Freelance hardware designer Augusto Marinucci has developed a homebrew watch which uses a custom-built ePaper screen, encased in poured resin to create something simultaneously low-power and extremely robust: the Solid State Tank.
"Solid State Tank (SST) is a [ePaper] wrist watch inspired by [the] CW&T Solid State Watch and the Cartier Tank," Marinucci explains of his eye-catching creation. "The basic idea is a clear case tank style watch with a DIY [ePaper] screen to be potted in clear epoxy. The goal for the battery life is the same as the Casio F-91W: 7 years."
The housing of the Solid State Tank is directly inspired by the Cartier Tank, a traditional and high-priced analog wristwatch Marinucci modeled then 3D-printed with mounting points for a pair of custom PCBs. The first of these is the watch itself, a logic board with a Microchip PIC16 microcontroller, a Micro Crystal RV-3028-C7 real-time clock (RTC), and a power supply based on a surface-mounted CR1616 battery.
"To minimize power consumption the power section of the watch uses two load switches in parallel to keep the MCU [Microcontroller Unit] in the off state for most of the time," Marinucci explains. "The first load switch is connected to the clock output of the RTC chip that can be programmed to output a 1Hz clock every minute. The second load switch is controlled by the MCU and is used to keep the supply on as long as it's needed."
It's the display that really catches your eye, though. Rather than opting for the traditional LCD or even LED technology, Marinucci built a custom ePaper display — using electrophoretic technology to draw power only when shifting between states once per minute, and being fully sunlight-readable. "Very simple," Marinucci says of the display board, "as it is only pads and two connectors."
To ensure the device's longevity, the custom display is cut with scissors then attached to the driving board with adhesive ― the edge using conductive adhesive for a common electrode. The edges of the display are then sealed with epoxy resin which hardens under ultraviolet light — and after programming and insertion into the 3D-printed case, the whole assembly is then treated to a bath in the same resin to provide a fully waterproof watch capable of withstanding some considerable abuse.
"In average I was able to achieve 495nA," Marinucci says of the watch's power draw, "[which] converts with a 50mAh battery in[to] a ideal lifetime of 11.5 years. In reality it will be less due to temperature changes and current peaks that damage the battery and increase the internal resistance. To set the time I used a quick Python script (don't judge), that send the current time to the watch via a simple USB to TTL converter."
Marinucci's full write-up is available on his website.