How to use an oximeter and heart-rate monitor with Arduino

An oximeter is a device that measures the amount of oxygen carried by a person’s red blood cells. Oxygen provides energy for our bodies, supports the immune system, helps replace cells that degrade, and more. Low blood oxygen levels can indicate certain health issues. These levels can also decrease as we age, so it can be helpful to monitor the oxygen in our blood.

The conventional method for doing so is via an aerial blood gas (ABG) test, which is a blood test that measures oxygen, carbon dioxide, and acidity levels. The results of an ABG typically take a few hours.

Newer, advanced techniques for testing the amount of oxygen saturation in one’s blood are currently under development. The most promising so far is through pulse oximetry, a non-invasive method whereby a set wavelength of light is emitted through one’s skin (typically, this is a clip device that one places on one’s finger — just make sure it’s not on a tattoo, which would absorb part of the light).

A photodiode is used to measure the amount of light that’s absorbed by the blood. This method can rapidly detect even small changes in how efficiently oxygen is being carried through one’s body and has a small error ratio of only two percent.

Small-scale oximeters are available that can be interfaced with low-end controllers for do-it-yourself (DIY) projects. SparkFun Electronics offers an oximeter and heart-rate monitor sensor, which comes mounted on a small board. Users only need to press a finger on the sensor module for a reading.

Communication with an external controller is easily done with an I2C protocol, which is what we use for this project, along with Arduino Nano. The I2C address for this sensor is set at 0x55 hex.

You’ll note that Arduino Nano is 3.3-volt tolerant and the above oximeter is the same, so this is ideal. There are two additional pins on the sensor: one is a reset and the another one is a multi-function input/output (MFIO). The reset pin is on active low and, as its name indicates, resets the module.

The MFIO is an important pin as it places the module in a data-acquisition mode. The module is composed of two IC’s. One is the MAX3010, which is an oximeter and heart-rate monitoring sensor. The second one is a MAX32664, which is responsible for converting the data received from the other sensor.

The hub is comprised of a cortex M4 microcontroller that receives data from the MAX3010 apply conversion algorithm and outputs this data to an external controller.

Circuit diagram

The I2C interface pins are connected with Arduino Nano’s A4 and A5 pins. Arduino’s digital pins 4 and 5 are of are used for the reset and MFIO. The sensor board is powered through Arduino’s 3.3v power-out pins.

The code
First, import the SparkFun sensor hub library. It’ll make this project much easier. You’ll also need to download the wire library to initialize the I2C interface.

Next, define the variables for the Arduino pins. Each pin’s name and number must be passed to the hub library for proper initialization of the HUB IC (MAX32664). Then, a variable named body of type biodata must be declared. This variable talks with the sensor IC (MAX3010).

In the setup loop, start the serial monitor of Arduino at 115,200 bps and activate the I2C pins. The hub should begin and its status is analyzed for any possible errors (result variable).

Initialize the sensor. The bioHub.configBPM() function initializes the sensor module. A variable named MODE_ONE is passed in the function, which is discussed below. After proper sensor initialization, it’s ideal to allow rest for stabilization so we inserted a four-second delay.

In the main loop, data is read via the sensor module. The sensor passes the data to the HUB, which prepares this data (including the heartrate, oxygen levels, etc.) for the external controller.

The external controller can put the reading request at any instance.