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How Do Sensors Work Inside Fitness Trackers?

Fitness trackers have become one of the most common wearable technologies around the globe, and the key to that is the versatility offered by this technology. Fitness trackers come in various shapes and forms, all tailored to the consumers’ basic needs such as armbands, wristbands, chest-straps, clip-ons, rings, and even bras with built-in heart monitors. Companies now even opt for embedded fitness trackers in smartwatches allowing for even more options than just counting steps.

The real question here is, how can something so small do so much? The essence of these gadgets is the built-in sensors for literally everything, using every bit of information that they can get, such as distance traveled, heartbeat, sleep tracking, calorie consumption, pressure difference, and even fall or crash detection. Each of these metrics is measured either individually by a specific sensor or is the product of several readings; we will be discussing these sensors in detail.

3 axis accelerometer, Altimeter, Barometer & Gyroscope

Now, these guys are mainly related to movement calculation, and through complex algorithms, they analyze movement to ascertain what activities you’re engaging in and alter the heart rate interpretation accordingly.

3 axis accelerometer sensors are the base of all fitness trackers, interpreting the body’s orientation, inclination, and tilt through sensing gravity as well as linear accelerations. Along with IMUs, the sensors track movement in each direction.

Gyro sensors, also known as angular rate sensors or angular velocity sensors, are devices that sense angular velocity. The more axes, the better. A 3 axis gyroscope can be paired up with a 3 axis accelerometer to provide a ‘6 degree of freedom’ motion tracking system. Most reliable fitness trackers use this combination to get a better grip on the 3D workout motions that an individual may perform.

A barometer relies on atmospheric pressure to carry out its job, which is quite similar in one aspect to the altimeter, yet, it also determines the weather of the day through the amount of pressure exerted by the atmosphere on it.

An altimeter is on the more developed and complex side than a barometer as it’s designed for climbing mountains and isn’t even offered in that many fitness trackers. Still, the foundation of both sensors remains the same, which is altitude determination through a change in atmospheric pressure.

Both the altimeter and the barometer contribute to the burnt-calorie count throughout the day.

Heart-rate, Tissue Oxygenation, Respiration-rate & Sleep patterns

Bioimpedance sensor

Through the use of two outer electrodes to drive tiny electrical energy and two inner electrodes to pick up changes in voltage, they measure tissue-resistance to the current, giving a lot of information regarding several metrics. Combining these metrics forms an image of the body composition which the fitness trackers translate into heart-rate and respiration-rate. It also gives an idea about how much body component ( muscle, fat, or water), needs to be lost or gained. This is the same technology used in smart scales.

Electrocardiogram (ECG) sensor

An ECG sensor records the pathway of the tiny electrical impulses traveling the heart muscle. Any alteration in this pathway or the frequency of these impulses can be catastrophic, so, through constant monitoring, it’s a lot easier to pick up these small changes so that the situation can be handled early-on and with as little damage possible.

Optical heart rate sensor

As the frequency of heart-beats increases, blood tends to rush in the blood vessels faster and faster. What an optical heart rate sensor does is that it shines a light on these blood vessels, mainly capillaries, and detects the reflection of that light. The more blood in the capillaries, the less the reflection of the light and the more the heartbeat. Now, this sensor is a lot more accurate in chest-straps, but of course, it’s a lot more practical to wear a watch than a belt around one’s chest.

Pulse oximeter

Oxygenation of distal body parts is very important; it’s measured through pulse oximeter sensors that emit light; usually, LEDs, into the tissue, and have a photodetector collect light reflected or transmitted from the skin. Oxygenated blood has a vivid red color, while de-oxygenated blood sends back a bluish picture.

Place Detection And Connection


A Compass is surely a must in all fitness trackers; it’s simply a magnetized needle that points to the North.

Global Positioning System (GPS), satellites are the answer to this one, through signals transmitted back and forth, the satellite calculates the distance between itself and the fitness tracker and through very specific calculations determines your exact place in the world.

A magnetometer measures magnetic fields, primarily that of the earth. It can be used as a compass and to pinpoint your exact movements.

If the smartwatch on your wrist happens to be LTE enabled, that simply means that is has a built-in cellular connection, making it independent of your smartphone.

Other Sensors

A temperature sensor that provides a reading of your body or the external temperature.

UV sensor which tells the fitness tracker when you might be absorbing dangerous UV radiation and warns you to seek shade if you are likely to burn.

Ambient light sensors detect the time of day by separating the forms of light visible to the human eye and processing them to adapt the screen to the amount of light needed by you. This is exceptionally helpful in protecting the user’s eyes and pocket to be real as it expands the battery’s life-span.

A capacitive sensor is another way to save the users’ batteries by waking the device’s screen only when worn or looked at.

In The End

The Fitness Trackers industry has taken quite a few leaps in the past years, and although the gaps between results of various devices is evident proof of the long way that this industry has to cross, we cannot deny that being able to know how much cardio you have done, calories you’ve burnt, the weather for the day and how much sun-ray you took all from one small device is quite awesome.