This week, I wanted to start tackling elements of my final project of building an EEG. Since the EEG will involve inputting voltage from the leads coming from a user's head and the output will be sounds and graphs based on the signal, I had previously decided to combine both the output week's and input week's assignments. This post covers the input part. The concepts I learned about included: floating gates, gate resistors, low-pass filters, class D amplifiers. In order to optimally combine the input and output week, I decided to create an optimized board that pulled the best of the speaker board and the microphone (see previous post for more on that). This is mostly a lot of notes sketched on paper and comparisons between different schematics for building EEG, EMG, and ECG specific circuits - all of these are in similar frequencies, but ECG and EMG have much higher power (partly because they are not muffled by the skull). There's an overview of major differences between these signals here. I was trying to combine the best of all worlds by looking across all of these, but I needed to understand how people have approached each in the past. These notes start with understanding how the mic and stereo board combo would work and then branch off into ECG and EMG and EEG schematics. Finally, I wanted to understand how much bigger (and thus likely easier to make sense out of and create circuits for) biological signals get handled: ECG and EMG. A site describing a simple ECG board that provided some great inspiration was actually written by former How to Make (almost) Anything students Adam Marblestone and Charles Facchia.
Also, How to Make Anything TA Jeff (Jean-Francoise Duval) let me look at an EMG design he made. It involved an instrumentation amplifier - which is what I ended up going with (see my final project page). It had a gain of 25 (useful to know, since EEG is a much smaller signal than EMG is).