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Separate host-adapter PCB (Arduino Nano ESP32-class host, 3.3 V)
  ├─ 3V3/PWR ───────────────┬────────────> VLED = 3.3 V (bulk pulse reservoir)
  │                          ├─ TPS79318 LDO ──> 1V8_AFE
  │                          └─ SPI/control pull-ups and MISO translator 3.3 V side
  ├─ GND ─────────────────────────────────────> common ground plane
  └─ Adapter FFC connector
       SCLK, MOSI, CS_N ───────────────────────> MAX86141 3.3 V-tolerant inputs
       MAX86141 SDO (1.8 V) ─> SN74LVC1T45 ───> MISO (3.3 V)
       INT_N <──────── open-drain + 3.3 V pull-up
       RESET_N ───────> active-low hardware reset by controlling LDO EN

MAX86141 optical AFE
  ├─ LED1/2/3 current sinks ─> SFH 7018A green/red/IR cathodes
  ├─ PD1_IN <─────────────── VEMD5080X01 cathode
  ├─ PD2_IN tied to PD_GND (unused second channel)
  └─ PD_GND ── R4 0Ω net tie ──> PCB GND (single point; nets remain distinct for guarded routing)

Sensor-board J1: Molex 0512810898, 8p 0.5 mm dual-contact non-ZIF right-angle SMT
  pin 1..8: 3V3_PWR, GND, SCLK, MOSI, MISO, CS_N, RESET_N, INT_N
  │
  └── 50 mm, 8-position, 0.5 mm FFC, same-side contacts ──> separate adapter-board connector
      Adapter connector orientation will be matched for final straight-through 1:1 pin order.

Optical components are reserved for PCB-bottom skin-contact placement in Stage 3; no PCB sizing or layout is part of Stage 1.