For the first version of the watch I want to create a
functioning simple watch with minimal components needed to have it work. This version
isn’t going to have a Bluetooth module, but would (assuming I don’t change my
mind about anything else) feature similar other base components to the final
version. The purpose of creating this basic version is to test out the
platform and try making a few simple apps that don’t require Bluetooth.
Here is the current Part List:
1.
3.3V 8mhz Arduino Pro Mini
2.
DS3231 Real Time Clock
3.
I2C 128x64 OLED
4.
TP4056 Battery Charging and Protection Circuit
5.
3.7v 500mAh LIPO Battery
6.
Power Switch
7.
Button
8.
FTDI Programming Board
Arduino
I chose the 3.3V Arduino Pro Mini for 3 main reasons: size,
power consumption, and cost. The lack of a USB Serial
converter and the use of a 3.3v volt regulator makes this board easy to power
using a lithium ion battery. It already consumes little power and can be easily
modified to use even less (http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/).
Additionally the use of a low dropout 3.3V regulator on board means the normal
lithium ion battery operating range should keep the board stable. This combined
with a small footprint and a very low price (as low as $2-3 for a clone) makes
it a great candidate in my opinion.
Real Time Clock
For this watch, I am also using a DS3231 Real Time Clock.
Since the board does not feature Bluetooth to sync time with the connected
phone, I have to rely on internal time keeping methods. At first I considered
using Atmega’s internal clock and using delays with a counter to keep track
of time but I abandoned that idea for a couple of reasons. First, this means the
microprocessor always hat to be turned on or it would lose track of time. I wanted to feature a fully off mode for the watch to conserve power so
this was a big hindrance. Second, using delays for timing lead me to
losing close to 2 minutes a day. I’ve read about people achieving more accurate
results but since this is primarily a watch I wanted it to be able to keep
track of time accurately.
Although the DS3231 is a bit overkill for my purposes (as it
even features an internal thermistor to adjust for temperature affecting it’s
time keeping) it most importantly has it’s own on-board battery to keep track
of time when the watch is off. This means I could turn the watch on and off as
often as I would like without worrying about losing track of time. The only
downside of this is that the unit itself has a fairly big footprint.
Display
The display choice was straight forward. I wanted something
that was bright, had good contrast, allowed for 2D graphics and did this while
consuming as little power as possible. The widely available OLED modules fit
this role perfectly. They are inexpensive and can be easily found on sites like
Ebay or Aliexpress. OLED displays have no backlight and, as only the pixels used
light up, the displays are sharp, vivid and power efficient. I settled for a
1.3” 128x64 I2C based module.
Battery and Charger
I used a 500mAh battery for this project. I estimated the
power draw to be somewhere around 10mAh, so I would get around two full days of
use. This ended up being an overestimation. In order to charge the battery I am using a TP4056 charging and protection module. It is a little bulky but does the job well.
Button and Power
Switch
For my first prototype I settled with one switch and one
button. I wanted to have a switch to completely power down the watch. In the future,
however, I would use at least two buttons (or a touch screen?) as navigating
menus with one button becomes slows and frustrating.
In the next post I’ll show all the electronics assembled.
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