LND-8CDU

LND-8CDU - A LocoNet turnout decoder with 8 pulsed outputs

Video:

This is a firmware for LocoNet-only accessory decoder for 4 twin-coil turnouts. Outputs are pulsed with the help of built-in capacitor unit (CDU) that provides short bursts of current needed to operate a coil turnout.

Features:

1. 8 1A outputs for 4 twin-coil turnouts

2. Configurable pulse duration

3. Configurable number of pulses for each switch command.

4. Limiting current draw when charging capacitor

5. No PC required for configuration (only a LocoNet throttle).

The firmware is written using PlatformIO build system and Arduino framework. VS Code is recommended as an IDE. The code can also be built with Arduino IDE with minimal changes (some files need to be moved around and some libraries need to be installed).

The project requires LocoNet library to be installed. PlatformIO will install the required version automatically.

With minimal changes the firmware can provide continuous (not-pulsed) output to drive high-power devices like DC-motors (TODO).

Hardware

The firmware is designed to be used with this decoder board: https://oshwlab.com/positron96/loconet-decoder.

The board is based on Arduino Pro Mini (5V 16kB variant, 5V 32kB will also work), typical DIY LocoNet circuitry and contains a large 4700uF capacitor (it has place for 2 of them for even more charge). Outputs are driven with 2 ULN2803 ICs. Each IC has 8 500 mA outputs. To provide more current to coils, every 2 outputs are connected in parallel to provide 1A of current for 8 coils. This allows to drive 4 twin-coil turnouts.

The board can be powered either via separate power source or via LocoNet plug. Special handling is implemented for the case when power source cannot provide enough power to charge capacitor. In this case voltage drop is measured by Arduino and charging load is limited via PWMing transitor. This is especially relevant in case of powering from LocoNet bus. In case this is not relevant, the corresponding circuitry can be omitted altogether. Also there is jumper to bypass it in case it does not work well (I have not tested this functionality on final board, only on prototype).

Configuration

Table 1. Available configuration variables

Var	Meaning (Default value)
1	Address of first turnout. (6)
7	Duration of pulse, in ms. Max: 255. (10)
8	Pulse count for each open/close action. Recommended<5, max 255 [1]. (default: 1)

[1] The decoder does not process new messages while it is doing pulses, so too many pulses for one turnout may delay or disrupt operation of next turnout.

The decoder can be configured in 2 ways:

Using throttle

TL;DR: To set variable 1 to 20: go to config mode via long button press, throw turnout 1, throw turnout 20. Watch the LED for hints.

1. Connect a throttle and decoder to command station. Command station is not needed for communication, but something needs to power the LocoNet bus, decoder and throttle.

2. Hold button (marked BT1) on the decoder for 3s (when you press the button, the blinking LED (marked LED1) on the board will turn off. After 3 seconds, it will start blinking fast with 0.4s interval to make it easier to decide when to release the button).

3. While the LED is blinking fast, the decoder is expecting variable number to be input with a throttle. To change variable with number N, simply throw a turnout with address N. After that, the LED will start blinking even faster (0.15s interval).

4. While the LED is blinking faster, the decoder is expecting variable value to be input with a throttle. Again, to set var value to M, just throw a turnout with address M. The decoder will store the new value in EEPROM and LED will start blinking normally.

To change another value, start procedure from pt. 1.

If new settings are not working, reset the board with a reset button on the Arduino and file an issue.

Using SVs

TODO.

The decoder accepts LocoNet System Variable (SVs) messages. You can read and write SVs with JMRI, but decoder XML definition for JMRI is not created yet.