Maison/ESP32/DCC-Loco/Hardware/README.md

Hardware Design Files

This folder is reserved for KiCad hardware design files for the DCC Locomotive Decoder.

Planned Contents

• Schematic: Complete circuit schematic (.kicad_sch)
• PCB Layout: Printed circuit board design (.kicad_pcb)
• Bill of Materials: Component list (BOM.csv)
• Gerber Files: Manufacturing files
• 3D Models: Component models
• Assembly Drawings: Assembly instructions

Current Status

🚧 Under Development

Hardware design files will be added in future releases.

Design Goals

• Compact form factor suitable for HO/N scale locomotives
• Single or dual-sided PCB (TBD)
• Through-hole or SMD components (TBD)
• Easy assembly and testing
• Robust protection circuits
• Proper EMI/EMC considerations

Sections

The PCB will include the following sections:

1. Power Supply

   • Track power input with TVS protection
   • Schottky diode bridge rectifier (4x SS54: 5A, 40V)
   • Bulk filtering capacitors (470µF-1000µF electrolytic)
   • 3.3V LDO regulator for ESP32-H2 logic
   • Separate motor power feed to TB67H450FNG VM pin
   • Ceramic bypass capacitors (0.1µF near ICs)

2. DCC Input Stage

   • Optocoupler isolation
   • Signal conditioning
   • Protection diodes

3. Motor Driver

   • TB67H450FNG H-bridge
   • Current sense circuit (0.1Ω shunt resistor)
   • Bootstrap capacitors (if needed for gate drive)
   • Flyback diodes (usually internal to TB67H450FNG)
   • Bulk motor power capacitor (100µF near VM pin)

4. Microcontroller

   • ESP32-H2 module or bare chip
   • Programming header
   • Reset and boot buttons

5. LED Output

   • WS2812 connector
   • Level shifter (if needed)
   • Power filtering

6. RailCom

   • RailCom transmitter circuit
   • Cutout detection
   • Track coupling circuit

7. Accessory Outputs

   • 2x N-FET drivers
   • Screw terminals or connectors
   • Protection circuits

8. Configuration

   • Configuration button
   • Status LED
   • Optional programming port

Component Selection

Key Components

• Bridge Rectifier: 4x SS54 Schottky diodes (5A, 40V, SMA/DO-214AC) or SS56 (5A, 60V)
  • Lower forward drop (~0.5V per diode, 1V total vs 2V for standard bridge)
  • Better efficiency = less heat
  • Fast switching for DCC frequency
  • Arrange in standard bridge configuration
• Microcontroller: ESP32-H2 (RISC-V, Zigbee/Thread)
• Motor Driver: Toshiba TB67H450FNG (dual H-bridge, 3.5A)
• Optocoupler: 6N137 (fast) or PC817 (general purpose)
• N-FETs: AO3400A (SOT-23, 4A, 44mΩ RDS(on) @ 2.5V)
  • For accessory outputs (max 350mA each)
  • Logic-level compatible with 3.3V GPIO
  • Low cost (~$0.05-0.10)
• Voltage Regulator: AMS1117-3.3 (800mA) or HT7333 (LDO, low dropout)
• Current Sense Resistor: 0.1Ω, 1W metal film or wire-wound
• LEDs: WS2812B or compatible addressable RGB LEDs

Connectors

• Motor: 2-pin screw terminal or JST-XH
• Track Input: 2-pin screw terminal
• LED Strip: 3-pin JST connector
• Accessories: 2x 2-pin screw terminals
• Programming: 6-pin header (GND, 3V3, TX, RX, IO0, EN)

Design Considerations

Power Supply Schematic

                DCC Track Input (12-18V AC/DC)
                          |
                          v
                    +-----+-----+
                    |    TVS    | P6KE24A bidirectional
                    |  Diode    |
                    +-----+-----+
                          |
            +-------------+-------------+
            |                           |
         Track+                      Track-
            |                           |
    +-------+-------+           +-------+-------+
    |      D1       |           |      D3       |
    |    SS54 5A    |           |    SS54 5A    |
    +-------+-------+           +-------+-------+
            |                           |
            +--------->DC+<-------------+
                       |
                       |  +----------------------------+
                       +--| 1000uF/25V Electrolytic   |
                       |  +----------------------------+
                       |
                       +---> TB67H450FNG VM (Motor Power)
                       |
                       |  +----------------------------+
                       +--| AMS1117-3.3 or HT7333 LDO |
                       |  +----------------------------+
                       |           |
                       |           +--| 100uF |---> 3.3V Logic
                       |           |
    +-------+-------+  |           |  +-------+-------+
    |      D2       |  |           |  |      D4       |
    |    SS54 5A    |  |           |  |    SS54 5A    |
    +-------+-------+  |           |  +-------+-------+
            |          |           |          |
            +--------->GND<--------+----------+
                       |
                    Common Ground

Bridge Configuration:

• Track inputs: Connect to DCC rails (polarity-independent)
• DC+ rail: 10-16V after rectification
• Forward drop: ~1V total (0.5V per diode pair)
• SS54 Schottky: 5A continuous, 40V rating
• Handles motor (1-3A) + logic (~200mA) simultaneously

Component Values:

• Bridge: 4x SS54 (SMA package)
• Bulk cap: 1000µF/25V electrolytic
• LDO input cap: 10µF ceramic
• LDO output cap: 100µF electrolytic + 0.1µF ceramic
• TVS: P6KE24A or 1.5KE24CA

RailCom Transmitter Schematic

                   ESP32-H2 GPIO10 (UART1 TX)
                          |
                          v
                    +-----+-----+
                    |    1kΩ    | Pull-up
                    +-----+-----+
                          |
                          +--------+
                          |        |
                    +-----+-----+  |
                    |  NPN BJT  |  |  BC817 or 2N3904
                    |   Q1      |<-+
                    +-----+-----+
                          |C
                          |
              +-----------+
              |           |
         +----+----+  +---+---+
         |   10Ω   |  | 100pF | Snubber
         +----+----+  +---+---+
              |           |
              +-----------+--------> To Track (via DCC cutout)
                          |
                          |
                    +-----+-----+
                    |   10kΩ    | Pull-down
                    +-----+-----+
                          |
                          v
                         GND

   RailCom Cutout Detection (Optional GPIO11)
   
        Track Signal ---+
                        |
                    +---+---+
                    |Voltage|  Resistor divider
                    |Divider|  22kΩ / 10kΩ
                    +---+---+
                        |
                        +---> GPIO11 (Cutout Detect)
                        |
                    +---+---+
                    | 0.1µF | Filter capacitor
                    +---+---+
                        |
                       GND

RailCom Operation:

1. Cutout Detection: DCC command station creates ~450µs cutout window
2. Channel Timing:
   • Channel 1: 26-177µs (address broadcast)
   • Channel 2: 193-454µs (status data)
3. Transmit: UART TX at 250kbaud during cutout
4. Encoding: 4-to-8 bit encoding per RailCom spec

Components:

• Q1: BC817 NPN (SOT-23) or 2N3904
• R1: 1kΩ base resistor
• R2: 10Ω series resistor (current limit)
• R3: 10kΩ pull-down
• C1: 100pF snubber capacitor
• Cutout divider: 22kΩ + 10kΩ (scales track voltage to 3.3V)

Important Notes:

• RailCom transmits ONLY during DCC cutout window
• Requires command station with RailCom support
• Cutout detection is optional (can use timing from last DCC packet)
• Q1 must switch fast enough for 250kbaud (BC817: fT=100MHz)

Thermal Management

• Adequate copper pour for motor driver heat dissipation
• Thermal vias under motor driver IC
• Consider adding heatsink mounting holes
• Keep power traces wide (minimum 2mm for motor power)
• Bridge diodes: Place on copper pour for heat spreading

Layout Guidelines

• Keep DCC input traces short and isolated
• Star ground topology for power
• Separate analog and digital grounds near ADC
• Shield sensitive signals (DCC input, current sense)
• Keep high-speed traces short (WS2812 data <10cm)
• Proper decoupling capacitors near ICs (0.1µF within 5mm)
• Wide traces for rectifier output (2-3mm minimum)

Protection

• TVS diode on track input (P6KE24A bidirectional)

• Schottky diodes provide inherent fast response

• Reverse polarity protection on track input

• TVS diodes on all external connections

• Overcurrent protection on motor output

• ESD protection on user-accessible pins

Testing

• Test points for key signals (DC+, 3.3V, DCC signal, motor outputs)
• LED indicators for power (3.3V rail), DCC signal presence, status
• Easy access to programming pins
• Measure bridge rectifier forward drop (should be ~1V under load)
• Current sense test point for motor current monitoring

Future Enhancements

• Dual motor driver option
• Sound module integration (I2S DAC)
• Additional function outputs
• Servo outputs (2-4 channels)
• SUSI interface
• Optional Bluetooth antenna

Contributing

If you'd like to contribute to the hardware design:

1. Use KiCad 7.0 or newer
2. Follow IPC design standards
3. Include clear documentation
4. Provide design rationale for key decisions
5. Test thoroughly before sharing

References

• NMRA DCC Standards
• TB67H450FNG Datasheet
• ESP32-H2 Datasheet
• RailCom Specification

License

Hardware designs will be released under CERN Open Hardware License v2 - Permissive (CERN-OHL-P).

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Status: Planned for future release Last Updated: 2026-01-15