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)
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
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
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
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 |
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
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
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
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
Application Notes
- psybridge: An adventure game that is played while sleeping by using the player's brainwaves as input. (on progress)
Acknowledgments
Some pictures are from OpenEEG website.