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Strapless Heart Rate Monitor Teardown

Who Else Wants to See the Insides of a Strapless Heart Rate Monitor?

Strapless Heart Rate Monitor Electronic Components

Strapless HRM Components

Do you have a strapless heart rate monitor and wonder how it works?

Well, my strapless heart rate monitor just broke (at least I thought it did) so I decided to tear it apart…guess what? Once I took the back bit off it started to work again. Pretty weird. Pretty cool. Let’s take a look at the video I put together.

In it, we’ll be looking at the separate components within the strapless heart rate monitor and trying to get a heart rate reading with just the components and no watch. It should work because the watch simply makes it easier to activate the internal components (with the button and no loose parts) but is not required for the components to pickup your HR.

*Note: I apologize in advance. There is a few times that I fumble over my words and it makes the video look un-professional. Despite this fact, I hope it satisfies your little curiosity bug. I was looking for this video online for ages – couldn’t find it – so just decided to make it on my own.  

Let’s Tear Apart My Strapless Heart Rate Monitor – Look Inside – and See If It Still Works

In-Depth Explanation in Text 

How does the strapless heart rate monitor work?

Strapless Heart Rate Monitor Circuit Board

Strapless HRM Circuit Board

At the heart of the strapless monitor is the electronic components which I’ll just call the circuit board. This is the smarts that turn a tiny voltage on your skin into a digital output of your heart rate in beats per minute.

The reason why you need to touch the button on the watch face is because that is what closes the electric circuit and provides the external power source. You are providing the voltage that allows the circuit to function. Without you providing the external source the circuit is broken and will not do anything.

To the best of my knowledge this circuit board performs 3 functions with regards to heart rate sensing:

Amplifier circuit: You need to increase the voltage provided by your body so some of the components in the circuit can work.

Since the voltage on your skin at your wrist from your heart beat is tiny (like 0.001 Volts) you need to amplify that so that the voltage is in the correct range for the analog to digital converter to work.

We want to get that 0.001 Volts to be about 1 Volt. So we need a total gain of 1000. This is achieved in 2 steps.

Why 2 steps? Because there will be a small DC voltage loss across your skin. We need to perform this amplification in 2 steps. One that will allow compensation for the small loss and another to jump up the compensated voltage to the 1 Volt range.

I believe this would be done in two steps with the first having a gain of 5 that gets fed back through an integrator. Then that slightly amplified and compensated voltage would pass through again but through a gain of 200 to push it to around 1 Volt.

Analog to digital converter (ADC): Once the voltage is in the range of -5V to +5V the ADC outputs the digital signal to the software (computer program code) that will turn that digital signal into the number you see displayed on the watch.

Software program: So now you have a number proportional to the magnitude of that voltage which represents your heart beat. All you need to do now is detect when this happens and calculate and display the correct number in beats per minute on the screen.

How does touching a button on a watch output a heart rate?

Okay, that was a pretty science’y explanation. Lets break it down:

Picking up your heart’s electric impulse (signal): 

The watch has 2 electrodes to pick up the electrical signal your heart produces and give that electricity to the electric circuit within. Once it is flowing into the circuit the circuit needs to make it stronger so that the ADC can turn it into a real number that happens to correspond with that voltage (calibrated by the engineers who designed this).

  • Electrode 1 is within the back of the watch. Has a copper foil sticker just inside the bit touching your skin. To transfer the electricity from that to the circuit is a copper spring.
  • Electrode 2 is under that button you touch on the watch face. Once you touch that button a pin is activated and you close the electric circuit so that you can get your HR displayed on the screen.

This sounds like it takes a lot of power – does it?

Nope. In fact, the entire heart rate monitoring part of the watch circuit is only 1mA. The button cell battery has a power output ability of 200mAh. So, during a workout let’s say you touch and hold the button for 20 seconds at a time and 10 times per workout. You workout 3 times per week. At that rate (pretty minimal use) the watch battery would last almost 23 years. However, given the watch is always on, the indigo light probably gets used a bit etc etc. The User Manual says 2 years.

Where did I get all this information?

Here are the sources

ECG Primer 1.0 – This is where I learned about the electronics within the heart rate monitor. What is happening within the circuit when you close the circuit and allow the electrodes to do their thang. That guy made his own ECG machine complete with the real-time graph.

Ground (electricity) – Wikipedia is always a great point of reference when brushing up on Physics equations and laws that back them.

Analog-to-digital converter – Again a wikipedia page. When reading through the ECG Primer 1.0 report I had to get a better understanding how the ADC worked. This is the page that provided the information for me to wrap my head around it.

Open-circuit voltage – The page that explains why you  need to touch the button on the watch face for it to output your heart rate.

S-Pulse Technology – The watch I took apart had the term S Pulse Technology on the back so naturally I looked it up. Salutron is the company that owns the patent and so this page talks about the technology and how it works.

Other than that I was able to remember most of it from Mechanical Engineering studies…

Please let me know in the comments if I need to make a correction.

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