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OpenEEG Electrodes

Introduction

Passive electrodes have no inbuilt circuitry.

Saline Electrodes

Saline electrodes' main component is a sponge full of salt water which is next to your skin.

DIY electrodes (Dan Griffiths)
Nelo's Electrodes

Paste/Gel electrodes

These are the traditional kind. Basically, you find some bare skin on the appropriate part of your head (ie. the bits usually covered with hair), and paste these things to your head.

You may want to try the Electrode Howto

Electrodes

Joe Street's Electrodes
The Electrode Store has a variety. Someone recommended WBT-DSC as cheap. You'll want to read the following before ordering from The Electrode Store: http://sourceforge.net/mailarchive/forum.php?thread_id=6810985&forum_id=10061

Paste/Gel

You can get some from the Electrode Store listed above. Additionally, Andreas Robinson once made the following comment on the mailing list, in response to a message about homemade alternatives to 10-20 electrode paste:

About electrode paste: I can't help you there. I am using essentially the same thing: home made starch/gelatine gel with salt water. Cheap, but without any adhesive properties.

Other materials for electrodes

Jim Peters says:

I don't know much about electrodes, but this is what I've heard. You can get gold, tin, silver and silver/silver-chloride (Ag/AgCl). Silver / silver chloride are supposed to be best. Electrodes can be the normal round things or needle electrodes. The needle ones pierce the skin to get a better connection (I think). There are also disposable electrodes, which probably aren't much use for what we want because of the long-term cost. You will also need 10-20 paste (or some other conductive paste) to improve the connection between the electrode and the scalp.

Regarding DIY electrodes, I believe Andreas has made his own electrodes and paste. Also, Jim Meissner (from the Yahoo list) used brass buttons, which were the electrodes commonly used with the Mind Mirror, and he certainly managed to get good usable signals using them.

The electrodes I've got are from the Electrode Store (WBT-DSC):
http://www.electrodestore.com/EEG/EEG-p13.html
They are plastic with a silver coating and silver chloride on top, I believe. I don't know if other types are solid metal normally -- these are the first electrodes I've actually got hold of.

The other thing is that I'm planning to use shielded leads, which means making my own, and improvising my own connection to the electrodes, which isn't ideal. I believe shielded leads are required for the DRL circuit to work effectively. I could have bought the lead-set from the Electrode Store, and cut them all off at the connectors, but that seemed a waste.

Good skin contact

The results of using passive electrodes depend on a number of factors all being just right. One of these is the quality of the skin contact. The following are some tips on ensuring good skin contact.

Prepare the skin (ie. clean it thoroughly, preferably with alcohol (pure, not alcoholic drinks))
Sweating always produces artifacts in the signal, so ensure the test subject is cooled adequately.

Electrode Cabling

Each channel has two electrodes. Each channel has + and - inputs (electrodes), and calculates the difference of the input voltages. This cancels out or attenuates common mode signals like mains hum that (ideally) have the same signal strength on + and - inputs.

With 2 channels 5 electrodes (including DRL) are required (or at least 4 when 2 channels share one reference (-) electrode)

DRL is an output that acts as an active earth and suppresses mains hum (at least in theory) even more than a simple connection to VGND.

Shielding

You will need to use shielded cabling for all passive electrodes. The cables from the electrode store are not suitable. This is necessary to get the DRL working effectively. Andreas said:

First, any cheap shielded cable should work. Not too cheap though. Very low cost audio cabling can be worthless, that is have very little shielding.

Assume you want a 1.2 m electrode cable (longer than 1.8 m is not recommended). Have both leads for one EEG channel run in the same cable, sharing the same shield, for about 1 meter. Then have the cable branch into two leads the last 20 cm (or longer if you have a big head. ;-). The branches are also shielded of course, up to the electrode.

In short, it should look like a Y.

By doing this, you reduce the capacitive loading on the electrode leads, and get more managable cabling too.

Just remember to only attach the cable shield to the amplifier board, and not the electrode itself.