Listen to Your Heart - The Tech Behind Heart Rate Sensors

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This Valentine’s Day, “Listen to Your Heart.”

No, it’s not just a line from the aptly-named tune from Swedish rock band Roxette, but a growing trend in health and wellness. 

As far as health and performance is concerned, there are two major categories of data that must be transmitted: movement tracking (thanks to MEMs sensors) and biological data. When it comes to bio data, the heart rate monitor is the most common but challenging to design.

The heart rate monitor has 2 major technologies: an infrared based sensor and electrocardiography. Electrocardiography records the electrical activity of the heart over a period of time using electrodes placed on a patient body. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle depolarizing during each heartbeat. This method is very accurate and able to detect abnormalities of the heart but is inconvenient in many consumer applications due to the electrodes that must be stuck to the skin.

A second solution is based on photoplethysmography. This complex word describes a simple fact: blood is red because it reflects red light and absorbs green light. The contraction and relaxation of cardiac muscles causes blood to flow in and out of the heart, modifying blood volume flow periodically in the veins. The light absorption varies with the flow and an electronics circuit measures these changes.

Within photoplethysmography , two methodologies are available: the transmission method and the reflection method. The transmission method is used in medical equipment where you clip your finger in a small device. Obviously, this is not very convenient for a fitness device and the solution for wearables is the reflection method.

It can be designed discretely with dedicated Infrared or green LED sending light, a photodiode receiving the reflective signal, an amplifier and an ADC, and a microcontroller to process the signal. The received signal is only in the range of tens of microvolt. Therefore the signal must be amplified by a low power operational amplifier like the TSU104 from STMicroelectronics.

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STMicroelectronics Operational Amplifiers - Op Amps View

 Another option is an integrated solution. Silicon Labs offers the Si114x family. The Si1147-M01 integrates an optical sensor module that provides a highly integrated optomechanical solution for proximity, ambient light, heart rate, UV index and all signal processing capabilities.

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Silicon Labs Sensor Development Boards and Kits View


The design to measure heartbeat rate is challenging due to many sources of noise: muscle contraction, unstable body contact, skin color, tattoos, hair, etc. 

Once the heart rate is known, the number of burnt calories can be calculated. Indeed, the calorie count burnt during a workout depends on the average heart rate and workout duration. The other terms used in calorie computations are constant. Hence, wearables can easily display calories burnt.

If you’re looking to craft your own fitness tracker, several equations exist to estimate burned calories depending on the type of physical activities. For instance, weight lifting and cardio exercise use different formula. This most standard equation for cardio exercise comes from the Journal of Sports Science and provides a formula for each gender:

For Men:
Calories Burned = [(Age x 0.2017) + (Weight x 0.199) + (Heart Rate x 0.6309) — 55.0969] x Time / 4.184.

For Women:
Calories Burned = [(Age x 0.074) -(Weight x 0.126) + (Heart Rate x 0.4472) — 20.4022] x Time / 4.184.

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