Pulse Sensor – Working Principle & Its Applications

What is a Pulse Sensor?

A pulse wave is a change in the volume of a blood vessel that occurs when the heart pumps blood, and a detector that monitors this volume change is called a Pulse Sensor.

First, there are four main ways to measure heart rate

Electrocardiogram – An electrocardiogram (ECG) is a simple test that can be used to check your heart’s rhythm and electrical activity. Sensors attached to the skin are used to detect the electrical signals produced by your heart each time it beats.
Photoelectric Pulse Wave – Photoplethysmography (PPG), also known as the pulse oximeter waveform, is a simple and low-cost optical technique that can be utilized to detect blood volume changes in the microvascular bed of tissue, it uses a light source and a photodetector at the surface of the skin to measure the volumetric variations of blood circulation.
Blood Pressure Measurement – For a manual blood pressure measurement, the care provider places a stethoscope over the major artery in the upper arm (brachial artery) to listen to blood flow.
Phonocardiography – A phonocardiogram (or PCG) is a plot of high-fidelity recording of the sounds and murmurs made by the heart with the help of a machine called the phonocardiograph; thus, phonocardiography is the recording of all the sounds made by the heart during a cardiac cycle.

Pulse sensors use the photoelectric method

Pulse sensors using the photoelectric pulse wave method are classified into 2 types depending on the measurement method

Transmission type pulse sensor

Transmission types measure pulse waves by emitting red or infrared light from the body surface and detecting the change in blood flow during heartbeats as a change in the amount of light transmitted through the body.
This method is limited to areas where light can easily penetrate, such as the fingertip or earlobe.

Reflection type pulse sensor

Reflection-type pulse sensors (Optical Sensors for Heart Rate Monitor) emit infrared, red, or green light (~550nm) towards the body and measure the amount of light reflected using a photodiode or phototransistor. Oxygenated hemoglobin present in the blood of the arteries has the characteristic of absorbing incident light, so by sensing the blood flow rate (change in blood vessel volume) that changes following heart contractions over time we are able to measure the pulse wave signal.
Also, since reflected light is measured, the range of suitable areas is not limited as with transmission-type pulse sensors.

Pulse wave measurement using red or infrared light can be affected by infrared rays contained in sunlight (i.e. outdoors), preventing stable operation. For this reason, usage indoors or semi-indoors is recommended.

For pulse wave measurement outdoors (i.e. by smart watches), a green light source that has a high absorption rate in hemoglobin and less susceptibility to ambient light is preferred, so ROHM utilizes green LEDs as transmission light sources.

This is a heart rate/biometric pulse rate detecting sensor
This is a plug & play-type sensor
The operating voltage of this sensor is +5V/+3.3V
Consumption of current is 4mA
Length is 0.625”
Width is 0.125
Integral Amplification
Circuit for cancellation of noise
Thus, this is all about an overview of the pulse sensor and its working. The stability & accuracy of this sensor drop far behind. An alternative to this kind of sensor is an easy pulse sensor because it is extremely stable. Apart from this sensor, a pulse oximeter sensor like MAX30100 is used to measure the heart rate with the blood oxygen.