This article uses modulation tricks to try to get many inputs all on one audio channel.
However, USB sound cards, available for around a dollar[1] have a nice high input impedance and can directly be connected to EEG probes for direct sampling of microvolt level signals. No circuitry needed!
You can easily connect hundreds of them to the same system and get hundreds of input channels with great electrical isolation. The way USB works, they also have perfectly synchronised clocking too!
They use modulation tricks to get multiple inputs on one audio channel. Sound-cards with more than 4 audio channels are not cheap, and if you gang multiple sound cards together you end up with clock drift.
If you want more than 4 channels that don't drift from each other with consumer hardware, then you'll need to multiplex (And indeed, they offer non-multiplexing 4-channel options).
Anyways - search 'USB sound card' and sort by cheapest with free delivery and it is the one that costs $1.23. The comments show a teardown and it has an epoxy blob and a handful of passives on a small circuit board with two 3.5 mm jacks and a usb plug.
It's obvious that a sound card is the most affordable oscilloscope, the obvious downside being quite limited sample rate. You would expect that an oscilloscope would go into MHz. But if it works for you, fine.
I remember an article about ball mice disassembled and used as a cheap reliable rotation sensor somewhere in oil pipeline industry.
OpenBCI is usually a good place to start for BCI DIY projects. I use the Low-cost Biosensing Starter Bundle[1], it comes with EEG/EMG/ECG hardware & software.
I have some (cringe) unboxing & assembling vids if you want to see what comes in the box[2]. (Recommend watching it on 2x speed till I get better at editing)
Ions mean there's some positive electric charge. When you move some ions to one side of membrane, you have charge imbalance. When you have a charge imbalance between two sides of some insulator (cellular membrane), you have voltage potential. You can use that potential to do work (like light a diode, turn a motor or do some calculations).
However, USB sound cards, available for around a dollar[1] have a nice high input impedance and can directly be connected to EEG probes for direct sampling of microvolt level signals. No circuitry needed!
You can easily connect hundreds of them to the same system and get hundreds of input channels with great electrical isolation. The way USB works, they also have perfectly synchronised clocking too!
[1]: https://www.aliexpress.com/item/1005004134024654.html