From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-02-01 01:32:42
--- In buildcheapeeg_at_yahoogroups.com, Jim Peters <jim_at_uazu.net> wrote:
Hi Jim,
> I've now built the BrainMaster input stage (using AD620 + OP90), and
> it seems to be working.
Cool!
> 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.
Oops. Well, that was to be expected I guess...
> 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.
>
[snip]
>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 sounds like you have built a nice EMG meter. :-) Perhaps you need
to increase the gain a bit? Do you know what the total gain is?
(sound card * amplifier).
> 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 ?
As long as the hum doesn't saturate your sound card and cause
clipping, don't bother. But try to get some real paste, so that the
resistance is lowered more. The pros clean the scalp too, by rubbing
with alcohol and so on.
> 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 ?
I don't think so. Most of the hum is probably picked up by you.
> Can I improvise better electrodes ? I'm still looking for a UK
> supplier of Ag/AgCl electrodes.
You could also go to a goldsmith and buy a couple of 18mm silver
discs and chloridize them.
Well, that was advice from someone without practical experience! :-).
Jim Meissner is probably the right person to answer your questions...
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:37 BST