Introduction
Pulse Oximeter is a non-invasive medical diagnostic device used to detect the oxygen saturation of the blood. Heart rate meter detects the number of beats per minute of the patient, normally referred to as bpm. The pulse oximeter is designed using an infrared and a red LED, projected alternatively on the finger and detection of the transmitted light by a photodiode/phototransistor. The output of the photodiode is given to a transimpedance amplifier, and further amplified and filtered before giving to the Atmega644 microcontroller. The ADC in the microcontroller will convert it into digital form, and later display the value of SpO2 and heart rate on a 16*2 LCD screen. The goal of the project is to develop a low cost pulse oximeter and heart rate meter powered by solar energy. The idea was conceptualized due to the fact that the oxygen saturation of blood is one of the most important parameter to be monitored, and the pulse oximeter being low cost and solar powered, allows it to be accessed by the developing and under developed nations.

High Level Design/Background
Pulse oximetry has become a standard procedure for the measurement of blood-oxygen saturation in hospitals, clinics, etc. Pulse oximeter can directly detect hypoxemia, deficiency of oxygen saturation in the arterial blood. Early detection of hypoxemia can reduce the gas poisoning by CO2 or CO, tissue damage, etc. Thus, the oxygen saturation of the blood can quickly and accurately be monitored non-invasively using pulse oximeter. Pulse oximeter works on the principal of absorption and reflectance/transmittance of light by multiple components like skin, muscle and blood vessel. Absorption due to tissue, skin or muscle remains fairly constant, whereas absorption due to arterial blood varies. Arteries expand due to the pumping of the heart, expanding the arteries and inturn increasing the tissue between the LEDs and the photodiode, thus increasing the light absorption. Using this principle, heart rate can be detected. Absorption of oxyhemoglobin and the deoxygenated hemoglobin form differs significantly with wavelengths (i.e.) oxygen is transported in the blood by hemoglobin, and, depending on the binding of oxygen to the hemoglobin, absorption of light takes place at two wavelengths as shown below.

Figure 1: Absorption of oxygenated and non-oxygenated hemoglobin at different wavelength (http://www.oximetry.org/pulseox/principles.htm)

Light from two LEDs with different wavelengths i.e. 660 (RED) and 940 nm (IR) are made to fall on the finger. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through. The ratio of absorption at the two wavelengths is used to determine the fraction of saturated hemoglobin. Pulse Oximetry can be done using two methods, reflectance oximetry and transmittance oximetry. In case of reflectance oximetry, the two LEDs and the photodiode are on the same side. Here, the light moves through the skin, muscle and blood vessel, and is reflected back from the bone. Reflectance oximetry has low signal to noise ratio and difficult to set up. In case of transmittance oximetry, the two LEDs and the photodiode are on the opposite side of the finger. Here, the transmitted light is detected by the photo diode, and is found to have higher signal to noise ratio.

Figure 2: Transmittance oximetry v/s Reflectance oximetry (http://www.medgadget.com/2007/11/reflectance_pulse_oximetry_from_spo_medical.html)

#define F_CPU 16000000UL //16MHz crystal

#include <inttypes.h>

#include <avr/io.h>

#include <avr/interrupt.h>

#include <stdio.h>

#include <avr/eeprom.h>

#include <util/delay.h>

#include <stdlib.h>

#include <string.h>

#include <avr/pgmspace.h>

#include "uart.h" // serial communication library

#include "lcd_lib.h" //lcd library

const int8_t LCD_initialize[] PROGMEM ="\0";

int8_t lcd_buffer[17]; // LCD display buffer

int8_t lcd_buffer1[17]; // LCD display buffer

FILE uart_str = FDEV_SETUP_STREAM(uart_putchar, uart_getchar, _FDEV_SETUP_RW); //UART initialization

volatile int max, min,a,b,count,c,d,average,lastcount;

float HeartRate,Vmax,Vmin,x,y,z;

float value=0.0158;

void timer1_init()

{

TCCR1B |= (1 << WGM12)|(1 << CS12)|(0 << CS11)|(1 << CS10); // set up timer with prescaler = 1024 and CTC mode

TCNT1 = 0; // initialize counter

OCR1A = 155; // initialize compare value

TIMSK1 |= (1 << OCIE1A); // enable compare interrupt

}

void init_lcd(void)

{

LCDinit(); //initialize the display

LCDcursorOFF(); //no cursor

LCDclr(); //clear the display

LCDGotoXY(0,0); //go to x and y position on the lcd

CopyStringtoLCD(LCD_initialize, 0, 0);

}

int main()

{

ADMUX= (0<<REFS1)|(1<<REFS0)|(1<<ADLAR); //AVcc as reference voltage and Left shift bits

ADCSRA=(1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0)| (1<<ADATE); //Prescalar 128 and set bit for free running mode

ADCSRB= (0<<ADTS2)|(0<<ADTS1)|(0<<ADTS0); //Free running mode

init_lcd();

timer1_init(); //timer 1 initialization

average=60; //initialize average value for average crossing detection

max = 0; //initialize maximum =0

min = 255; //initialize minimum = 255

sei(); //Global interrupt enable

UCSR0B = _BV(TXEN0); //Serial transmission

UBRR0 = 103;

ADCSRA |= 1<<ADSC; //start conversion

stdout = stdin = stderr = &uart_str; //UART transmission

LCDclr(); //clear lcd

CopyStringtoLCD(LCD_initialize, 0, 0); //copy the string data to position x=0, position y=0

while (1)

{

z=1/(0.01*d); //calculation of frequency of ISR

HeartRate=z*60; //calculation of heart rate (beats per min)

fprintf(stdout,"HeartRate=%f \n",HeartRate); //UART transmission of heart rate

CopyStringtoLCD(LCD_initialize, 0, 0);

_delay_ms(2000);

sprintf(lcd_buffer,"HeartRate=%2.0f ",HeartRate); //sending heart rate values to lcd for display

LCDGotoXY(0, 0); // goto position x=0, position y=0

LCDstring(lcd_buffer, strlen(lcd_buffer));

sprintf(lcd_buffer1,"SP02=%f ",y); //sending SpO2 values to lcd for display

LCDGotoXY(0, 1); // goto position x=0, position y=1 (second row)

LCDstring(lcd_buffer1, strlen(lcd_buffer));

PORTD=0b00000001; //blinking led every 4 ms

_delay_ms(4);

PORTD=0b00000000;

_delay_ms(4);

PORTD=0b00000010;

_delay_ms(4);

PORTD=0b00000000;

_delay_ms(4);

}

}

ISR(TIMER1_COMPA_vect)

{

lastcount=count; // copying value of count into last count used for average crossing (becomes previous value_

count = ADCH; //copying value of ADCH into count (current value)

//we checking for maximum and minimum value for analog input

if(count>max)

{

max=count; //if current value in count is greater than maximum value, then copy it to maximum

}

else if (count<min)

{

min=count; //if current value in count is less than minimum value, then copy it to minimum

}

c++; //counter increments

if((lastcount>average) && (count<average)) //average crossing condition

{

average=((max+min)/2); //average calculation

a=max;

b=min;

fprintf(stdout,"a=%i\n",a); //UART transmission

fprintf(stdout,"b=%i\n",b);

x=value/(a-b); //calculation of ratio

fprintf(stdout,"x=%f\n",x);

y=(10.0002*x*x*x)-(52.887*x*x)+(26.871*x)+98.283; //calculation of SpO2

fprintf(stdout,"y=%f\n",y);

d=c; //copy counter value to d

c=0; //clear counter

fprintf(stdout,"d=%i\n",d);

max=0; //clear max

min=255; //clear min

}}