DIY EEG Troubleshooting

Thread Starter

Zanpoole

Joined Feb 13, 2022
10
Hello all, I am a computer and electrical engineering student and I am doing a project where I am making an EEG. I used a design that I found here: https://hackaday.io/project/167393-low-cost-eeg-circuit. I have checked the circuit that we made on a breadboard several times and everything is connected properly. As I tested from stage to stage I found that after the lowpass filter in the bandpass filter, the first 1uF capacitor seems to discharge down to zero. So after that stage, like the output of the variable amplifier, there is no output voltage. Any possible solutions?
1651353649926.png
 

MrChips

Joined Oct 2, 2009
26,089
We cannot see the front end of the circuit, the part that connects to the EEG electrodes.
If you are not using an INA (instrumentation amplifier) then you are off to a wrong start.
 

Audioguru again

Joined Oct 21, 2019
4,680
The bandpass filter has the same output level as its input level within its bandpass. C5 cuts 48.5Hz by -3dB.
Get rid or the error-prone breadboard and solder together a real circuit.

Here is most of your circuit doubled in size and with most extra spaces between parts removed, and the response of the bandpass filter without C5:
 

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Thread Starter

Zanpoole

Joined Feb 13, 2022
10
The bandpass filter has the same output level as its input level within its bandpass. C5 cuts 48.5Hz by -3dB.
Get rid or the error-prone breadboard and solder together a real circuit.

Here is most of your circuit doubled in size and with most extra spaces between parts removed, and the response of the bandpass filter without C5:
I am a little confused, you are saying without C5 but these circuits seem to be the same to me. I have very little time left with this semester coming to an end, so as much as I would love to solder it all on a PCB, I don't have time.
 

Audioguru again

Joined Oct 21, 2019
4,680
I simulated the lowpass and highpass filters in your bandpass filter but I did not show the additional lowpass filter made with U7, R9, R10 and C5. The schematic works fine.

You said that your breadboarded circuit does not work which is like most messy breadboarded circuits. Maybe the breadboard contacts make poor connections or maybe one of the hundreds of wires all over the place is in a wrong hole.
 

Thread Starter

Zanpoole

Joined Feb 13, 2022
10
I simulated the lowpass and highpass filters in your bandpass filter but I did not show the additional lowpass filter made with U7, R9, R10 and C5. The schematic works fine.

You said that your breadboarded circuit does not work which is like most messy breadboarded circuits. Maybe the breadboard contacts make poor connections or maybe one of the hundreds of wires all over the place is in a wrong hole.
Just to check, do I need to use the Vos trim pins on the opamp? This could be a very dumb question but my work with opamps is limited.
 

DickCappels

Joined Aug 21, 2008
8,707
An early mentor of mine once said to check the power supply voltages and the dc levels throughout the circuit, "If the DC voltages are ok then the AC will take care of itself."
 

Thread Starter

Zanpoole

Joined Feb 13, 2022
10
Also I found out a resistor was out of place and that’s why the capacitor was discharging. Now I just get a constant signal output that is unaffected by the electrodes. So now I need to somehow troubleshoot this.
 

Audioguru again

Joined Oct 21, 2019
4,680
"I found out a resistor was out of place". That is what everybody says when they make a circuit on a solderless breadboard.
Please describe the "constant signal output". Is it a low frequency hum or a high frequency squeal?

I simulated the opamps feeding the "bandpass filter" that I simulated before and they are another bandpass filter but much simpler.

It seems that the entire circuit works to amplify very low frequencies.
 

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Thread Starter

Zanpoole

Joined Feb 13, 2022
10
"I found out a resistor was out of place". That is what everybody says when they make a circuit on a solderless breadboard.
Please describe the "constant signal output". Is it a low frequency hum or a high frequency squeal?

I simulated the opamps feeding the "bandpass filter" that I simulated before and they are another bandpass filter but much simpler.

It seems that the entire circuit works to amplify very low frequencies.
I don't really know how to describe it but here is a picture from the oscilloscope.scope_0.png
 

Audioguru again

Joined Oct 21, 2019
4,680
It looks like some of the opamps are oscillating at 634.5kHz which frequently happens when a complicated circuit is made with a huge tangled mess of wires all over the place when built on a breadboard.
 

Thread Starter

Zanpoole

Joined Feb 13, 2022
10
It looks like some of the opamps are oscillating at 634.5kHz which frequently happens when a complicated circuit is made with a huge tangled mess of wires all over the place when built on a breadboard.
I understand that the breadboard isn’t ideal but with the timeline given for the project, a pcb isn’t an option.
 

DickCappels

Joined Aug 21, 2008
8,707
Do you have power supply bypass capacitors next to each op amp package or in the case of low power and low frequency every two op amp packages?

Also, what was the input doing when you captured the waveform in post #16. Were the leads dangling?
 
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