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STMicroelectronics optical heart rate monitoring solution

Build your own wearable optical heart rate monitor with the ST X-NUCLEO-IKA01A1 expansion board

The X-NUCLEO-IKA01A1 is a multifunctional board based on ST operational amplifiers and it embeds several configurations. In this design tip we will use two of them to build an optical heart rate monitor (OHRM). We will also show how to connect BLE expansion board X-NUCLEO-IDB05A01 to the heart rate monitor and how to send data to your smartphone. This design tip will make it easier to build a prototype of your own wearable optical heart rate monitor.

Main components
TSU104 Nanopower, rail-to-rail input and output, 5 V CMOS operational amplifiers
BlueNRG-MS Upgradable Bluetooth® Low Energy network processor
STM32L053R8 Ultra-low-power ARM Cortex-M0+ MCU with 64 Kbytes Flash, 32 MHz CPU, USB, LCD

Introduction to OHRM

Optical heart rate monitoring is a simple method to determine one’s heart rate without using electrodes. It uses the optical characteristics of hemoglobin, which attenuates light based on the concentration of oxygen in the blood. Each time the heart beats, it brings fresh blood with oxygen to the muscles. The muscles use the oxygen and decrease its concentration. Therefore we are able to determine the heart rate from the time dependency of oxygen concentration in the blood.

This simple principle can give us heart rate, but cannot measure the level of oxygen directly. To measure the oxygen concentration we need to add a second light source with a different wavelength. Thanks to this method, it is possible to determine peripheral oxygen saturation. This principle is called pulse oximetry and is commonly used in medical heart rate monitoring.

The LEDs are used to emit light through the skin. Hemoglobin has the highest attenuation for red light, so it is good to use it but green or infrared light can be used as well. The sensing element is usually a photodiode.

Block diagram

To build a complete optical heart rate monitor we need to know how to do signal conditioning and which boards can be used. The overall block diagram of the system is shown bellow:

STMicroelectronics X--NUCLEO-IKA01A1

STMicroelectronics OHRM solution

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