Kai's Build Log

Hello World!

Like many others, I actually have EE education, but ended up working a software career and now I want to go back to doing hardware. So here I am refreshing my rusty skills, and maybe picking up some new ones. The funny thing about university education is it’s so very theoretical, so I know (or at least used to) the ins and outs of how a transistor functions, but I’ve never actually biased one! So that’s one thing I’ll try to address in this log.

Lately I’ve been involved in wearables, so I thought I’d continue on that path and start off by designing a minimalist smartwatch/activity bracelet, something I can throw together using off-the-shelf parts.


A good place to start from would be defining the functionality. I want the device to tell me time, step count, and heart rate. I want it to have a touchscreen and haptic feedback. It should have a rechargeable battery and Bluetooth connectivity. I want all the parts to be available from Mouser (except for the battery which is a restricted item, but I’ll find something similar locally). Because the supply chains are still like they used to be, I’m not going to be building a physical prototype in the immediate future. So if the components are normally stocked, that’ll be good enough for me, even if there is no current stock.

The display proved to be the most difficult part to find. But eventually I came across a 80 x 160 pixel IPS panel, with capacitive touch sensor. These displays are common and cheap, but finding one with a digitizer was not so simple. The screen itself is driven with SPI and the touch panel connects via I2C (sadly there’s no access to the raw sensor data, but I’ll settle for the x and y coordinates).

For the processor I’ll go with STM32WB55. My day job has had me program Nordic chips so that would be an obvious choice, but I want to try something new.

For battery charging, I picked BQ25601D. There’s a ton of chips that do more or less the same, and I don’t have much experience in batteries and their management. But this IC seems to do the job, and it has power path management functionality, which is nice to have.

Haptic driver will be DRV8601. Simple and compact. I’ve used DRV2605 before, and it’s too complex for what I need and want, with libraries of effects and multiple functional modes. 8601 has PWM input and that’s about it.

Step counting will be with LIS2DE12TR, a 3-axis accelerometer. I’m still thinking if it could be worth finding an IMU with magnetic sensor, for 6 or 9 axes of measurement (since accelerometers have drift), but let’s keep it simple for time being.

Heart rate I will read optically. This requires a few LEDs and some circuitry to drive them. I haven’t worked out the details yet, but I think I’ll skip ICs and let the MCU do the heavy lifting.

All of the above will require support components and so on, but I’ll see to them on a subcircuit basis, when I get to that, one function at a time.

Until next time,

re: heart rate monitor
you might want to know about this

it’s not from mouser though. does mouser sell heart rate monitors?

sounds like a cool project. is there a project page posted publicly, perhaps?

Thanks for the tip Ozzloy! The link is indeed very similar to what I had in mind. Years ago I ran into this white paper or application note, that detailed the use of LEDs in heart rate monitoring and what wavelengths were best for which purposes. This was before all smart devices started to have the functionality built in. It also seems that green light is the most popular these days because it apparently reflects better from blood or something to that effect, while IR would go deeper into the tissue, if I understood the paper correctly. This pdf looks like a good overview on the topic.

As for project page, I haven’t made one yet so this thread contains pretty much everything I have done so far.