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OpenEEG Frequently Asked Questions
1. General questions
Q: Where do I ask questions?
A: Join the mailing list and ask away! If your question is not specific to the project, but about EEG, biofeedback or electronics in general, please try a search engine first.
Q: How do I unsubscribe or change my mailing-list options?
A: Go to the mailinglist signup page, scroll down to the subscriber options and enter your e-mail address. You will be presented with a configuration page where you can change your preferences or unsubscribe.
Q: I am reading the message digests. How do I extract any attachments?
A: Here are a couple of suggestions you may want to try:Windows: Try the Files/Decode function in Total Commander (a "swiss army knife" type file utility), or save the encoded data blocks in separate files with a .b64 extension. Then open them in Winzip. Linux: Use Munpack for example. From the man-page: The munpack program reads each RFC-822 message filename and writes all non-text MIME parts or split-uuencoded files as files.
Q: Are there any rules for the mailing-list I should be aware of?
A: Yes, a few:
Q: Has anyone found a source for cheap quality electrodes?
A: People have ordered electrodes from The Electrode Store, specifically those marked WBT-DSC which are 9mm plastic electrodes covered with silver-chloride. You will either have to buy or make your own cables for these.You can also check out Nelo's photos and Joe's photos for examples of home made electrodes and cabling.
Q: I do not have any previous electronics knowledge, what should I do?
A: This is a tricky project. There are quite a few parts and it all costs a fair bit of money.So, if you have never used a soldering iron, and will have to buy tools on top of everything else, try building a few simpler and cheaper electronics projects first, to get a feel for what it is like. I'm sure you can find stores near you that sells electronics kits and tools, through a simple web search. You can also go to the library and read a couple of electronics magazines + look in a few beginner's books ... If you then feel that this might be too hard or take too long because it is beside what you actually want to do (play with your brain), put out a request for help on the mailing list to find out if someone is willing to build one for you, for a fee. Alternatively, it's possible to order them from Olimex (boards only, not a full assembly).
Q: I don't know how to solder.
A: Everyday Practical Electronics, has a Soldering FAQ that may be worth a visit.Their home page is also worth a visit.
Q: Why not use a sound card to capture the EEG? Everyone has one, so it should be easier...
A: It is not as simple as you might expect at first.Human ears are said to be sensitive to frequencies between 20 Hz and 20 kHz. Therefore many (all?) sound cards are made to filter out signals below 20 Hz and above 20 kHz. Since much of the EEG signal is below 20 Hz, a lot of it would be lost if captured with a sound card. You can work your way around this by modulating the signal first. An FM-based design is in the works.
Q: Why is putting both boards in the same metal box a safety issue?
A: It is because you need to let the power and output signals pass through the box wall somewhere. There you have to ensure that the connectors are isolated from the box, or you will end up connecting isolated ground to PC ground, which is a bad thing. Even if the connector housings are made of plastic, you risk lowering the isolation voltage.It is possible to use a metal box, if you create wide enough gaps between the connectors and the metal wall. Problem is, you still end up with an outer surface connected to isolated ground, so it is safer to avoid the subject entirely. It is recommended that you put the amplifier board in a metal box, which is in turn placed in a plastic box along with the digital board. Look here for an example.
Q: Can I tell if a part is a diode or a resistor etc, just by looking at the parts list?
A: Yes. Each type has a different prefix in its part number. IC107 for example, is an IC.The prefixes are: C = capacitor, D = diode or LED, IC = integrated circuit, J = connector, L = inductor, P = potentiometer or trim potentiometer, Q = transistor, R = resistor, X = crystal.
Q: I have a bunch of 10% resistors, can I use them rather than the 1% kind?
A: No, unfortunately you need 1% tolerance in most places. The exceptions are the large resistor on the DRL output, and the resistors near the currently unused digital inputs on the microcontroller.However, you can measure the resistances of your 10% resistors with an ohmmeter, and pick out the ones that actually have 1% or better tolerance.
Q: What does X7R and C0G mean?
A: The capacitance of all ceramic capacitors changes with temperature, voltage, frequency and over time. The marking specifies what performance you can expect.C0G (sometimes marked as NP0) caps are very stable and are manufactured in sizes up to 10nF, while X7R caps are less stable, and manufactured in sizes up to 1uF. A third common type is Z5U, which is very low grade (though cheap), and suitable only for decoupling. There are many other types available, so if you come across one, just ask on the list if they are usable. You can replace X7Rs with C0G caps, since C0G are of higher quality, but you should avoid any other substitutes unless you know for sure that you are getting a higer-quality part.
4. ModularEEG general questions
Q: What do I need to get started on a ModularEEG?
A: Start by reading the documentation on website.Then, go to the SourceForge page and download the ModularEEG project zip-file. You will find schematics, board layouts and parts lists for a bare EEG device inside. Read the parts list and decide where you want to buy the parts from. There are already lists for four distributors, but you can order from somewhere else of course. You will probably have to order parts from two places. Factory made printed circuit boards can be ordered from Olimex. More information on this is found in the ModularEEG zip-file in a file named "How to order PCBs.txt". You will also need to make / buy a few other items that may not be mentioned in the documentation (yet). These are:
Q: What postprocessing do I have to do before making the modEEG pcbs?
A: The modEEG pcbs are double sided with (plated through) vias and make heavy use of "copper pouring".Therefore the EAGLE *.brd files have to be postprocessed before printing in the EAGLE CAD software with the "ratsnest" command. Make sure Menu->options->set->misc->"Ratsnest processes polygones" is checked. If the option is unchecked or no ratsnest is done, you do not get all the important electrical connections that are implicitly done by the large so called "copper pouring" polygons and the plated through vias - that are electrical vertical connections from top to bottom layer. Instead with "ratsnest" you get many thin yellow air-wires that indicate missing connections between nodes (pins, pads) and would have to be soldered manually if you should decide to use no copper pouring (what would be absolutely no good idea!). The openEEG developer teams decision to use double layer and copper pouring is well founded, because it is essential for reduction of RF interference and mains hum capacitively coupled into the circuit. A modEEG built from pcbs without copper pouring after manual fixing all not routed airwires (on both pcbs) will probably have sub optimal electrical characteristics and performance (more noise and hum).
Q: What does it take to build a ModularEEG, in terms of time, money, skill etc.
A: The current estimate is around 200 USD for the electronics. Parts like a box, electrodes, cables and batteries will raise the cost, but by how much depends largely on what you assemble yourself (e.g cables) or buy. One builder has reported 400 euros for a complete system. This cost includes 146 euros for electrodes and gel and 15 euros in shipping costs. (1 euro was roughly equal to 1 USD at the time of writing.)Building takes a few weekends of your time. You need to have some experience with soldering electronics and be a tinkerer in general.
5. ModularEEG electronics questions
Q: There are two types of 10nF capacitors. One is marked X7R and the other 5%. Are they the same? Can I replace the 5% type with X7R?
A: The ceramic capacitors (marked X7R) are more likely to have 10% or 20% tolerance which is not good enough. You could use a ceramic type if you know it is 5%, but all the 5% capacitors are actually supposed to be made of plastic.
Q: There are two types of 47uF tantalum capacitors. One is marked 1 ohm, and the others are unmarked. Can I replace the 1 ohm type and use the unmarked type instead?
A: In theory no. 1ohm means that the ESR (equivalent series resistance) is 1 ohm. The unmarked, "regular" 47uF capacitors, are cheaper but have much higher ESR and in this case, that difference is important.The 1 ohm-ESR capacitor helps reduce "ringing" in the VGND plane, if the current-load on the power supply changes suddenly and rapidly. This is much like when you hit a tuning fork. The impact will cause it to start ringing too. The difference is that the tuning fork is designed to maintain the ringing for as long as possible while on VGND, it dies out in mere milliseconds. Now, this may not make any practical difference in real life, because the VGND plane does not have any big power-sucking circuits that could cause this kind of impact. So if you have problems finding the 1 ohm ESR capacitor, try a regular tantalum.
Q: Is there a different or more common name for pinhead/pinhd connectors? Many electronic component shops do not seem to know that term.
A: Sometimes these are called Berg pins or Molex pins.Showing them a picture might be more helpful:
Q: I am having trouble finding capacitor X, can I replace it with capacitor Y?
A: It depends. Not all capacitors are created equal, and perform differently.The capacitors with 5% value tolerance for example, can not be replaced by their 10% counterpart, because they are all part of a tuned filter. The same goes for most resistors. You can replace some of the tantalum capacitors. You can use aluminium electrolytics for those near the MAX232, if they are rated for 16V or higher. You may also be able to replace all the 47uF capacitors with aluminium electrolytics, but that may make the device more sensitive to noise. You can most likely not replace the 47uF capacitor marked "1 ohm" (Capacitors have some resistance too and sometimes it is important).
Q: Why are there filters on the amplifier boards? Isn't it better to do all signal processing in software?
A: The filters are there to filter out any signals above half the sampling frequency. If we don't do this we risk getting aliasing. (Explanation of the term from Dictionary.com: The static distortion in digital sound caused by a low sampling rate.)Now, to be perfectly honest - the filter in the ModularEEG is not good enough to do this properly with a sample rate of 256 Hz. However, increasing the sampling frequency to 768 Hz would prevent aliasing in the 0 - 50 Hz frequency range where the EEG is. Fortunately, EEG signals are not very strong above 50Hz, so an imperfect filter should suffice. At least, we hope so, because there is no room for more IC's on the boards!