One problem with LED wristwatches is that the battery ultimately must be charged. The “Perpetual” Wristwatch (PWW) from Serhii Trush is completely different. Twelve photodiodes on the watch face act as a photo voltaic panel to recharge the built-in battery. This characteristic means the watch may run perpetually!
PWW’s printed circuit board has a 35-millimeter diameter. Nevertheless, Trush recommends a thinner 0.8-millimeter board. (PCBs are sometimes 1.6 millimeters thick.) The board accommodates 12 crimson LEDs, 12 BPW35S Photodiodes, passives, and a Microchip ATtiny13A microcontroller.
Inside PWW is a 60 milliamp-hour LIR240 Li-Ion battery. Nevertheless, there are not any ports to cost the battery. As an alternative, the 12 BPW35S photodiodes act as a photo voltaic panel! Trush says the diodes can obtain as much as two milliamps every in direct daylight. A Zener diode protects the battery from over-voltage whereas charging.
The LEDs are normally off, which saves power. The microcontroller wakes up each 500 milliseconds to replace a time counter, which takes 100 nanoseconds. Whereas asleep, the watch attracts as much as 5 microamps.
One tradeoff with PWW’s design is that it’s essential to press a button to show the time. First, the watch face reveals the present hour, after which it reveals an LED for the present minute in five-minute increments.
Because the ATtiny13A has a restricted pin depend, Trush wired them with the Charlieplexing approach. It permits pairs of GPIO pins to manage three LEDs. For instance, PWW’s design makes use of 4 pins for the 12 LEDs.
The PWW GitHub repo has a lot of the mission’s design recordsdata. It accommodates gerbers for the PCBs, a PDF of the schematic, and the Arduino code. It doesn’t embody a case for the electronics. Nevertheless, you may mannequin one utilizing the 3D fashions of the assembled PCB.
The mission’s {hardware} and firmware are open-sourced. Try the “Perpetual” Wristwatch mission web page for detailed directions on programming the ATtiny13A. (You will want an in-circuit programmer.)