Logbook 7/31

 On Monday Mario and I met at his gym and he gave me the breadboard with the amp circuit built on it. He didn’t have a potentiometer so just included the last two needed resistors on the side so I could put them once I got a potentiometer. On Tuesday in Addison I went down to the lab and ran some tests.

I inputted a 100mV sinewave signal to the side where the electrodes would attach, and attached a scope probe to the output. The circuit requires +9v, -9V, and ground, luckily they do have a power supply in Addison capable of such.   I was sad to find out that the output was VERY fuzzy, and only around 100uV amplitude. I tried first at frequency of 10hz, then at larger frequencies, and the amplitude increased to 1mV as the frequency rose.

This indicates our filter is backwards, being a highpass instead of lowpass, but I don’t understand why this would be the case. I put the scope probe at different spots on the chain of amplifiers (ordered as follows: 60hz, lowpass, highpass, high gain amp). The signal became weaker after every section! There was no signal at all after the high gain amp, something was simply killing it.

I triple checked the circuit with the schematic and could findno problems.

I am going to try to get a high gain amplifier working and disregarding the filters for now. Amplification in the most import part. Hopefully Mustafa will be able to meet with me this week.

Logbook 7/24

We met on friday to begin buildling the filtering circuit for the brainwaves. This circuit has been a bottleneck in our progress because it is behind schedule, and nothing else can be done until we have it at least half-working! We met in Mario's lab and brought out the schematics, Mario has many resistors and it was a pain digging to find the right one.

Mario plans on finishing it monday afternoon and handing it over to me by the evening. On tuesday I want to run some tests on it.

We did order a new opamp chip more similar to the one they use on instructables, where we got our schematic. This one is dual supply, so we should be able to get the negative swing of the signal.  Because we have to use the instrumental amplifier first, which MUST be done using negative and positive voltage, we figure itd be best to just make the second opamp be the same. Otherwise, with biasing and all, it may be a mess. 

http://www.instructables.com/id/DIY-EEG-and-ECG-Circuit/

op amp: TL084CN (quad)

instrumental amp: ad620

Logbook 7/17

We got in our lithium battery 'charger' that we ordered this week. Although we dont plan to implement this piece til the end, we though ahead and realized that most users of our device may not have an easy wasy to charge their product unless we provide them with a hookup. The module we purchased connects via usb and regulates current to the the lithium battery (we assume all consumers will have access to a 5V usb jack). Tte device is the size of a small cracker.  Furthermore, we still havent gotten our filter put together, but Mustafa has been doing simulations on different filter designs. Hopefully we get something working by next week.

After we get the filter/amplifier working, we can examine the signals that we get and possibly use a frequency to voltage IC chip to detect frequencies rather than our complex sampling code. Unfortunatly, the oscilliscopes in the Elab do not have spectrum analyzers built into them (or at least nobody knows how to find the spectrum analyer function) so we will still need to use the existing arduino sampling code to at least harvest some data to graph and examine.

 double pole (more components, better rolloff)

single pole, inverting and non inverting

http://www.radio-electronics.com/info/circuits/opamp_low_pass_filter/op_amp_lowpassfilter.php

**since our signal is bipolar, whether we invert or not doesnt matter much unless we use a DC bias.

schematic for DC offset to an op amp

http://electronics.stackexchange.com/questions/34071/how-do-i-correct-the-offset-voltage-of-op-amps-which-have-no-explicit-offset-nul

Logbook 7/12

This week I did further research about our preamplifier circuit. After discussing things with Professor Krauss, I have determined that we probably will have to use at least some negative voltage. Brain waves are bipolar, and although we could use a DC offset, i feel like if we use that directly from the scalp we risk swamping the already faint signal with noise. I found a IC chip, the  ICL7660 that will easily just convert the +5v that we have to a -5 V.  It cannot provide high current, but we'd barely need any to power the op amp chips. I purchased 10 of these chips for 1.76 from china. We do not need this chips to construct the filter, we can just use a DC power supply, we'll only need the chips when we build the final prototype.

We'll add a DC offset right before the signal gets sampled by the microcontroller.

Hopefully we can construct this circuit ASAP using the schematic from instructables as is, we will have to include credit to the author of that article.

http://pdfserv.maximintegrated.com/en/ds/ICL7660-MAX1044.pdf

datasheet above.