Modular EEG

Initial report by Younghyun Chung, 09, May, 2007

The OpenEEG project is about low cost hardware and software of brainwave detection and analyzation. ModularEEG is a cheap and open schematic hardware project of OpenEEG. However, building the hardware is not easy so this report arranges and condenses the instructions. (EEG: Electro-Encephalo-Gram, that is Electronic-Brain-Record)

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Sources

Printed circuit boards(PCB's) or pre-built boards by Olimex, $162.00

 - Ordering Code: AR180903A/ASM - Set of analog and digital assembled boards)
 - Process: 
  1. Send request email. (they have email filter for yahoo, hotmail, gmail, etc. use @nyu.edu accounts or others)
  2. Receive PO form
  3. Fax the PO form

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Applications

1. Clinical use - monitoring and diagnose

 epilepsy, syncope(fainting), sleep disorder, eating disorder, coma, brain death,

2. Research use

 Theories to explain sleep

3. Neurofeedback - Brain Computer Interface

 - psybridge
 - Mindball EEG Game
 - Some article about projects of neuroksy.com 

4. Assistive Technology

 - Brain-fed switch
 - By pass physical disabilities with nerve-based signals

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Characteristics of Brainwaves

• Major types

 - Delta (~ 4 Hz): very young, brain disease, stage 3 and 4 sleep
 - Theta (4 ~ 8 Hz): drowsiness, childhood
 - Alpha (8 ~ 12 Hz): relaxed
 - Beta (12 ~ 25 Hz): active, busy, anxious
 - Gamma (26 – 100 Hz): higher mental activity - perception, problem solving, fear, consciousness

• Signal contains noise
• 1-2 mV on the surface of brain, 100 µV on the scalp

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Electrical Characteristics

- Modular Design Document - 2.2 Technical specifications

Example EEG graph (courtesy of Nelo).

- General specification

Number of channels	2 - 6
Resolution	four 10 bits, two 8 bits
Input Voltage Resolution	0.5 uV
Input Voltage Full Scale	+/-256 uV
Wideband noise	~ 1 uVp-p
Supply Current	(5V or 9 - 12V supply) 70 mA (2 channels)
Isolation voltage	2500V (1 minute)
Continuous isolation voltage	480V

- Supply Current

Digital board	52 mA
Digital board + serial cable connected	56 mA
Total current (including 2 channel amp)	73 mA
Estimated total for 4 channel setup	77 mA
DC/DC converter efficiency	~78% ( =>15mA, 2 ch)

- EEG Amplifier specification

Gain	7812.5 (nominally)
Offset handling capability	TBD > 200 mVdc
Low frequency CMRR	TBD > 100 dB (?)
Highpass filter	2nd order, fc = 0.4Hz
Extra features	250 uV(+/-10%) test signal.
.	DRL circuit - adds > 40dB CMRR at 50/60Hz.

- AD-converter specification (AT90S4433)

Number of channels	6
Resolution	10 bits
Effective number of bits	9.1 bits (< 100Hz)
Signal to Noise Ratio	58.7dB (max possible 60dB)
Integral Nonlinearity	+/- 0.5 LSB (typ)
Differential Nonlinearity	+/- 0.5 LSB (typ)
Offset Error	1 LSB (typ)
Absolute Accuracy (ADC clock = 200kHz)	1 LSB (typ), 2 LSB max
Gain error (Voltage reference error)	1% (typ), 2.25% max

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Component of hardware

Amplifier

1. Electrode

 - 2 electrode for 1 signal
 - To avoid drowning in noise

2. Protection Circuit

 - Protects circuit from electrostatic discharge(ESD)
 - Protects user from failing circuitary

3. Instrumentation amplifier

 - Measures V-diff btw two location on the scalp
 - Common-mode signal from inputs = ( (+)input + (-)input ) /2

4. Amplifier + high pass filter

 - Amplify about 40 times
 - Removes DC-voltage offsets: polarizable electrode can accumulate electric charge

5. DRL circuit (Driven Right Leg)

 - Reduce common-mode noise by canceling it out
 - Attenuates mains hum up to 100 times

6. Low pass filter

 - Prevents aliasing effects when signal is digitezed

Microcontroller

 - Electrically isolated from PC and external power

Electrode

1. Detects electrical current

2. Place them on your head

 - Passive Electrode
  = Just wire and electrode
  = Need skin preparation conductive gel 
 - Active Electrode
  = Built-in circuitry which amplifies the electrical current
  = Improves signal quality and No skin preparation

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Microcontroller Connections

Preparing parts

(Serial cable for connecting to a PC, 9V battery holder, RCA sockets for connecting the electrodes, respectively)

(above parts and DRL(Driven Right Leg; Reduce common-mode noise by canceling it out, Attenuates mains hum up to 100 times), etc)

Give a list of the pins, and a pin diagram as appropriate. Detail the function of each pin in a short paragraph following the list.

Combining

 - plastic housing for the whole unit
 - metal housing for the analog board
 - Connect two boards with 34-pin IDC cable (PCB-to-PCB cable)

 - serial cable socket for connecting the cabel to the unit
 - RCA socket for connecting the electrodes
 - socket for connecting DRL

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Code Sample

Signal Testing using Hyperterminal

 - check the USB port first
 - setting: 57600 baud, 8 data bits, No parity, 1 stop bit, Flow control off

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Typical Behavior

Test result using BioExplorer - more software available in this page

 - left: nervous status. Beta wave(12~25 Hz, reddish) is dominant. 
 - right: after some meditation. Since not sleeping, beta wave is still high, but Alpha wave(8~12 Hz, blueish) is growing

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Application Notes

- psybridge: An adventure game that is played while sleeping by using the player's brainwaves as input. (on progress)

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Acknowledgments

Some pictures are from OpenEEG website.