Download Introduction to pulse oximeters

Introduction
Pulse oximeters are basically oximeters, measuring instruments that measure the oxygen in the
arterial blood that also measures your pulse. It is used to detect pulse as well as saturated oxygen
levels in a variety of circumstances including surgery, other procedures involving sedation, medical
conditions like heart attack and asthma, sleep apnea, effectiveness of lung medications and many
other applications.
Though these pulse oximeters may seem compatible, they are in fact limited. In some cases, their
readings can be affected by body movement, reducing accuracy as well as overhead lights and other
light sources.
How it works
Haemoglobin, a protein in your blood, absorbs visible red light when carrying oxygen and absorbs
infrared light when not carrying oxygen.
A clip like device called a probe is placed on body parts like a finger or ear lobe. This probe contains a
light source, like an LED, a light detector and a microprocessor, which compares and calculates the
differences in the oxygen rich and oxygen poor haemoglobin.
What we will use
The Pulse Oximeters we will make detect heart rates by using LEDs to shine light on the top of your
finger and a light sensor (made from a photodiode) sensing the light that is not absorbed by the
haemoglobin protein in your blood. Haemoglobin not carrying oxygen absorbs infrared light so we
will be using LEDs that infrared light that is not visible our eyes.
With each pulse, there is a slight increase in the volume of blood flowing through the arteries.
Associated with this small increase in blood volume, the volume of oxygen rich haemoglobin also
increases. By sensing this increase, we can record each heartbeat.
Limitations- body movement can affect readings making them less accurate, interference from other
light sources eg bulbs
http://www.turnermedical.com/PULSE_OXIMETER_INFORMATION_s/25.htm
The LED
There is a voltage of 3V going over the whole circuit and the LED has a voltage of 1.2 V so the the
resistor would have a voltage of 1.8V running through it.
I = 20mA = 0.02A (Current)
VT=3V (Total Voltage)
VLED=1.2V (Voltage running through LED)
VR=1.8V (Voltage running through resistor)
R = 90 ohms (Resistance)
The Circuit
There are two main circuits, one which powered the light sensor and another that powered the LED. The
circuit with the LED is simple and consists of the LED, a 3V battery and a 90 ohm resistor.
The circuit for the light sensor consists of a photo diode, a resistor, a battery and an IC that contains two
Op Amps though we will only use one. The negative input of the IC (Pin 2) which is then connected to the
output pin (Pin 1). Pin 2 also connected to the photo diode which, along with the negative input, Pin 3,
connected to the ground pin, Pin 4. The battery is connected to Pin 8.
Pins in an LM358P IC
http://us.100y.com.tw/ChanPin.asp?MNo=19283
Circuit for the light sensor (ours is a little different as we use a 100Kilo Ohm resistor and an IC with two
Op Amps instead of a single Op Amp.
The light sensor
How breadboards are connected
http://www.roguescience.org/wordpress/building-a-midi-out-controller/add-a-led/22-breadboards/
http://www.atariarchives.org/ecp/preface.php
5mm Infrared Transmitting LED
5MM EMITTING DIODELooks like a 5mm LED & has a blue transparent lens.Specifications:- Forward
Current (If): 50mA max - Peak forward current (Ip): 1.2A- Forward Voltage (VF): 1.2V @ 20mA Reverse Voltage (VR): 5V max- Power Dissipation (Pd): 100mW max - Viewing Angle: 30°- Peak
Spectral Wavelength(IR): 940nm @ 20mA * Spectral Bandwidth (DI): 50nm@20mA- Material: GaAs
http://www.jidonline.com/viewimage.asp?img=JInterdiscipDentistry_2011_1_1_14_77191_f4.jpg
Programing (CODE)
delayMs(50);
LED1Toggle(); //Toggle with LED 1
result=ReadADC(4,POS_REF_BATT); //Define Result
current_value=result; //Changes current value to show result
y = current_value-prev_value+20; // y axis shows the current value minus previous value plus 20
if (y<0) // is y is less than 0
y = 0;//Than y must equal 0
else if(y > 47)//if y is more than 47
y = 47;//Than just let y=47
BufferWritePixel(x, y , PIXEL_ON);//Draw pixel x and y values
x++; //x increase by one
}
if(x > 83)//if x is less than 83
{
BufferClear();//Then reset scren
BufferWriteMessage("Back", LEFT, 0,40, NORMAL); //Write ‘Back’ on left of screen
x = 0; //make x equal 0
}
prev_value=current_value; //the previous value is what used to be the current value
writeBufferToScreen();//Write everything to screen