The Charliewatch is an "analog" watch with two rings of Charliplexed LEDs - 60 for the minutes and seconds, and 12 for the hours. The CR2032 battery lasts around three months since the CPU is in a deep sleep most of the time, and only wakes up every 16 miliseconds to PWM the LEDs for a few clock cycles. It has many animations that trigger when the hands line up or are in other patterns to keep it fun.
The 3D printed case is designed in OpenSCAD and probably requires some tweaking to get a good friction fit with your printer. The 22mm NATO watch strap does not require any pins, and I've been surprised that the printed lugs have lasted for years without breaking. The strap also holds the watch and sapphire crystal against the face of the case, which makes for a more compact and screwless package.
I also tried a fully 3D printed band as well, but found it was itchy and not as flexible. The pin joints failed after a month of wearing, so I abandonded this and went back to the NATO strap design.
Source code is
github.com/osresearch/charliewatch and should be buildable
msp430-gcc and installable with the
(TODO: Document how to install all of these tools along with
There are six vias to connect a MSP-EXP430FR4133 "Launchpad" programmer, which can flash the CPU as well as provide serial communication for debugging during firmware development. The pins are staggered so that it is possible to have the cable hold in place while programming, which is important for doing energy trace measurements.
- RST / TDIO
- RXD on programmer
- TXD on programmer
- TST / TCK
Bill of Materials
- LEDs: 0603 amber. The white LEDs in the first version have a 3V forward voltage, so they go dim too quickly as the battery runs down. Using low voltage LEDs allows the system to run the battery down lower and keep working.
- Battery is CR2032, estimate 200mAh of capacity, or 2450, with 600 mAh. Need to change the footprint for the larger holder.
- 32.768 KHz crystal (X1)
75K Ohm pullup resistor (R1)
CPU datasheet: http://www.ti.com/lit/ds/slas554i/slas554i.pdf
- Bootloader app note: http://www.ti.com/lit/ug/slau319t/slau319t.pdf
More photos in this album
|Pink and orange||White|
|v0.1 vs v0.2 size comparison||Applying solder paste with stencil secured by programming pins|
|Preparing to solder||Placing LEDs with tweezers through a microscope|