Pulse Necklace 15Nov2009

Hack Notes, pulse choker, Nov 15th, 2009

TENS Pad Electrodes

We tested out the new TENS pads today. We didn't buy lead connectors, which were expensive (e.g. $10 for a pair), so we ended up using the leads from the multimeter and attached those by alligators to our Arduino + Open ECG project chip. The readings were quite comparable to that of the red dot, though slightly noisy. The noise might just be due to our code being optimized for use of red dot electrodes. Further work could be done.

We also experimented with removing the connectors on the pads. We found strands of white and black thread inside. Only the black threads conducted though. When we connected the cut TENS pads with exposed threads to our circuit, the LEDs flashed randomly. Hence, we'll stick with the red dots for now.

Op-Amp Experiment

Chung-Hay bought a couple AD 620 op-amp chips to experiment with bypassing the Open ECG chip at filtering/amplifying the bio-electric signal. We worked off of the lab document BE 513 Lab, in addition to the AD 620 data sheet. The sample circuit for detecting ECG in the AD 620 data sheet included two other op-amp components. We have yet to figure out what the op-amp circuit attached to the gain-control resistor does... Chung-Hay is going to seek EE assistance on that front.

To look at the output signal from AD 620, we resorted to a virtual oscilloscope - link and code are in the next section. Unfortunately, we saw only noise from the oscilloscope. We tried switching the gain-control resistor from 98 ohms (two 47 ohm resistors) to 1 kohm. That only lowered the amplitude. We were using red dot electrodes, so the ECG signal was entering the chip reliably. More work to be done on this front.

Oscilloscope Code

We got great code for a virtual Arduino oscilloscope from Poorman's oscilloscope and just changed the serial port name to correspond to either Eric's or mine cable. Below is the Arduino code:

// The Arduino code.

#define ANALOG_IN 2

void setup() {
  Serial.begin(9600); 
}

void loop() {
  int val = analogRead(ANALOG_IN);
  Serial.print( 0xff, BYTE);
  Serial.print( (val >> 8) & 0xff, BYTE);
  Serial.print( val & 0xff, BYTE);
}

And here is the Processing code doing all the computation:

/*
 * Oscilloscope
 * Gives a visual rendering of analog pin 0 in realtime.
 * 
 * This project is part of Accrochages
 * See http://accrochages.drone.ws
 * 
 * (c) 2008 Sofian Audry (info@sofianaudry.com)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */ 
import processing.serial.*;

Serial port;  // Create object from Serial class
int val;      // Data received from the serial port
int[] values;
// Change this to the portname your Arduino board: 
// Have USB cable connected. Open Arduino app. 
// Go to tools -> Serial Port -> Find /dev/tty.usbserial-<blah> option
//String portname = "/dev/tty.usbserial-A9007Llr"; // Chung-Hay's
String portname = "/dev/cu.usbserial-A8007U5u"; //Eric's

void setup() 
{
  size(640, 480);
  // Open the port that the board is connected to and use the same speed (9600 bps)
  //port = new Serial(this, Serial.list()[0], 9600);
  port = new Serial(this, portname, 9600);
  values = new int[width];
  smooth();
}

int getY(int val) {
  return (int)(val / 1023.0f * height) - 1;
}

void draw()
{
  while (port.available() >= 3) {
    if (port.read() == 0xff) {
      val = (port.read() << 8) | (port.read());
    }
  }
  for (int i=0; i<width-1; i++)
    values[i] = values[i+1];
  values[width-1] = val;
  background(0);
  stroke(255);
  for (int x=1; x<width; x++) {
    line(width-x,   height-1-getY(values[x-1]), 
         width-1-x, height-1-getY(values[x]));
  }
}