S1E4: How are Heartbeats detected in a Fitness Watch?
Introduction
Numerous technologies are used in a fitness watch and they all boil down to fundamental physics and nature. And the way heartbeat is detected in a fitness watch is no exception!
To detect a heartbeat, Apple watches and Fitbits use PPG (Photoplethysmography) sensor. It’s a complicated term that has a rather simple explanation. The following is the exact setup used in a Fitbit. Let’s jump in!
Relevant Components of PPG sensor
LEDs
Two Infrared LED lights are mounted in the fitness watch to generate periodic light pulses directed toward our skin.
Photodiodes
Photodiodes are light-based sensors that produce a current when illuminated by light. They are placed between the LEDs and are used to absorb the photons (light) reflected from our arteries and generate a current based on the intensity of light.
Circuitry
A circuit is designed to process the information received by the photodiode and to manage the LEDs in generating successive light pulses apart from handling other functions.
Figure 1: Components used in a fitness watch and their interaction with our body. Source: US11096601 B2
Working Principle
The basic principle used in detecting a heartbeat is that blood is red in color because it absorbs green light and reflects red light.
Heartbeat is detected using this simple fact in the following way.
Two green light LEDs are used to flash light hundreds of times per second on our skin. The light passes through the skin to a blood vessel such as the artery or vein in our wrist.
A portion of the light is reflected by the artery and is absorbed by the photodiode placed in between the LEDs. The higher the intensity of the light absorbed, the higher the current generated by the photodiode.
When our heart beats, the blood flow in our wrists is greater. This means that the green light absorption is greater when the heart beats and is lesser between the beats.
This fact in turn changes the light absorption intensity observed on the photodiode across time. Every rise and fall in this signal is a heartbeat detected!
Heart rate measuring algorithm
My blog post coming out next Wednesday is about how this data is processed to compute our heart rate.
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