, etc). If you have some alphanumeric information to relay on your watch face, it would make sense to look to these first; the tide prediction concept in the previous section, for example, imagined labeling the displayed prediction with “HT” for high tide and “LT” for low tide.

If you have no data to put in these segments, they can also serve as a useful spot to title your watch face, so that the wearer is clear what information they are looking at. The “Temperature” watch face, for example, uses these digits to display “TE”. This distinguishes it from the “Temperature Log” watch face, which titles itself “TL”.

Note that you are not limited to keeping the same characters in this position the whole time your watch face is on screen! The “Temperature Log” watch face displays “TL” most of the time, but when the wearer enters timestamp mode, it displays the word “AT” in these positions, signaling that the data point was logged “at” the displayed time and date.

Rendering: a temperature logging app, showing the logged temperature of 68.7 degrees Fahrenheit, and a 0 at the top right. A second screen to the right displays “At 12:00” with a 25 at the top right

Consider: when the wearer presses the “Mode” button and sees the watch face for the first time, “TL” tells them which watch face they are viewing. Once they have that information, they know they will remain in this watch face until they press “Mode” again, so you can feel free to change it after a short delay or in response to a button press. It’s done its job, and given the wearer the context they need to use the watch in the current mode.

Of course, you could use these segments in a completely different way. The LIS2DH accelerometer test face (which we’ll look at a bit later) uses these two positions to indicate the presence of an interrupt on the X or Y axes, lighting up “Y” and/or “X” when an interrupt is asserted, and remaining blank otherwise.

Limitations of the Weekday Digits

On a stock Casio F‑91W, the digits in these two positions normally display one of just nine values: abbreviations for the seven days of the week, the text “AL” for alarm mode, or the text “ST” for stopwatch mode. If you break this down, position 0 will only ever display one of the characters A, F, M, S, T or W, and position 1 will only display the letters A, E, H, L, O, R, T, or U. The Sensor Watch aims to display many more characters in these positions, and in the case of position 0, we can do it! With the exception of an uppercase R, the digit in position 0 can display any character in the Sensor Watch character set.

TODO: A diagram of the F‑91W’s Weekday Digits with position’s 1 shared segments highlighted in red.

Position 1 is a bit more trouble. Looking closely at the position 1 letters above, you’ll notice that segments B/C and E/F never have to operate independently (as they would to, say, display the letter S or the number 2). Thus, to save pins on the LCD, the designers of the F‑91W tied these two pairs of segments together. This means you can only display a limited set of characters in position 1: the letters A, B, C, D, E, F, H, I, J, L, N, O, R, T, U and X, and the numbers 0, 1, 3, 7 and 8.

All of this is to say that you may need to think about terminology that makes sense for your watch face, and tweak it to fit the constraints. For example, the “Settings” watch face requires changing a setting related to Movement’s low power mode, but the ‘P’ in “LP” cannot be displayed in position 1. To address this limitation, the setting is instead called “Low Energy” mode, which abbreviates to “LE”.

The theme of limitations will come up in each of these groups.

The Day Digits

On a stock Casio F‑91W, these two digits normally display the day of the month. This makes this position very useful for displaying values from 0-39:

  • The Clock and World Clock face use this area to display the day of the month.
  • The TOTP watch face uses this area for a 30-second countdown, indicating the validity window for the displayed code
  • The Temperature Log uses this area to display the index of the logged measurement, from 0 to 35

TODO: Three watch faces showing these uses.

Limitations of the Day digits

While the digit in position 3 is a full-featured 7-segment display that can display any number and just about any letter, the digit in position 2 is the most limited digit on the watch: it can only display the numbers 1, 2 and 3. This constraint relates to the fact that it is missing one segment (segment F), and that segments A, D and G are all tied together.

TODO: A diagram of the F‑91W’s Weekday Digits with position’s 2 shared segments highlighted in red.

There’s not much to say about these digits because they are pretty limited: if you have a single letter or a number from 0-39, you can display it here. If not, you can leave these digits blank.

The Clock Digits

The bottom row of digits, positions 4-9, comprise the clock display on a stock F‑91W. This is the main event: you can display a ton of information here. But before we look at some of the ways we might want to use this display, let’s take a look at some of the subtler properties of these digits:

TODO: Two watch faces showing 88:88 88 and 88 88 88.

  • First, note that the digits in positions 8-9, the seconds, are slightly smaller than the rest. This makes it obvious even without a colon that they are the seconds on a clock.
  • Next, note the way that the colon separates positions 5 and 6 in the display on the left. This clarifies the separation of the hours and the minutes on the clock.
  • Now look at the display on the right, where the colon is turned off. Note that in the absence of a colon, these digits are not evenly spaced: there is a gap between them.

TODO: Three watch faces: HI: 87.2°, 10 20 30, Sunset.

These details are useful to keep in mind as you think about the data you want to display, as these subtleties lend themselves to some non-obvious strategies. For example:

  • Consider how the visual weighting of the seconds digits could make them useful for displaying the decimal part of a floating point number, as shown in the display on the left (87.2°).
  • Also on the left: consider how the colon might be useful both as a separator in a clock display, and as a punctuation mark separating two concepts, like the high temperature label and the measured temperature.
  • Consider how the spacing of these digits might let you treat this line not as one six-digit display, but as three distinct two-digit displays. For example: 10, 20 and 30 could represent three separate data points in the display in the middle.
  • Also consider how the wearer’s eye would read across the display if a full word were placed there, as in the display on the right. Do your eyes notice the gap, or does the word “Sunset” read legibly?

You saw this guideline before in the “Big Picture” section, but it bears repeating: over time, the wearer will become familiar with the layout of your watch face, so don’t be afraid to use these digits in an unorthodox way.

Limitations of the Clock digits

As you might have expected, there are some limitations to the clock digits. The first limitation applies to all of them: these are 7-segment displays meant to display numbers, not letters. As such, characters like ‘M’ and ‘W’ are not available; they will render as an ‘N’ or a ‘U’ respectively. Additionally, some letters aren’t available in their preferred case; uppercase ‘R’, for example, will appear as a lowercase ‘r’. Finally, some of these characters may look a bit awkward. ‘K’, for example, requires a little bit of imagination, and ‘X’ requires a lot of imagination; the screens below are meant to display the words “Ranked” and “PIXEL”:

TODO: Two watch faces displaying the words “ranked” and “pixel”

The second limitation has to do specifically with digits 4 and 6: the top and bottom segments of these digits (segments A and D) are tied together:

TODO: The shared segments on the clock display

The Sensor Watch display library will try to work around this limitation in some cases. For example, some letters may have their case changed: in displaying the word “Apogee” below, the Sensor Watch will render the “a” in lowercase and the ‘O’ in uppercase — “apOgee” — to work around the constraints on positions 4 and 6. This is because position 4 cannot turn off segment D without also turning off segment A, as would be required for an uppercase ‘A’, and position 6 cannot turn on segment D without also turning on segment A, as would be required for a lowercase ‘o’.

TODO: The word “Apogee”, mangled on the left, but displayed correctly on the right

Other characters may take on slightly different forms in these positions: for example, a 7 in position 4 or 6 will render as a short “lowercase” 7, and characters that require a segment on the top or bottom may be shifted up or down to make the best use of the position.

TODO: Two watch faces: and the word 7-up twice

With some extra care, you can sometimes dodge this limitation. In the two screens above, for example, you can see that displaying the name of this popular soft drink in positions 4-7 results in a lowercase 7, and a U that’s shifted up to the top half of the digit. But by shifting it over to positions 5-8, both the 7 and the U get displayed normally.

TODO: “Hi Eide”, “Hi  tid”, “TI HIGH”

Note however that in some cases, you cannot dodge this limitation. Letters that can only be displayed by energizing segments A and D independently will not display correctly. In the above example, the lowercase ‘t’ in ‘Hi tide’ will not work in position 4 or 6. It will display as an uppercase E, “Hi Eide”, because segment D also controls the state of segment A. You will have to figure out another way to convey this concept, either by shifting the ‘t’ to a position that can display it, or re-imagining the way the information is presented to the wearer.

Regardless of case, the following characters are fully unsupported in positions 4 and 6: F, K, P, Q, T, X and Y, as well as the underscore and a few other punctuation marks. In addition, the decimal point appears as a hyphen; this may lead to confusion when displaying negative numbers, so you may have to work around this as well.

The Indicator Segments

In addition to the three major segmented display areas, the watch has five indicator segments: “Signal”, “Bell”, “PM”, “24H” and “LAP”. You can energize these indicator segments to convey information or status to the user. In the stock F‑91W, these have some fairly obvious uses: Signal and Bell indicate whether the hourly signal and alarm are active, and PM and 24H relate to the clock display. “LAP” only operates in stopwatch mode, where it indicates when the stopwatch is showing a lap time. Still, when looking at these indicators, you should consider what concepts people might have associated with these icons.

TODO: All indicators on

The “Signal” indicator takes the form of a series of bars moving from thick to thin. It does convey the sound waves of an hourly indicator, but consider other possible meanings: they could represent radio waves, sensor activity or the reading of data from an accessory board. The Temperature watch face, for example, energizes this segment to indicate that a new reading is being taken from the thermistor.

The “Bell” conveys the concept of an alarm bell, but consider alternate meanings like the presence of a notification or a warning condition. For example, a tide prediction face could energize this segment when high tide is very near — an important thing to know if you are walking along a remote stretch of beach!

The PM indicator has the narrowest use case: if you are displaying a time value that is in the afternoon, you should activate this segment. This guidance also applies to the 24H indicator. Also note that Movement has a global setting that will tell you whether the user has selected 24H mode; you should respect this whenever displaying time values.

Finally, the LAP indicator. This indicator has a very specific meaning for a stopwatch face, and almost no meaning anywhere else. The Stopwatch face uses this segment for its expected use case, but beyond that, it’s not a great semantic fit for most others. Having said that, if you need to indicate a rare warning or error condition, and none of the other indicators are a good fit, the LAP indicator might make sense. For example, the Simple Clock face energizes this segment when a low power condition is detected. It’s awkward, but it works.

Putting It All Together

While the previous sections outlined how each of these bits of the display worked on their own, the fact is, you will mix and match these strategies. We’ve seen how the Temperature and Temperature Log watch faces use the Weekday digits to display their name (TE, TL) and a data point. But a Sunrise/Sunset face could use those characters to display something about the data point: that this is a sunSEt at 5:42 PM on the 5th, and a sunRIse at 6:24 AM on the 6th:

TODO: sunrise/sunset face

The accelerometer test face uses all of these areas in a radically different way. It uses the top row to indicate the presence of a Y, X or Z interrupt, and a single digit in the top right to count the last ten seconds to the logging of the next data point. The bottom line counts interrupts per minute, and displays the most recent three minutes’ counts in three two-digit groups on the clock line:

TODO: accelerometer face

There’s also nothing that says you have to use these displays for digits at all. Here we imagine using the 12 vertical bars in the clock display to show an activity log over the last 12 hours, where the presence of a short bar indicates some activity in that hour, and a tall bar indicates more. You can imagine toggling this watch face between different timeframes, which could be displayed in the top right: 6/12/24, or 1d/2d/3d.

TODO: activity log face

Final Notes

The Sensor Watch display is both powerful and limited: we can push it to do things that a stock F‑91W couldn’t dream of, but it also inherits some of the limitations inherent to the donor watch. Still, armed with these guidelines, you should feel empowered to build useful and information-rich watch faces. The important things to remember:

  • Keep in mind how the segments are grouped, and think about how you can make the information you want to display fit into those groups.
  • Be mindful of the limitations of various digits and groups, and prepare to be flexible when naming things or laying out text.
  • Don’t be afraid of information-dense displays or unorthodox uses of the LCD. The wearer using your watch face will develop a mental model for the information you’re presenting to them.
  • Use indicators to signal various conditions, and keep in mind creative ways of using them to convey meaning to the wearer.

5.2 - Big Picture Stuff

The Casio F‑91W, the donor watch on which Sensor Watch is based, is one of Casio’s smallest and simplest wristwatch designs. It is designed to show very basic information: the date and time. As we walk through the device’s hardware capabilities, we can see how it is tailored carefully to that use case:

  • It has a small segment liquid crystal display with ten digits.
  • It has a dim backlight on the left side of the display that mostly illuminates the hours and minutes.
  • It has a buzzer for firing a single alarm, as well as an hourly alert.
  • The three buttons on the watch offer very basic inputs:
    • the “Light” button illuminates the watch face,
    • the “Mode” button cycles through the four modes that the watch offers,
    • and the “Alarm” button is the only truly multi-function button: it turns the alarms on or off when the alarm face is active, and swaps the main 12/24 hour mode when the main clock is active.

Rendering: a Casio F‑91W displaying the time and date (just after noon on Thursday the 9th)

Sensor Watch inherits many of the same hardware limitations of the F‑91W: the screen is small, the layout is designed to display no more than ten characters at a time, and we have only those three buttons to work with (one of which is dedicated to switching modes). This leads to some broad guidelines that will be useful to think about when designing your watch face:

  • Your watch face’s main interface should focus on presenting a single screen of useful information.
  • If possible, your most important information should appear on the left side of the screen, since the LED will illuminate the left side more brightly than the right.
  • You should feel free to use every character position and every indicator segment to its maximum potential! You have ten characters to work with; use them to the fullest.
  • If you have more than one screen of information to display, you should hide it behind a press of either the “Alarm” or “Light” buttons…
  • …because your watch face should be prepared to resign immediately when the wearer presses the “Mode” button. (More on this in the “buttons” section!)

Let’s look at some of the watch faces that come with Sensor Watch to see how these guidelines are implemented in practice.

The Temperature Watch Face

Rendering: a temperature display app, showing the current temperature of 68.7 degrees Fahrenheit

This is the Temperature watch face. It displays the current temperature, as read from a thermistor voltage divider on a Sensor Watch accessory board.

This watch face is relatively simple. It’s dominated by a single large piece of information: the current temperature. The wearer is able to absorb this information quickly and easily. The temperature data itself is left-justified. If the LED only illuminated the first two digits, the wearer would at least know that the temperature is in the range of 68 degrees. The fractional part, while useful, is not essential to a quick read of the conditions.

Also note that the “signal” indicator segment, used on the main watch face to indicate the status of the hourly beep, is here repurposed to indicate when a new reading is being taken. Don’t be afraid to use these indicators for your own purposes! Whereas the bell indicator might signal that an alarm is enabled on an alarm watch face, you may want to use it to signal an error or warning condition on another face. This is totally fine: the wearer will over time grow familiar with the layout of your watch face, and make the correct associations between icons and concepts.

The temperature display face is relatively simple; let’s take a look at a more complex watch face.

The Temperature Log Watch Face

Rendering: a temperature logging app, showing the logged temperature of 68.7 degrees Fahrenheit, and a 0 at the top right

This watch face stores a 36-hour log of temperature values, which the wearer can read back to get a sense of high and low temperatures over time. This seems at first glance like it might be a formidable challenge for a watch face with only ten digits, but by thinking carefully about the design, we can fit a lot of information on this small screen.

Let’s revisit the first guideline. This screen displays a useful, information-dense readout of the most recent temperature logged: the “TL” indicates the mode (Temperature Log), and the “0” at the top right indicates that this is the zeroth (i.e. most recent) reading taken. The whole bottom line is dedicated to a large, clear display of the logged temperature.

For clarity, it would be nice for this screen to display not just the index of the temperature reading, but also the time that the temperature reading was taken. There isn’t room for that on the display, but by pressing the “Light” button, the wearer can temporarily move the watch face into a mode that displays the timestamp of the reading:

Rendering: Same as before, but a second screen to the right displays “At 12:00” with a 25 at the top right

This mode simply displays the timestamp for a second or so — this reading was taken at 12:00 AM on the 25th day of the month — and then returns the wearer to the temperature reading at that timestamp. This is a simple way to stack two aspects of the data point into one screen. If you had multiple aspects (say, temperature, humidity and timestamp) you might consider using the light button to cycle through those three aspects of the data point.

This handles the most recent temperature and timestamp. But what about our 36 hour log? The answer there lies in the other button, the “Alarm” button. Pressing the “Alarm” button moves one index back in the list of readings:

Rendering: the app shows a logged temperature of 67.9 degrees Fahrenheit, and a 1 at the top right. To the right, a second screen displays “At 11:00 PM” with a 24 at the top right

Once again, we’ve managed to fit more information onto the screen by hiding it behind a button press. The index at the top right makes clear that this is data point 1 (not data point 0), and again, pressing the “Light” button on this screen will display the timestamp for this reading: 11:00 PM on the 24th. Repeated presses of the “Alarm” button move the wearer back in time. At any time, the wearer can press the “Light” button to view the timestamp. And of course, the wearer can press “Mode” at any time to move from the temperature log face to the next face in the list.

Note that while the wearer is likely to develop familiarity with your watch face’s unique visual layout over time, they will likely expect the buttons to behave consistently between watch faces. They will always expect the “Mode” button, for example, to advance the watch face, and for the “Alarm” button to toggle settings or change values. Generally speaking, you should use the “Alarm” button to move through time or advance a value, and use the “Light” button to move between sub-modes within the watch face (and illuminate the LED, as appropriate). The “Time Set” watch face, for example, uses the “Light” button to move between setting the hour, minute and second, and uses the Alarm button to change the value in each field.

Wrapping Up

When designing watch face visuals for the Sensor Watch, you should aim to present one screen of content with clarity:

  • Don’t shy from information density; use as many digits and indicators as you need.
  • Try to fit more crucial information on the left side of the screen, and put less pressing information on the right.
  • Don’t worry about how segments or digits are used on other watch faces; when your watch face is on screen, you are in control.

When designing interactions for the Sensor Watch, you should aim for consistency with other watch faces:

  • Your watch face should resign immediately when the “Mode” button is pressed and pass control to the next watch face in the list.
  • If your watch face has multiple modes, you should use the “Light” button to toggle or advance through them.
  • If your watch face presents multiple data points or advances through multiple values, you should use the “Alarm” button to advance through those.

6 - Contribution Guidelines

In the interest of creating a welcoming and constructive space, we have adopted the Contributor Covenant Code of Conduct:

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We as members, contributors, and leaders pledge to make participation in our community a harassment-free experience for everyone, regardless of age, body size, visible or invisible disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, caste, color, religion, or sexual identity and orientation.

We pledge to act and interact in ways that contribute to an open, welcoming, diverse, inclusive, and healthy community.

Our Standards

Examples of behavior that contributes to a positive environment for our community include:

  • Demonstrating empathy and kindness toward other people
  • Being respectful of differing opinions, viewpoints, and experiences
  • Giving and gracefully accepting constructive feedback
  • Accepting responsibility and apologizing to those affected by our mistakes, and learning from the experience
  • Focusing on what is best not just for us as individuals, but for the overall community

Examples of unacceptable behavior include:

  • The use of sexualized language or imagery, and sexual attention or advances of any kind
  • Trolling, insulting or derogatory comments, and personal or political attacks
  • Public or private harassment
  • Publishing others’ private information, such as a physical or email address, without their explicit permission
  • Other conduct which could reasonably be considered inappropriate in a professional setting

Enforcement Responsibilities

Community leaders are responsible for clarifying and enforcing our standards of acceptable behavior and will take appropriate and fair corrective action in response to any behavior that they deem inappropriate, threatening, offensive, or harmful.

Community leaders have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, and will communicate reasons for moderation decisions when appropriate.


This Code of Conduct applies within all community spaces, and also applies when an individual is officially representing the community in public spaces. Examples of representing our community include using an official e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event.


Instances of abusive, harassing, or otherwise unacceptable behavior may be reported to the community leaders responsible for enforcement by contacting @josecastillo on Twitter. All complaints will be reviewed and investigated promptly and fairly.

All community leaders are obligated to respect the privacy and security of the reporter of any incident.