The Unicorn Naked comes with the same dry electrodes and electrode leads (wires) as the Unicorn Hybrid Black. However, there is no real advantage of the Naked version over the Hybrid Black version if you want to use them with the same electrodes, unless you insist on a 3-D printed headset to hold the electrodes in place instead of a cap. We figured that the small size and weight, and it being wireless, made it an ideal candidate for developing a wearable system for EEG research on infants and children. At the same time, we sometimes have other applications where a small and wireless ExG system would be useful, for example to record EMG from the muscles or ECG from the heart.
This first image shows the result, further down on the page you can read more on the details and design considerations.
Connector and cables
The connector on the Unicorn Naked PCB board is a crimp-style 20 pin connector with 1 mm pitch that looks a lot lot like the JST SH connector. I am not sure it is exactly this JST SH connector, as that only seems to be available in white whereas the one on the Unicorn Naked is black. It is a non-locking connector that is relatively easily to connect and disconnect, where it not that it is very small. It being a non-locked connector also means that in real applications with movement and some pulling on the cables it might accidentally disconnect.
The development and testing of new EEG electrodes is not so easy with this very small PCB connector, nor with the electrode leads that are specifically designed for the g.SAHARA dry electrodes. Prototyping and testing requires frequent switching between electrodes and cables. Considering a final EEG system that consists of a single head-mounted amplifier that needs to be switched between different EEG caps to accommodate the variable head sizes of participants, it is also relevant that switching can be done fast and without risk of damaging the PCB board. So the connector needs to be sturdy and should allow for frequent connecting/disconnecting.
I decided on using a 2×5 shrouded male header (also known as a box header) with 2.54 mm pitch for the amplifier side and 2×5 female IDC ribbon cable connectors for the electrode leads. Compared to the very small PCB connector on the Unicorn Naked board, the 2.54 mm pitch connectors are much easier to handle in a DIY situation. Furthermore, they are standard, cheap and readily available, which also facilitates the construction of other electrode cable bundles, for example for EMG and ECG.
The Unicorn github page includes the design for a 3-D printed enclosure for the Unicorn Naked. It comes with a clip that allows for flexible mounting on clothes or so, and it keeps the front side of the PCB exposed. The back of the enclosure keeps the PCB board connector free, so that you can swap the cable bundle.
As I decided on using a 2×5 shrouded male header, I need a different 3-D printed enclosure. I have made the fit of the PCB board and battery very tight to keep the overall size as small as possible. The 3-D design consists of three parts and uses a combination of screws and a bit of superglue to hold them together.
Mounting on cap
Except for “microphone boom” LED strip over the right ear, the housing of the Unicorn Hybrid Black itself is very minimalistic and lightweight. A nice feature is that it has a magnetic mount to the counterpart that is at the back of the caps. The magnetic mount is sturdy and will in the long run hold better than velcro. The magnetic mount also has the advantage that – when applying the EEG cap on yourself – you can shortly take the amplifier off and look in the mirror (or using a phone or laptop webcam as makeshift mirror) to see whether the status light is blinking, and then pop it back on again.