It's been quite a long time coming but I've finally got some updates on this project. I've been repeatedly delayed due to a vacation, massive computer failures, waiting for a new computer (then loading all the software etc.) and finally an illness. None of this has completely stopped me from working on this project but it all has definitely slowed me down.
After experimenting with the PulseSensorAmped (a great project and the documentation is really great) I decided to go back to using the MAXREFDES117. The main reasons for this decision are:
- more pre-processing in the breakout
- easier control of the light intensity settings, etc.
In addition I've decided to switch to a different circuit for GSR/EDC that makes some aspects of GSR measurement and interpretation simpler:
- self adjusting to background skin resistance (tonic?)
- easier to use
Circuit was pulled from research paper (link), with the addition of the precision voltage divider. In the same paper, the authors, looked at other testing sites for GSR/EDA that were comparable to the palm and fingers but allowed the subject greater freedom of movement. There testing revealed that the palmar wrist area proximal (near) to the hand worked as well as the fingers and palm. They even had success with a subject using a wrist strap for a week of continuous readings while performing normal daily activities.
In initial tests I used the above circuit along with the MAXREFDES117 breakout board, both connected to the Adafruit Feather Bluefruit, to develop a simple program to make readings of GSR and HRV and send them back to the computer for interpretation. (The ultimate goal, obviously, is to send the information via BLE back to a processor).
I put this together after working out a number of bugs while building the circuits on a solder-less breadboard and testing different configurations.
One major issue, turned out to be the lack of insulation on the face of the MAXREFDES117 board that contacts the skin (used by itself it would not be an issue, but when coupled with GSR measurements it added current and potential the the GSR circuit). Once I insulated all the possible skin contacts in the MAXREFDES117 all the GSR data returned to normal.
I fabricated one initial (singled sided) board that mounts directly onto the Adafruit Feather Bluedfruit so that I could start more comprehensive testing of individual GSR and HRV changes in relation to emotional stimulation. After designing the board, etching it, populating it, re-flowing it, drilling out holes, soldering jumper wires, soldering pins to attach to the Adafruit Feather... something went wrong!
It turned out that either during etching, re-flow, drilling or hand soldering - a number of thinner tracks were either cut or pulled up. So, I had to re-etch a new board, populate it and drill out the holes for the connecting pins to the Feather board and the smaller holes for the jumper wires. This is where I'm at right now. I'll have more to post in the afternoon (once I have the jumpers and header pins done)!!!