Saturday, January 18, 2014

EEG Electrode Adapter - Version 2

As many people are aware, many commercially-available EEG electrodes use an unusual connector called a "touch-proof" connector.  If your EEG system does not use these connectors, you need to either replace the connectors on the electrodes, or you need to make an adapter.  While it is a sensible choice to cut up your electrodes, I prefer to make an adapter.  My previous version of such an adapter worked really well, but it was a bit fragile.  So, I decided to try again.  I also decided to work with a friend of mine who's a little smarter about these kinds of things.  Here's what we came up with.

My Revised EEG Electrode Adapter ("V2") attached to an OpenBCI Board

Problems with the Previous Version:  The picture below shows my previous version of the adapter.  The good part was that the adapter was really small.  I liked that.  The bad part of this adapter (in addition to the fact that I never finished all of the connections) was that it was quite fragile. Specifically, the soldering of the wire to the female connector had no strain relief, which meant that mating the connector always threatened to break the wiring.  Another problem was that I was always confusing which electrode I had plugged into which input on the OpenBCI board.  I really needed to remake the adapter so that it was easier to see which were the "special" connections (SRB, Bias, and 8P) versus all of the "normal" connections (1N-8N).  These are the deficiencies that I focused on in this revision.

My First Attempt at an EEG Electrode Adapter ("V1")
Revised Approach:  With this iteration, my friend and I decided that it would be best if the female touch-proof connectors were mounted to some sort of rigid frame instead of merely being attached to the ends of the wires.  This would give the strain relief that we needed when mating and de-mating the connectors.  To address my other issue -- getting the connections confused -- I decided to use color-coded connectors, instead of just the black connectors in my first build.  Easy.  Ok, let's go!

Parts:  The parts are all the same as in the first build.  The female touch proof connectors are P/N 36145 from Plastics One.  The ribbon cable is just a set of female-female jumper wires from Adafruit (P/N 266).  This time, we also used a piece of scrap plastic channel that we had lying around, probably originally from McMaster-Carr.  And, as you'll see, I used a thin scrap piece of plastic sheet and a wide piece of shink tube, both from our generally pile of "goodies for a future project".  None of these pieces (except for the female touch-proof connectors) are particularly critical, so use what you have.

Assembly:  My buddy who came up with the idea of using the plastic U-channel as a mounting plate did all of the of the assembly.  He started with drilling a bunch of holes in the U-channel into which he pressed in the female connectors.

Touch-Proof Connectors Mated to the U-Channel -- For Strength!

He then took the purchased wires, pulled off (as a group) the number of wires that we needed and cut the existing connectors off one end.

Inexpensive Jumper Wires Used for My Wire Bundle

To keep the individual wires from splitting themselves off the ribbon, he reinforced the ribbon with a flat piece of plastic and a big piece of shrink tube.  Here's him preparing the items.  The red thing is the big shrink tube and the yellow-ish thing is the scrap bit of flat plastic that he'll use for the reinforcement.


Preparing the Shrink Tube (Red) and Scrap Plastic (Yellow)
to Reinforce the Wire Bundle

With the wire bundle prepared, he soldered the individual wires to the back of the touch proof connectors.  He used some normal size shrink tube to protect the individual solder joints on the back of the connectors.  Very nice.

Connecting the Individual Wires to the Back of the
Touch-Proof Connectors.

And that's all there is to it.  A picture of the completed unit is below.  You can also see it attached to an OpenBCI board at the top.  In the picture below, you can see how reinforcing the ribbon cable was an important feature for keeping the wire bundle from falling apart.

The Finished Adapter.

Pin-Out:  For anyone wondering why I used the unusual color scheme for the connectors, the idea is to clearly indicate that some of the electrodes have different functions.  So, if you use this adapter to connect to the OpenBCI V2 board as shown in the zoom'd picture below, or like the one at the top of this post, the order of the connections is this, from left to right:

   Red: Bias  (aka. driven ground)
   Blue: 8N  (the regular input for channel 8)
   Red: 8P  (the reference input for channel 8, if selected in software)
   Blue: 
       7N  (the regular input for channel 7)
       ...
       1N  (the regular input for channel 1)
   Red: SRB2  (the reference input for all channels)

Further Revisions:  After working with this revised piece for a bit, I found that there is still room for improvement.  For example, in connecting the adapter to the OpenBCI board, it is important to connect the wires in the right order.  The right order is shown below.  At first, I thought it was a good idea that I kept the individual female pin connectors on each wire of the adapter.  I thought that I'd like the freedom and fllexibility that this might provide.  I was wrong.  I should have swapped out the individual female pin connectors for a ganged female connector that would have kept them all in the right order all of the time.  I was wrong.  (So, to the EEG hacker that I'm handing this adapter off to, sorry for the annoyance!).

The Correct Order for the Individual Wires.

So that's the story of the hacking of this electrode adapter.  The real solution, of course, would be to have the connectors on the electrodes and on the EEG system (OpenBCI, in my case) be the same.   The easiest thing would be to put female touch proof connectors on the OpenBCI board.  But, the PCB-mount version are almost $2/each, even in quantity!  This is completely incompatible with the price of the OpenBCI board.  So, the next option would be to buy electrodes with a connector system that we could afford.  That would be a really nice solution to this problem of making kludge-y adapters.

11 comments:

  1. I'm working on a project of my own for facial muscle EMG and EOG to work along with a pressure sensor and IMU to control a Wii video game. My project will be targeted for my dad who has ALS and is loosing arm/finger strength and currently has no leg or core strength. I love the work that you and others like you (EEGmouse) are doing. I have been interested in this field for a long time, but the time has come and I am being forced to spend my spare time on it. : )

    I'd love to exchange notes on the ADS1299, although it may be a one way exchange at first! Having your open design has helped me and I have been sharing my design with the people who have assisted me so far.

    I don't know if you can contact me personally by this comment. If you can't, let me know and I will give you my e-mail address.

    P.S. My dad has never played a video game in his life, and the only computer he has ever used is an iPad. I was taking some measurements this weekend and he demanded to know what I was doing. He told me "But I don't play video games". I replied "Not yet!" : ) Trying to expand the family activities accomplishable from a chair...

    Thanks,
    Grant

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    1. hi this is krishna i am also working on ADS1299 EEG kit
      i have lots of doubts can u help me if you can please mail me at ksktulasi@gmail.com

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  2. I found your blog yesterday and couldn't stop reading until I read the entire blog! For someone with technical ind IT skills who are curious about Neurofeedback, "brain hacking", personal performance enhancement via the neurofeedback technol, your blog is both inspiring and instrumental!

    There are several cheap Chinese made medical grade EEG systems on ebay, the Contec KT88 series, and the electrodes and lead wires for these machines are very cheap on ebay, I was wondering if they can be used as an alternative to your DYI electrodes:

    http://www.ebay.com/itm/TEN-PCS-Silver-Plating-EEG-Electrodes-For-CONTEC-KT88-KT88-2400-KT88-3200-/301069734934

    http://www.ebay.com/itm/like/181307271566

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  3. Explorer, hi.

    I looked at the Contec a couple years ago. Big downside is it's sample rate, 100 Hz. OpenBCI is running at 250 or 500 Hz.

    The sensor system you are looking at is 'saline' based, salt solution vs. paste or gel. This is a nice advantage and avoids cleanup mess. This sensor design originally came from Europe where it is popular. You purchase the set of sensors along with a 'cap' system consisting of left right 'endplates' with holes, and elastic straps that stretch between plates across the 10-20 lines. Usually a chin strap also to hold it all down.

    The sensors have a felt pad that stretches across the top flat round post, held on by an o-ring. You moisten the pad with saline solution, then tuck under the corresponding strap at the desired 10-20 position. Lever-action holds it down in place on the skin.

    ANY passive sensor system can be used with EEG, as long as you use all of the same metal type, silver in this case. You would also get some silver ear clips, Contec might have those, or there are some decent ones on MVAP, 2 for about $30.

    I use a saline system myself, with Ag-Ag/Cl sensors. But not with these rubber straps, instead I use velcro double sided tape. I need to put together some photos and web page on this.

    Cheers,

    William

    PS a photo of the cap in place on head,
    http://www.pamel.hr/index.php/en/proizvodi/eeg-kape-elektrode-pribor/2012-11-05-09-59-48

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  4. Hi William,

    Thanks so much for your reply!

    Like you said, the downside of KT88-1016 is its sample rate, 100 Hz, but I read (from the specs) that the 24 and 32 channel version have 200 Hz sample rate.

    The biggest advantages of KT88 system are the low cost, large number of channels, and it is medical grade device made by a large Chinese medical device manufacturer for use in Chinese medical hospitals, they might not have the same standard of medical devices used in US, but I would think they have higher quality than most DYI EEG devices found in US.

    I am by no means downplay the OPENBCI, but to the exact contrast, I want to buy and use the OPENBCI device because it is open source and there are BCI communities who would develop new applications for the OPENBCI device (as oppose to KT88 which has very limited support for third party apps and open source apps) , but I had missed the deadline for Kickstarter funding, and OPENBCI is not returning my inquiry about whether I can still pre-order the device to support OPNBCI, or when I may expect a consumer release of OPENBCI device, which forced me to pursue the DYI route and KT88 route.

    Any comments and suggestions would be greatly appreciated, and thanks again for your help.

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  5. Hey, great discussion, folks! I wasn't aware of these products. Cool stuff. Thanks, William, for sharing your knowledge about them. With so little experience with them myself, I can't really comment on those products vs OpenBCI.

    I also can't really help with letting you know about the expected ship dates for the OpenBCI kickstarter boards. It's all in Joel and Conor's hands at this point. I think that they were saying an April/May delivery in their Kickstarter? I'm not sure. If history is any guide, I'd assume that additional boards will be offered for sale just after the kickstarter commitments are satisfied.

    Sorry I can't be more help. Thanks so much for this discussion, though. I'd love to hear about your experiences. Maybe you should start your own brain exploration blog!

    Chip

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  6. Hi William,

    Can you share more details about your setup of velcro double sided tape head straps with with Ag-Ag/Cl sensors?

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  7. Explorer, send me an email so I have your address, and I'll reply to you and Chip with some photos and descriptions of the velcro saline system I am using. It's simple DIY components. Easier to include photos and working links that way. I use Ag-Ag/Cl because of the InfraLow Frequency protocols I use. Metals drift too much in that range (DC offset movement.) croft attt lightfield dottt com

    I should perhaps do a web page or blog entry when I have more time.

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  8. Hi Chip,
    I made mine. Thanks for a great tutorial. Check it out here:
    http://eegexplore.blogspot.com/2014/05/making-eeg-electrode-adapter.html

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    Replies
    1. I checked it out. Nice work! Also, thanks for pointing out that Plastics1 changed their website. I'll update my links!

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