Relative Contact Load and PPG Modulation in a Low-LED-Drive Reflective Platform: A Single-Participant Feasibility Study

Reflective photoplethysmography (PPG) is sensitive to sensor–skin contact conditions. This engineering feasibility study examined synchronized relative-load voltage and Red/IR PPG in one participant using a MAX30101 sensor, nRF52833 microcontroller, and adjacent force-sensitive resistor (FSR). Five repeated trials followed a fixed low–medium–firm–medium-return sequence. Mean FSR voltages were 500.0 ± 22.1, 1179.8 ± 87.6, and 1996.6 ± 71.3 mV for the ascending stages. The 15 trial-stage summaries were repeated observations from the same participant. Higher FSR voltage was associated with lower IR AC/DC (within-trial r = −0.886; descriptive trial-fixed-effect slope, −0.0223 percentage points per 100 mV). Mean IR AC/DC was 0.560 ± 0.152% at low load, 0.218 ± 0.011% at firm load, and 0.381 ± 0.099% during medium return, compared with 0.330 ± 0.074% at the initial medium stage. Preliminary artifact controls documented setup-specific waveform changes. The adjacent FSR provides an uncalibrated spatial proxy rather than optical-site force or pressure. Fixed loading order prevents separation of load from time, adaptation, and loading history. These observations establish acquisition feasibility in the tested participant and arrangement, without establishing a causal pressure–response relationship, externally validated quality thresholds, or performance across users and devices.