April 14, 2025

Good morning !

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Alright — let’s jump right back into where we left off yesterday!

Let’s talk about the heart rate sensor.

The very first step is to design the electronic circuit.

Now, there are all kinds of electronic components out there.

Today, we’re going to focus on two amazing ones: the “LED” and the “phototransistor.”

Each of them costs just a few cents — that’s right, just a few cents!

And yet, these tiny components have the power to do incredible things.

Let’s take a moment to appreciate just how great they really are!

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You probably already know this — when you give electricity to an LED, it lights up.

But what exactly is a phototransistor?

Well, it works in the opposite way!

Instead of lighting up when it gets electricity, it reacts to light — it generates an electrical signal when light hits it.

So… what does this have to do with a heart rate sensor?

Great question — and that’s exactly what we’re going to explore next!

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And now — surprise! — we're suddenly diving into an academic paper!

At this point, most of you probably have no idea what it’s talking about.

But here’s the amazing part…

Just a few days from now, you’ll actually understand it.

Can you believe it!?

Seriously — this is happening!

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Here comes the paper.

It may look intimidating now, but soon, it’ll all make sense.

https://www.analog.com/en/resources/technical-articles/guidelines-to-enhancing-the-heartrate-monitoring-performance-of-biosensing-wearables.html

The paper above is from Analog Devices, a well-known tech company in the U.S.

Starting now, we’re going to break it down — piece by piece.

And just to remind you…

In just a few days, you’ll be able to understand it all!

Yes — you really will!

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We’ll set aside the Abstract and Introduction for now.

What really matters is the “Optical Sensor Operation” section.

That’s the key part we’ll focus on.

Alright — let’s dive in and start reading!

This is where the fun begins.

Stay sharp — we’re about to uncover how it really works!

Take a look at Figure 1: "The principle of reflective optical-pulse measurements."

See the part labeled **“PHOTODETECTOR”?

That’s what we call a phototransistor!

It’s the key component that detects the tiny changes in light reflected from your skin — changes caused by your pulse.

Cool, right?

Alright — now let’s dive even deeper.

We’re going to break down the details step by step, so you can really understand how it all works.

Get ready — things are about to get seriously interesting!

Let’s imagine this:

You place your finger on top of the LED and the photodetector.

Now, when you send electricity to the LED — it lights up.

So what happens next?

That light hits your finger, right?

That light hits the blood vessels just beneath the surface of your finger.

And then… something amazing happens.

The light gets reflected by the blood vessels.

Why? Because blood absorbs and reflects certain colors of light!

When you shine a specific color of light through your skin, some of it gets reflected back from the blood vessels — especially if that light matches the absorption/reflection properties of blood.

Now here’s the exciting part:

Your blood vessels expand and contract slightly with every heartbeat.

That tiny change — that’s your pulse!

Whoa — it’s getting a bit tricky, right?

Don’t worry!This is where things really get interesting.

But before we go further, let’s slow down and organize what we’ve learned so far.

Let’s break it down, step by step.

When more blood flows through the vessels,the distance between the vessels and the skin surface becomes shorter.

As a result, more light is reflected back.

When only a small amount of blood flows through the vessels,the distance between the vessels and the skin surface becomes longer.

As a result, the amount of reflected light becomes smaller.

It’s because less light reaches the surface when the blood vessels are deeper.

But when your heart goes thump! — and more blood flows —the vessels expand and move closer to the skin.

As a result, more light is reflected back to the sensor.

This reflected light is received by the phototransistor.

The phototransistor converts light into an electrical signal.

・The more light it receives — the stronger the signal.

・The less light it receives — the weaker the signal.

That’s it!

Light in — signal out!

We need to make this electrical signal strong enough for the microcontroller to understand.

This process of making the signal bigger is called amplification.

In short —we amplify the signal so the microcontroller can read it clearly!

Alright — tomorrow, we’ll dive into the world of amplification!

But maaan… I can’t wait that long!!

See you then! 😆