1Acquire a B20 ECG Fitness Tracker or Comparable
First thing first, you need to get the materials. Here are some links for purchasing the B20:
There are a bunch of other inexpensive Chinese fitness trackers with both an ADS1292 ECG chip and a Nordic nRF52832 SoC. Be careful to select the right one, or even better, hack a different model and add your work to the project GitHub repo.
You can also run all the code posted for this project with an Arduino compatible microcontroller and an ADS1292 breakout like the one offered by Protocentral.
2Disassemble the B20 and Solder Wires for Programming and Serial Data Transfer
For instructions on actually programming the device, see the project GitHub repository README: https://github.com/curtpw/nRF5x-device-reverse-engineering and videos from this project which also involves hacking Nordic nRF52832 MCU based fitness trackers: https://hackaday.io/project/144350-hacking-wearables-for-mental-health-and-more
3Desolder Channel 1 and Right Leg Drive Spring Pins
The spring-loaded pogo pins connect to the two meta plates/electrodes at the bottom of the unmodified fitness tracker enclosure.
Those big pogo pins suck up a lot of heat so you'll have to leave your iron sitting next to them for a couple seconds. Make sure you don't accidentally de-solder anything else.
4Solder Lead Wires to Channel 1 and Right Leg Drive Solder Pads
I used fairly large wire for the electrode leads with lots of surface contact when soldering to the pads.
5Trim and Punch Holes Through Wrist Band to Fit on Glasses
This will probably have to be customized for individual glasses, but I'm sure all kinds of variations are possible. This just happened to be easiest for me.
There are many ways to put holes in things. This one worked for me.
The legs of my glasses got in the way of charging with the fitness tracker body installed, but since the electrode leads are modular with convenient plugs its easy to just pull the fitness tracker body out of the rubber strap for charging.
This is actually quite comfortable with a little adjusting. It is completely out of the way of my face.
6Extract Miniature Audio Plugs from Ear Buds and Prep For Use As Electrode Lead Plugs
Yup, I used earphones.
I was looking for plugs and couldn't find any I likes. Then I found these earphones and didn't look back. They work quite well.
This pulls right off.
There is a chance I could have done this without a Dremel cut-off wheel but I didn't want to take any chances.
Female end of the plug epoxied to what remains of the aluminum earbud enclosure.
Ready for installation
7Carve Out Holes in the Enclosure for Electrode Plugs, Programming Wires and SMD Switch
This really does require a Dremel.
1) Electrode plug 1
2) Electrode plug 2
3) Programming/GPIO wires
8Install the Power Switch
These little SMD switches are handy. I solder wires onto them before installing.
I epoxy the switch in place, being careful not to jam it with excess epoxy.
9Mount PCB In Modified Enclosure
Fitting all the wires in their assigned holes....
I trim down the electrode wires for soldering. Test to see if everything fits inside the enclosure when the cap is fixed on top. The cap and enclosure body should fit together without any pressing or pushing. I had to adjust the position of the battery inside the enclosure to get a good fit.
10Mount EEG Electrode Plugs on Enclosure, Finish With Bondo and Paint
Placing the female ends of the earbud plugs by positioning the combined plugs relative to the enclosure body.
I twist the wires together making sure to get lots of contact. There should be enough space carved out of the enclosure for all this to get tucked away and covered in Bondo.
When I soldered the headphone plugs to the leads from the PCB, I was also burning off enamel. This means use lots of flux and lots of solder. I let the solder 'cook' on the wire strands for a little while just to make sure all the enamel had been brought off.
Bondo! My wife hates when I use this stuff in the apartment. I use Bondo to cover over the female electrode plugs and strengthen their placement against the plastic enclosure body.
After Bondo application....
After Bondo has been shaped by a Dremel cutter, a hobby knife and some sand paper. I put rolls of masking tape inside the plugs to keep paint out.
I sprayed black spray paint onto a piece of paper and then brushed it onto the Bondo.