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Project Specification
Design Summary
Status: Draft
Manufacturing target: Prototype
Software / firmware: Controlled from A-MAIN ESP32 and MCP23017 I/O expansion
Scope
Purpose
This board is the Auxiliary Module PCB for the PROD-TRIBO tribology test stand. It expands the A-MAIN controller with PWM load control, two stepper motor channels, a precision 5 V analog reference, isolated 24 V status feedback, RTC, SD card access, and external harness connectivity.
In scope
  • 12 V power input and star-ground entry
  • PCA9685-based PWM/control distribution
  • Five low-side MOSFET load drivers for RGB LEDs and two chamber fans
  • One low-side MOSFET relay driver for K3
  • Two A4988-based stepper motor driver channels
  • REF195-derived 5.000 V analog reference output for ADS1256/sensors
  • Isolated 24 V PELV feedback using three optocouplers
  • RTC and SD module interfaces
  • External connectors J1 through J12 per TRIBO-ELEK-012
Out of scope
  • PCB placement/routing optimization
  • Resolution of all TBD items in the originating specification
  • Functional safety certification for K3 path
System context
The auxiliary board sits between the A-MAIN ESP32 controller and the external loads, motors, and instrumentation harnesses of the tribology test stand. It receives power and logic signals from A-MAIN, distributes PWM and stepper control, provides an accurate analog reference to the ADS1256 measurement chain, and isolates selected 24 V cabinet signals before returning them to 3.3 V logic.
Key interfaces
  • Power input: +12V and GND at J1, +3V3/+5V/I2C/IO_B4 at J2
  • SPI and logic interface: J3 to ADS1256 and SD module
  • Analog/instrument harness: J4 with +5V, SPI, DRDY, and +5V_REF
  • Chamber power/load interface: J5
  • Stepper motor power stage outputs: J6 and J7
  • Stepper logic control: J8
  • TEC PWM/dir signals: J9
  • K3 relay output: J10
  • Isolated 24 V input side: J11
  • 3.3 V logic feedback output side: J12
Architecture
Diagram



  
    
  
  
    
  





















12V and logic inputs
PCA9685 PWM and logic distribution
REF195 node_5V analog reference
LED fan and K3 MOSFET drivers
Stepper driver section U2 U3
RTC SD ADS1256 and control headers
24V PELV optocoupler inputs
PC817 isolated feedback
Chamber loads and K3 relay
M4 and M5 motors
Requirements
Functional
  • The board shall provide a PCA9685 PWM controller at I2C address 0x40 with VCC at 3.3 V and V+ at 5 V.
  • The board shall drive five low-side MOSFET outputs for LED_R, LED_G, LED_B, FAN_M3a, and FAN_M3b.
  • The board shall drive a K3 relay coil using a dedicated low-side MOSFET controlled by IO_B4.
  • The board shall host two A4988 stepper driver modules for M4 and M5.
  • The board shall generate a precision +5V_REF rail from +12V using REF195 for ADS1256 AVDD and sensor excitation.
  • The board shall expose SPI to the ADS1256 harness and the SD module with independent chip selects.
  • The board shall expose RTC access over the shared I2C bus.
  • The board shall isolate three 24 V PELV feedback signals using PC817 optocouplers and present active-low 3.3 V logic outputs.
Electrical
  • Input power: +12V from J1, +3V3/+5V from J2.
  • Key rails: +12V, +5V, +5V_REF, +3V3, GND, isolated PELV_GND.
  • REF195 load current target is approximately 22 mA and shall remain below 30 mA.
  • Optocoupler LED series resistance shall be 3.3 kΩ at 1/4 W, producing approximately 6.9 mA from 24 V.
  • MOSFET gate resistors shall be 100 Ω for Q1-Q5 and 1 kΩ for Q6.
  • MOSFET gate pull-down resistors shall be 10 kΩ.
  • A4988 VMOT bulk capacitance shall be 100 µF per driver.
  • REF195 input and output capacitance shall be 10 µF each.
  • I2C pull-up pads should be provided only if not already present on A-MAIN.
Mechanical / environmental
  • MOSFETs use TO-220 packages.
  • Stepper drivers are socketed Pololu-style carrier modules.
  • Maintain at least 2 mm creepage between PELV and 3.3 V domains.
  • No copper pour under the optocoupler isolation barrier.
  • Provide mounting clearance for M3 standoffs and at least 5 mm to cabinet housing.
Key constraints
  • Star ground shall be established at J1.2.
  • REF195 analog ground return shall go separately to the star point and avoid motor currents.
  • Motor and +12V high-current paths require wider traces than logic/reference paths.
  • The 24 V PELV input domain shall remain galvanically isolated from PCB logic ground.
  • The board shall preserve unresolved TBD items in documentation without blocking schematic completion.
  • Prefer surface-mount passives; connectors and power/mechanical parts may remain through-hole.
Dependencies and risks
Dependencies
  • A-MAIN board provides +3V3, +5V, I2C, SPI chip-select host logic, and IO_B4.
  • ADS1256 and sensor chain consume +5V_REF and are external to this board.
  • Motor and relay operating details depend on external harnesses and final system settings.
Key risks
  • Missing confirmation of SD_CS assignment, I2C pull-up ownership, and some optocoupler source semantics.
  • M5 stepper current setpoint remains TBD pending motor data.
  • Incorrect grounding around REF195 could degrade analog reference performance.
Validation
Success criteria
  • Complete schematic includes all major functional blocks and connectors from TRIBO-ELEK-012.
  • Nets represent the documented power, control, motor, and isolated interface connectivity.
  • Open items remain clearly documented where the specification is incomplete.
Planned checks
  • Schematic completeness review against the extracted block list and netlist.
  • Review for missing decoupling, pull-downs, pull-ups, and connector wiring.
  • Verify separated analog and isolated domains are preserved in schematic intent.
Release-facing notes
Expected deliverables
  • Schematic
  • BOM-ready component selection for all placeable parts available in the Flux library
  • Documented open questions and assumptions
Special release notes
  • I2C pull-up components should be included as optional placeholders unless A-MAIN ownership is confirmed.
  • K3 flyback diode is intentionally omitted per specification due to avalanche-rated MOSFET assumption.
  • ADS1256 reset and sync are assumed hardwired on the external module.
Change notes / open questions
Changes in this revision
  • Converted TRIBO-ELEK-012 into project-level requirements and architecture.
  • Captured board interfaces, constraints, and unresolved TBD items for schematic execution.
Open questions
  • TBD-1: SD_CS GPIO number assignment at A-MAIN
  • TBD-3: Final A4988 microstep setting for U2 and U3
  • TBD-4: Final VREF/current target for M5 oil pump driver
  • TBD-6: Whether A-MAIN already provides I2C pull-ups
  • TBD-8: Whether Arctic P12 PWM pin 4 needs on-board handling
  • TBD-9: Whether HW-111 SQW/INT is needed
  • TBD-11: Confirm full REF195 output load budget
  • TBD-15: Exact OC1 source definition
  • TBD-16: Exact OC3 door-contact polarity

  • Design Summary

  • Scope

  • System context

  • Requirements

  • Functional

  • Electrical

  • Mechanical / environmental

  • Key constraints

  • Dependencies and risks

  • Validation

  • Release-facing notes

  • Change notes / open questions