modularEEG build tips

Table of Contents

• Miscellaneous (has subsections)
• Parts Sourcing

Miscellaneous

modularEEG build tips 
---------------------

NOTE: THIS FILE WAS LAST UPDATED IN MAY-2003
(In April 2005 it was sorted into categories, but otherwise left untouched)

Introduction
------------

This is a collection of bits of information, build tips and so on,
that have appeared on the openEEG mailing list regarding the
modularEEG.  Eventually these will be formatted up into a proper build
document.  They are in latest-first order, which means the most
accurate information should be nearest the top!

Authors are indicated with initials:

AR == Andreas Robinson
JH == Joerg Hansmann
... == Summary of points made in discussion

Miscellaneous
-------------

[AR] (Circuit reasoning)

  > - There seems to be lots of capacitors in parallel in
  >   the power circuit (and also elsewhere). Examples are
  >   (C122, C124), (C117, C118, C119) or (C111, C112, C113).
  >   Why is this? Even a 5% tolerance on 47uF (C118) - 2350 nF -
  >   is way more than 100nF (C117) or 10 nF (C119)
  
  There are multiple capacitors because each capacitor covers a part of the
  spectrum. A 47uF capacitor is only effective with low frequencies up to a
  few hundred kHz, a 100nF cap handles frequencies up to 10MHz or so
  (guesstimate) while the 10nF capacitor should be good up to maybe 100MHz.
  That's the theory anyway.


[JH+AR] (circuit reasoning)

  > can anyone tell me what all all the resistors, capacitors and
  > diodes (R102, R105, C101, C108, IC101 etc.) are doing beside the
  > AT90S4433 chip?  what is their function?
  
  Generating a stable 4V reference (TL431) for the ADC in the AT90S4433.
  Generating 2V for the virtual ground in the analog section.
  Generating a lowpass filtered +5V on pin 20 (AVCC)

  The discrete parts to the left of the microcontroller are used for
  several things, but most of them are dedicated to creating a 4.0V DC
  signal for the AD-converter.


[JH] (re DCDC converter and isolation)

  Summarized I wanted to say:

  1) Bypassing the DCDC converter is not a good idea.

  2) A modularEEG with bypassed DCDC should never be operated on a PC
    connected to mains.

  3) Operating a modularEEG with bypassed DCDC with a laptop is much
    safer than with a PC connected to mains, but it is still not a
    good idea (regarding user safety)

  4) Using the recommended DCDC converter type is the best solution.


[...] 
 
  People building version 0.04 who plan to supply regulated 5V directly
  to the digital board, bypassing the regulator (e.g. using power from
  a PS/2 port or USB port), might wish to add the crowbar diode that
  Andreas added to version 0.05.
 
[JH] regarding software available for version 0.04

  Electric Guru is compatible with the modularEEG.
  
    http://www.realization.org/page/topics/electric_guru.htm

[JH] (battery power vs. plugpack)

  > Maybe the emphasis on battery power should be added to the openeeg
  > warning?

  The openEEG design uses 2 optocouplers and one DCDC converter for
  isolation. In this configuration battery operation would not provide
  much advantage. You can as well use the +5V from the PCs gameport,
  keyboard or USB port.
  
  BTW: When using power from the PC, a ca. 500mA fuse should be used
  near to the PC so that in case of a shortcircuit on the openEEG pcb
  the PC powersupply will be protected.

  (The recommended fuse rating was later revised to 250mA)

[JH] (overview)

  Each pcb has the dimensions 100mm x 80mm.
  
  The minimum configuration consists of 2 pcbs (1 amplifier + 1
  digital board).  If layed side by side these 2 pcbs together would
  need 100mm x 160mm.
  
  The maximum configuration could be 3 amplifier boards + 1 digital,
  perhaps later + some additional pcbs for measuring electrode
  impedance.

  > Out side of the obvious cost issue, are there any advantages to a
  > metal vs.  Plastic box?  shielding, grounding, static
  > discharge...Input please.

  I would prefer plastic for safety reasons. However an inner shield
  made of copper foil might be necessary.

FIRMWARE SECTION
----------------

[JH] (firmware)

  > The fix: I had to flash it again. I used the same rom version
  > (v.03) as before so something weird was up with it.
  
  If I remember right there was a faulty firmware version that could
  not read the ADC correctly. So better use the current firmware in
  the V0.06 zip file.

[AR]

  Plans for the programming cable are included in the modular eeg
  zip. All the software tools needed for programming are
  free. (www.avrfreaks.com has tons of information on where to get
  them). A good starting point is of course the current firmware...


SOLDERING TIPS
--------------

[AR] (soldering tips)

  People are bound to solder in parts in the wrong places (I do. Every
  time). Removing them and putting in new parts is *not* trivial with
  factory-made PCBs.
 
  So if you happen to do something like that, I'll give you a couple of
  suggestions and then you can take a few pictures when doing it.
 
  For example: if you want to remove the DCDC converter once in place,
  you must probably use a hot-air gun, with a nozzle just big enough to
  only heat the places you want hot.

[AR] (soldering tips)

  The green circles without tracks connecting to them are "ground
  vias". They don't have thermal-reliefs which makes it hard to
  desolder wires from them with a normal (wimpy) soldering
  iron. That's why you should solder in socket strips while you are
  assembling and testing, so that you do not need to solder the wires
  in right away.

// END Hardware Build Tips //

    

Parts Sourcing

modularEEG build tips 
---------------------

NOTE: THIS FILE WAS LAST UPDATED IN MAY-2003
(In April 2005 it was extracted from the build tips, but otherwise left untouched)

Introduction
------------

This is a collection of bits of information, build tips and so on,
that have appeared on the openEEG mailing list regarding the
modularEEG.  Eventually these will be formatted up into a proper build
document.  They are in latest-first order, which means the most
accurate information should be nearest the top!

Authors are indicated with initials:

AR == Andreas Robinson
DG == Dan Griffiths
... == Summary of points made in discussion

PART SOURCING
-------------

[DG] (NZ low-ESR supplier; see Newark for US)

  I think I have found a supplier for the elusive low ESR capacitor.

  47uF 35V 0.8 Ohm ESR @ 100kHz
  
  www.farnell.com
  Order code: 966990
  Price: NZ $9.04
  
  Manufacturer: AVX
  Manufacturer list number: MDT476M035
  
  http://www.farnell.com/datasheets/337.pdf

[DG] (New Zealand ordering)

  I have chosen to source the remaining parts from Dick Smith
  Electronics (http://www.dse.co.nz) and RS Components
  (http://www.rs-components.co.nz).  Dick Smith are cheaper than RS
  because they do not impose bulk minimums on parts.

[...]

  Ordering a 28-pin socket for the Atmel chip is not a good idea,
  because it is too wide.  Order two 14-pin sockets instead.

[AR]

  These two suppliers have most parts, if you order from both:
 
    www.reichelt.de
    www.rs-components.com
 
  This supplier has most parts, but shipping is expensive for people
  outside North America:
 
    www.digikey.com

{out of mailing list order}
[DG] (on which components to use)

  I am using the NMV0505SA from rs-components (NZ$47.89, RS stock
  no. 1920826) without any problems. From memory I measured 5.6 volts
  on pin 8 of IC103. So I did not have to add extra resistance to the
  circuit.
  
  You can find a 47uF tantalum 0.8 ohm ESR at www.farnell.com
  (NZ$9.04, order code: 966990).
  

{out of mailing list order}
[AR] (re using v0.06 parts-list for earlier board designs)

  * Some parts from Newark did not fit the v0.04 / v0.05 PCBs so the
  layout was modified for v0.06.  If you have a v0.05 board or
  earlier, the 1uF (both kinds), 47uF (both kinds) and 10uF capacitors
  from Newark may not fit.
  
  Addendum: the 1uF film capacitor will not fit on a 0.04 or 0.05
  boards without some pin bending - the lead pitch is just 5mm. (10mm
  still works fine)
  
  So, in these cases you either try to squeeze them in (no guarantees
  given), or find parts elsewhere. (Digikey is a good bet, though they
  are more expensive.)

// END Hardware Build Tips //