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Re: [buildcheapeeg] NaffEEG

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  • Joerg Hansmann
    Hi Jim, ... From: Jim Peters To: Sent: Friday, February 01, 2002 2:13 AM Subject: [buildcheapeeg] NaffEEG ... 10
    Message 1 of 6 , Feb 1, 2002
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      Hi Jim,

      ----- Original Message -----
      From: Jim Peters <jim@...>
      To: <buildcheapeeg@yahoogroups.com>
      Sent: Friday, February 01, 2002 2:13 AM
      Subject: [buildcheapeeg] NaffEEG


      > I've now built the BrainMaster input stage (using AD620 + OP90), and
      > it seems to be working. I had to buy 5 of the polypropylene
      > capacitors, so I picked a good matching pair anyway, just in case it
      > made a difference.

      10 nF capacitors ?
      (see other thread about BM-input stage...)


      > I'm now getting 50Hz hum, no problem at all! I'm sampling at 5000Hz
      > (5kHz), and piping this through to my filterbank display that I posted
      > PNGs of earlier.
      >
      > I've improvised some electrodes from 1 pence pieces tinned with solder
      > on one side. I also experimented with little wrappers for them dipped
      > in saline. (However, I'm guessing now that probably it's not a good
      > idea to use electrodes with lead (Pb) in them).

      I would like to know what electrode impedances you get with this low-cost-
      solution. My guess is that without proper electrolyte (e.g. 10/20 paste wax)
      the impedance is far too high. (1k to 50 kOhms is best matched with
      AD620 IIRR)
      Has there been a tendency, that the results were somewhat better when you
      used saline as electrolyte ?


      > I'm not seeing any brain-waves yet, though.

      What total gain has your circuit ?

      Have you tried to calibrate it with e.g. a simple 555 oscillating
      at 10Hz and a voltage divider that gives an output of e.g 50uV ?

      > With electrodes on my forehead (ground in the middle, other two either
      > side), I get a clean trace with 50Hz hum in the middle. If I tap the
      > left or right electrode, I get a pulse, but the middle one doesn't
      > give one, or it only gives a little one. This seems to be more or
      > less as expected.

      Yes. I agree.

      > If I frown or clench my teeth, I get patterns appearing, and they peak
      > around two to four times the amplitude of the 50Hz hum.

      It should be EMG voltage that looks like white (or whatever color ..) noise
      when 50Hz is absent.

      > So, what do I need to do to improve this situation ? Do I need to buy
      > some proper electrodes and paste, or should I look at reducing this
      > hum first ?

      You can do both.

      Try to disconnect the electrode cables and tie the electrode inputs
      together to your analog ground. The hum should be gone now and you should
      see the amplifier noise. (post picture, I would like to see it)

      If you still have hum, place the circuit in a metal box connected to
      analog ground. That has helped when I had 50Hz hum in RS232EEG.

      > At the moment the electrode connection is passing through about 3m of
      > microphone cable, with about 8 inches unshielded at the electrode end.
      > Would shortening this cable make a significant improvement ?

      Probably not.

      > Can I improvise better electrodes ? I'm still looking for a UK
      > supplier of Ag/AgCl electrodes.

      Buy thin silver metal sheet, cut disks with 1 cm diameter and use this
      as electrodes with 10/20 paste wax.


      BTW: What isolation do you use ? Optocoupler ?
      Isolation is not only good for your health, it also reduces common mode
      hum voltage to about 1/10th ...


      Regards,

      Joerg
    • Jim Meissner
      Dear Jim Peters: I looks like you got some real good information from Andreas and Joerg. As Joerg pointed out, you need to build a microvolt calibrated source.
      Message 2 of 6 , Feb 2, 2002
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        Dear Jim Peters:
         
        I looks like you got some real good information from Andreas and Joerg.
         
        As Joerg pointed out, you need to build a microvolt calibrated source.  He uses a 555 to generate a square wave of known amplitude.  In my case I used a 8038 sinewave generator chip.  The sine wave lets you test your frequency response,  the FFT accuracy, and the Nyquist rejection problem.  You need to build something so you know what you are doing.
         
        50 Hz pickup:  Most standard EEG system do not uses shielded wire.  They eliminate the problem with a sharp cutoff filter and a notch filter set to 50 or 60 Hz.  That limits the top frequency to about 35 Hz.  For my research I wanted to see all the data so that made shielding mandatory.  My experience was that I had to put the input stage in its own shielded box ( see my website for details ).  All the power lines in and the audio out lines are bypassed with 0.01 uf capacitors.   I used 4 standard RCA female connectors on the input to the box.  I found a ready made audio RCA cable with four shielded wires molded together.  I cut off one end and connected them to the electrodes.   I tried to keep the unshielded part to be less than 1/4 inch, not just for shielding but also to reduce charge current coupling.  I built a shield for the electrodes by using a double sided PC board.  One side holds the electrode connector and the other is the ground.
         
        Electrode types:  Whatever type of electrode you use, you have to clean the skin to get rid of the oil.  A paper towel with rubbing alcohol will work.  Some people actually abrade the skin to get lower ohms.  My first electrodes were made from the black conductive foam that IC chips are shipped in to reduce electrostatic charge damage.  I used a needle to pull a # 30 wire through the foam a few times and then solder that to the shielded wire.  That worked quite well.  Using salt water improves the performance to some degree, but salt water dries out too fast and is not a good solution.  Conductive paste works better. 
         
        Over the years I have been using the Mind Mirror electrode system.  You live in England?  You should get some from them.  Also get the book by Maxwell Cade "The Awakened Mind".
         
        Probe Placement:  It is quite critical as to where you place your electrodes.  The professional 10-20 system ( skull cap ) is very specific so that the data is to be meaningful.  As an amateur have followed the Mind Mirror probe placement.
         
        This is a quote from an earlier Email about probe placement.............
        My first introduction to EEG measurement was meeting Dr. Edith Jurka and working with her Mind Mirror.  She had studied with Maxwell Cade in England.  I noticed that the book "The Awakened Mind" by Maxwell Cade is listed on your webpage.  I strongly recommend that you read that.  The book by Anna Wise is much quoted, but to me it is a very poor copy of the Cade book.  The Cade book suits the engineer in me while the Wise book is too "right" brained and full of new-age fluff. 
         
        With the majority of my EEG testing over the years, I have stayed with the Cade probe placement.  Now I suppose you want to know what that is?  Well, the ground goes to the middle of the forehead.  Then there is a pair of right and a pair of left channel electrodes.  The technical placement is F1 - O1 and F2 - O2.  On the forehead, locate the scalp hair line or where it used to be.  Put the right channel electrode directly over the right eye at the hair line, and similarly for the left channel.  The occipital region is a little more difficult to describe.  There is a place on the back of the head called the innion (sp?).  That is a little bony protrusion at the back of the head, in the center at the top of the neck and where the scull starts.  You should be able to feel it.  Draw an imaginary line one inch above this.  The rear electrodes go on this line and are spaced the same as the eye distance. Getting through the hair and making good low ohm connections is a serious challenge.
        End of quote.
         
        If you have trouble locating the Mind Mirror electrodes, let me know and I will try to tell you how to make them.
         
        Probe material:  I would be careful about putting lead solder near your skin. 
        The Mind Mirror buttons are nickel plated brass.  The conductive paste is quite corrosive and after a few cleanings with Scotch Brite, they are essentially brass buttons.  I am aware that this may not sit well with the purists on this group, but for my research I did not find it necessary to go to chlorinated silver electrodes.
         
        If you need any more info please let me know.
         
         
        Juergen P. (Jim) Meissner
        Check out my Website at www.MeissnerResearch.com
        Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
         
         
        ----- Original Message -----
        Sent: Thursday, January 31, 2002 8:13 PM
        Subject: [buildcheapeeg] NaffEEG

        I've now built the BrainMaster input stage (using AD620 + OP90), and
        it seems to be working.  I had to buy 5 of the polypropylene
        capacitors, so I picked a good matching pair anyway, just in case it
        made a difference.

        I'm now getting 50Hz hum, no problem at all!  I'm sampling at 5000Hz
        (5kHz), and piping this through to my filterbank display that I posted
        PNGs of earlier.

        I've improvised some electrodes from 1 pence pieces tinned with solder
        on one side.  I also experimented with little wrappers for them dipped
        in saline.  (However, I'm guessing now that probably it's not a good
        idea to use electrodes with lead (Pb) in them).

        I'm not seeing any brain-waves yet, though.

        With electrodes on my forehead (ground in the middle, other two either
        side), I get a clean trace with 50Hz hum in the middle.  If I tap the
        left or right electrode, I get a pulse, but the middle one doesn't
        give one, or it only gives a little one.  This seems to be more or
        less as expected.

        If I frown or clench my teeth, I get patterns appearing, and they peak
        around two to four times the amplitude of the 50Hz hum.

        So, what do I need to do to improve this situation ?  Do I need to buy
        some proper electrodes and paste, or should I look at reducing this
        hum first ?

        At the moment the electrode connection is passing through about 3m of
        microphone cable, with about 8 inches unshielded at the electrode end.
        Would shortening this cable make a significant improvement ?

        Can I improvise better electrodes ?  I'm still looking for a UK
        supplier of Ag/AgCl electrodes.

        Jim

        --
        Jim Peters                  (_)/=\~/_(_)                        Uazú
                                  (_)  /=\  ~/_  (_)
        jim@                  (_)    /=\    ~/_    (_)                  www.
        uazu.net           (_) ____ /=\ ____ ~/_ ____ (_)           uazu.net


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      • Jim Peters
        ... Yes, sorry for not replying yet to anyone. I m working through the tests that Joerg outlined, and I ve put the circuit in a metal box. Thanks for the
        Message 3 of 6 , Feb 2, 2002
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          Jim Meissner wrote:
          > I looks like you got some real good information from Andreas and Joerg.

          Yes, sorry for not replying yet to anyone. I'm working through the
          tests that Joerg outlined, and I've put the circuit in a metal box.
          Thanks for the advice on the electrodes, all of you.

          > Over the years I have been using the Mind Mirror electrode system.
          > You live in England? You should get some from them. Also get the
          > book by Maxwell Cade "The Awakened Mind".

          I'll ring them Monday to get the book, and see what they say about
          electrodes. The site is here, in case anyone else needs to find them:

          http://home.clara.net/biomind/

          I'm realizing that I may have to start again with the electrode
          cabling, looking to minimize any possible pickup of hum. I copied the
          cabling idea from the book "How to build a lie detector, brain wave
          monitor and other secret parapsychological electronics projects" --
          but they used filters, so perhaps they didn't need to be quite so
          careful.

          Yes, Jim, I did already dig out your electrode placement instructions
          posted earlier. However, without electrode paste I didn't think I'd
          have much luck with the rear placement positions.

          To Joerg -- re isolation, I'm using no isolation at all -- the AD620
          output goes straight to the laptop sound card input. I did consider
          that isolation might have some effect on the noise and hum, so I
          disconnected everything from the laptop (power, modem, network,
          hi-fi), leaving it running just on batteries, and there was no obvious
          difference. I'll try again once I've got everything more sorted.

          For a signal generator, I can use my other PC with a potential
          divider. I can measure the AC voltage before the divider and
          calculate the resultant.

          I'll give some more specific feedback once I've got somewhere with the
          tests and maybe bought/built myself some better electrodes + paste.

          Cheers -

          Jim

          --
          Jim Peters (_)/=\~/_(_) Uazú
          (_) /=\ ~/_ (_)
          jim@ (_) /=\ ~/_ (_) www.
          uazu.net (_) ____ /=\ ____ ~/_ ____ (_) uazu.net
        • Jim Meissner
          Dear Jim Peters: Had another thought. That s the second one today! I modified the probe placement of the ground. Instead of one electrode in the center of
          Message 4 of 6 , Feb 2, 2002
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            Dear Jim Peters:
             
            Had another thought.  That's the second one today!
             
            I modified the probe placement of the ground.  Instead of one electrode in the center of the forehead, I use two ground electrodes, one on each temple near the hair line.  The reason was that if you get a little sloppy with the paste, you inadvertently short out the forehead electrodes.  With just one of me it is hard to see what you are doing and the extra spacing really helps.  I have all the electrodes mounted on a headband with a Velcro fastener.
             
            ( BTW if you are really serious you will shave your head or at least the back where the probes go!  )
             
            Another way to reduce the hum pickup is to find a quiet spot in your house where there is no wiring.
             
            Juergen P. (Jim) Meissner
            Check out my Website at www.MeissnerResearch.com
            Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
             
             
            ----- Original Message -----
            Sent: Saturday, February 02, 2002 3:58 PM
            Subject: Re: [buildcheapeeg] NaffEEG

            Jim Meissner wrote:
            > I looks like you got some real good information from Andreas and Joerg.

            Yes, sorry for not replying yet to anyone.  I'm working through the
            tests that Joerg outlined, and I've put the circuit in a metal box.
            Thanks for the advice on the electrodes, all of you.

            > Over the years I have been using the Mind Mirror electrode system.
            > You live in England?  You should get some from them.  Also get the
            > book by Maxwell Cade "The Awakened Mind".

            I'll ring them Monday to get the book, and see what they say about
            electrodes.  The site is here, in case anyone else needs to find them:

              http://home.clara.net/biomind/

            I'm realizing that I may have to start again with the electrode
            cabling, looking to minimize any possible pickup of hum.  I copied the
            cabling idea from the book "How to build a lie detector, brain wave
            monitor and other secret parapsychological electronics projects" --
            but they used filters, so perhaps they didn't need to be quite so
            careful.

            Yes, Jim, I did already dig out your electrode placement instructions
            posted earlier.  However, without electrode paste I didn't think I'd
            have much luck with the rear placement positions.

            To Joerg -- re isolation, I'm using no isolation at all -- the AD620
            output goes straight to the laptop sound card input.  I did consider
            that isolation might have some effect on the noise and hum, so I
            disconnected everything from the laptop (power, modem, network,
            hi-fi), leaving it running just on batteries, and there was no obvious
            difference.  I'll try again once I've got everything more sorted.

            For a signal generator, I can use my other PC with a potential
            divider.  I can measure the AC voltage before the divider and
            calculate the resultant.

            I'll give some more specific feedback once I've got somewhere with the
            tests and maybe bought/built myself some better electrodes + paste.

            Cheers -

            Jim

            --
            Jim Peters                  (_)/=\~/_(_)                        Uazú
                                      (_)  /=\  ~/_  (_)
            jim@                  (_)    /=\    ~/_    (_)                  www.
            uazu.net           (_) ____ /=\ ____ ~/_ ____ (_)           uazu.net


            To unsubscribe from this group, send an email to:
            buildcheapeeg-unsubscribe@egroups.com



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