KUdos RPi Expansion HAT

Project Overview

The KUdos RPi Expansion HAT is a custom PCB I designed to interface a Raspberry Pi Zero 2W with the KUdos VEX-U robot electronics stack. The board provides two I2C ports, native RS485 communication, Limelight camera integration, and power management for the coprocessor system.

This HAT is part of a fully custom electronics stack that the KUdos team developed for VEX-U competition, which is rare in the VEX ecosystem where most teams use stock electronics.

Role: PCB designer, including schematic capture, layout, and fabrication coordination.

Tools: KiCad, JLCPCB

KUdos RPi Expansion HAT

Features

Dual I2C Ports: Two independent I2C buses for sensor integration
RS485 Interface: Native RS485 for communication with the main robot controller
Limelight Integration: Dedicated connector for Limelight vision camera
USB-C Input: Power and data via USB-C
On/Off Control: Hardware power switch
Toggle Switch: Configurable GPIO toggle
HAT Form Factor: Mounts directly on Raspberry Pi Zero 2W GPIO header

Schematic

KUdos RPi HAT Schematic

Design Details

RS485 Transceiver

The board uses an SP3485EN-L_TR RS485 transceiver for communication with the VEX V5 brain:

DI/RO: Data in/out connected to Pi's TX/RX pins
RE/DE: Direction control via PNP transistor (Q1 - MMBT5401)
A/B: Differential pair output to J4 connector
ESD Protection: SMAJ12CA TVS diodes (D2, D3, D4) clamp transients on the RS485 lines

The direction switching uses a transistor rather than direct GPIO to ensure proper timing and drive strength.

Dual I2C Ports

Two 4-pin I2C connectors (J2, J3) with identical pinout:

Pin	Signal
1	GND
2	3V3
3	SDA
4	SCL

Each port has TVS dual diodes (D5, D11 - D_TVS_Dual_AAC) for ESD protection on the data lines. Both ports share the same I2C bus from the Pi.

Power Circuit

USB-C Input (P1): VBUS provides 5V power
Power Switch (SW2): SPDT switch between VBUS and VIN rail
Power LED (D1): Indicates when board is powered, with R1 current limiting resistor
3V3 Rail: Sourced from Pi's 3.3V regulator via GPIO header

GPIO Switch

SW1 is a general-purpose SPDT switch directly connected to a Pi GPIO pin. Used for robot configuration (e.g., selecting Blue vs Gold robot mode).

Component Selection

Component	Part	Rationale
RS485 Transceiver	SP3485EN-L_TR	3.3V compatible, low power, half-duplex
Direction Transistor	MMBT5401	PNP for active-low RE control
RS485 TVS	SMAJ12CA	12V standoff, handles ESD and voltage spikes
I2C TVS	D_TVS_Dual_AAC	Dual-line protection in single package

Connectors

The main motivation for this board was replacing unreliable header connections with proper locking connectors:

Connector	Type	Purpose
J1	2x20 Header	Pi GPIO interface
J2, J3	4-pin JST	I2C sensors (Pinpoint)
J4	4-pin JST	RS485 to VEX brain
P1	USB-C	Power input

Fabrication

PCB Fab: JLCPCB (2-layer board)
Assembly: Hand-soldered SMD components
Status: Designed and ordered, but not tested before Worlds due to shipping delays