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Digital Inputs

The regulator has two different styles of digital input architectures, depending on the pin in question. They are all exposed in the Ethernet Connectors (see Connectors & Wiring).

  1. Optocoupler Style — Activated by a positive voltage input (5–54 V applied to the input pin)
  2. Ground Style — Activated by shorting the input pin to ground

The two styles serve different installation scenarios: opto-style inputs are right for signals coming from elsewhere in the boat's 12/24/48 V system (ignition, BMS contacts, mode-switch outputs), where galvanic isolation matters and the signal source already has its own ground reference. Ground-style inputs are right for local switches mounted near the regulator that simply close to its own GND — they consume less idle power, eliminate the optocoupler aging concern, and are appropriate when the signal source is electrically local.

Optocoupler Channel Map (5 channels)

Function Opto IC Series Diode ESP32 GPIO
IGNITION (engine wake) U5 D19 GPIO1
HIGH/LOW (charging mode) U10 D25 GPIO39
BMSLogic (BMS enable) U16 D27 GPIO42
FORCEFLOAT (force float mode) U17 D28 GPIO40
EXTRA_OPTICAL_GPIO1 (spare) U26 D34 GPIO8

All five channels are identical optically and electrically — only the destination GPIO differs. The schematic uses U1 in the example below as a generic representative; substitute the relevant designator from the table above for the channel of interest.

Note (audit 2026-05-23): Earlier doc revisions used "U1" as a placeholder designator for the optocoupler. U1 in the V9 schematic is actually the LSM6DSOX IMU — see LSM6DSOX IMU. The optocouplers are U5/U10/U16/U17/U26. Example schematic diagrams below retain "U1" as a generic placeholder.

Optocoupler Style Digital Inputs

Optocoupler Schematic

Design Requirements

  • Input voltage range: 5V to 54V on any of 6 digital input channels that use optical isolators
  • Temperature range: -40°F to 180°F (-40°C to 82°C)
  • Lifetime target: 10+ years at typical voltages (~12V), 3-6 years at maximum voltage (54V)

How an Optocoupler Works

The VO615A optocoupler provides electrical isolation between input circuit and ESP32 GPIO using light transmission:

Input Side (LED)

  1. Input voltage applied across current limiting resistor and IR LED
  2. Forward current flows through LED, producing infrared light
  3. Light intensity proportional to forward current

Output (ESP32 GPIO) Side (Phototransistor)

  1. IR light strikes phototransistor base region
  2. Light creates electron-hole pairs, acting as base current
  3. Transistor turns on, allowing collector current to flow
  4. Collector current = LED current × CTR (Current Transfer Ratio)
  5. When sufficient collector current flows, voltage at collector drops low enough to register as logic LOW on ESP32
  6. Voltage otherwise pulled high (3.3V) by pull up resistor to 3.3V Supply

Isolation Benefits

  • Protection: High voltage transients on input side cannot damage ESP32
  • Flexibility: Can accept input signals from 5V to 60V (lower life at higher voltages)

Component Selection

Optocoupler: VO615A-9

  • CTR: 200-400% minimum (vs 50-600% for base VO615A)
  • Reason: Higher CTR enables reliable switching at low input voltages (5V), minimal extra cost

Current Limiting Resistor: 6.8kΩ, 1W, 2512 SMD

  • Calculation basis:
  • At 5V: IF = (5V - 1.43V) / 6.8kΩ = 0.53 mA
  • At 54V: IF = (54V - 1.43V) / 6.8kΩ = 7.7 mA
  • Power rating: 404mW at 54V requires 1W resistor for safety margin

Pull-up Resistor: 13kΩ, 0603 SMD

  • Logic HIGH: When LED is OFF, phototransistor is OFF, collector pulled to 3.3V through 13kΩ
  • Logic LOW: When LED is ON, phototransistor conducts, pulling collector toward ground
  • Switching threshold: ESP32 typically switches at ~1.65V (50% of 3.3V)

Performance Analysis

Input Voltage LED Current vs Min (1mA) Lifetime Power (Watts) Power (mA@12V)
5V 0.53 mA 53% of min 20+ years 1.9 mW 0.16 mA
12V 1.57 mA 157% of min 20+ years 17 mW 1.4 mA
28V 3.91 mA 391% of min 10+ years 104 mW 8.7 mA
54V 7.7 mA 770% of min 3-6 years 404 mW 33.7 mA

Power consumption notes: - Power is consumed only when Input Signal is active (ie external switch is ON) - No power consumption when Input Signal = 0V (ie external switch OFF) - LED power = Forward current × 1.43V (LED forward voltage) - Resistor power = Forward current² × 6.8kΩ

Protection Diode: 1N4148W (SOD-123)

Function: Reverse Voltage Protection

The 1N4148W diode (SOD-123 package, LCSC C81598 — V9 BOM lists one per opto channel at D19, D25, D27, D28, D34) provides critical protection for the optocoupler LED:

  • Normal operation: Diode is reverse biased across LED, no current flows
  • Reverse voltage protection: If Input Signal goes negative, diode conducts and clamps reverse voltage to ~0.7V
  • Prevents LED damage: VO615A LED can only handle 6V reverse voltage maximum

Why Critical

  • Load dump transients: Long wires / disconnection can create voltage spikes
  • Electrical noise: Marine electrical systems are electrically noisy environments

Specifications & Failure Limits

  • Forward voltage: 0.7V typical, 1.25V maximum at 10mA
  • Reverse voltage rating: 75V maximum
  • Forward current rating: 300mA continuous, 2A peak (1µs pulse)
  • Power dissipation: 350mW at 25°C

Failure Analysis

The diode will fail if: - Forward current > 300mA continuous: At maximum negative transient - Reverse voltage > 75V: If positive Input Signal exceeds 75V (well above our 54-60V max) - Power dissipation > 350mW: P = If × 0.7V, so fails at If > 500mA

The 1N4148WSX provides robust protection for all expected marine transient conditions.

System Power Consumption

Condition LED Side ESP32 Side Total Power Total (mA@12V)
Input OFF 0 mW 0.83 mW 0.83 mW 0.07 mA
Input ON (12V) 17 mW ~0 mW 17 mW 1.4 mA
Input ON (54V) 404 mW ~0 mW 404 mW 33.7 mA

LED Current Guidelines

  • 1-3mA: Conservative operation (20+ year lifetime)
  • 3-8mA: Moderate operation (5-10 year lifetime)
  • 8-15mA: Aggressive operation (1-5 year lifetime)

Design Validation

  • 5V switching: 0.53mA with 200% CTR provides reliable switching despite being below 1mA datasheet specification
  • 54V survival: 7.7mA current provides marginal 3-6 year life if 100% duty cycle
  • Temperature performance: VO615A-9's high CTR provides margin for temperature derating across -40°F to 180°F range

Schematic Implementation

Input Signal ──[6.8kΩ, 1W]──[LED|──── GND
                                 |
                                 |  (Optical coupling)
                                 |
ESP32_GPIO ──[13kΩ]──3.3V        |
        └─────────────[Collector|Emitter]──── GND

Bill of Materials (Optocoupler Style — per channel)

Reference (per channel) Part Number Description Package LCSC
U5 / U10 / U16 / U17 / U26 VO615A-9X017T Optocoupler, CTR 200–400 % SMDIP-4 C145478
R65 / R74 / R75 / R67 / R76 / R87 (6.8 kΩ pool) — Current-limit resistor, 1 W 2512 C26073
R62 / R63 (13 kΩ pool) — Pull-up resistor to 3V3 0603 C22797
D19 / D25 / D27 / D28 / D34 1N4148W Reverse-bias protection diode SOD-123 C81598

The 6.8 kΩ resistor pool (R65–R87 range) is shared across all five opto LED-drive series-resistors plus the buzzer-circuit gate pull-up (R65). The 13 kΩ pool (R62, R63) is shared between the two HCPL2531 output pull-ups for VeDirect / NMEA0183.

Ground Style Digital Inputs

Schematic

3.3V ──[10kΩ]──┬──[C51:1µF]──GND
               │
               ├──[D32:ESD]──GND
               │
               ├── Output Signal to ESP32 GPIO
               │
               └──[R5:500Ω]── Input Signal

Design Requirements

  • Input method: Short to ground activation
  • Temperature range: -40°F to 180°F (-40°C to 82°C)
  • Debouncing: Hardware RC filter for switch bounce elimination
  • Protection: ESD protection for exposed inputs

How Ground Style Inputs Work

This design uses RC debounce filtering:

Normal Operation (Switch Open)

  1. Input Signal is not connected (floating or open)
  2. No current flows to Ground
  3. Pull-up resistor R57 (10kΩ) pulls GPIO pin to 3.3V
  4. ESP32 GPIO reads logic HIGH (3.3V)

Activated Operation (Switch Closed to Ground)

  1. Input Signal is shorted to ground by external switch/contact
  2. Current flows from 3.3V through R57 (10kΩ) to ground via the switch
  3. Junction voltage: V_out = 0V (direct connection to ground)
  4. ESP32 GPIO reads logic LOW (0V << 0.8V threshold)

Component Analysis

Pull-up Resistor: R57 (10kΩ)

  • Function: Pulls Output Signal HIGH when Input Signal is not grounded
  • Power when input open: P = 3.3V²/10kΩ = 1.089 mW
  • Power when input grounded: P = 3.3V²/10kΩ = 1.089 mW (continuous)

Debounce Capacitor: C51 (1µF)

  • Function: Filters switch bounce and electrical noise
  • Time constant: τ = R57 × C51 = 10kΩ × 1µF = 10ms
  • Settling time: ~3τ = 30ms for clean switching
  • Noise immunity: Filters high-frequency electrical noise

ESD Protection Diode: D32 (ESD5Z25.0T1G)

  • Function: Clamps overvoltage transients to protect ESP32 GPIO
  • Clamp voltage: 25V typical
  • Application: Protects against static discharge and voltage spikes on input wiring

Performance Analysis

Switching Characteristics

  • Rise time: Limited by RC time constant = 10ms
  • Fall time: Much faster, limited by R5 and ESP32 input capacitance
  • Debounce time: ~30ms eliminates typical switch bounce

Power Consumption

Condition Current Power Power (mA@12V)
Input Open 0.33 mA 1.089 mW 0.09 mA
Input Grounded 0.33 mA 1.089 mW 0.09 mA

Power consumption notes: - Continuous power draw regardless of input state (due to pull-up resistor) - Very low power consumption suitable for battery applications - Power draw is essentially constant whether switch is open or closed

Voltage Levels

  • Logic HIGH: 3.3V (input open/floating)
  • Logic LOW: 0.157V (input grounded via voltage divider)
  • Switching threshold: ESP32 switches at ~0.8V, providing excellent noise margin
  • Noise margin HIGH: 3.3V - 0.8V = 2.5V
  • Noise margin LOW: 0.8V - 0.157V = 0.64V

ESP32-S3 Internal ESD Protection and Real ESD Event Analysis

ESP32-S3 Internal Protection Specifications

From the ESP32-S3 datasheet: - Human Body Model (HBM) ESD tolerance: ±2000V - Charged Device Model (CDM) ESD tolerance: ±1000V
- Absolute maximum input voltage: 3.6V (VDD + 0.3V) - Internal protection: Snapback devices on all GPIO pins

What Actually Happens During an ESD Event

Scenario: 8kV HBM discharge to Input Signal wire

  1. Initial ESD pulse: 8000V spike with ~1.5kΩ source resistance (HBM model)
  2. External TVS diode (D32) activates:
  3. Clamps voltage to ~25V within nanoseconds
  4. Current through D32: I = (8000V - 25V) / 1500Ω = ~5.3A initially
  5. D32 can handle 2A peak current for 1µs pulses - adequate for HBM discharge
  6. ESP32 internal protection response:
  7. 25V still exceeds the 3.6V absolute maximum
  8. Internal snapback devices activate at ~7-10V
  9. Snapback devices clamp GPIO voltage to ~5-7V
  10. Current flows through internal protection to chip ground
  11. Current path: ESD energy → D32 → ESP32 internal snapback → chip ground
  12. Result: GPIO survives due to dual protection layers

Why 25V TVS Clamp Voltage Works

The key insight: The ESP32's internal ESD protection is designed to handle exactly this scenario. The internal snapback devices are specifically designed for ESD events where voltage exceeds normal operating range.

Espressif's official position (from ESP32 forum): "The ESP32 has internal snapback devices in order to handle ESD on all pins. The external ESD protector is simply a belts-and-braces approach to make sure someone doesn't accidentally zap through the internal protections."

Design Validation for ESD Protection

IEC 61000-4-2 Contact Discharge Test (typical requirement): - Test voltage: ±8kV - With D32 + ESP32 internal protection: PASS - Reasoning: Dual protection layers handle energy dissipation effectively

Real-world ESD sources: - Human walking on carpet: 2-15kV - Handling in dry environment: 5-25kV
- Protection margin: Adequate for typical applications

The design provides robust ESD protection through the combination of external TVS diode current limiting and ESP32's proven internal snapback protection, without requiring additional current limiting resistors that would compromise digital functionality.

Design Validation

  • Noise immunity: 1µF capacitor filters electrical noise effectively
  • ESD protection: 25V clamp diode protects against static discharge
  • Current limiting: No current limiting resistor - relies on ESP32 internal protection
  • Low power: <1.1mW continuous power consumption
  • Reliable switching: Large noise margins ensure reliable operation

Bill of Materials (Ground Style — per channel)

Reference (representative) Part Number Description Package Notes
R57 (and other 10 kΩ pull-ups in the 10k pool) RC0603 10 kΩ 1 % Pull-up to 3V3 0603 Shared 10 kΩ pool across the board
C51 (and other 1 µF pool members) GRM188R71H105KAS5D or equiv. RC debounce cap 0603 Shared 1 µF pool
Series input resistor 4.7 kΩ (R4/R5/R6/R7 — 0402) Fault current limit on input lead 0402 Not all channels populate this — verify per schematic
D32 (and PESD pool) PESD5V0S1BA TVS clamp to GND SOD-323 DNP in V9 — see V9 BOM dnp list. ESP32 internal ESD clamps are relied upon instead.

V9 population caveat: Per the V9 as-ordered BOM, the PESD5V0S1BA sensor-protection diodes (D32, D35, D38, D39, D40, D41, D42, D43, D44, D45, D46, D47, D48, D49, D51) are not populated. The ground-style inputs rely on the ESP32's internal snapback ESD protection alone. Adding the external TVS on a future revision is the recommended path if higher ESD margin is required (see the "ESD event analysis" section above — current design is adequate for HBM but minimal margin remains).

Comparison: Optocoupler vs Ground Style

Feature Optocoupler Style Ground Style
Input Range 5V to 54V Ground short only
Power (Active) 1.4–33.7 mA @ 12V 0.09 mA @ 12V
Power (Inactive) 0.07 mA @ 12V 0.09 mA @ 12V
Component Count 4 components 4 components
Cost Higher (optocoupler) Lower (passive)
Complexity Medium Low
Noise Immunity Excellent Good
Applications High voltage, isolation Simple switches, low voltage