What is PPG(Photoplethysmography) sensor? how does it work?

 A PPG (Photoplethysmography) sensor is a non-invasive optical sensor used to measure various physiological parameters, most commonly heart rate and blood oxygen saturation (SpO2). It works by utilizing light to measure the changes in blood volume and oxygenation in the microvascular tissue.

The basic principle of PPG involves illuminating the skin with a light source, typically an LED (Light-Emitting Diode), and measuring the changes in light absorption or reflection caused by the pulsatile nature of blood flow. The sensor consists of a light source and a photodetector placed on opposite sides of the skin, typically on a fingertip, earlobe, or wrist.

The light emitted by the LED passes through the skin and reaches the underlying blood vessels. When the heart beats, there is a cyclic change in blood volume due to the arterial blood flow. As the arteries expand and contract with each heartbeat, the amount of light absorbed or reflected by the blood changes. Basically PPG circuit is a light sensor circuit.

The photodetector in the PPG sensor captures the light that is transmitted or reflected back from the tissue. The changes in light intensity detected by the photodetector are then converted into an electrical signal, which is further processed to extract meaningful physiological information. 

The following shows circuit diagram of a typical DIY implementation of a PPG sensor with Arduino. It displays the beat per minutes(BPM) of heart rate on serial monitor.

PPG with Arduino circuit diagram

A simpler and similar circuit was demonstrated in the tutorial photodiode light sensor circuit with Arduino. Instead of displaying the heart beat rate(the BPM) on serial monitor, we can use HC-05 Bluetooth module with Arduino or IRF540N arduino circuit to transfer BPM data wirelessly to PC or mobile phone.

In a typical PPG waveform, there are two primary components: the AC (alternating current) component and the DC (direct current) component. The AC component represents the pulsatile changes in blood volume associated with each heartbeat, while the DC component represents the baseline level of blood volume.

The PPG signal is then analyzed to extract heart rate and SpO2 information. Heart rate can be determined by measuring the time interval between successive peaks in the AC component of the PPG waveform. SpO2, which indicates the oxygen saturation level in the blood, is estimated based on the ratio of oxygenated to deoxygenated hemoglobin as inferred from the characteristics of the PPG signal.

PPG sensors are widely used in medical devices, such as pulse oximeters and fitness trackers, to monitor vital signs and track physical activity. They offer a non-invasive and convenient method for continuous monitoring of heart rate and oxygen saturation in real-time. Additionally, PPG sensors have found applications in areas like sleep monitoring, stress assessment, and cardiovascular health monitoring.

Video demonstration of PPG sensor with Arduino

Following video demonstrates how a PPG sensor with Arduino circuit works.

It is important to note that while PPG sensors provide useful information about physiological parameters, they are not intended for diagnosis or medical treatment. In case of any health concerns, it is always recommended to consult a healthcare professional for proper evaluation and guidance. See next PPG Sensor with Arduino: A Gateway to Health Monitoring.

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