Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a low-cost, open-source PPG acquisition and HRV analysis platform based on ESP32. The system is practical and educationally valuable, but several aspects should be clarified or enhanced:

1. Please clarify whether the ESP32 and PSoC modules were synchronized during validation. If not, discuss how physiological variability was controlled.
2. Provide more details or references about the “validated PSoC module” used for comparison (e.g., infrared LED, phototransistor, analog front-end specs).
3. Given ESP32’s built-in WiFi support, consider enabling TCP/UDP-based wireless streaming to improve system portability and real-world usability.
4. Add a physical diagram of the system to show the connections between the sensor, amplifier, ESP32, and GUI to help readers understand the system.
5. Please describe sensor placement (e.g., location, angle, fixation method) and techniques used to reduce motion artifacts or light interference.
6. Suggest including additional HRV metrics (e.g., LF/HF, Poincaré plots, sample entropy) to expand the system’s analytical depth.

Author Response

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The article titled “ESP32-Powered PPG Signal Acquisition and HRV Analysis System: Open-Source Hardware and Software for Research and Education” developed an open-source photoplethysmography (PPG) acquisition platform for high-speed analog signal acquisition, real-time control, and wireless communication. The study is interesting and exhibits some direct social impact. However, there are several major concerns that the author needs to address. Additionally, a major concern is whether the topic fits well within the core scope of the hardware journal, as the primary focus of this paper is not the hardware technologies.

 

1. The related works section provides a relatively comprehensive overview of existing systems; however, it would be beneficial to also highlight the strengths and specific advantages of this system.
2. The manuscript should further clarify what sets this work apart from existing PPG acquisition tools. Please elaborate on the unique advantages, innovations, or technical breakthroughs of the proposed platform in comparison to others.
3. More detailed information about the hardware design is necessary. For instance, the authors should explain the working principle of the PPG sensor, describe the signal acquisition process, and specify the experimental setup. Including schematic diagrams and real-world images of the complete system and sensor placement would greatly enhance clarity and reproducibility.
4. The paper mentions comparative verification with the PSoC module but lacks specific experimental data. It is recommended to add detailed comparative results.
5. Please provide further details on the ADC configuration, such as resolution, sampling rate, and input voltage range, as well as the filter design parameters, including type, order, and cutoff frequency. These factors are crucial for assessing signal integrity and the accuracy of HRV metrics.
6. How reliable is the whole system? How long can the system be measured and can it provide long-term testing?
7. It is recommended to include comparative experiments involving participants of different ages and physiological conditions to further validate the overall performance and robustness of the system.

Author Response

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This work presents a valuable contribution by providing an open-source, real-time data acquisition solution for PPG measurement, complete with GUI for data visualization. The system appears to have significant potential advantages, including high wearability, high integration, and low cost, which could make it a configurable and accessible tool for researchers and educators. However, the manuscript could be significantly strengthened by better presenting and supporting these potential advantages and by addressing several key areas for improvement.

1. Introduction of Related Work: The overall flow of the introduction and related works section needs improvement. Instead of simply listing ten representative systems, the authors should clearly articulate the limitations of these existing systems. The discussion should then transition to explicitly state how the proposed system addresses these shortcomings and accomplishes the challenges posed by current technology.
2. Consistency of Related Works: The section on related works lacks consistency in its presentation. The authors should revise this section to ensure a uniform flow, consistent English, standardized citation format, and a consistent naming convention and feature description for each system discussed.
3. Operating Instructions: The authors should update the instructions to explicitly include pip install scipy as a required step for the software to function correctly.
4. Supplementary Materials: The supplementary materials should include the binary executable file (gui.exe) for the GUI talked in the main text, as it was not found in the provided files.
5. Flow Diagram: The flow diagram in Figure 1 could be improved for clarity. A more detailed and clearly labeled diagram would better illustrate the system's architecture and the flow of data.
6. Hardware Description: An image of the physical device is needed. It would also be beneficial to include a discussion on whether this specific placement and distance of LED and PD affects the performance of the system.
7. Validation Details: While the paper mentions validation with a PSoC-based module, it fails to provide any explanatory details or results of this validation process. 
8. The abstract mentions the calculation of HRV metrics such as RMSSD and SDNN. However, the paper lacks any further analysis, or results related to these metrics. The authors should either include HRV computation or revise the abstract to avoid overstating.

Author Response

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed the main concerns. I offer the following two suggestions before final acceptance:

1. Although the authors chose to use a UART connection for simplicity and teaching stability, it would be helpful to include examples from past studies that used ESP32’s built-in WiFi (such as sending data via TCP or UDP). This would make the system’s technical background more complete and give readers clearer ideas on how to upgrade to wireless transmission in the future.
2. While the current system provides basic RR interval and HR analysis, we recommend adding a few fundamental PPG waveform features, such as pulse amplitude variability, rise/fall time, or signal quality metrics. These additions would enhance the platform's educational and analytical value without significantly increasing system complexity.

Author Response

ROUND 2: Reviewer 1

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The author has addressed all my concern.

Author Response

ROUND 2: Reviewer 2

 

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript and addressed most of the previous comments, which has improved the overall quality of the work.

However, a few points remain:

1. Figure 3 – Including a scale bar would improve clarity.

2. Supplementary Materials – The authors mentioned updated source code and a video in their response, but I was not able to locate these in the revised supplementary files. Please ensure that the supplementary code and video files are properly included and accessible.

With these minor issues addressed, I believe the manuscript will be ready for acceptance.

Author Response

ROUND 2: Reviewer 3

 

We sincerely thank the reviewers for their valuable and constructive comments. Their timely observations and insightful suggestions were instrumental in refining the manuscript and have significantly improved the clarity, structure, and overall presentation of the article.

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

No further suggestions.