I started a project a few weeks ago, to build a pulse oximeter from scratch using an Arduino. Today was the first time I tested it out and saw how the sensor works.

The setup is just an Arduino Uno and a sensor called the MAX30102 on a breadboard. The sensor shines a red LED and an infrared LED into your finger and measures how much light comes back. Apparently oxygen-carrying blood and oxygen-poor blood absorb these two colors differently, so by comparing them you can figure out blood oxygen saturation. Each heartbeat also pushes a pulse of blood through your finger, which briefly changes how much light gets through, that’s how it counts heart rate.

The wiring took about twenty minutes. Four wires for power, ground, and two for something called I2C, which is how the sensor talks to the Arduino. Then I uploaded a basic example program from the SparkFun library and opened the Serial Monitor.

When I put my finger on the sensor, numbers started coming out. They kept shifting as I changed how my finger was placed, based on how it was blocking the light.

I tried it out with different temperatures and positions, and I have some questions.

• The library does the heart rate math for me, but I don’t really understand how. I want to look at the signal and try to count it myself.
• The temperature/position of your finger seems to change how the numbers come out, why?