STM

STM32 Mini PLC v1.0

A compact industrial-style PLC board designed in KiCad, built around an STM32F407 microcontroller. It provides 8 optically isolated digital inputs, 8 relay outputs, RS-485 communication and USB-C serial/bootloader access, all powered from 24 VDC.

Status: Hardware design v1.0 complete. The board has not been manufactured or tested, and no firmware has been written yet.

Table of contents

• Key features
• Board gallery
• Specifications
• System overview
• Sub-systems
• Connectors
• PCB design
• Repository structure
• Project status and roadmap
• Tools
• License and credits

Key features

• STM32F407 microcontroller as the processing core
• 8 optically isolated digital inputs (optocoupler per channel)
• 8 relay outputs, MOSFET-driven, with freewheeling diodes
• RS-485 interface on a DB9 connector (suitable for Modbus RTU)
• USB-C with CH340C USB-to-UART bridge for serial debugging and bootloader flashing
• 5-pin SWD header for ST-LINK programming and debugging
• 24 VDC industrial supply with fuse and reverse-polarity protection
• Status LEDs on inputs and outputs
• Reset button

Board gallery

3D render: top angle

3D render: side view

3D render: back side

Back side showing input labels (X1 to X8), copper routing and relay output pads.

PCB layout (KiCad PCB editor)

Red traces are the top copper layer and blue traces are the bottom copper layer. The logic section (power, STM32, inputs, communication) is at the top, and the relay and output section is at the bottom.

Specifications

System overview

flowchart LR
    PWR[24 VDC input<br/>J1, F1, D1] --> BUCK[Buck converter U1]
    BUCK --> LDO[3.3 V regulator U2]
    LDO --> MCU[STM32F407<br/>U13]
    IN[Field inputs X1-X8<br/>J2] --> OPTO[Optocouplers<br/>U4-U11]
    OPTO --> MCU
    MCU --> FET[MOSFET drivers<br/>Q2-Q9]
    FET --> RLY[Relays K2-K9]
    RLY --> OUT[Outputs Y1-Y8<br/>J3]
    MCU <--> RS[RS-485 transceiver U12<br/>DB9 J4]
    MCU <--> USB[CH340C U14<br/>USB-C J19]
    MCU <--> SWD[SWD header J5]

Sub-systems

Connectors

PCB design

• Logic section (power, MCU, optocoupled inputs, communication) kept separate from the relay section
• Relays placed along the bottom edge with output terminals on the same side
• Input terminal block along the top edge
• Mounting holes in all four corners
• Silkscreen labels for all terminals and ratings (Input: 24 V DC, Relay outputs: 250 VAC / 30 VDC, 5 A)

Project status and roadmap

• Schematic design
• PCB layout
• 3D model review
• Design rule check and final review
• Manufacture the board
• Write firmware
• Hardware testing

Specifications above come from the design, not from measurements on a built board.

Tools

• KiCad (schematic, PCB layout, 3D viewer)

Designed by: Noraiz