EE Summer School

Pulse Oximeter Project

The development of medical devices has driven significant progress within the fields of medicine and healthcare. One of these core fundamental developments is that of pulse oximetry, which allows medical professionals to monitor a patients heart-rate and blood-oxygen levels.

During this guided summer school, you will have opportunity to build your own pulse oximeter system, experiencing some of the skills that engineering students must develop while studying at university. These include component selection, circuit prototyping, programming embedded systems, and soldering. By the end of the week, you will have built a functioning system of your own, and gained insight into how they operate.

Get Started Arduino Nano Docs

Lab 1
Lab 2
Taking it Further

Technical Timetable

The technical part of this summer school will happen within the Electronic and Electrical Engineering Department. The days will be broken down as follows:

Monday

  • 15:00 - 16:00 - 2 East Foyer - Tour of the department and introduction to the project.

Tuesday

  • 10:15-12:45 - 2 East Undergraduate Labs - Lab 1 Morning Session.
  • 14:00-16:30 - 2 East Undergraduate Labs - Lab 1 Afternoon Session.

Wednesday

  • 10:15-12:45 - 2 East Undergraduate Labs - Lab 2 Morning Session.
  • 14:00-16:30 - 2 East Undergraduate Labs - Lab 2 Afternoon Session.

Other non-technical activities will be happening outside of these hours as outlined in your personal timetables.

Intro to Pulse Oximetry

The ability to take medical readings non-invasively is key in our modern medical practices today. Measurements like weight, temperature, heart-rate, blood-pressure and blood-oxygen levels are all used to provide the datapoints that help diagnose and track a patients health. Pulse oximetry is a critical part of this process, providing a rapid and reliable reading for both heart-rate and blood-oxygen levels.

The key principle behind this technology is found in the way that oxyhaemoglobin (oxygenated blood) and deoxyhaemoglobin (deoxygenated blood) absorb and reflect light slightly differently. These differences are shown in .

The absorption spectra for oxyhaemoglobin and deoxyhaemoglobin over the range 600 nm to 1000nm. Note that the y axis is logarithmic. Figure reproduced from Metcalfe et al.<span class='ref'><span class='refnum'></span><span class='refbody'>A Cost-Effective Pulse Oximeter Designed in Response to the COVID-19 Pandemic - Benjamin Metcalfe, Pejman Iravani, Jonathan Graham-Harper-Cater, Richard Bowman, Julian Stirling, Peter Wilson - <a href='https://openhardware.metajnl.com/articles/10.5334/joh.26'>DOI: 10.5334/joh.26</a></span></span>
The absorption spectra for oxyhaemoglobin and deoxyhaemoglobin over the range 600 nm to 1000nm. Note that the y axis is logarithmic. Figure reproduced from Metcalfe et al.A Cost-Effective Pulse Oximeter Designed in Response to the COVID-19 Pandemic - Benjamin Metcalfe, Pejman Iravani, Jonathan Graham-Harper-Cater, Richard Bowman, Julian Stirling, Peter Wilson - DOI: 10.5334/joh.26

Two key wavelengths of light are identified in , the Red and Infrared spectrum. Under infrared light, the absorption of the two types of blood are very similar, however under red light deoxyhaemoglobin absorbs significantly more light than oxyhaemoglobin. By measuring the absorption of light at these two wavelengths and comparing the results, we can therefore work out the relative levels of oxygen in the blood. As both these wavelengths can penetrate skin, it is possible to perform this measurement using sensors external to the body.

The Method

There are two key methods available when implementing this process. Which of these methods is chosen will vary how the hardware needs to operate:

  1. Reflectivity: In this mode, we use Red and IR light sources to shine the selected wavelengths into the subject. The light reflected back is measured by sensors located next to the light sources. This information may then be used to calculate how much of the light was absorbed (and also transmitted through the subject), as any light reflected and seen by the sensors has not been absorbed by the blood (or other organs).

  2. Transmission: In this mode, Red and IR light sources are used to shine the selected wavelengths through the subject. Sensors are then placed on the opposite side of the subject to measure the light transmission levels. Any light absorbed by the blood (or other organs) will be missing in the sensors data.

Typically, these measurements are taken in places where the skin is relatively thin, with minimal organs to get in the way. This typically includes the finger tip or earlobe.

We will be designing a Reflectivity system, as this allows both the sensors and light emitters to be located close to one another, greatly simplifying the design process.

The Maths

Once the levels of reflected red and IR light have been measured, the ratio of the two wavelengths present in our measurements is then used to determine the blood-oxygen levels of the subject A Cost-Effective Pulse Oximeter Designed in Response to the COVID-19 Pandemic - Benjamin Metcalfe, Pejman Iravani, Jonathan Graham-Harper-Cater, Richard Bowman, Julian Stirling, Peter Wilson - DOI: 10.5334/joh.26.

Mathematically this ratio, $R$, is defined as:

\[\begin{equation} \label{eq:exampleEquation} R = \frac{AC_{660}/DC_{660}}{AC_{940}/DC_{940}} \end{equation}\]

The value, $R$, is then calibrated using an empirical calibration curve (based on data measured using other measurement techniques).

Typical calibration curve relating the R ratio to SpO2, note that for R = 1 SpO2 = 85%. Inset: example received signal for a 660 nm wavelength demonstrating the pulsatile AC component and the DC component caused by absorption in other tissues such as the nail and bone. Figure reproduced from Metcalfe et al.<span class='ref'><span class='refnum'></span><span class='refbody'>A Cost-Effective Pulse Oximeter Designed in Response to the COVID-19 Pandemic - Benjamin Metcalfe, Pejman Iravani, Jonathan Graham-Harper-Cater, Richard Bowman, Julian Stirling, Peter Wilson - <a href='https://openhardware.metajnl.com/articles/10.5334/joh.26'>DOI: 10.5334/joh.26</a></span></span>
Typical calibration curve relating the R ratio to SpO2, note that for R = 1 SpO2 = 85%. Inset: example received signal for a 660 nm wavelength demonstrating the pulsatile AC component and the DC component caused by absorption in other tissues such as the nail and bone. Figure reproduced from Metcalfe et al.A Cost-Effective Pulse Oximeter Designed in Response to the COVID-19 Pandemic - Benjamin Metcalfe, Pejman Iravani, Jonathan Graham-Harper-Cater, Richard Bowman, Julian Stirling, Peter Wilson - DOI: 10.5334/joh.26

The Exercise

In this project, you will build your own pulse oximeter. Using an Arduino Nano as the brains of the device. This will include soldering the components onto the board, selecting suitable component sizes (such as resistor values) and programming some of the board operation.

Portions of the design and programming will be provided to ensure that you can achieve a working solution in the time-frame of the summer school. Obviously, there will be space to take things further, such as adding additional code to achieve new function. The Pulse Oximeter board (shown in ) also includes some spare LEDs, and a screen, allowing you to come up with and implement your own features.

The pulse oximeter board that you will build during this project.
The pulse oximeter board that you will build during this project.

At the end of this project, you will get to take the pulse oximeter home, meaning you can go on to use the Arduino Nano and Pulse Ox board in future projects of your own creation. There will be further project ideas and information provided on the last page of this lab script.

The Demonstrators

Throughout this week you will have support from a handful of friendly demonstrators and lecturers within the department. Feel free to ask them if you need any help or have any queries at any point.

We hope that you enjoy this week, and we welcome feedback if you have any to give.



Citations