Active Electrode Building Guide

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IMPORTANT NOTE: Please do not contact me about the active electrodes. You will get more help if you send an email to the OpenEEG mailing list. Also, check out Olimex if you're interested in buying a preassembled active electrode.

This guide should help you to build Active Electrodes for the modularEEG, as proposed by Jarek Foltynsky here. The active electrodes described here correspond to the modified circuit diagram suggested by Joerg Hansmann:


Active Electrodes V2 by Jarek Foltynsky.	Modified Active Electrodes V2 by Joerg Hansmann.

This guide proposes a tradeoff between building simplicity and signal quality. The guarding box has been omitted and replaced with silicone for encapsulation, because making robust and very small mini-boxes is not simple at all. These are instructions for building 4 active electrodes (2 channels). If you want to try another approach using batteries, give Joe Street's AEs a try.

Please be warned that making AEs is not a simple task. I myself needed to make several prototypes to arrive at this design. Several people on the openeeg-list have reported problems with the electrodes described on this page, but I don't know why. They worked perfectly for me.

In a previous design, I tried a rectangular pin array, but I didn't work. Someone on the list pointed out that this can be due to the particular pin material. That's why this approach uses only silver wires. Also, the thin pin arrays can hurt your skin if you use them for too long.

Step 1: First of all, I decided to etch the boards myself because industrial etching is expensive and requires a careful final design. So this is actually my latest design so far, and it worked well. To start, download the active electrode template from [this link]. The template includes 40 active electrode patterns for etching.

Step 2: Print the pattern, using the darkest laser printer settings, onto a sheet of polyester paper. You can also try to use other types of paper, but polyester works fine. Don't touch the board-part of the pattern paper with fingers, or with anything else, before or after it's been printed.

Step 3: Cut the pattern out, leaving a bit of extra paper, all the way around the pattern.

Step 4: Scrub a copper plate with an abrasive pad. Lay the pattern flat, face-down, onto it.

Step 5: Use a regular handheld household electric clothes iron. Temperature setting should be something beetwen "Linen" and "Polyester", i.e. very hot, and without steam. Place the iron onto the pattern. Hold it very firmly, and press as much as you can for at least 30 seconds or more. Change position, and repeat, just to avoid a non-uniformity problem. The whole process shouldn't take you more than 5 minutes.

Step 6: Wash the copper plate and the pattern with cold water. Then, remove the pattern. The copper plate should look like this.

Step 7: Make any necessary corrections, using a permanent ink or other etch-resistant marker pen.

Step 8: Use a saw and cut the boards. Be careful.

Step 9: This is how your boards should look like.

Step 10: Etch the boards. I use Ferric Chloride and some water in a plastic food container. I recommend to heat the closed container in your microoven, so etching is much faster. Don't touch the etchant, and don't get it on anything else.

Step 11: The etched boards.

Step 12: Remove the toner with some Acetone, or some Laquer Thinner solvent.

Step 13: Finish the edges with sandpaper.

Step 14: The boards with rounded corners.

Step 15: Drill the holes with a dremel tool. Use some goog PCB bits, about 1mm/0.035 inches.

Step 16: The boards are ready now for soldering.

Step 17: Another picture :-).

Step 18: Now, you'll need: The 4 boards; 4 x 200k, 4 x 10k and 4 x 100Ohm resistors; 8 x 100pF and 4 x 10nF capacitors; 4 x TLC272 opamps; some silver wire of 1mm diameter.

Step 19: Solder the parts as described [here].

Step 20: Place the board, parts looking down, over a thick coin. Cut 18 silver sticks and place them in the holes. They all should touch the underlying table. Then, solder them. Using 1mm wire, the contact area should make more than 14 square milimeters.

Step 21: Finish the ends with sandpaper. They should make a flat surface like in this picture.

Step 21: The almost finished active electrodes.

Step 22: Now, get some double cables with shielding. One of them should carry the current to the opamp (wire +, shielding -), and the other the output signal. Connect the red wire to the arrow, and its shielding nowhere, as specified by the figure. My cables are 1m long.

Note: If you want to use a little guarding box (e.g. made of copper plates) for the electrodes, then connect it to the remaining connector.