315 lines
10 KiB
Markdown
315 lines
10 KiB
Markdown
# Hardware Design Files
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This folder is reserved for KiCad hardware design files for the DCC Locomotive Decoder.
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## Planned Contents
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- **Schematic**: Complete circuit schematic (`.kicad_sch`)
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- **PCB Layout**: Printed circuit board design (`.kicad_pcb`)
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- **Bill of Materials**: Component list (BOM.csv)
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- **Gerber Files**: Manufacturing files
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- **3D Models**: Component models
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- **Assembly Drawings**: Assembly instructions
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## Current Status
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🚧 **Under Development**
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Hardware design files will be added in future releases.
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## Design Goals
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- Compact form factor suitable for HO/N scale locomotives
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- Single or dual-sided PCB (TBD)
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- Through-hole or SMD components (TBD)
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- Easy assembly and testing
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- Robust protection circuits
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- Proper EMI/EMC considerations
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## Sections
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The PCB will include the following sections:
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1. **Power Supply**
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- Track power input with TVS protection
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- Schottky diode bridge rectifier (4x SS54: 5A, 40V)
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- Bulk filtering capacitors (470µF-1000µF electrolytic)
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- 3.3V LDO regulator for ESP32-H2 logic
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- Separate motor power feed to TB67H450FNG VM pin
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- Ceramic bypass capacitors (0.1µF near ICs)
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2. **DCC Input Stage**
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- Optocoupler isolation
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- Signal conditioning
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- Protection diodes
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3. **Motor Driver**
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- TB67H450FNG H-bridge
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- Current sense circuit (0.1Ω shunt resistor)
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- Bootstrap capacitors (if needed for gate drive)
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- Flyback diodes (usually internal to TB67H450FNG)
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- Bulk motor power capacitor (100µF near VM pin)
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4. **Microcontroller**
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- ESP32-H2 module or bare chip
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- Programming header
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- Reset and boot buttons
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5. **LED Output**
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- WS2812 connector
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- Level shifter (if needed)
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- Power filtering
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6. **RailCom**
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- RailCom transmitter circuit
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- Cutout detection
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- Track coupling circuit
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7. **Accessory Outputs**
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- 2x N-FET drivers
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- Screw terminals or connectors
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- Protection circuits
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8. **Configuration**
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- Configuration button
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- Status LED
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- Optional programming port
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## Component Selection
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### Key Components
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- **Bridge Rectifier**: 4x SS54 Schottky diodes (5A, 40V, SMA/DO-214AC) or SS56 (5A, 60V)
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- Lower forward drop (~0.5V per diode, 1V total vs 2V for standard bridge)
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- Better efficiency = less heat
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- Fast switching for DCC frequency
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- Arrange in standard bridge configuration
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- **Microcontroller**: ESP32-H2 (RISC-V, Zigbee/Thread)
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- **Motor Driver**: Toshiba TB67H450FNG (dual H-bridge, 3.5A)
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- **Optocoupler**: 6N137 (fast) or PC817 (general purpose)
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- **N-FETs**: AO3400A (SOT-23, 4A, 44mΩ RDS(on) @ 2.5V)
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- For accessory outputs (max 350mA each)
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- Logic-level compatible with 3.3V GPIO
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- Low cost (~$0.05-0.10)
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- **Voltage Regulator**: AMS1117-3.3 (800mA) or HT7333 (LDO, low dropout)
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- **Current Sense Resistor**: 0.1Ω, 1W metal film or wire-wound
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- **LEDs**: WS2812B or compatible addressable RGB LEDs
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### Connectors
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- **Motor**: 2-pin screw terminal or JST-XH
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- **Track Input**: 2-pin screw terminal
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- **LED Strip**: 3-pin JST connector
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- **Accessories**: 2x 2-pin screw terminals
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- **Programming**: 6-pin header (GND, 3V3, TX, RX, IO0, EN)
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## Design Considerations
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### Power Supply Schematic
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```ditaa {cmd=true args=["-E"]}
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DCC Track Input (12-18V AC/DC)
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v
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+-----+-----+
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| TVS | P6KE24A bidirectional
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| Diode |
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+-----+-----+
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+-------------+-------------+
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Track+ Track-
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+-------+-------+ +-------+-------+
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| D1 | | D3 |
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| SS54 5A | | SS54 5A |
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+-------+-------+ +-------+-------+
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+--------->DC+<-------------+
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| +----------------------------+
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+--| 1000uF/25V Electrolytic |
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| +----------------------------+
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+---> TB67H450FNG VM (Motor Power)
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| +----------------------------+
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+--| AMS1117-3.3 or HT7333 LDO |
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| +----------------------------+
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| +--| 100uF |---> 3.3V Logic
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+-------+-------+ | | +-------+-------+
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| D2 | | | | D4 |
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| SS54 5A | | | | SS54 5A |
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+-------+-------+ | | +-------+-------+
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+--------->GND<--------+----------+
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Common Ground
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```
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**Bridge Configuration:**
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- Track inputs: Connect to DCC rails (polarity-independent)
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- DC+ rail: 10-16V after rectification
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- Forward drop: ~1V total (0.5V per diode pair)
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- SS54 Schottky: 5A continuous, 40V rating
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- Handles motor (1-3A) + logic (~200mA) simultaneously
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**Component Values:**
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- Bridge: 4x SS54 (SMA package)
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- Bulk cap: 1000µF/25V electrolytic
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- LDO input cap: 10µF ceramic
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- LDO output cap: 100µF electrolytic + 0.1µF ceramic
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- TVS: P6KE24A or 1.5KE24CA
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### RailCom Transmitter Schematic
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```ditaa {cmd=true args=["-E"]}
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ESP32-H2 GPIO10 (UART1 TX)
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v
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+-----+-----+
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| 1kΩ | Pull-up
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+-----+-----+
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+--------+
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+-----+-----+ |
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| NPN BJT | | BC817 or 2N3904
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| Q1 |<-+
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+-----+-----+
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|C
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+-----------+
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+----+----+ +---+---+
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| 10Ω | | 100pF | Snubber
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+----+----+ +---+---+
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+-----------+--------> To Track (via DCC cutout)
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+-----+-----+
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| 10kΩ | Pull-down
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+-----+-----+
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v
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GND
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RailCom Cutout Detection (Optional GPIO11)
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Track Signal ---+
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+---+---+
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|Voltage| Resistor divider
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|Divider| 22kΩ / 10kΩ
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+---+---+
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+---> GPIO11 (Cutout Detect)
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+---+---+
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| 0.1µF | Filter capacitor
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+---+---+
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GND
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```
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**RailCom Operation:**
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1. **Cutout Detection**: DCC command station creates ~450µs cutout window
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2. **Channel Timing**:
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- Channel 1: 26-177µs (address broadcast)
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- Channel 2: 193-454µs (status data)
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3. **Transmit**: UART TX at 250kbaud during cutout
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4. **Encoding**: 4-to-8 bit encoding per RailCom spec
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**Components:**
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- Q1: BC817 NPN (SOT-23) or 2N3904
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- R1: 1kΩ base resistor
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- R2: 10Ω series resistor (current limit)
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- R3: 10kΩ pull-down
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- C1: 100pF snubber capacitor
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- Cutout divider: 22kΩ + 10kΩ (scales track voltage to 3.3V)
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**Important Notes:**
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- RailCom transmits ONLY during DCC cutout window
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- Requires command station with RailCom support
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- Cutout detection is optional (can use timing from last DCC packet)
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- Q1 must switch fast enough for 250kbaud (BC817: fT=100MHz)
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### Thermal Management
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- Adequate copper pour for motor driver heat dissipation
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- Thermal vias under motor driver IC
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- Consider adding heatsink mounting holes
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- Keep power traces wide (minimum 2mm for motor power)
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- Bridge diodes: Place on copper pour for heat spreading
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### Layout Guidelines
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- Keep DCC input traces short and isolated
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- Star ground topology for power
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- Separate analog and digital grounds near ADC
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- Shield sensitive signals (DCC input, current sense)
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- Keep high-speed traces short (WS2812 data <10cm)
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- Proper decoupling capacitors near ICs (0.1µF within 5mm)
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- Wide traces for rectifier output (2-3mm minimum)
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### Protection
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- TVS diode on track input (P6KE24A bidirectional)
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- Schottky diodes provide inherent fast response
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- Reverse polarity protection on track input
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- TVS diodes on all external connections
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- Overcurrent protection on motor output
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- ESD protection on user-accessible pins
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### Testing
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- Test points for key signals (DC+, 3.3V, DCC signal, motor outputs)
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- LED indicators for power (3.3V rail), DCC signal presence, status
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- Easy access to programming pins
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- Measure bridge rectifier forward drop (should be ~1V under load)
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- Current sense test point for motor current monitoring
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## Future Enhancements
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- Dual motor driver option
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- Sound module integration (I2S DAC)
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- Additional function outputs
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- Servo outputs (2-4 channels)
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- SUSI interface
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- Optional Bluetooth antenna
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## Contributing
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If you'd like to contribute to the hardware design:
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1. Use KiCad 7.0 or newer
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2. Follow IPC design standards
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3. Include clear documentation
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4. Provide design rationale for key decisions
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5. Test thoroughly before sharing
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## References
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- [NMRA DCC Standards](https://www.nmra.org/dcc-standards)
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- [TB67H450FNG Datasheet](https://toshiba.semicon-storage.com/ap-en/semiconductor/product/motor-driver-ics/brushed-dc-motor-driver-ics/detail.TB67H450FNG.html)
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- [ESP32-H2 Datasheet](https://www.espressif.com/sites/default/files/documentation/esp32-h2_datasheet_en.pdf)
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- [RailCom Specification](https://www.lenz-elektronik.de/railcom/)
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## License
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Hardware designs will be released under CERN Open Hardware License v2 - Permissive (CERN-OHL-P).
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---
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**Status**: Planned for future release
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**Last Updated**: 2026-01-15
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