From: Jim Peters (jim_at_uazu.net)
Date: 2002-02-01 01:13:19
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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