This is not where I expected to be in my timeline by this point.
I’m far behind. I thought by now that I’d have all three circuits up and running, but that’s not the case.
I had a lot of issues with the first circuit, and it just isn’t working. I’ve tried:
- Looking for shorts/continuity errors
- Removing the last 60Hz notch filter stage entirely (due to commenters on Instructables saying that it wasn’t working)
- Using a Mac instead of a PC (due to soundcard frequency discrepancies)
- Having others check my layout and connections (no issues!)
- Plugging it in via Arduino instead of audio cable (still no signal)
- And building a second circuit
but nothing has worked, and I realize that it’s time to let go of this circuit if I plan to finish this project to the extent that I planned.
I’ve been using the internet as a very important resource, with the most helpful so far being an electronics forum. I’ve been posting in the forum since I started this project. The fact that the schematic originated from Instructables was upsetting to many members of the community, and I was advised to use a different schematic and make my own, which I am now in the progress of creating. The forum has been extremely helpful – members are quick with feedback and understanding of my beginner background. I really understand how this circuit works and how I need to make it happen now, and it’s really exciting!
I’ve gotten advice to follow the basic stages of the first schematic with an AD260AN amplifier stage, a bandpass filter (high and low pass filters together), a notch filter centered around 60Hz (to cut “buzz” from external electronics), and a final gain adjustable amplifier.
The AD260AN amplifier stage is simple. It consists of the instrumentation amplifier chip, a resistor, and my two “active” electrodes (active meaning not my reference/ground electrode. All the electrodes I bought are passive, which basically means they don’t have circuitry attached to them. In the simplest sense, they’re pieces of metal attached to a wire). The chip takes the difference between the electrode inputs, such that if one electrode was inputting a signal of 20uV and the other 13uV, the difference of 7uV would process through the chip and be amplified at its output.
The bandpass filter is a high pass and a low pass filter in succession. I’m focusing on alpha and beta brainwaves, which range from about 7Hz-30Hz. High pass filters take input and filter out frequencies lower than the designated frequency, and low pass filters do the opposite. So, my high pass filter’s cutoff frequency is 7Hz, which basically means that any signal below 7Hz is attenuated. The low pass filter has a cutoff frequency of 30 Hz, so signals above 30Hz are attenuated.
Notch filters reject a certain frequency and filter it out, which in this case is 60Hz, the power line/utility frequency we use for alternating current power in the US. This filter helps clean up the data a bit more before amplifying it once more.
The adjustable gain amplifier will amplify the signals to prepare them for computer input and processing. The gain for this section (where gain is the ratio between output voltage and input voltage) is controlled through a potentiometer, which has a dial that can be turned to have precise control over the signal’s amplification.
From my calculations so far, it does seem like I’m going to need to buy new parts. However, the parts I need are resistors and capacitors, which are extremely cheap and range from costing $0.01 to $0.10 each, so I’m not worried about purchasing.
I was given the idea to rewrite and rework the Instructable that inspired this project by a classmate, and I really like the idea. The author of the Instructable I used abandoned it a while ago, and much of the comments consisted of very confused people who are trying to build the circuit but can’t figure it out. This also ties into my plan to make everything I create open source, so others can experience EEGs and generative art.
I’m determined to make this work.