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A project log for nRF52 SmartWatch

nRF52 Smartwatch from Scratch

Jeff CooperJeff Cooper 04/17/2021 at 18:360 Comments

As usual, pictures first:

The project is slowly moving again! The reason for the long time between updates is primarily that rev. 5 didn't work, I got discouraged, and I stopped working on it for a while. Pictured, jammed into the case, is rev. 6, which has a few improvements:

  1. An IPX8 splashproof USB port. This took some fidgeting to get the fit right, but now the USB port is (theoretically) resistant to splashes coming in to the USB port. This lets me leave the watch on while I wash my hands without worrying. The bottom of the case is still snap-fit PLA without an O-ring, so the whole thing is almost certainly not actually waterproof.
  2. The flashlight system has been massively simplified. The MOSFET is now on the main board, with just two holes to solder wires in. Only the LED and its resistor are on the daughterboard, which is mounted in the side of the case, and they're connected using some thin wires that I pulled out from a deconstructed ethernet cable.
  3. I've (at least temporarily) given up on buttons on the hand side of the watch. In addition to overcomplicating the internal wiring, they also just don't look good.

The flashlight feature is stupidly bright. At full brightness, it's significantly brighter than my cell phone's "flashlight" feature. After using it to navigate around my dark bedroom at night, I may have to dial it back just so I don't blind myself. It also draws somewhere around an amp, which is not a great thing to do with a 500mAh battery (a 2C draw). It also gets extremely hot; I'm hesitant to leave it on for more than 30 seconds at a time for fear of melting the solder joint. This explains why the breakout boards available for the XM-L2 are chunks of aluminum bigger than my watch itself... the next revision will probably use an aluminum daughterboard PCB that can act as a heatsink, and the final form for the watch might involve a machined aluminum case rather than the 3D printed version.

The flashlight mounting also needs some work. The XM-L2 lens isn't, as I initially thought, hard plastic. It's soft, and I managed to mar it with the tip of my tweezers when I was trying to mount it. It still works, but it doesn't look great.

The remaining question is battery life. I haven't done any profiling on a rev. 6 board yet (it's a pain to take in and out of the case, so I plan to assemble a second one for testing at some point). My empirical testing has so far indicated that battery life is pretty decent; I've been wearing it nonstop since Thursday evening (when these pictures were taken) without charging it. It's now Saturday afternoon, and the MAX17262 reports 94% remaining (which I don't especially trust), and a battery voltage of 4.0V (which I do), down from 4.2V at full charge.

Now that the watch basically works mechanically, it's time to turn to software. The next few updates, whenever they are, will likely be focused on the watch UI and/or the mobile app rather than the hardware. If I get a good power profiling setup, I may focus on some power optimization too.

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