Wire Harness Testing Schematic and PCB Design -- 2

1) Project goal Design a single PCB device for testing cable harnesses up to 25 wires, where both ends of the same harness are connected to the board: Side A: 25 lines (A1..A25) Side B: 25 lines (B1..B25) Total: 50 signal lines on the PCB. The detailed test logic (continuity/shorts/mapping) will be implemented in firmware later. For this phase, the key deliverable is robust hardware (schematic + PCB) prepared for such testing. 2) Scope of work (hardware only) Electrical schematic PCB layout Production files: Gerbers + drill BOM (with specs and acceptable alternatives) Assembly files (pick&place/positions) preferred Short mapping table: MCP23017 pins → A/B line numbers (Firmware is NOT required in this phase. Optional “I2C scan / basic blink” is a bonus.) 3) Main hardware requirements MCU ESP32-WROOM (preferred for availability and support) I/O expansion Use MCP23017 (I2C) I/O expanders — I already have these and want them used Quantity: minimum 4× MCP23017 (64 I/O total → spare pins) Unique I2C addresses via A0/A1/A2 (e.g. 0x20, 0x21, 0x22, 0x23) Power input Supply input: 9–32 V DC On-board step-down regulation to stable 3.3V (for ESP32 + MCP23017) Input protection: reverse polarity protection TVS on input fuse / polyfuse proper filtering and EMI-aware layout 4) Connectors (mandatory) Pluggable / detachable terminal blocks, 3.5 mm pitch Two connector groups: Side A: 25 pins (A1..A25) Side B: 25 pins (B1..B25) Clear silkscreen labeling on PCB: A1..A25 and B1..B25 5) User interface (mandatory) TEST push button (momentary) – will trigger test in future firmware LEDs: Green LED = PASS/OK Red LED = FAIL/ERROR Power LED (POWER) 6) ESP32 pin assignment (required – proposal) I2C (MCP23017) SDA = GPIO21 SCL = GPIO22 I2C pull-ups: 4.7kΩ to 3.3V on SDA and SCL TEST button GPIO27 (input) Pull-up (10k to 3.3V or internal pull-up), button to GND Optional debounce capacitor ~100nF LEDs GREEN = GPIO25 RED = GPIO26 POWER LED: always-on from 3.3V/5V rail (does not need MCU control) Note: avoid ESP32 strapping pins in ways that can affect boot (GPIO0/2/12/15 etc.) for LEDs/buttons if it risks boot issues. 7) Critical requirement: protect ALL A/B lines from accidental 12V / 24V On the A and B connectors there may be accidental external DC voltage present (12V or 24V) depending on what the harness is connected to. The design must ensure: ESP32 and MCP23017 are not damaged input voltages are clamped/limited to safe levels device remains functional after such an event (within reasonable energy/time assumptions) Minimum required protection per each line (A1..A25 and B1..B25) 330Ω series resistor on every line (mandatory; placed close to the connector) Additional protection components sized so that 12/24V will not harm the electronics (designer must calculate currents): clamp to 3.3V/GND (ESD/TVS/diode clamping) with proper current limiting, and/or buffering/isolation stage if it’s more reliable/cost-effective The schematic must include a short note specifying the assumed abuse case, e.g. “Accidental 24V DC applied to a single line for a few seconds” and show the design rationale (safe clamp current). 8) USB programming / service (mandatory) On-board USB connector (USB-C preferred) for ESP32 programming and service UART Add a USB-to-UART IC (e.g., CP2102/CP2104 or CH340, designer to choose proven/available part) Preferred: auto-programming support via control lines to: EN/RESET BOOT (GPIO0) so flashing can be done without manual button sequences Add ESD protection for USB D+/D- lines (good practice) 9) PCB requirements 4 mounting holes EMI-aware routing (switching regulator kept away from sensitive signal routing; proper ground strategy) Clear documentation in silkscreen and in the project notes 10) Deliverables KiCad/Altium/Eagle(Fusion360) project: schematic + PCB Gerbers + drill files BOM with specifications and alternatives Pick&place / assembly position files (preferred) MCP23017 → A/B lines mapping table

Project ID: 40258520