Sensor Watch is a bridge between eras. It takes an iconic 30-year-old design from a golden age of digital watches, and pairs it with a modern, powerful microcontroller. This small circuit board, less than an inch in diameter, replaces the original quartz movement in a Casio F-91W or A158W watch to put the capabilities of an ultra-low-power ARM Cortex M0+ microcontroller on your wrist.

Sensor Watch Lite is everything you loved about Sensor Watch, streamlined into one simple board.

Sensor Watch is not like most smart watches. It makes a different set of engineering tradeoffs, to achieve a different set of goals:

• Instead of a high-resolution TFT LCD, Sensor Watch repurposes the monochrome segment LCD on the Casio F-91W and A158W. This gives it an always-on display that consumes mere microamperes of power.
• By avoiding power hungry features like Wi-Fi and Bluetooth, Sensor Watch can run for over a year on a single 100 mAh coin cell, eliminating the need for frequent recharging.
• The lack of an external charging port, coupled with reuse of the famously water resistant F-91W and A158W enclosures, makes this a hackable wristwatch that you can wear while surfing or swimming.

What Can It Do?

The community Sensor Watch firmware (called Movement) comes pre-loaded on every Sensor Watch Lite board, and includes useful features for timekeeping and keeping tabs on the sun and moon:

• The Clock face allows Sensor Watch to function like, y'know, a watch, displaying the time and date.
• The World Clock face allows you add a time display for any number of time zones around the world.
• The Sunrise/Sunset face displays the next sunrise and sunset times for your location.
• The Moon Phase face displays the current phase of the moon, and lets you play the month forward to see moon phases in the future.
• The Timer Face allows you to time events up to 40 days long with one-second resolution.

But this is just the beginning. Several alternate firmware images offer additional features:

• The Backpacker firmware adds a digital thermometer and a temperature logger, which lets you track overnight low temperatures outside your tent.
• The Stargazer firmware adds an astronomy watch face, capable of calculating right ascension, declination, altitude and azimuth for the sun, the moon and all the planets.
• The Athlete firmware adds a countdown timer and exercise counter, as well as an old-school pulsometer complication scaled for 30 beats.

You can also build your own firmware to take advantage of even more watch faces:

• There’s a TOTP generator for carrying around two-factor auth codes
• A tachymeter for computing average speed based on distance and travel time
• A beat time clock for showing decimal time according to the Swatch Internet Time system
• There’s even a tarot complication, which uses the SAM L22’s hardware random number generator to give you a reading on your wrist
• And many more!

More importantly: Sensor Watch is open source and easily hackable, which means you can write the apps that make sense for YOU. Do you want a watch face that can predict satellite passes? A transit face programmed with train arrivals for your nearest subway station? An astrology face that can tell you if Mercury is in retrograde? These are all watch faces that you could write for Sensor Watch.

Sensing, More Simply

For the first run of Sensor Watch, we included a nine-pin connector for interfacing with sensor boards. But the truth is, we only had a temperature sensor board available at first, and the vast majority of folks only ever made use of that board.

For Sensor Watch Lite, we’re bringing that temperature sensor onto the Sensor Watch board itself, and dispensing with the complexity of the nine-pin connector. This allows us to hit an affordable price point. The old setup ran you $40: $36 for the main board plus $4 for the temperature sensor, whereas the new board with integrated temperature sensor is just $39!

We’re also breaking out two test points from the nine-pin connector, pins A1 and A4. You can use these pads as analog inputs, digital IO (with PWM), or a UART for serial commiunications. Pin A4 can even wake from the ultra-low-power BACKUP mode.

The Microchip SAM L22: Big Power in a Small Package

The SAM L22 microcontroller at the heart of Sensor Watch is an ARM Cortex M0+ chip with 256 KB of Flash and 32 KB of RAM, running at up to 32 MHz. It’s similar in many ways to the SAM D21 you’d find in a Feather M0 or Arduino Zero, with many of the same versatile peripherals:

• An integrated USB peripheral and UF2 bootloader let you plug the board into your computer and program it by dragging firmware onto it, just like a thumb drive.
• The real-time clock peripheral, paired with a 32.768 KHz crystal, allow for accurate timekeeping and configurable wake-up options.
  • Moreover: its frequency correction function, paired with the onboard temperature sensor, allow us to make this a temperature compensated crystal oscillator, entirely in software!
• The integrated 12-bit ADC, with oversampling to 16 bits of resolution, lets you read the temperature as well as analog values from pads A1 and A4.
• The SERCOM peripheral lets you interface with UART-oriented devices on pads A1 and A4.
• Four timer/counter peripherals allow for versatile use cases like pulse-width modulation, frequency counting and configurable periodic callbacks. This is in addition to the TCC peripheral that drives the red/green backlight and piezo buzzer ¹.

¹ The piezo buzzer is the only piece that requires soldering. You will need to remove a metal piece from your donor F-91W or A158W and solder it to the Sensor Watch board. If you don’t feel comfortable doing this, all other features of Sensor Watch will function identically; the watch just won’t beep.

The Segment LCD: a Low-Power Hero

In addition to these familiar peripherals, the SAM L22 packs one less familiar one: a segment LCD controller. This controller speaks the native language of the F-91W/A158W display glass, and it’s the key to the unique low power capabilities of Sensor Watch:

• Unlike a TFT, with its layers of color filters, the segment LCD glass is readable without backlighting.
• Unlike an OLED, which relies on light-emitting diodes, segments do not consume significantly more power when on versus off.
• Unlike an e-paper display, which requires current to move ink particles, updating the segment LCD glass does not consume significantly more current than keeping an image on the screen.

This is not to say there aren’t tradeoffs: the segment LCD in the F-91W and A158W was only designed to display the time, so not every number or letter works in every position. It also cannot display arbitrary images like an e-paper display or LCD matrix. Still, with a bit of creativity, you can create a watch face that displays quite a lot of information, and carry it with you all year long.

Features & Specifications

• ARM Cortex M0+ microcontroller running at up to 32 MHz
• 256 KB of on-chip Flash, with up to 8 KB EEPROM emulation area
• 32 KB of RAM with full retention in low-power standby mode
• 32.768 kHz crystal for real-time clock functionality with alarm support
• Red & green PWM’able LED backlight
• Temperature sensor
• On-board USB Micro B connector
• Reset button with double-tap UF2 bootloader
• Controller for ten digit segment LCD, plus five indicator segments
• Edge-plated contacts for three interrupt-capable buttons
• Connection pad for piezo buzzer (requires light soldering)
• Open Source

The A1 and A4 test points offer some additional functionality:

• Two interrupt-capable digital inputs, with internal pull-up or pull-down resistors
• Two digital outputs with PWM capabilities
• Two analog inputs
• One UART TX/RX pair
• One external wake input that can wake from the ultra-low-power BACKUP mode

Comparisons

² $39 Sensor Watch Lite board + the cost of an F-91W

Support & Documentation

• Sensor Watch GitHub Repository
• Sensor Watch Documentation
• Low-Level Watch Library Documentation
• Watch Interface Guidelines
• Sensor Watch Discord
• README for Movement, the community Sensor Watch firmware

And please feel free to reach out using using the Ask a technical question link below!