The OpenBCI Ultracortex headset is a nice piece of hardware, but rather expensive (350-800 USD) and does not always provide a good fit with the subjects in terms of comfort and signal quality.
Using loose wires with cup electrodes offers flexibility and attaching a few of them with Ten20 paste to the scalp is not such a big deal. The advantage of cup electrodes combined with Ten20 paste is that signal quality is usually better and that a single set of 8+2 electrodes can be used to cover all head sizes.
Low-cost cup electrodes with Dupont connectors are available in the OpenBCI shop, but also on Ebay where they usually come with DIN 42 802 "touch-proof" connectors. Better quality (i.e. thicker gold-plating and more flexible leads) sets are available from EEG vendors for about 100 USD for a set of 10 (e.g. here at MedCaT).
To facilitate the use of cup electrodes, we designed a head sweatband with the OpenBCI board attached to the back using Velcro. The electrode leads are cut to length and wired to a 11x2 Dupont connector. If you cannot find a 11x2 Dupont connector, you can take a longer one and cut it to length. After some sanding with fine sandpaper it will look just fine.
The OpenBCI board comes with a right-angle 11x2 male header soldered to it. SRB is on the left, followed by N1P-N8P, BIAS and AGND. The outer most pads on the PCB (AVSS on the left and AVDD on the right) are not connected to the header.
We have connected the reference electrode lead to the SRB1 pin (the upper pin), 8 electrode leads to P1-P8 (the upper pins) and the ground electrode lead to one of the AGND pins, skipping the BIAS pin. This allows for a conventional common reference montage. The electrodes leads are cut to length to easily reach the following standard 10-20 electrode sites on the scalp:
- pin SRB1 - 31 cm - white - left mastoid (M1) or alternatively AFz
- pin P1 - 11 cm - grey - electrode site O1
- pin P2 - 18 cm - purple - electrode site P3
- pin P3 - 24 cm - blue - electrode site C3
- pin P4 - 31 cm - green - electrode site F3
- pin P5 - 31 cm - yellow - electrode site F4
- pin P6 - 24 cm - orange - electrode site C4
- pin P7 - 18 cm - red - electrode site P4
- pin P8 - 11 cm - brown - electrode site O2
- pin AGND - 31 cm - black - right mastoid (M2) or alternatively Fpz
The leads are long enough so that they can reach one electrode site further to the front of the head. The longest ones (connected to the middle pins P4 and P5) can also reach around the head and be used for EOG.
Warning: in the schematics of the OpenBCI cyton board the SRB1 and SBB2 labels are swapped; in the TI ADS1299 datasheet you can see that SRB1 is pin 17 on the corner, and hence following the PCB layout connected to the top pin of the double cyton header.
Note that SRB1 is on the upper side with the "P" pins and SRB2 is on the lower side with the "N" pins. Contrary to the two photo's included in this documentation, the white wire connected to SRB1 should be in one line with the colored wires connected to the upper "P" pins. For the AGND you can use either upper or lower pin.
PS Matt Millman has a very interesting page with information about Dupont and other crimp terminals.