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

Figures

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  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).

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  Figure 2. Block diagram of the proposed hybrid PPG sensor system.

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  Figure 3. The layer structure of the OPD device.

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  Figure 4. (a) Block diagram of the driving circuit containing TIA, filter, and amplifier. (b) Customized PCB including the driving circuit and LED.

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  Figure 5. (a) FreeCAD design of the device holder. (b) Image of the 3D-printed device holder.

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  Figure 6. The steps of arranging the PPG sensor system.

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  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.

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  Figure 8. Obtained PPG signal with hybrid PPG sensor.

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  Figure 9. Process flow chart of heart rate estimation.

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  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.