BLVD Card Assembly Guide

Table of contents

1. BLVD Card Assembly Guide
   1. Introduction
      1. Key Features:
      2. When to Use It:
      3. Common Causes of Low Voltage in Model Railway Layouts
      4. System Overview
   2. Assembly and Component Placement
   3. Tools Required
   4. Testing and Verification
      1. Visual Inspection
      2. Connectivity Testing
      3. Power-Up Tests
      4. Functional Testing
         1. HW Communications Testing
         2. MCP IC Testing
         3. Track Block Testing
   5. Safety Precautions
   6. Troubleshooting
   7. Appendences
      1. Specifications
      2. How It Works
      3. Protection
      4. References

Introduction

See the How to Use Assembly Guides for detailed instructions.

The Block Low Voltage Detection (BLVD) Card is a vital component for model railway enthusiasts using the LCC Fusion Project. This card continuously monitors the voltage levels of individual track blocks to ensure they remain within safe operating limits. It is designed to detect when the voltage drops below a predefined threshold, indicating potential issues such as poor connections, excessive load, or power supply problems.

Click to listen to an audio explanation.

flowchart LR; 
subgraph layout ["Train Layout"];
direction LR;
track["Track Block (4x)"];
bb[Block Breakout Board];
c["BLVD Card (16x)"];
nodeCard[Node Card];
can["CAN Network"]

track --> |"Voltage"|bb;
bb --> |"Low Voltage (yes/no)"|c;
c --> |"GPIO on/off"|nodeCard;
nodeCard--> |"Events (Low Voltage)"|can;
end;
classDef lSalmonStyle fill:#FFA07A,stroke:#333,stroke-width:2px,font-size:24px;
class c lSalmonStyle;
classDef lightGrayStyle fill:#d3d3d3,stroke:#333,stroke-width:2px,font-size:24px;
class layout lightGrayStyle; 

Key Features:

• Voltage Monitoring: Continuously measures the voltage across track blocks.
• Low Voltage Indicators: When configured with the LCC Configuration Tool, LCC Events are generated when a block’s voltage drops below 12 V. In addition, an LED status indicator on the card turns on.
• Comparator Circuit: Utilizes an LM393 comparator to compare block voltage against a stable reference voltage.
• Visual Indicators: Supports LED indicators for each block, providing real-time status.
• MCP23017 Integration: Interfaces with MCP23017 GPIO expander to report low voltage conditions to a central controller.
• Protection: Includes TVS diode protection to safeguard against voltage spikes.

When to Use It:

• Track Power Monitoring: Essential for ensuring consistent power delivery to locomotives.
• Fault Detection: Quickly identifies and alerts to low voltage conditions that could affect train operation.
• Maintenance: Aids in diagnosing and troubleshooting electrical issues within the layout.

By integrating the BLVD Card into your LCC Fusion Project, you can enhance the reliability and performance of your model railway system, ensuring smooth and uninterrupted operation of your locomotives and other track components.

The BLVD Card hardware configuration includes:

• communication address; I2C bus (0 or 1) and address offset (0-7)

Common Causes of Low Voltage in Model Railway Layouts

1. Short Circuits: Direct connections between V+ and GND, often due to derailments or wiring issues, can cause significant voltage drops.
2. High Current Draw: Multiple locomotives or accessories drawing excessive current from the same track block can lower voltage levels.
3. Poor Connections: Loose, corroded, or poorly soldered connections increase resistance, causing voltage drops.
4. Long Wire Runs: Extended wiring increases resistance, leading to voltage loss over distance.
5. Faulty Power Supply: A malfunctioning or underpowered supply can fail to maintain stable voltage levels.
6. Dirty Tracks: Dust and dirt on the tracks can impede electrical contact, resulting in lower voltage at the rails.

System Overview

Here’s an explanation of the diagram below.

flowchart LR; 
can["CAN Network"];
subgraph layout ["Train Layout"];
  direction LR;
    subgraph blvd ["BLVD Card (16x)"];
    direction LR;
    comparator["Voltage Comparator <br/> (LM393 IC)"];
    expander["GPIO Expander <br/> (mcp23017)"];
    end
  track["Track Block (4x)"] --> bb["Block Breakout Board"];
  bb --> |"Voltage <br/> (level)"| comparator;
  comparator --> |"GPIO Input <br/> (High/Low)"| expander
  expander -->|"GPIO Output<br/>(High/Low)"| n[Node Card];
  n -->|"LCC Event<br/>(Normal/Low)"| can;
end;
classDef lSalmonStyle fill:#FFA07A,stroke:#333,stroke-width:2px,font-size:24px;
class blvd lSalmonStyle;
classDef lightGrayStyle fill:#d3d3d3,stroke:#333,stroke-width:2px,font-size:24px;
class layout lightGrayStyle; 

## Terminology

• Bridge Rectifier

• Bus

• Cleaning PCB

• Component

• Current Limiting Resistor

• DCC Signal

• Decoupling Capacitor

• Edge Card Connector

• ESD Protection Diode

• Ferrite Bead

• GPIO Expander

• HW Communications Address

• HW Communications Bus

• I2C (Inter-Integrated Circuit)

• I2C Bus

• jumper_caps - not defined

• LCC Configuration Tool

• lcc_event_id - not defined

• LCC Event Monitoring Tool

• LCC Fusion Breakout Boards

• LCC Fusion Cards

• lcc_fusion_node_bus - not defined
• lcc_fusion_node_card - not defined

• LCC Fusion Project

• low_voltage_detection - not defined

• MCP23017

• Network Cable

• Stencil

• Track Bus

• TVS Diode

For other terms, please refer to the full Terminology Guide.

Assembly and Component Placement

This section combines both the component specifications and the assembly instructions to ensure a smooth assembly process. Below is a comprehensive list of components, their placement on the PCB, and orientation details to assist you during assembly.

High-Level Steps for Assembly:

• PCB for the card can be ordered from any PCB fabricator using these Gerber Files.
• Clean PCB with alcohol to remove residue. See Cleaning_PCB for details.
• See also: Soldering Tips
• PCB Components - listing of components used for PCB assembly
• PCB Parts - listing of parts used for PCB assembly

Below is a list of the PCB components used for this card (see diagram before reference):

Component Identifier	Count	Type	Value	Package	Purpose	Orientation
Bridge Rectifiers
BR1-BR8	8	Bridge Rectifier	MB6F	SOP-4	Converts DCC pulsating AC-like waveform into a DC-like pulsating waveform for current detection.	Position IC’s indent to PCB right edge)
Capacitors
C1 - C8	8	Capacitor-Polymer Solid	100uF, 35 V	6.3x5.8mm SMD	Smooths DC signal.	Cathode is position to PCB’s top edge
C9	1	Capacitor-Ceramic	0.1uF (100nF), 50 V	1206 X7R	Conditions/filters the current for the IC (U1).	None
Diodes
D1-D8	8	TVS Diode	SMBJ18A	SMB SMD	Protects from high-voltage transients (>29 V).	Cathode end has a white line and positioned towards PCB left edge
D9	1	ESD Diode	PESD1CAN	SOT-23 SMD	Provides I2C data bus electrostatic discharge (ESD) protection.	None
ZD1	1	Zener Diode	18 V	1206/DO-213 SMD	Creates a reference voltage determining lower track block voltage.	Cathode end has a white line and positioned towards PCB left edge
Filters & Noise Suppression
FB1, FB2	2	Ferrite Bead	BLM31PG121SN1L	1206 SMD	Suppresses noise on I2C data lines.	None
Connectors
J1, J2	2	RJ45 Socket	8P8C	PTH	Provides network cable (CAT5/6) connection to (1 or 2) Block Breakout Boards.	Fits only one way
Resistors
R1-R8	8	Resistor	1kΩ	1206 SMD	Limits current to the MCP23017 GPIO pins and block low voltage LEDs.	None
R9, R10, R11	3	Resistor	10kΩ	1206 SMD	Limits current to SW1 and MCP23017 for the I2C address.	None
R12	1	Resistor	10kΩ	1206 SMD	Current limiting for MCP23017 reset.	None
R13	1	Resistor	200Ω	1206 SMD	Current limiting for diode (reference voltage).	None
R14	1	Resistor	1kΩ	1206 SMD	Current limiting for power LED.	None
R15, R17, R19, R21, R23, R25, R27, R29	8	Resistor	4.7kΩ	1206 SMD	Voltage Divider to track is 3 V for comparator input	None
R16, R18, R20, R22, R24, R26, R28, R30	8	Resistor	1kΩ	1206 SMD	Voltage Divider to track is 3 V for comparator input	None
Selectors & Indicators
JP1, JP2	2	Male Header	3P, 0.1” spacing	PTH	Used for COMM BUS selection (I2C hardware bus) for either BUS A or BUS B.	None
SH1, SH2	2	Jumper Cap	2.54mm	—	Used with I2C Bus and Vcc selections. Recommend tall caps for ease of use.	None
SW1	1	DIP / Slide Switch	3P, 2.54mm	PTH	Used for COMM ADDR selection (I2C address offset, 0-7).	Position ON towards PCB top edge
LED1 - LED8	8	LED	Green	1206 SMD	Used as block low voltage indicators, green when voltage is >12 V.	Reference back of LED, position cathode towards PCB bottom edge
LED9	1	LED	Red	1206 SMD	Power status indicator.	Reference back of LED, position cathode towards PCB bottom edge
ICs
U1-U4	4	IC (Voltage Comparator)	LM393 or LM2903N	SO-8, SMD	Used for detecting low voltage (<12.1 V).	Small dot (pin 1) on package is positioned to PCB top/left edges
U5	1	I/O Expander (MCP23017 IC)	SSOP28	SMD	Controls 16 GPIO pins using I2C serial interface.	Dent on package positioned towards PCB bottom edge

Tools Required

For a list of recommended tools, refer to List of recommended tools.

Testing and Verification

Configure the BOD Card:

1. Select the COMM BUS by positioning (2) Jumper Caps on either A or B male header pins (JP1, JP2)

2. Select the COMM ADDRESS switch (SW1) by sliding each of the 3 switches to either the ON or OFF position. Setting a switch to ON increments the address by 1, 2, or 4 for an address range of 0 to 7. Up to 8 devices can then be configured for A and 8 for B.

The following test and verifications of the card should be performed after a through inspection of the card’s soldering. Check all of the PTH component pins and SMD pads. Make sure there are no solder bridges between pins and pads.

Visual Inspection

1. Initial Check: Examine the board for any obvious issues like missing components, solder bridges, or components that are misaligned or not fully seated.

2. Solder Joint Inspection: Use a magnifying glass or a microscope to inspect solder joints. Look for cold solder joints, insufficient or excessive solder, or any shorts between pads.

3. Component Orientation: the IC’s are correctly oriented according to the PCB silkscreen or schematic.

Connectivity Testing

Continuity Check: Use a multimeter in continuity mode to check for shorts between power rails and ground, and to ensure there are no open circuits in critical connections.

Power-Up Tests

1. Assembly a tested Power Module to the LCC Fusion Node Card.
2. Apply Power to the Power Module and verify the following:
   • Check for Hot Components: Feel for components that are overheating, which could indicate a problem like a short circuit or incorrect component.

Functional Testing

Functional testing of the card can be performed after completing the power-up testing. Testing consists of testing network communiations, followed by the sensing of current in a layout track block.

HW Communications Testing

Communications testing verifies the LCC Node can communicate with the BOD Card’s I2C IC via the Node Bus Hub connections. This is performed using the LCC Fusion Node Card’s testing firmware and a serial monitor.

1. Insert the BOD Card into a Node Bus Hub along with a LCC Fusion Node Card.

2. Install LCC Fusion Project firmware that includes serial monitor for testing.

3. Verify that the I2C connection between the LCC Fusion Node Card and the BOD Card work.

   See Testing I2C Cards for details on how to test the communications for a I2C enabled card.

MCP IC Testing

After hardware communication has been established, to verify the MCP IC, use the Node’s serial monitor to simulate an input from each of card’s input pins as follows:

1. In the serial monitor’s input, enter M for the LCC Node’s serial monitor Main Menu.
2. Select [1] Node Management and [1] Device Testing Management
3. Select [5] Simlulate MCP Device Input
4. From the list of MCP devices, select the device # for the BOD Card being tested
5. Enter the number for the pin to tested.
6. Using the JMRI Event Monitor, verify the correct Event ID was issued.

Track Block Testing

After validating the LCC Fusion Node Card can communicate (find) the BOD Card, test each of the BOD Card block connections.

Track block detection verification is performed using a Block Breakout Board connected to a track block.

1. Connect a network cable (CAT5/6) to one of the BOD Card’s RJ45 connector.

2. Connect the other end of the network cable to (1) Block Breakout Board.
3. Connect one of the Test Track rails directly to the Track Bus V+ connection.
4. Connect the other Test Track rails to one of the Block Breakout Board block connections (BLK1 - BLK4)
5. Connect the Test Track Bus GND to the Block Breakout Board TRACK BUS GND connection
6. Configure each of the card’s device lines using an LCC CDI Configuration Tool.
7. Open an LCC Event Monitoring tool (e.g. JMRI Event Monitoring tool).

8. Test each of the Breakout Board device connections using a locomotive (one at a time)

   1. draw current on the track by either
      • placing a 10KΩ resistor across the track, or
      • placing a locomotive on to the track
   2. Turn on the Track Power (let locomotive idle)
   3. Validate the configure Event ID for Block Occupied is sent by viewing the Event Monitoring tool
9. Repeat testing for all (8) BOD Card connections as follows:
   1. Connect the Block Breakout Board to the first RJ45 socket (J1) for blocks 1-4.
   2. Reconnect to 2nd RJ46 socket (J2) for blocks 5-8.

Safety Precautions

• See Safety Precautions.

Troubleshooting

• See I2C Trouble Shooting.

Appendences

Specifications

Characteristic	Value
Max Blocks Monitored	8
Maximum Number of Cards per LCC Fusion Node Cluster	161
Low Voltage Threshold	12 V
Max Track Voltage (determined by LM393 IC)	36 V

1. The LCC Fusion Node Cluster can support up to 16 cards, distributed across two I2C hardware buses, with a maximum of 8 cards per bus.
   • Note: total includes all cards using the I2C address range of 0x20 (MCP23017 IC).

How It Works

The Block Low Voltage Detection (BLVD) Card continuously monitors each track block’s voltage and isolates the block via its relay when an undervoltage (indicative of a short or overload) occurs. Here’s an overview of its key stages:

1. DCC Rectification

   • Track V+ and GND feed a KBL406 bridge rectifier, converting the bipolar DCC waveform into pulsed DC.

2. Smoothing

   • A 100 µF capacitor across the rectifier output smooths the pulses into a stable DC level for comparison.

3. Reference Generation

   • To comply with the NMRA DCC specification (which requires track voltage to remain above 12 V RMS for reliable decoder operation), the comparator threshold is set to detect when the DCC voltage drops below 12 V RMS.

   • A Zener diode provides the reference. By default, an 18 V Zener is used, which—when paired with the divider network below—yields a trip point at ~12 V RMS. Adjust the divider if you need to fine‑tune exactly 12 V:

     R_top = 4.7 kΩ
     R_bot = 1 kΩ
     Scale ≈ 1 k / (4.7 k + 1 k) ≈ 0.175
     

     • Zener input: 18 V → 3.15 V at comparator inverting input
     • Track-sense divider: 12 V RMS (≈17 V peak) → 2.98 V at comparator non-inverting input

   • The Zener is biased from the 12 V rail through a 200 Ω resistor to ensure ~5 mA knee current, keeping the reference stable.

4. Comparator Action

   • The LM393 compares the smoothed DC (non‑inverting input) against the Zener reference (inverting input).
   • When the track voltage falls below the reference, the comparator’s open‑collector output pulls LOW, signaling a fault.

5. Logic Interface

   • The LM393 output ties to an MCP23017 GPIO pin configured as an input with pull‑up to 3.3 V, preventing the line from floating.
   • On a LOW output, the MCP23017 registers the fault and your ESP32 firmware generates the corresponding “Low Voltage” LCC event.

6. Visual & Surge Protection

   • An optional LED (anode to 3.3 V, cathode through a 1 kΩ resistor to the comparator output) lights when the comparator output is LOW.
   • A TVS diode across Track V+ and GND shields the circuit from voltage spikes and transients.

This coordinated operation ensures reliable, configurable undervoltage detection and clear isolation of overloaded track blocks, while providing both visual and networked alerts to enhance layout safety and maintenance.

Protection

To ensure the reliable operation and longevity of your Block Occupancy Detection (BOD) card, several protection components have been integrated. These components safeguard the BOD Card from overcurrent, voltage spikes, and electrical noise. Below is a brief overview of each protection element and its role:

Protected Component	Protection Component	Function	Specifications	Location
Entire Circuit	TVS Diode SMBJ18A	Protects from high-voltage transients by clamping voltage spikes, preventing them from reaching sensitive components.	Stand-off Voltage: 5V Clamping Voltage: 29 V	Across the Vcc and GND lines of the track voltage
MCP23017 GPIO	10k ohm Resistor	Further limits current between the optocoupler output and the MCP23017 GPIO pin.	Value: 10k ohms	Between the optocoupler output and MCP23017 input
MCP23017	Decoupling Capacitor 0.1 µF	Filters out high-frequency noise and transient voltage spikes from the power supply, ensuring a stable voltage for the MCP23017.	Value: 0.1 µF	Across Vcc and GND near the MCP23017
I2C Lines from LCC Fusion Node Bus Hub	Ferrite Bead BLM31PG121SN1L	Provides high-frequency noise suppression on the I2C lines.	Impedance: 120 Ω at 100 MHz	In series with the SDA and SCL lines of the I2C bus
I2C Lines from LCC Fusion Node Bus Hub	ESD Protection Diode PESD1CAN	Protects the I2C lines from electrostatic discharge and voltage spikes.	Reverse Stand-off Voltage (Vr): 24 V Clamping Voltage (Vc): 40 V	Across the SDA and SCL lines from the card’s edge connector to GND, near the MCP23017.
LEDs	Current Limiting	Protects LEDs from current overload.	Value: 120 Ω	In series with LED

References