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. 2024 Nov 29;24(23):7634.
doi: 10.3390/s24237634.

A Hybrid Photoplethysmography (PPG) Sensor System Design for Heart Rate Monitoring

Farjana Akter Jhuma  1 , Kentaro Harada  2 , Muhamad Affiq Bin Misran  1 , Hin-Wai Mo  2 , Hiroshi Fujimoto  2 , Reiji Hattori  1
Affiliations

A Hybrid Photoplethysmography (PPG) Sensor System Design for Heart Rate Monitoring

Farjana Akter Jhuma et al. Sensors (Basel). .

Abstract

A photoplethysmography (PPG) sensor is a cost-effective and efficacious way of measuring health conditions such as heart rate, oxygen saturation, and respiration rate. In this work, we present a hybrid PPG sensor system working in a reflective mode with an optoelectronic module, i.e., the combination of an inorganic light-emitting diode (LED) and a circular-shaped organic photodetector (OPD) surrounding the LED for efficient light harvest followed by the proper driving circuit for accurate PPG signal acquisition. The performance of the hybrid sensor system was confirmed by the heart rate detection process from the PPG using fast Fourier transform analysis. The PPG signal obtained with a 50% LED duty cycle and 250 Hz sampling rate resulted in accurate heart rate monitoring with an acceptable range of error. The effects of the LED duty cycle and the LED luminous intensity were found to be crucial to the heart rate accuracy and to the power consumption, i.e., indispensable factors for the hybrid sensor.

Keywords: LED duty cycle; LED luminous intensity; heart rate; light-emitting diode (LED); organic photodetector (OPD); photoplethysmography (PPG).

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Conflict of interest statement

Authors Kentaro Harada, Hin-Wai Mo and Hiroshi Fujimoto were employed by the company OPERA Solutions Inc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
(a) Schematic representation of the amplitude over time to explain PPG signal generation through tissues. (b) Schematic illustrations of transmissive mode PPG (LED and OPD are placed on the opposite side of the sensing location). (c) Schematic illustrations of reflective mode PPG (both LED and OPD are placed side by side on the sensing location).
Figure 2
Figure 2
Block diagram of the proposed hybrid PPG sensor system.
Figure 3
Figure 3
The layer structure of the OPD device.
Figure 4
Figure 4
(a) Block diagram of the driving circuit containing TIA, filter, and amplifier. (b) Customized PCB including the driving circuit and LED.
Figure 5
Figure 5
(a) FreeCAD design of the device holder. (b) Image of the 3D-printed device holder.
Figure 6
Figure 6
The steps of arranging the PPG sensor system.
Figure 7
Figure 7
(a) Schematic representation image of the OPD and actual image of prepared D1 device. (b–d) J–V characteristics of the prepared OPD devices D1, D2, and D3, respectively. The black and red solid lines indicate the dark and photocurrent densities, respectively. The photocurrent was obtained under a red LED illumination with a peak wavelength of 624 nm and a luminous intensity of 150.06 mcd. (e,f) On/off ratio of the OPD at different biasing voltages under the above-mentioned red and green LED illumination conditions.
Figure 8
Figure 8
Obtained PPG signal with hybrid PPG sensor.
Figure 9
Figure 9
Process flow chart of heart rate estimation.
Figure 10
Figure 10
(a) PPG signal before filtering (some disturbance can be seen at the signal peak due to the disturbance introduced while measuring from fingertip). (b) PPG signal after applying a moving average filter with a window size 3 to smooth the signal. (c) PPG signal after moving average and Butterworth bandpass filter with a cut-off frequency of 0.5 Hz–16 Hz to remove unwanted frequency signals.
See this image and copyright information in PMC

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