Community /

Active electrodes for OpenBCI – 2nd part

[Update 6/29] – TComp has generously shared additional instructions for assembling these active electrodes which can be found via this link

In a previous post (https://openbci.com/community/posts/active-electrodes-for-openbci) I had described my solution on how to build low-cost active electrodes for the OpenBCI headset. The DYI active electrodes based on previous works from Joerg Hansmann and Jarek Foltinsky at the OpenEEG project (http://openeeg.sourceforge.net/doc/hw/ae.html) worked very well and significantly reduced the impedance of the circuit.

The goal of the second part of the project was to find a way of attaching the active electrodes to the existing Ultracortex Mark III headset without any modification of the components. To achieve this, I built a simple assembly by using an M2 long screw that was passed through a hole in the upper side of the 3D printed enclosure and then through the center hole of the electrode holder. All components are then tightened with a double nut and spring washer on the upper side of the electrode holder. A 3D printed pin with a blind hole at the bottom was pressed on the nuts; it served as a guiding pin going through the upper hole of the octabolt.

Active electrode – octabolt assembly
Spring mechanism with guiding pin

The system is spring loaded in the same way as the original passive electrodes assembly suggested by the OpenBCI team. The octabolt will need to be unscrewed out of the headset in order to make space for the active electrode assembly.

OpenBCI headset with active electrodes
Active electrode setup for SMR capturing

The 3 wire shielded cables from the active electrode assemblies is then connected through a special connector blocks to the Cyton board. The 5V power comes from the AVDD (+2.5V) and AVSS (-2.5V) pinouts on Cyton. The signal wire is goes to the signal channels in the same way as regular passive electrodes, while the shield is connected to the ground. The small internal Li-Ion battery (as recommended in the OpenBCI setup) was replaced by a set of larger bateries since the active electrode setup will require more power than just the board alone.

Board connections of active electrodes

All electrodes in the system present significant lower impedance than the original passive electrodes and performed very well when tested with EMG signals such as eye blinks or teeth grinding.

Circuit impedances
Eye blinks
Teeth grinding

5 Comments

neurojoe

Wow! This is awesome. Any other documentation you could share for those looking to replicate your setup?

TComp

Hi neurojoe, thanks for kind words. The first part of my project explains how to build the active electrodes. Unfortunately, the link in this post seems to be incorrect; the correct one is https://openbci.com/community/posts/active-electrodes-for-openbci. Also, I have prepared some detailed instructions on how to build them, but it is a PDF file and I am not sure how to upload it here.

neurojoe

If you see still an option to “edit post” up at the top by the post date, there should be an option in the wordpress editor to add a file, similar to adding images.

Or maybe:
**upload it to google drive / github / dropbox and add a public link here

**email it to me at joseph#openbci.com

I fixed the link to your previous post at the top

IanR

Wow this is great! Would it be possible to modify this setup to include the Daisy and/or the WiFi board?

TComp

I do not have the Daisy module or the WiFi board, but I believe it will work fine by just plugging the connector block on the top board. The signal output of the active electrodes will still have to be plugged into the Cyton or Daisy pins, while the job of the connector block is only to get the power voltage from AVSS & AVDD and the ground line from GND pin and feed them to the electrodes. This means it should work with both Daisy and WiFi without any modification.

Leave a Reply