# Portable Sensor System for Registration, Processing and Mathematical Analysis of PPG Signals
## 2 Portable Sensor System for Registration, Processing and Analysis of PPG Signals
### Portable Sensor Device for Registration of PPG Signals  
The portable sensory device (Figure 1) is used to record changes in blood volume by a sensor placed on the patient's finger and/or ear. PPG technology is an optical measurement technique, whereby, using an invisible infrared light source transmitted to human tissue and a photodetector, the intensity of the light transmitted or reflected by the tissues is recorded.  
Figure 1: The photoplethysmographic data capture device.  
The created photoplethysmographic device is managed from a microcontroller, as for the specific conversion a microcontroller from the LPC546 xx MCU family (manufactured by NXP Semiconductors), was used. The PPG device can work with discrete sensors that are external to the device (in the form of a pinch that is placed on the index finger of the hand or the ears).  
The AFE4490 integrated circuit (manufactured by Texas Instruments) is used to process the signals that they receive, which is used to convert the signals and control the LEDs. This integrated circuit has two separate channels, which allows for the use of two external sensors (pinch) at the same time. Standard Nellcor-compliant PPG finger and ear sensors are used (Figure 2). In addition to the ability to work with external sensors, the device has a built-in integrated sensor (using the MAX30102 scheme, manufactured by Maxim Integrated), which allows for short-term measurements of heart rate and oxygen content in the blood.  
Several types of memory are built into the PPG device, such as for RAM, the internal memory of the microcontroller is used. The device has a real-time clock (DS3231/manufactured by Maxim Integrated) powered by a separate battery, which can track and record the exact astronomical time of the study (recording of photoplethysmographic signals of the investigated individual). An expansion connector (if necessary to add new functionality), a Bluetooth module, and more module can be installed.  
The device's PCB is four-layer and it is designed with an open source KiCAD PCB design system [41].  
Power is provided by a Li-ion rechargeable battery and a buck-boost SMPS converter (TPS63001, manufactured by Texas Instruments) and a charger circuit (BQ24075, manufactured by Texas Instruments) for battery management and charging, when the device is connected to a PC through a USB connector.  
The device has two buttons. One for switching ON/OFF, while using a smart button on/off controller (STM6601/STMicroelectronics) and other that can be used by a software.  
Figure 2: Photo of sensors used.  
The created PPG based device can transmit the logged data over a period of time (from a few seconds to several hours) or continuously to the software web based system for processing and analyzing the received data.  
PPG sensing technology uses the properties of the light to detect changes in the arterial blood volume in a closed area (tip of the finger, ear, nose, wrist, etc.). The principle of operation of PPG sensors is to record changes that occur in the light intensity upon reflection or transmission through the tissue. These differences reflect the changes in the blood perfusion of the human tissue. The PPG sensors can operate in two modes: transmission mode and reflection mode. In the reflection method, the light source and the photodetector are placed on the same side of the fabric, while two different sides of the fabric are placed in the transmission method.  
The experimental device works in two modes: transmission mode, with integrated sensor, which is external to the device and in the form of a pinch; reflection mode via a discrete sensor that is integrated into the device.  
For the discrete sensor, the red LED operates at a wavelength of 660 nm; the infrared LED operates at wavelength-880 nm. For the integrated sensor (max30102), the red LED operates at a wavelength of 660 nm; the infrared LED operates at wavelength-880 nm. Standard Nellcor compatible PPG sensors for finger and ear are used [42].