Galaxy S5 Heart rate monitor a.k.a Pulse Oximeter [The Age of personal sensors]

This Year so far has been tagged as the year of "Smart wearables" - those smart watches or wrist bands that contain sensors to measure and transmit  physical information like heart rate, blood pressure, body temperature. number of steps walked etc.

But the Galaxy S5 has become the first smartphone to have placed a dedicated heart rate sensor. Many observers consider this new addition as mere gimmick and not really meaningful for an average buyer. Moreover the accuracy of the measurement results is been questioned by some who have tested it.

Actually the so called Heart rate monitor on the phone is a "Pulse Oximeter" sensor package. Let's see how it works.

Before beginning to explain the working of a standard medical "Pulse Oximeter" and comparing it with the so called "Heart rate monitor" on the Samsung Galaxy S5 smartphone, we need to look at what is available on the phone's package.

The Heart rate monitor is situated next to the LED camera flash on the back of the phone [see below pic]. It contains an Red/Infra-red LED and a Sensor [photo-diode] to capture the reflected light from the user's finger. The variations in the reflected light is processed by a Micro-controller/DSP situated on the back of the circuit board [see below pic].

LED and Photo-Diode

Pulse Oximeter CSP chip by MAXIM

We are not sure if the sensor package contains both IR and Red light LEDs. A standard Transmissive type Pulse oximeter contains both IR and Red LED lights. A pulse Oximeter is used to measure the oxygen saturation levels in the blood and as a added bonus gives out the pulse rate (heart rate) of the user too.

Meanwhile it is clear from testing that Galaxy S5 package is only able to calculate the pulse rate and not the oxygen saturation levels in blood. The package is based on "Reflectance" type measurement - which means the light reflects from the blood flow happening inside the finger. With each heart beat the heart contracts, which momentarily increases arterial blood volume across the measuring site. So with each heart beat, the  amount of light reflected would vary considerably and the sensor output would look like a waveform containing peaks and troughs. The micro-controller can process and calculate the peaks in this waveform signal to count the heart beats per minute. This is then displayed on the display as say 74 BPM (beats per minute).

Transmissive type Pulse Oximeter

Now in a standard medical pulse oximeter, the oxygen saturation levels (SpO2) is measured. Oxygenated hemoglobin in the blood absorbs more IR light and allows more Red light to pass through. Deoxygenated hemoglobin absorbs more Red light and allows more IR light to pass through.

Conventional Pulse Oximeter measurement

After both types of light is detected by the Photo-detector, the R/IR ratio is calculated. The R/IR is compared to a "Look-up" table to convert the R/IR ratio to a SpO2 value.      

Typical Transmissive pulse oximeter measurement circuitry

                                                           
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