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RS485 Node Design with STM32

flux

0:42

Planning.

Thinking deeply about your request.

Creating a high-level execution plan.

Drafting a refined execution plan.

Structuring the execution plan.

Your goal is a compact 24 V-powered RS485 node with a small STM32, 3 bistable 230 V / 16 A relays, 3 I/O inputs, and 1 DS18B20.
  • I’ve created the execution plan.
  • It covers:
    1. Power from 24 V over Cat5
    2. RS485 interface + STM32
    3. Relay drivers, inputs, and DS18B20
    4. Safety separation for mains vs low-voltage circuitry
Click Start on the plan bar to begin. Let me know if you want any requirement changed first.
Plan
Goal
Design a compact RS485 node integrating a low-power STM32 microcontroller, powered via 24 V over the same Cat5 cable used for RS485, with three electrically-driven mechanically bistable (latching) relays rated 230 V / 16 A, three digital I/O inputs, and one DS18B20 temperature sensor. Ensure safe separation between 230 VAC mains and SELV logic/power.
What’s been completed
  • Updated project title/description and added key project properties (24 V, RS485, 6 mm isolation boundary guidance).
  • Created and stored a Mermaid system architecture diagram in project property System Architecture and shared it.
  • Selected and added major components to the schematic:
    • MCU: STM32G031G8U6 (U3)
    • RS485 transceiver: THVD1451D (U2)
    • Buck regulator: MCP16301x-xCH (U1)
    • Temperature sensor: DS18B20+ (U4)
    • Optocouplers for relay control isolation: PC817X1CSP9F (Q1–Q3)
    • Latching relay candidate in library: Panasonic ADJ42024 (K1–K3)
    • Coil switch MOSFETs: DMN6075S-7 (Q4–Q6)
    • Flyback diodes: PMEG4010EH,115 (D1–D3)
    • Connectors: 282837-2 (J1, 24V in), 282837-3 (J2 RS485, J3 inputs, J4 DS18B20)
    • Key passives added: RS485 termination/bias resistors, input resistors/pulldowns, DS18B20 pull-up, decoupling/bulk capacitors, contact snubber RC parts.
  • Identified library gap: exact “bistable/latching 230 VAC 16 A” relays are not clearly labeled; web research found candidate families (Omron G5RL latching; TE RT series; Panasonic ADJ/DJ). Library contains ADJ42024 and G5RL-1A-E-TV8 DC24 but latching status must be confirmed.
Implementation plan (remaining work only)
  1. Confirm relay MPN meets requirements and update if needed
    1. Verify via datasheet whether ADJ42024 is mechanically bistable/latching and is rated for 230 VAC / 16 A (and coil type/voltage).
    2. If ADJ42024 is not compliant, replace K1–K3 with a confirmed latching 16 A relay MPN (e.g., Omron G5RL-K1-E DC24 or TE RT latching variant) by finding an existing Flux library part or switching to the closest available library part and documenting the intended MPN for later library import.
  2. Wire the 24 V input + protection + buck regulator (U1) to create a stable 3.3 V rail
    1. Connect J1 pins to 24V_IN and GND, then wire U1:VIN/GND/EN/SW/BOOST/VFB per datasheet.
    2. Add any missing buck-required parts not yet present (inductor, catch diode if required, feedback divider if adjustable, bootstrap cap if required) and connect C1 and one of C2–C4 and a 100 nF (C5–C12) as input/output decoupling.
  3. Wire STM32G031 (U3) minimal bring-up circuitry
    1. Connect U3 VDD/VDDA to 3V3 and VSS/VSSA to ^GND.
    2. Assign and connect at least one 100 nF decoupler per VDD/VDDA pin and one bulk cap near MCU.
    3. Add and wire BOOT0 pull-down and NRST RC + reset access (button or header) and add SWD header (SWDIO/SWCLK/GND/3V3, optional NRST/SWO).
  4. Wire RS485 interface (U2) including bus termination/bias and connector (J2)
    1. Connect U2 VCC to 3V3, GND to ^GND, and D/R to an STM32 UART TX/RX.
    2. Connect U2 A/B pins to J2 and add ESD protection and optional common-mode choke if desired.
    3. Wire DE/RE control (Z/Y pins as appropriate for this transceiver) to an MCU GPIO; set R1 (120 Ω) and R2/R3 (560 Ω bias) as optional/conditional population (end-of-line only).
  5. Wire DS18B20 (U4) and/or external sensor connector (J4)
    1. Connect J4 to 3V3, GND, and 1-Wire DQ net; connect U4 in parallel if on-board sensor is desired.
    2. Connect R4 (4.7 kΩ) pull-up to 3V3 on the DQ net.
  6. Wire three digital inputs (J3) with conditioning to STM32 GPIOs
    1. Connect J3 pins to DI1/DI2/DI3 (and add a dedicated GND pin if required; current J3 is 3-pin).
    2. Wire R5–R7 series resistors into MCU pins and R8–R10 pull-downs to ^GND.
    3. Decide input electrical standard (dry contact to GND vs 24 V logic) and add missing protection/level shifting (TVS, divider, opto) accordingly.
  7. Complete relay coil driver wiring and mains contact interfacing
    1. For each relay channel, connect MCU GPIO → R11–R13 → PC817 LED (Q1–Q3) and return to ^GND.
    2. On isolated/coil side, wire optocoupler transistor outputs to MOSFET gates through R14–R16 with pull-downs R17–R19.
    3. Wire MOSFET sources to coil return (GND_COIL), drains to relay coil pin(s), and place D1–D3 across each coil (cathode to +24V_COIL).
    4. Identify relay pinout for K1–K3 (coil vs contacts) and connect COM/NO/NC to suitable mains terminal blocks (not yet added).
    5. Wire RC snubbers (C13–C15 with R20–R22) across COM–NO per channel.
  8. Add mains-side connectors and safety annotations
    1. Add terminal blocks for each relay contact set (COM/NO/NC or COM/NO depending on requirement) sized for 16 A.
    2. Add schematic notes/zones for HV vs LV and specify 6 mm creepage/clearance and recommended PCB slot boundary.
  9. Schematic completeness review
    1. Run ERC and resolve missing power pins, floating pins, and incomplete connections.
    2. Ensure all external connectors have basic protection (TVS/ESD) as appropriate for industrial cabling.
  10. Prepare for PCB (after schematic is fully wired)
  11. Review footprints for all parts, especially relays and terminal blocks.
  12. Set board outline and plan HV/LV component placement zones before routing.
Add isolation requirement
Use 3.3V logic
Specify Cat5 pinout
47 days

Started implementing a plan

#Goal

- Design a compact RS485 node that integrates a low-power STM32 microcontroller, powered via 24 V over the same Cat5 cable used for RS485 communications, with three electrically driven mechanically bistable relays (rated 230 V / 16 A), three digital I/O inputs, and one DS18B20 temperature sensor. The node will safely interface high voltage loads with sensitive low-voltage logic while conforming to safety and size constraints.

#Implementation plan

1. **Project Definition & Documentation**

1. Update the project description to reflect the node's functional scope including power, communications, relay control, and sensor interfacing.

2. **Power Supply Subsystem**

1. Add a 24 V input port sourced from the Cat5 cable.

2. Design and add a buck converter/regulator to generate the required low-voltage rail (e.g., 3.3 V or 5 V) for the STM32 and peripheral circuitry.

3. **RS485 Communication Interface**

1. Select and add an RS485 transceiver component (e.g., MAX485 or equivalent) to the schematic.

2. Connect the transceiver to the UART pins of the STM32 MCU.

4. **STM32 Microcontroller Integration**

1. Select and add a compact STM32 microcontroller component from the Flux parts library.

2. Configure power and signal connections to support node operations.

5. **Relay Control Circuitry**

1. Add driver circuits (including transistor drivers, snubbers, and isolation elements as required) for three mechanically bistable relays rated at 230 V/16 A.

2. Connect the relay driver outputs to specific digital output pins of the STM32.

6. **Digital I/O Inputs and Temperature Sensing**

1. Add three digital input circuits with proper signal conditioning for the I/O inputs.

2. Add the DS18B20 temperature sensor circuit along with its required pull-up resistor and connect it to a digital input pin on the STM32.

7. **Safety, Layout, and Mechanical Constraints**

1. Demarcate high voltage (230 V) and low voltage (MCU and sensor) sections in the schematic for clarity and safety.

2. Annotate components and nets to reflect safety clearances and compact layout considerations.

8. **Final Review and Documentation**

1. Review the schematic to ensure all functional blocks are complete and properly interconnected.

2. Document the design decisions and verify that the power distribution, RS485 bus, relay drivers, digital inputs, and sensor circuits meet their specifications.

#Success criteria

- The schematic includes a clearly defined power conversion from 24 V with proper regulation for the STM32 and peripherals.

- A functional RS485 interface linked to the STM32 is present.

- Relay control circuits for three 230 V/16 A bistable relays are properly designed and connected.

- Digital I/O inputs and a DS18B20 temperature sensor circuit are integrated and connected to the MCU.

- The design clearly demarcates high voltage and low voltage circuitry, ensuring safety and manufacturability.

47 days

Execution stopped due to insufficient credits.

12:47

47 days

Updated 5 document properties

0:01

Set Operating Voltage to "24V"

Set Connectivity to "RS485"

Set Domain to "Industrial control"

Set Compliance to "230V mains isolation boundary: 6mm creepage/clearance"

Set Power Requirements to "24V input over Cat5 -> 3.3V logic rail"

47 days
​
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Reviews

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Prickly Ivory Robot Vacuum thumbnail
Compact 24 V Cat5-powered RS485 node with STM32, 3 bistable relays, 3 DI, DS18B20

Properties

Diagram



  
    
  
  
    
  
































"Cat5 Cable"
"24V Power Pair"
"RS485 Pair"
"Input Protection"
"Buck node_24V to 3.3V"
"STM32 MCU 3.3V"
"RS485 Transceiver"
"3x Bistable Relays node_230VAC node_16A"
"Relay Drivers"
"Optional Isolation"
"3x Digital Inputs"
"DS18B20 1-Wire Temp"
"AC Mains Terminals"
"Low Voltage Zone"

Pricing & Availability

Distributor

Qty 1

Arrow

$9.28–$12.08

Digi-Key

$63.77–$65.61

LCSC

$241.12

Mouser

$20.97

TME

$2.43

Verical

$6.59–$14.20

Controls

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