Highlights

Prepare for Presentation

By : Noctis King

Presentation: Arduino Program Part

    In this week, we prepared for the presentation and I would introduction the Arduino program. This part would talk about the logic and idea how the program calculated the heart rate.


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UPDATE: Week 9

By : x

This week we presented our final design to our advisory. Our product is ready and we are getting desired results. We have found a small box that can house our circuit. From the  circuit we made last week we swamped out the 3.5mm female audio port with a much smaller one that now fits on the board.

About the Moving Average

By : Noctis King

About the Moving Average

      In our new program, it used the moving average theory to calculate the heart rate. So, I will introduce what is moving average theory and how it use in the program. 
      The moving average theory is a calculation to analyze, count data by creating a series of averages value of different subsets form the whole data. 
      At first, people would set a number of subsets, this number is always same.Use this group of subsets.to calculate the first element of the moving average. Then, the subset would shifting forward but the number of group of the subsets do not change. Use the new group of subsets to get the second element of the moving average. This process is repeated over the entire data series.
      In the program, we use the moving average to calculate the heart rate. We only have three subsets and every time we only gay two peaks and use this two peas to get the heart rate. However, we get the subsets every frequently, it can ensure the accuracy of the heart rate. 




UPDATE: Week 8

By : x
This week we rushed to get the device to its final stage. We worked on making the circuit smaller and fitting all the components tighter. We have now incorporated Arduino Nano and the sensor into one die of a circuit board. We have fitted all the components together and made it as compact as possible. The power issues we were have with Bluetooth before has also been solved.
The circuit is now very stable with the 9V DC battery power source. We have also incorporated a 3.5 mm audio cable for  plug and play capability.
On the software side, we are transferring the Processing code to Arduino code so that the heart rate calculation is done on the Arduino and doesn't require a computer. The Arduino will then broadcast the result via Bluetooth.
For the Android Application, it has been setup to receive Heart rate in Beats per Minute from Arduino via Bluetooth.
The Project is almost complete. We will be looking into the aesthetics of the design next.

Hardware update

By : Lam Ho Wang
Hardware update:  From the breadboard to soldered circuit board
The group integrated the circuit with the arduino board and bluetooth by soldering.
The new circuit has a 9V power supply and the 9V is split to two, 6V for the bluetooth and 9 V for the arduino nano3.0
The new circuit gives a much more stable signal than the old one. The reason may be the 9V supply gives more stable voltage than USB does.




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ECG:Coding

By : Noctis King
A new version of 

collection heart rate program

      In this week, we updated and changed the collection and graphing program. this new program could graph and calculated the heart rate more accurately. The reason why calculated more accurately than last one because this program used the "moving average" theory to calculate the heart rate. 
      The last one just use a some if{} sentences to compare, count and find the gaps between one peak and another. So it would make the heart rate very unstable and disordered. 
      However, this new program was used the "moving average" theory to calculate and count to find out the correct time between two peaks. It not only find one but also find two gaps. By using two data to calculate the heart rate would more accurately than only use one.
      The new program would collect enough sample data first and calculated the biggest range from those data. Then, the program would scale those data and made than easy to calculate and compared.
      After scaling the data, the program would begin to filter the data to ready for computing the heart rate. 
      Computing the heart rate was the most important part in the program. The program would find the peak and minima value from the data which after filter again. Then, it would begin to find three successive peaks. After a series judge, save three successive peaks, then, used the "moving average" theory to calculate the heart rate. 
      The program would still looping the calculate the heart rate until the user close it.
    



UPDATE: Week 7

By : x
This week we are completely focused on a Android application development. We have successfully connected the Bluetooth to the Arduino board and tested the communication. All the Bluetooth connection is working well and the data was successfully received. But, we ran into a problem immediately when testing it with the heart beat sensor. When ever we connected the Bluetooth module to the circuit we noticed unwanted signal outputted by the circuit. As shown in the image below this signal was consistent and regular when the sensor was idle.

We were suggested that the problem might have been because of bad ground in the circuit, but that didn't solve our problem either. We have currently decided to use separate power supply for the Bluetooth module and the Arduino board.

How does pulse sensor works ??

By : Lam Ho Wang
How does the IR pulse sensor works ??

Principle of photoplethysmogram (PPG) Measurement !!

          The pulse sensor is made using an infra-red emitter and detector. Light emitted by an infra-red emitter enters the skin and is diffusely scattered. The peak wavelength emitted by the emitter is typically in the range on 900nm < λ < 960nm. The detector placed on the other end captures a part of this diffused light. As light propagates through the tissues, a fraction of its intensity is absorbed by pulsating blood. Hence, when blood volume increases during the cardiac systole the captured intensity decreases creating a pulse-like photoplethysmogram (PPG) signal which reflects the blood volume changes in the arterioles.  Furthermore, light absorptivity of haemoglobin (Hb) is significantly greater than that of oxyhemoglobin (HbO2) at 660nm. As arterial blood have high concentration of HbO2 and contrastingly, venous blood have high concentration of Hb, the sensor is able to detect the variation in both the systole and diastole stages.

UPDATE: Week 6

By : x
This we we worked on filtering the signal form the heart beat sensor. The signal received from the sensor went through hardware filtering and now we want to accurately count the beats per minute. We started by trying to detect the peaks on the graph. On Processing we started by coding an array that received 200 data points. As each data point is spaced by 15 millisecond, we got readings from the sensor for 3 seconds. We determined the high and lows on this signal and removed everything that is above or below our range. This range were determined to be greater that 800 or less than 500. Anything that falls in this range is an error caused by moving the sensor and not the signal of heart beat as we wanted. We are getting close to accurate reading and the reading doesn't go too low and too high.
We also tested with a sensor form http://pulsesensor.com/. This is the sensor that works on the same principle as the designed sensor but is smaller in size as all the circuit is printed onto its back.
Finally we are working on the Final Lab Report draft. We are diving the work among each other and will combine and form the report.

Tutorial: Making the ECG

By : x
This is our custom built heart beat monitor. It uses a Infra-Red(IR) emitter and detector to detect heart beat. The concept is to detect a change in Infra Red detected by the IR detector with each pulse. You can see the two led at the left corner of the circuit board. The darker one is the emitter and the transparent one is the detector. Both placed in a foam to minimize the ambient infra red detection.
The electrical signal needs to be amplified so we are using Op-Amp 741 and then its output is filtered using three stage filters to minimize the noise. This signal is then fed into a single analog input in the Arduino.

The Arduino board is programmed to read the analog data and perform a serial output every 15ms. A Processing code is written on a PC to receive this data. The Prrocessing code contains a serial listener which get the data sent by the Arduino through the serial port. This data is then plotted on to the screen. For plotting a black screen is drawn and a line is made connecting adjacent data point as they come to give a running graph.

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ECG: Circuit

By : Lam Ho Wang

ECG: Circuit


Required Components:
  • Infrared emitter and detector
  • 1 x 15ohm, 1 x 10kohm, 2x 100ohm, 1x 1kihm resistors
  • 1 x 1uF, 2x 100uF, 1x 10uF capacitor
  • 1 x 741 Op-Amp

The Design has 4 sections, the IR emitter, IR detector, OPAMP, and fitter

IR Emitter  

The group used a 15ohm resistor, to obtain a bright IR source without burning the emitter.

IR detector 

The group use a IR detector to detector the IR reflect from the user body, with some tests the group found that fingers will give the best signal. 
The group used a 10 kohm resistor, to prevent burning the IR detector
The group has tried not using any resistor and 15 ohm resistor, which end up burning the IR detector.

OPAPM

The signal received from the IR detector is too low for observe and calculation, so an amplify is needed for a readable signal. 
The group used LM741 signal amplify from the ECE lab

Filtering 

The signal received after amplify has a lot of noise that makes the data inaccurate, so a RC filter is designed to reduce the noise after amplify. 
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ECG: Coding

By : Noctis King

The program of collection and graphing data

    This program ran on the Arduino and used to collect and graphing the heart rate data and calculated the BPM (heart rate/minute).
    There were three parts of this program: the collecting part, graphing part and calculating part. The collecting part was the most important part in these parts, because the data which collected from this part was necessary from next two part.
    The program would use the ECG which connected on the Arduino to detect the heart beat from the user. The user can set the different port if they wanted to change the port of ECG;
    After the program got the real-time data, it would start the graphing and calculating.
    In the graphing part, the program would group the heart beat on a new window.
    In the calculating part, the program would find out the data of changing and judging them weather a heart beat and counted the number of heart beat to calculated the BPM.

import processing.serial.*;

Serial myPort; 
int val, screen_increment, old_x=0, old_y=0;  
String inString;  
int lf = 10; 
double bpm,t,lbpm;
int lval=510;
void setup() 
{

  size(displayWidth-100, 1000);  //set the size of window
  
  String portName = Serial.list()[0]; //the port of input
  println(Serial.list());
  
  myPort = new Serial(this, portName, 115200);
  myPort.bufferUntil(lf);
  background(0,0,0);//the color of background of window
}

void draw()
{

}

void serialEvent(Serial myPort) { 

  inString = myPort.readString();
  inString = trim(inString);
  val = int(inString);//get the value of rate
  strokeWeight(5);
  stroke(255, 255, 255);
  println(val);
  
  line(old_x, old_y, screen_increment, 1000-val); //graph the hate rate
  
  old_x = screen_increment;
  old_y = 1000-val;
  
  screen_increment=screen_increment+2;
  if(screen_increment>(displayWidth-100))
  {
    background(0,0,0); 
    screen_increment=-50; 
  old_x = -50;
  old_y = 0;
  }
  if(val-lval>7) //calculate the BPM
  {
    t=t+1;
    lval=val;
  }
  if(val<lval)
  {
    lval=val;
    t=t+1;
  }
  if(val-lval>5)
  {
    bpm=60/(0.015*t);
    t=0;
  }
  
  if (bpm<130 && lbpm-bpm<40)
  {
  text(bpm+ "                                      BPM",600,200);
  //println(bpm);
  lbpm=bpm;
  }
}



UPDATE- Week 5

By : x

We are currently working on connecting all the parts of the ECG to breadboard. The circuit has been fully connected and we are seeing good results with signals. The custom designed ECG needed a lot of fine tuning with Op-Amp amplification and filtering techniques. This website http://www.kevindarrah.com/ was a big help. We also looked into other algorithms and one by http://pulsesensor.com/ works well. Combining these we were able get stable signals. Processing 2 was used to program a serial port listener which recieved data from the designed sensor and displayed a moving graph.

ACTIVITIES:

  • Click here to view building your own ECG tutorial by Ashish
  • Click here to view the circuit diagram and explanation by Marco
  • Click here to view the algorithm and code for Arduino and Processing by Nero


NEXT WEEK:
  • More work on signal processing
  • Work on Bluetooth connection
  • Work on Android Programming


Inventory:

By : Lam Ho Wang
Inventory:



Siliconix / Vishay TCRT1000 PHOTOINTERRUPTER REFLECTIVE PHOTOTRANSISTOR 4-PIN BULK

Price:$10.45



20 pcs Photo Light Sensitive Resistor Photoresistor Optoresistor 5mm GL5516 5516

Price: $7.87



pulsesensor Arduino Pulse Heart Rate Sensor - Black + White

Price: $35.24+$10 Shiping



Arduino Wireless Bluetooth Transceiver Module Slave 4Pin Serial + DuPont Cable

Price: $10.98

http://www.amazon.com/gp/product/B0093XAV4U/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1

20pcs 200mW 10mm Infrared IR 1-Chip LED Night Vision 850nm

Price: $6.99



Total Price:$10.45+$7.87+$35.24+$10 $10.98 +$6.99=81.53

Operational Amplifier

By : Lam Ho Wang
 Operational Amplifier 

What is operational amplifier ?
    An operational amplifier (or an op-amp) is an integrated circuit (IC) that operates as a voltage amplifier. An op-amp has a differential input. That is, it has two inputs of opposite polarity. An op-amp has a single output and a very high gain, which means that the output signal is much higher than input signal. An op-amp is often represented in a circuit diagram with the following symbol:

These amplifiers are called "operation" amplifiers because they were initially designed as an effective device for performing arithmetic operations in an analog circuit. The op-amp has many other applications in signal processing, measurement, and instrumentation.



Feed back

   An ideal opamp has infinite gain. It amplifies the difference in voltage between the + and - pins. Of course in reality this gain is not infinite, but still quite large.

Why feed back is needed ?

  The output of the opamp (at to some extents the input also) is constrained by the power supply, we can't get out more than the supply puts in.If we simply put signals into the opamp without feedback it would multiply them by infinity and get a binary output (it would saturate at the supply rails)So, we need some way of controlling the gain. That is what the feedback does.

Pros:
  1. Can increase or decrease input impedance (depending on type of feedback)
  2. Can increase or decrease output impedance (depending on type of feedback)
  3. Reduces distortion (increases linearity)
  4. Increases the bandwidth
  5. Desensitizes gain to component variations
  6. Can control step response of amplifier
Cons:
  1. May lead to instability if not designed carefully
  2. Amplifier gain decreases


The group use OP amp to increase the signal from the photo diode and connect to the arduino analog




LM741 The opamp that the group is using











Android Development

By : x
For our mobile application we have chosen Android as our development pattern. Android has seen major growth over the past few years and is now the most widely used mobile platform of the world.

Worldwide smartphone sales to end users by operating system in 2013
Android
  
79.0%
iOS
  
14.2%
Windows Phone
  
3.3%
BlackBerry
  
2.7%
Other
  
0.9%
Figure: Mobile OS Market Share as of 2nd quarter 2013 Gartner

To begin development on Android familiarity with Java is required. Many of the developer tools can be found in the official Android Developer Website - developer.android.com

To begin development start by downloading the official Android Software Development Kit (SDK) for the developer website. To write apps you can use any development environment of choice, for this project we will be using the official Android Studio provided on the developer website.

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UPDATE - Week 4

By : x

This is the final planned week for hardware development. We are still working on getting the hardware ready. We have collected most of the parts required for assembling the hardware. We are trying to get all the required components together and test our self made ECG. We have also ordered another ECG as a backup if our ECG doesn't function properly.
Marco is researching on ECG, op-amps and sensor. Ashish is working on Android application development and Xingyu is looking into Arduino Programming and algorithm development.


  • Click here to view the getting started with Android Development by Ashish
  • Click here to learn about op-amps by Marco


Concept Design: ECG

By : Lam Ho Wang
The group is planning to make our own ECG by using, an inferred LED, a photodiode, and OP amp. The sensor is based on the DIY pulse sensor posted on picture 2

Figure: ECG and Arduino Concept diagram
Figure: Circuit Diagram for the proposed ECG

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UPDATE - Week 3

By : x
The group looked into methods of making a ECG sensor. A simple sensor could be setup using IR and light sensitive resistor. IR sensor would be mounted on one lip and light sensitive resistor on the other lip of a clip.
More Arduino programming was done this week. The group worked with LEDs and accelerometer. The acceleromter and LEDs were connected to the Arduino board. There were 3 LEDs and each LED corresponded to the X, Y and Z direction of the accelerometer movement. The associated LED blinked when substantive change was detected on its corresponding direction.
  • To read the tutorial of how to connect the LEDs to the Arduino click here
  • To view the Arduino program click here
  • To look through our ECG concept click here


LEDs and Accelerometer

By : x
Arduino Tutorial:
This tutorial will show you how to connect LEDs into Arduino Uno, and how to extract data from accelerometer and make the LEDs react to the change.

First you need to  connect the LEDs to the Arduino board. We will be using three LEDs and bread board to setup the LEDS. You may use any port; here we use port 13, 10 and 4 for X, Y and Z direction associated LEDs respectively. Watch the video if you need help setting up the LEDs.


Now follow the tutorial by Xingyu Huang on how to program using the Arduino development environment to receive data from acclerometer and design an algorithm which effectively translate this change to LED output.

Here is how your hardware would react if everything is setup correctly.

Program: Accelerometer

By : Noctis King

The program of LEDs and Accelerator

    In this week, our group made a monitor by using LEDs to react the change of accelerator.
    Now, we will show and explain how this program work.


const int groundpin = 18;             // analog input pin 4 -- ground
const int powerpin = 19;              // analog input pin 5 -- voltage
const int xpin = A0;                  // x-axis of the accelerometer
const int ypin = A1;                  // y-axis
const int zpin = A2;                  // z-axis (only on 3-axis models)
int ledx=13;                          // the pin of x-axis LED
int ledy=10;                          // the pin of y-axis LED
int ledz=4;                           // the pin of z-axis LED

void setup()
{
  // initialize the serial communications:
  Serial.begin(9600); // set the data rate in bits per second
  
  //initialize the digital pin as an output.
  digitalWrite(groundpin, LOW); 
  digitalWrite(powerpin, HIGH); 
  pinMode(ledx, OUTPUT); 
  pinMode(ledy,OUTPUT); 
  pinMode(ledz,OUTPUT); 
}

void loop()
{
  // set the initial value of accelerator
  int ax = analogRead(xpin); // the initial value of x-axis
  int ay = analogRead(ypin); // the initial value of y-axis
  int az = analogRead(zpin); // the initial value of z-axis
  
  while (true) // loop, tracking and reacting the change 
  {
    int Tax = analogRead(xpin); // reading the x-axis value after changing
    // determining the changing margin of x-axis' value 
    // if value change bigger than 50, the LED will blink
    // if not, it will closs
    if (abs(Tax - ax) > 50)  
    {
      digitalWrite(ledx, HIGH); 
    }
    else
    {
      digitalWrite(ledx, LOW);
    }
    int Tay = analogRead(ypin);
    if (abs(Tay - ay) > 50)
    {
      digitalWrite(ledy, HIGH);
    }
    else
    {
      digitalWrite(ledy, LOW);
    }
    int Taz = analogRead(zpin);
    if (abs(Taz - az) > 50)
    {
      digitalWrite(ledz, HIGH);
    }
    else
    {
      digitalWrite(ledz, LOW);
    }
  }
  
  
  // delay before next reading:
  delay(100);
}

UPDATE - Week 2

By : x
This week activities mostly involved researching and experimenting with the Arduino Uno board. The team also discussed on the type of ECG monitor to be used. We stumbled on the possibility of creating our own ECG monitor. We also looked for other components that would be required for making the hardware - the bluetooth shield and battery.

The following activities were conducted to get familiar with Arduino:

  1. Researching on Arudino -->(Basic Insight by Ho Wang Lam)
  2. Setting up the development environment --> (Tutorial by Ashish Shrestha)
  3. Writing up some fun script --> (SOS signal by Xingyu Huang)

Arduino !!

By : Lam Ho Wang
Arduino official website:
“ARDUINO IS AN OPEN-SOURCE ELECTRONICS PROTOTYPING PLATFORM BASED ON FLEXIBLE, EASY-TO-USE HARDWARE AND SOFTWARE. IT'S INTENDED FOR ARTISTS, DESIGNERS, HOBBYISTS AND ANYONE INTERESTED IN CREATING INTERACTIVE OBJECTS OR ENVIRONMENTS.“

Feature:
1 It is a circuit design platform base on CC open source
2 The software is free to download, and open to everyone
3 Arduino allow to use ICSP to load bootloader on the IC chip
4 Provide USB hub and DC 5V
5 Support a lot of program such as: Adobe Flash, Max/MSP, VVVV, Pure Data, C, ect….

Why Arduino?
Arduino simplify a lot of process of working with microcontroller, the advantage of it are:
1.       Inexpensive – Arduino is relatively inexpensive compared to other circuit. The cost can be lower than 50$
2.       Cross platform – Arduino support Window, Mac OSX and Linux
3.       Extensible software

Extensible hardware-Arduino is based on Amel’s ATMEGA8 and ATMEGA168 microcontrollers. 

Making a SOS signal

By : Noctis King

Making a SOS signal

     The Arduino program is basic on C++, so it is to learn and program. I make a SOS signal program by using the LED which on the Arduino board. 



int led = 13; //set the LED connected to pin 13

void setup() {                
  pinMode(led, OUTPUT); //set the pin as output     
}

//The signal of SOS is  three short, three long, 
//and three more short flashes of light, so that is
//the LED action. Using loop to loop this process.
void loop() {
  digitalWrite(led, HIGH);   
  delay(1000);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(1000);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(1000);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(3000);  
  digitalWrite(led, LOW);   
  delay(1000); 
  digitalWrite(led, HIGH);   
  delay(3000);  
  digitalWrite(led, LOW);   
  delay(1000); 
  digitalWrite(led, HIGH);   
  delay(3000);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(500);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(500);  
  digitalWrite(led, LOW);   
  delay(1000);
  digitalWrite(led, HIGH);   
  delay(500);  
  digitalWrite(led, LOW);   
  delay(1000);  
}

Getting Started with Arduino

By : x
This tutorial is a beginners guide to setting up Arduino development environment. We will be using Arduino Uno for this project. To follow the official getting started with Arduino tutorial click here.

  1. Getting the hardware: You can obtain the Arduino board from the official store or through Amazon, and also other vendors.
  2. Getting the software: Download the required software (development environment) to program the Arduino board. The supported operating system as of now are Windows, Linux and MacOS.
  3. Connecting to Arduino: After installing the software,  
    1. Connect the Arduino board to the computer with the provided USB cable.
    2. Now go to Tools>Board and select Arduino Uno.
    3. In the Tools>Serial Port select the associated COM port.(You can find this by unplugging the USB cable. The COM port that disappeared is the one.)
  4. Uploading your first program: Several example scripts are provided. For this tutorial we will be using a simple script that makes the LED 13 on the board blink.
    1. Go to File>Examples>1.Basics and open ‘Blink’. This will load the Blink script.
    2. Now click ‘Upload’ button on the development environment.
    3. If uploading is successful than you should see LED 13 blinking. If it failed make sure that the Arduino is connected and associated COM port is selected.

UPDATE - Week 1

By : x

The project idea was formed by the group on 1st April on the first lab session. The idea was a combination of the resources and topics available in the lab section 106. The lab topics covered a few aspects of bio-medical applications and the group decided on an appropriate project that would use the available skillset of the group. So, this meant finding a bio-medical application that required electrical and computer engineers.
As a result we decided to use the provided simple 2- lead wearable ECG monitor and Arduino board to make a remote heart monitoring system. The idea behind this was to make a portable heart monitoring device that kept track of the rhythm of patient’s heart. The benefit of doing so would mean that less complicated devices would be involved, and the patient will be able to carry on with normal activity.
Major updates:
·        Technical: A Bluetooth shield will be used for the wireless transmission, and the Bluetooth would be paired to a smartphone with built-in Bluetooth. A mobile application would be made to display and process the information received.
·        Software: Android platform will be used to design the mobile application. The preferred programming language for this platform is Java. Some member have previous programming experiences and they will build on those to program on Android.
·        Feasibility: The Bluetooth shield would set us back by about $30.00.
·        Next week work: More research and work on Arduino is required since none of the group members are familiar with it. We will also be looking into Bluetooth hardware.

Project Overview

By : Lam Ho Wang
The Problem:
Heart health problem has been a big issues for many years. There are lot of heart health monitor product in the market to help them keep track their health. However there are patient, who can not take care themselves even when they know the heart problem is coming. If the patient is alone, and he does not has the ability to ask for any assistance. The heart problem can be fatal to the patient.

The Design Objective: 1.Design a sensor that can sense heart beat
                                      2. Design algorithm to accurately calculate heart beat
                                      3. Design a mobile application that connects to the sensor
                                      4. Make the design minimal and aesthetically pleasing


The workflow of  heart rate monitor

    The monitor will check the heart rate of users. after record a number of times, it will send those record to the phone by using Bluetooth or WiFi.
    After the phone get the records, it will arrange the records and compare with the older records and try to find out weather user get ill. 
    The application will help  doctors and hospitals track one or more patients at the same time. The application also can notice not only the hospitals and doctors, but also the people who setting in the application if the patients' heart rate becomes abnormal. It can help patients get treatment more quickly.

FAQs

By : x

Your Questions Our Answers

[If you have any question/suggestions please post on the comment section below]


1. What is this blog about?
- This blog is our documentation and progress timeline as we go through various stage of development of this project.
2. Who are involved?
- We are a group of three freshman engineering student at Drexel University. More about us here.
3. What is the objective?
- We are building a remote and portable heart rate monitor using Arduino. More about this here.
3. How does the sensor work?


The sensor detects the change in amount of Infra-red adsorbed by the blood as it pulsates. This change relates to the heart beat.  More about this here.
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