Working of a heart rate monitor explained | Today I learned

Curious minds are the best minds. People who question everything are my favourite kind of people. Let us talk about curiosity questions behind heart rate monitor.

Owning a gadget  becomes more meaningful when you know each and everything about it. How do the elements work and what they do exactly on the device.

We all know about heart rate monitors on our smart devices. Be it phones, fitness bands, fitness watches, smart watches, heart rate monitor devices.

Here we won’t be talking abut professional heart rate monitors used by doctors but the everyday use heart rate monitors that give more meaning and stats to our workouts, jogs, runs, step counting etc.

Working of heart rate monitor explained | Today I learned

Components of a heart rate monitor


     1.Light Sensor

They can be of many types. Depends on the company. The type here is not necessary, almost all have similar principles.

     2.Light Source

The light source is typically a LED. Most companies use Green colour LED. Why the green colour you ask? Well, there is an explanation for that as well. Read on.

Principle of the heart rate monitor

It works on three principles of different domains. The machine principle, the body principle and the light principle.

  • The machine principle :

The heart rate monitor works on the principle of absorption and reflection of light. The sensor detects the amount of light reflected and based on that information we get to know our heart rate.

  • The body principle :

Our veins have capillaries which contain blood. The red colour of the blood is because they absorb all other colours and reflect red. This information is vital when I explain the working of the heart rate monitor.

  • The light principle :

Light has property to get reflected or absorbed. This property helps us see the colour we want to see. All colours are absorbed by black and similarly white reflects all the colours.

Working of the heart rate monitor

Working of heart rate monitor explained | Today I learnedWhat does the light source and the sensor do?

The light source sends green light through our skin into the blood. The blood being red absorbs the green light thrown at it. Photodetector sensor senses this absorption.

What does the sensor actually sense?

More the absorption of light lesser is the light reflected and vice versa. This reflection of less or more light is detected by the photodetector. The light reflected from the blood capillaries.

How does this absorption measure our heart rate?

Absorption is directly proportional to the blood present/blood density. More the blood more is the absorption i.e heart rate is higher. Density of blood varies and so does light absorption by the capillaries.

Less absorption means less blood capillaries to absorb light i.e heart rate is lower.

What happens if our heart rate decreases or increases?

When the heart rate monitor shows 73 it that means blood is being pumped into our veins at a rate of 73.  More blood being pumped will show higher heart rate. More blood means more capillaries and more capillaries means more absorption of the light.

That’s it. Optical heart rate monitors are that simple. They just use simple principle of light and voila!

Where can you find optical heart rate monitors?

Working of heart rate monitor explained | Today I learned

  • ECG or EKG machines used by hospitals detect most accurate heart rate. That method is too bulky and far from practical.
  • Professional athletes use strap on bands that have same electrodes found in ECG whereas normal consumer feels awkward and not practical with this method too.
  • Smart watches, fitness bands and smart phones use optical heart rate monitors thereby a close to accurate heart rate is available at no extra baggage or cost.

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Main drawbacks :

Inaccuracy constitutes the skin texture, less amount of light reflected, processing.

Skin texture

Tattoos or dark skin or others absorb a lot of light resulting in poor results.


Bones, muscles also reflect or absorb light which makes the processing difficult as not all light is from or to blood.

Reflected light

The light reflected to the sensor is very less. This poses a problem too.


Now you can tell this to your friends and family. You know what you are wearing and you now know how it works. That is the kind of confidence we all dream of don’t we?

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