The main incentive behind this project was to see how much I could cram, in terms of both hardware and software, into a wristwatch-like device that is no larger than the display itself. An OLED display was chosen for being only 1.5mm thick and not requiring a backlight (each pixel produces its own light), but mostly because they look cool.
The watch was originally going to have a 0.96″ display, but this proved too difficult to get all the things I wanted underneath it. Going up a size to 1.3″ was perfect.
On the hardware side the watch contains an Atmel ATmega328P microcontroller, 2.5V regulator, Maxim DS3231M RTC, 1.3″ 128×64 monochrome OLED, 2 LEDs (red and green), a buzzer sounder, 3 way switch for navigation, powered by a 150mAh LiPo battery which can be charged via USB and 2 PCBs (though one PCB is just used as a raiser for the OLED).
The ATmega328P uses its internal 8MHz oscillator and runs on 2.5V from a linear regulator. Its current draw is around 1.5mA when active and 100nA in sleep mode.
The DS3231M RTC is an excellent chip, housed in a small 8 pin package which includes a built-in temperature compensated MEMS resonator with an accuracy of ±5ppm (± 2 minutes 40 seconds per year). Only a decoupling capacitor and a few extra pull-up resistors were required. The RTC is wired up so that instead of having power applied to the VCC pin, it’s applied to the Vbat pin which reduces its current draw from around 100uA down to 2.5uA.
Unfortunately this chip seems to be very hard to get hold of at a reasonable price if you’re not in the US. I had to get mine as samples.
The battery charging circuit uses a Microchip MCP73832 along with some additional components for load sharing, where the battery can charge without the rest of the watch interfering with it.
You might have noticed in the schematic that the LEDs are directly connected to the microcontroller without any resistors. The internal MOSFETs of the microcontroller have an on resistance of around 40Ω, so with a 2.5V supply voltage and LEDs with 2Vf, around 20mA ends up through the LEDs. I would have liked to have a blue LED, but the voltage drop for those are usually more than 3V which would have required some additional resistors and a MOSFET.
For more detail: DIY Digital Wristwatch