Lightning mode Designed for networking. hold the main button down for five seconds to activate lightning mode. While in this mode, the watch waits a random amount of time (1-5 minutes) before Lighting up a cascade of letters in such a way that it looks like lightning raced Across your wrist. Simple, but eye-catching. This little trick is the ultimate icebreaker.
Extreme power efficiency every level of the word watch’s design was built with power efficiency in mind. The green LEDs were picked not only for their elegant look, but also because they are extremely energy efficient. The use of onboard chips was minimized, and the CPU sleeps when not in use. As a result, the expected life expectancy of the single CR2032 that powers this watch is about 2 years. For more on energy-saving techniques used in this design, please scroll down to the engineering considerations section.
Ease of use to change the time, simply press and hold the main button for two seconds. The current time will flash on-and-off when you have entered time-change mode. Now you can simply click the main button to increment the time. Once you are satisfied with the time setting, hold the main button down for two seconds to return to the default mode. You can now tap the button at anytime to show the time.
Waterproofing the screen The screen is laser cut from a 3mm thin piece of plywood. Since it has holes in it that lead straight to the circuit board, it needs to be waterproofed. To accomplish this, an epoxy resin is used. This resin needs to have an extremely low viscosity(similar to water) otherwise it will not pour nicely into the letter holes in the screen. Furthermore, all epoxies need to be degassed to produce crystal clear results upon hardening. Degassing cannot be done with heat for this design because it will cause the wood to degas- releasing gasses into the epoxy, which bubble up and obscure the light coming through the letter holes. Therefore, degassing must be done in a pressure chamber (positive or negative pressure above 30 PSI works perfectly)
neodymium magnets twelve tiny (2mm h x 1 mm d ) cylindrical magnets were used to hold the screen onto the rest of the watch body. This was because the addition of screws (which adds a theme of ruggedness and complexity) would have ruined the design of the watch, which is meant to appear elegant and minimalistic. Furthermore, tiny screw holes are impossible to model with FLA 3D printing. These screen magnets pair with a matching set of magnets built into the body of the watch
Blinds Blinds were produced to enclose every single LED on the circuit board, ensuring that light would not leak from an LED into an adjacent letter. The Blinds are the square compartments of a 3mm square mesh that was 3D printed and sandwiched between the circuit board and the screen. The height of the mesh is slightly greater than the maximum height of the LEDs and lays flush on the bottom side of the screen, ensuring that the light from an LED cannot reach it’s neighboring LED’s letter.
Charlieplexing was used to decrease manufacutiring costs. This board has over a hundred LEDs, which need to be individually controlled. Conventionally, this would require atleast a hundred pins. By using charlieplexing, the required number of pins was decreased to 28. Charlieplexing is a method for controlling 2(n+1) LEDs using n pins. This can be accomplished because microcontroller pins can go into a high-impedance mode where the input resistance is effectively infinite. By putting some pins in high impedance mode, its possible to route current along a network of LEDs such that only a single LED is turned on.
Modularized Charlieplexing One major problem with the use of charliplexing for this design is that only a single LED within a charliplex network can be turned at any given time. Given that this design must light up long phrases like 'It is half past twelve', lighting each letter after another would produce a flickering display. Therefore, instead of networking all the LEDs together into one network, all the LEDs are networked into groups of phrases that are mutually exclusive. For example, 'one', 'two', 'three' ....'twelve' are all grouped plexed together into an 'hour' network because 'two' will never lightup when 'four' does. Similarily 'oclock', 'past' and 'to' form an 'indicator' group. Since 'two' and 'oclock' are in different charliplexes, the system can cycle through turning on the LEDs associated with 'two' and 'oclock' at the same time. That is to say, two LEDs can be turned on at the same time. This eliminates flickering.
External RTC clock running on a custom library, this design utilizes a ultra-low power RTC unit to improve accuracy and save power. The unit is used to keep track of the time, with a greater accuracy than is possible with the native clock circuit in the microcontroller. Since the microcontroller is not used to keep time, it can go into a deep-sleep mode that reduces power consumption by more than 99.99 percent. Finally, the RTC also provides interrupts that allow lightning mode to work.
Minimalistic RTC library A minimalistic library was coded to control the RTC. This library includes functions for communicating with the RTC using I2C. These Functions include RTC_init() and change_time().
Minimalistic IO Expander Library a minimalistic library was coded to control the MCP2008 IO Expansion module. This library includes functions for communication using I2C.