As stated in my last post, I am currently working on making a pulse oximeter from scratch using an Arduino. Today I further experimented with the sensor, a MAX30102, to see how it works.

I started with wiring up the sensor to the Arduino Uno, four wires for power, ground, and 2 for the I2C, which is how the sensor communicates with the Arduino. I then tried to test the sensor to start off, but there was an issue. When I attempted to run the code, an error popped up saying that the computer wasn’t able to communicate with the port. I tried to change the port, board, and the cable, none of which worked. After that, I tried rebooting my computer to see if anything would change, and a new port popped up. After trying that one, the code finally uploaded to the Arduino.

While looking at the code, I noticed a part of the code that was called the baud rate. In the serial monitor, a bunch of gibberish was showing instead of the numbers that showed the light levels. While researching, I realized the baud rate in the code didn’t match the one that it was set to. The baud rate controls the speed that the Arduino and sensor communicate. After matching them up to 115200, the sensor began to show data. After looking at the numbers, I noticed there were 3 categories. R, IR, and G (representing red, infrared, and green light). The red and infrared numbers were around 100000 and 110000 respectively. The green number however, was very low (around 50-100). After doing some exploration, this is because the MAX30102 doesn’t have a green LED in comparison with the other models.

After sorting out all of the technical and hardware issues, I finally got to testing the sensor with different conditions. When trying the sensor with colder fingers and in less sensitive areas, I noticed the numbers for the R and IR light levels were lower or more inconsistent. I learned that the temperature can affect the numbers by shrinking the blood vessels to conserve heat, and that causes inconsistency in measurements due to there a weaker signal. This can make the numbers smaller or more inconsistent. The position can affect it in the same way due to how the sensor calculates blood oxygen levels, and will give more accurate results in areas with more vessels. One other thing that I found interesting was how age can affect the numbers. When trying it on my parents, the numbers were lower by about 10000 each. I think it might be because of age or skin condition, but I’ll have to do some more research.

This session answered a lot of my questions, but I have some new ones.

• I understand how the sensor gets the numbers, but what do they actually mean?
• How do these numbers affect blood oxygen, does it have to do with the code?