12/12/2025 12 PM - Created iOS flutter app for device.

First I created the flutter app running on XCODE iOS. It is linked in the GitHub repository. Small tweaks in the algorithm and filtering have been done. I know you want to see the process but I started this during the fall and I don't have anything to show for progress but I am constantly making UI tweaks, and new features. Last week I added the temperature feature that is on the max30102 to the UI.

12/12/2025 1:05 PM - Built Arduino IDE code for MCU nrf52840

Built code for MCU nrf. This code sends the data connected from SCL, SDA, and analog 0 pins and transmits it over BLE with several different UUIDs. I looked at online guides to the max30102 sensor and also the gsr sensor, combined it, and learned how to use the Arduino BLE library. My last update to the code was 2 weeks ago. Again, I started this before but I have many variations to this code I will show: The code is linked in GitHub.

12/12/2025 1:09 PM - Figure out XCode

I had to learn how to use VSCODE and XCODE together and made some tweaks to my iPhone to get the app running and installed. Also had to figure out BLE permissions. Also learned how to send data BLE with some random BLE apps.

12/12/2025 1:11 PM - Set up the schematic/researched parts.

Learned everything about schematics, parts, pins, pads, nets, etc. Built a schematic for the PCB including the max30102 and the gsr sensor. I also fiddled with different LDOs and battery protectors.

I started out with a double NMOS but then decided to proceed without them.

I watched a lot of youtube videos on a guide through kiCad

Below, you can see my progress and process:

12/12/2025 1:14 PM - Layed out the PCB and got rid of Schottky diode.

I learned how layers worked, how to use KiCad PCB editor. I somehow layed out the PCB very tightly with no DRC errors. All of the capacitors are nearby their pins and resistors are in proper place. In this session, I updated my PCB from my schematic and also cut out the D_Schottky Diode. Here is my process in screenshots:

12/12/2025 1:16 PM - Routed the PCB

I routed the SUPER tight PCB and used both top and bottom layers. I adjusted some pins in the schematic so I could get all the pins corrected with no interference or DRC errors.
I got this done in three long sessions:

OHH also I learned all about vias and track widths because I ran into an issue where the inner pins of the MCU were not able to be routed out, so what I did was reduced width requirements, then I made a smaller track, placed a via, and was able to get through that wall. Its screenshot is below

12/12/2025 11 PM - I am making a more comprehensive CAD in Fusion 360.

I downloaded fusion 360, got the hang of it, then I imported my PCB stl file from KiCad and designed the 2 rings, embedded the pcb, added the battery, cut holes, hid the pcb away, and added the electrodes and a connecting wire. See the hole thing in Git Repo

12/13/2025 9 AM - Find the cheapest possible options and make the BOM.csv

I had a google spreadsheet, but I am downloading it into a CSV. Also I reduced the price by almost 30 dollars and found the ABSOLUTE best price including shipping. I tried to use some Aliexpress parts but they didn't have the exact thing I needed.

12/13/2025 11 AM - Added pictures of PCB

I updated my readme to include direct images of my PCB and the 3D image of my PCB. For any more PCB details, there is a folder in my github repo that says Ring PCB and Schematic. There are more pictures including the schematic and the direct files of everything including gerber.

12/17/2025 - Touched up the PCB and rerouted some wires.

I rerouted some wires on my PCB just for more organization and improved the via layouts. A lot of my BCU tracks are on the horizontal axis while my FCU tracks are going up and down vertical axis.

1/5/2026 - Completely changed the PCB

Okay so the one big PCB was a scary and probably a big design flaw because it would be uncomfotable for the user to wear. Instead, I split the PCB into two main boards, one for each finger. Board A is the logic board with the MCU and whatnot. Board B is the sensor board. On the edges of each board are the interconnect pads with 5 pins each including VDD, GND, I2C pins.

1/6/2026 - Touched up the PCB

I fixed some of the ratlines BUT there are still some ratlines because there is an interconnect so just don't pay attention to those.
Also I fixed the GND plate..rerouted the whole thing better.

1/18/2026 - Version 2

Alright so in these 3 weeks, I made a completely new version that costs a bit less and is more efficient. Essentially the difference is that I would use a XIAO nrf52840 breakout board instead of my custom logic board. Then, I would have my compact, HIGHLY reviewed and refined sensor board with a new LDO, operational amplifier, and accelerometer. This would be PCBA by JLCPCB simply because its cheaper to have them assemble it than buy the rework station for assembly at home.

This new system is simple, small, and functional but it is not designed optimally. I would like to decrease thickness and make it a lot sleeker.
This sets us up for our final version: v3. I am going to use this new refined sensor board schematic, but I will integrate my own logic board on a flex PCB.

New Everything:

1/23/2026 - Version 3: Flex PCB Part 1

I copied and pasted my version 2 sensor board schematic into my new project. For the logic board, I will be using the Raytac MDBT50Q which is a nrf52840 board with a built in LDO and crystal. Next, I had to figure out how to do the charging and data usb system. First I added the port, then the MCP charger. Later, I added an ESD array for protection.In order to power the MCU with both the 5V USB system and the 3.7 volt LiPo, I created a 2 Schottky diode OR-ing system. This system allows the 5v usb power to take priority and power the VDDH pin when it is present. Otherwise, the 3.7 lipo powers the VDDH. The VDD pin is the LDO output for the sensors and what the nrf actually runs on. Also, there is a DC-DC mode I may use with the inductor.

1/24/2026 - Flex PCB - Part Two

Okay I made some edits and completely removed the charging block because it takes up way too much space and I can easily charge the battery externally. I measured out the length and width that I should use for the layout and properly layed everything out. All of the decoupling caps are where they should be and the distance between the sensor and mcu is appropraite. I used a hatched ground fill so it doesn't interfere with the flex.

2/2/2026 - Good news for the reviewers!! This project actually works

You dont have to be worried that this project is a waste of your money. I created a new PCB that just has the sensors. If you scroll down its labeled as version 2. So I ordered it with my own money just because I was neurotic, and everything works. So I'm 99% sure that my new Version 3, which the grant money is for, will work. Just to clear things up because this sounds confusing. The version 2 test board was ordered with my own money. The grant is for version 3, which is the full flex PCB including the MCU. Here are some pictures of it actually working.

2/7/2026 7:54 PM - Simulated the Rigid-Flex

In CAD and real life, I used paper to represent the flex polymide and cardboard as FPC stiffeners. I arranged the stiffeners in a way that it the flex could bend into a ring circle.

2/7/2026 7:58 PM - 3D Print Time

So I need to design a case that is perfect for my finger being mindful of the 1.55mm thick battery, flex pcb, stiffeners, and the subtracted radius from the way the stiffeners bend. It is modeled here and I also need to create a holding place for the battery to keep it even.