Joe's ModularEEG gallery

Joe Street's images of his ModularEEG, and home made electrodes.

You can reach him at ve3vxo at hotmail.com

Click on the images to get a larger view.

A four channel ModularEEG with head band and silver electrodes. This is an early version of the headband.  You can see two channels are connected to electrodes on either side of the band and the middle strap which runs over the top of the head is connected through a Y harness to the negative or indifferent inputs of both channels as a common reference.  This was before I added the extra heatshrink to the electrodes and I found I had some problems with salty paste getting back into the female pin on the cable.  Not good.  The extra cable seen on top of the box was just to show an example of an electrode plugged in.

The head band. Use your imagination here.  I also have a band which is just a single strip which goes over the head from ear to ear and an elastic strap which runs under the chin.  This is good for working with C3 and C4 sites and is a simple thing to put on.

An electrode cable made of a standard phono (cinch) connector and thin coaxial cable, and a silver electrode. The DIY electrode consists of a square piece of pure silver with a short length of 1mm solid wire soldered on the back. The solder connection is encapsulated in 5 minute epoxy right up to the wire insulation so that the electrode can be chlorided safely if desired.

Silver is pure and is about 1mm thick.  Check with a jeweler to buy it.  It is not expensive and will last forever.

The wire fits the RS-232 crimp pin on the end of the coax. The sharp corners of the electrode could be removed but I couldn't bring myself to waste the silver by trimming the corners and it has not been a problem or a discomfort so far.

A Y-adaptor used to tie the minus side of two inputs to a common electrode, for so-called monopolar montage. In this way signals from the two brain hemispheres can be measured against a common reference point such as the centerline on the top of the head.

The finished electrodes. (You may want to open the larger view in another window.)
The photo shows a cable with blue marking and the peice of velcro attached with cable ties.  This allows the cable to be positioned at any hole location on the headband and prevents head movements from disturbing the electrode contact with the skin.  The unplugged electrodes are to show how the heat shrink is done so that the electrode can be sealed well yet still removable.  The cable with the yellow tape is an example of the mounting on the perforated headband.  The 5 cent piece is in there only to show the size and well because I couldn't resist showing off the Canadian beaver. The cable although not visible continues off the right side of the image.

After quite a bit of fiddling and testing I settled on the electrode and cable arrangement seen here.  The cable is RG-174 miniature coaxial cable.  It is light, soft and flexible and has excelent shielding.  The RCA or phono plug is what I chose for the modeeg box and has proven to be reliable but requires that unused channels be shorted with a shorting plug since the jack is open circuit when the plug is not plugged in.  The electrode end of the cable has the shield trimmed back a few mm from the end and a female type RS-232 pin is soldered on the center conductor and then a piece of heat shrink tubing is shrunk over the pin and cable end.  Coloured heat shrink can be used to identify each end of the cable and is a good idea. I used coloured electrical tape but if I was doing it again I would use coloured heat shrink.

The electrode wire is a solid 1 mm wire and is soldered on the back of the electrode as shown.  After soldering push the insulation down close to the solder joint and put enough epoxy on the back that it forms a convex bulge so that the end of the wire insulation is inside the epoxy.  This is easily done since the surface tension of the epoxy prevents it from running off the electrode.  I used Lepages 5 minute epoxy for this.  Any hobby or hardware store should have something equivalent.

After the epoxy is cured strip the insulation from the electrode wire enough to insert in the female pin on the cable, about 8 mm or so.  Now cut a piece of heat shrink tubing the same length as the length of the wire from the electrode.  Mine are about 2 cm.  Put this heat shrink on the cable first and then plug in the electrode and slide the heat shrink down to the electrode and then shrink it.  Be careful not to over heat it near the pin which will cause it to fuse to the heatshrink on the cable.  You do not want this to happen because then you won't be able to remove the electrode from the cable which is the whole idea.  Once this is done you have a nice reasonably waterproof revovable connection.

I cut two tiny slits in a rectangular peice of velcro (with hooks) at each end, so that I can slip a small cable tie through the two slits and around the cable at each end of the velcro as shown.  The slits are only about 2 mm long and 3 mm apart so that the cable is firmly attached to the velcro.  This is an important point and releives strain and the weight of the cable so that the electrode is not disturbed.  When making headbands of this type it is also important to include a section of elastic webbing to join the two ends.  I do this by sewing a square of the velcro (with hooks) to each end of the elastic band so that it is adjustable for different sizes. Don't make the mistake of thinking that you can just use a strip of velcro to hold everything tight, it needs the dynamic element of the elastic to get reliable and constant electrode contact on the scalp.