# 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

![Power Supply Schematic](power-supply.png)

```ditaa {cmd=true args=["-E"]}
                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

![RailCom Transmitter Schematic](railcom.png)

```ditaa {cmd=true args=["-E"]}
                   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](https://www.nmra.org/dcc-standards)
- [TB67H450FNG Datasheet](https://toshiba.semicon-storage.com/ap-en/semiconductor/product/motor-driver-ics/brushed-dc-motor-driver-ics/detail.TB67H450FNG.html)
- [ESP32-H2 Datasheet](https://www.espressif.com/sites/default/files/documentation/esp32-h2_datasheet_en.pdf)
- [RailCom Specification](https://www.lenz-elektronik.de/railcom/)

## License

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

---

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