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By Tim Blythman Decoder Base Station Using DCC Remote Controller DCC Booster DCC Decoder for model locomotives DCC is a great way to control model trains in a realistic fashion. You need a DCC Decoder for every locomotive and a base station to produce DCC signals on the track. This DCC Decoder project provides the first essential part. We will follow up with a DCC Base Station project and a guide to getting started with DCC using these in our next issue. Image source: https://pixabay.com/photos/steam-locomotive-model-series-23-3627896/ F or many years, model railways were limited to a simple DC system. The two rails formed conductors that carried power from a controller directly to the motor of a model railway locomotive. The controller would simply provide a voltage and polarity to control the loco’s speed and direction. It’s simple and elegant, but only allows one train to be operated at a time. Of course, real railways run more than one train simultaneously and, over time, different technologies have been used to allow this. One wellknown example divides the track into sections called blocks. A switch panel routes power between the blocks and multiple controllers. As you can imagine, this can get very complex. In the late 1980s, Lenz Elektronik of Germany developed the early versions of what would later become digital command control or DCC. The US National Model Railroad Association (NMRA) took an interest and developed DCC as a standard under its auspices. The NMRA had previously worked on standards relating to matters such as wheels and trackwork for model railways, intending to foster 70 Silicon Chip compatibility and interoperability between manufacturers. With its promotion as a standard, DCC continues to be one of the most popular systems for operating model railways. The NMRA continues to develop and publish updates to the DCC standards and these, along with their other standards are available to download from siliconchip.au/link/ac7w We have published several articles related to DCC, including feature articles and projects for boosters, programmers and even a computer-based base station. However, this is the first DCC decoder we have published, which means that all you need to build a complete DCC system can now be found within Silicon Chip. The Complete Arduino DCC Controller can round out a working system (January 2020; siliconchip.au/ Article/12220), but it relies on a computer, so might be excessive for readers looking for a straightforward starter system. Thus, we have also designed a standalone Base Station based on a Raspberry Pi Pico 2 microcontroller and touch panel LCD, which we will introduce next month. Australia's electronics magazine That unit can control up to five locomotives when paired with a suitable power supply, and can also perform decoder programming; more than enough for most small layouts. If you are not familiar with DCC terminology, refer to our glossary later in this article. DCC operation The Arduino-based DCC Programmer from October 2018 (siliconchip. au/Article/11261) included a panel on how DCC works. Fig.1 is a comparison between so-called DC operation (using a controller providing an analog DC voltage) and DCC operation. Fig.1(a) shows DC operation. This project deals with the decoder shown in Fig.1(b). Its main role is receiving power and data as a varying voltage sent from the DCC base station, and decoding the data to drive the motor and function outputs. The latter might be incandescent globes or LEDs. Note that the motor and lights are not part of this project; they will be part of the locomotive to which you are adding the Decoder! Our Decoder and Base Station can siliconchip.com.au Features & Specifications 🛤 Suitable for N and HO/OO scales 🛤 Has support for a keep-alive capacitor 🛤 Long, short and consist addressing 🛤 Acceleration control and speed curve adjustment 🛤 Configurable lighting outputs and flashing effects 🛤 Voltage compensation option 🛤 Supports operations, paged, physical and direct programming modes 🛤 Supports 14-step, 28-step and 128-step speed control 🛤 Robust design with current limiting on all outputs Fig.1: (a) shows the typical wiring inside a DC-only (non-DCC) locomotive, while (b) shows how this changes when a DCC decoder is fitted. If two locomotives were on the track in (a), their motors would run together. The decoder allows the lights and motors in the two locomotives to run independently. The vertical red lines show where wires need to be cut to install the Decoder. be the electronic basis of an economical but complete DCC system. The Decoder design is robust, with current limiting provided for both the motor and function outputs. Commercial DCC decoders include a multitude of features, including sampled and synthesised sounds and complex lighting effects. Our Decoder provides some basic lighting effects and is an inexpensive option if you assemble it yourself. We’ve also designed the Decoder with the idea that our readers might want to extend or modify it by adding circuitry. Thus, the Decoder provides connections for rectified DC from the track as well as regulated 3.3V, something not commonly seen on commercial decoders. Alternatives These days, RF (radio frequency) controls have become more accessible and microcontrollers more powerful. We have published a few projects relating to RF control of model railways, including Les Kerr’s Battery-Powered Model Train from the January 2025 issue (siliconchip.au/Article/17607). siliconchip.com.au We have also seen systems based on WiFi. These typically use low-cost ESP8266 or ESP32 WiFi processor modules. Still, we are yet to see uniform standards for these sorts of systems, so you will likely be committed to building all the parts yourself if you choose one of these options. Our DCC Decoder and Base Station have both been designed according to the DCC standards, and tested with their commercial counterparts. So they can be used as part of an existing DCC system, or further expanded in the future with other products. The Decoder A locomotive decoder (also known as a mobile decoder) is intended to be used with a model railway locomotive or perhaps a motorised railcar. It could be fitted into the tender behind a steam locomotive, if there is space, or if that is where the motor is installed. 🛤 Size: 28 × 14 × 4mm 🛤 Input voltage: up to 17V peak 🛤 Motor current: up to 500mA 🛤 Function outputs: four, up to 100mA each There are other types of decoders, such as accessory decoders, that can operate line-side equipment, such as points motors and signal lights. An accessory decoder could even control lights or other effects in model buildings and such. A function-only decoder is another type of decoder that does not drive a motor, but can operate lights or other effects. A typical use would be for internal or marker lights in a carriage, or perhaps to control a smoke generator in a guard’s van to emulate a wood stove. Since a locomotive decoder can also control these functions, there is no reason our Decoder cannot be used as a This tiny DCC Decoder board can be fitted to a model railway locomotive, allowing it to be controlled by a DCC Base station. It’s only 14mm wide (shown here at twice actual size), so is suitable for HO/OO and N scale models. Australia's electronics magazine December 2025  71 function-only decoder in other rolling stock. Still, we expect that many readers new to DCC will be most interested in adding the Decoder to their locomotives as a starting point. As a rough guide to getting started, you will need a locomotive decoder for each locomotive or other item of motorised rolling stock, plus a base station to coordinate them all. This article will present the Decoder design; for those who already operate a DCC system, we’ll provide a wiring guide and some information about its operating parameters; particularly its configuration variables (CVs). This information should be enough to allow our Decoders to be customised and integrated into an existing DCC layout. For those new to DCC, we’ll provide a more complete user guide alongside the DCC Base Station in the next article. Circuit Fig.2 shows the circuit of our four-function DCC mobile Decoder. Some of the connections are marked with colours; these correspond to the wire colours used by the DCC standards. Each connector on the Decoder is just a PCB solder pad. With space at a premium, the expectation is that wires are soldered directly to the Decoder PCB. The DCC track signal is an AC square wave of about 12V-15V in amplitude (24V-30V peak-to-peak) with its frequency varying around 6kHz. The actual voltage is not too important, although our design limits it to about 7V minimum and 17V maximum. Just over 12V is typical, since this is the nominal operating voltage of the motors in many locomotives. The track connections to the Decoder (red and black) feed into bridge rectifier BR1 and then REG1, an MCP1703 3.3V LDO (low-dropout) regulator, providing our nominal 12V and 3.3V rails. The MCP1703 has a maximum operating input voltage of 16V; our 17V maximum track voltage is due to this, with a drop of at least 1V due to the bridge rectifier. Note that some DCC systems operate at up to 22V peak, so you should check this before using our Decoder with an existing system. Some systems can provide a wide range of supply voltages, so it may be possible to simply reduce it. The 3.3V rail powers IC1, a PIC16F18126 microcontroller, at its pins 1 and 14. This is the 14-pin sibling of the 20-pin PIC16F18146 micro we have used in numerous other projects. The firmware we have created will work in a PIC16F18124 or PIC16F18125 (with less memory), but we will supply the PIC16F18126 parts in the kit as it’s only slightly more expensive. The three capacitors provide the necessary bypassing for the regulator and microcontroller, while a pair of 100kW resistors connect pins 10 and 11 of the micro to the track connections. The 100kW resistors limit current flowing into the pins, and allow the micro to safely detect the polarity of the higher track voltage and thus receive the DCC signals. An 11:1 divider (10kW/1kW) on the 12V rail connects to pin 9 on IC1, allowing the voltage on this rail to be measured by the 3.3V micro. IC2 is a DRV8231 3A full-bridge motor driver IC, powered at its pins 1 and 5 from the 12V rail. Digital Fig.2: despite its simple design, the Decoder includes current limiting on its motor and function outputs. The 100W resistor provides charging current for an external capacitor that can feed power back to the circuit via D1, allowing the Decoder to continue operating through brief contact losses, as might occur with dirty track. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au signals from IC1 (pins 5 and 6) feed control pins 2 and 3 on IC2. Pins 6 and 8 are the outputs, which are connected directly to the motor pads on the PCB (orange and grey); it is expected that a brushed DC motor is connected here. IC2 sets our lower voltage operating limit. At about 4.5V, it disables its motor outputs, so we can guarantee operation down to 7V as required by the DCC standards. In practice, we expect that everything will work down to about 6V on the track. The 3.3V rail also connects to pin 4 of IC2 (Vref). This is used by the circuitry inside IC2 to provide a current-­ limiting feature. The motor current flows to ground via pin 7, so a 0.68W current measuring resistor (shunt) between pin 7 and ground develops a voltage proportional to the motor current. An amplifier internal to IC1 multiplies this by 10 and compares it to the Vref voltage. If Vref has been exceeded, the power to the motors is shut off briefly to limit the current. A current of 485mA develops 0.33V, sufficient to trip the current-limiting circuitry, hence the nominal 500mA motor current limit specification, which is well within the limits of this chip. The DRV8231 can also detect short circuits and, to limit damage, it provides a longer shut-off time in that case. The circuitry around N-channel Mosfets Q1-Q4 is for the function outputs. Q1-Q4 are in an open-drain configuration for switching 12V loads under the control of a 3.3V microcontroller. The 10kW gate pull-down resistors ensure that the Mosfets are switched off any time the micro is not driving the control signals from its pins 2, 3, 7 and 8. A typical load would be a light of some sort, such as a string of LEDs and their dropping resistors, or a small ‘grain of wheat’ incandescent globe, connected between the 12V pad (blue) and the function output (white, yellow, green or purple). The 10W resistor between each Mosfet source and ground provides the current-limiting feature. The 2N7002 Mosfets require a source-to-gate voltage of about 2.3V to fully switch on, so if more than 100mA flows through one of the 10W resistors, there is no longer sufficient voltage (out of the 3.3V logic level signals) to turn on the Mosfet, and the current is limited to this level. The Mosfet will not handle a short circuit to 12V indefinitely, even with current limiting, but will survive longer than if it were exposed to a direct short circuit. With four 100mA outputs, a 500mA motor plus some tens of milliamperes consumed by the microcontroller, the Decoder is also kept under the 1A limit of BR1. The remaining circuitry is a so-called keep-alive that allows another larger capacitor to be connected, so that the Decoder can operate over dirty track or in other cases of intermittent This Decoder is fitted with a full complement of the wires that have colours set out in the DCC standards. There is also a small capacitor fitted to the keep-alive connections. DCC power. The capacitor is charged via the 100W resistor, limiting the peak charging current. The power is returned to the circuit when needed via D1, a schottky diode. We haven’t specified a capacitor here, since the circuit is fully functional without it. If needed, the capacitor can be chosen based on the space available in the locomotive. It is completely optional and unnecessary in most cases. This is also a handy location to pick off 12V if needed for other circuitry. The pins needed for in-circuit serial programming (1, 4, 12, 13 and 14) are connected to the ICSP header pads, with the standard 10kW pullup resistors between pins 4 (MCLR) and 1 (Vdd); pin 14 is ground. The 3.3V power and ground connections on the ICSP header can also be used to power other circuitry if required. Connection diagrams Figs. 3 & 4 show these locations on the PCB. Firmware operation 1 double-sided 14 × 23mm PCB coded 09111241, 0.8mm thick 1 PIC16F18126-I/SL 8-bit microcontroller programmed with 0911124A.HEX, SOIC-14 (IC1) 1 DRV8231DDAR motor driver IC, SOIC-8 (IC2) 1 MCP1703A-3302 3.3V SOT-23 LDO regulator (REG1) 4 2N7002 SOT-23 N-channel Mosfets (Q1-Q4) 1 1A SMD bridge rectifier BR1 [MBS4 or CD-MMBL110S] 1 1N5819WS SOD-323 schottky diode (D1) 1 3cm length of 20mm diameter heatshrink tubing Capacitors (all SMD M2012/0805 size MLCC) 2 10μF 25V X5R 1 100nF 50V X7R Resistors (all SMD 1%, M2012/0805 size, ⅛W unless noted) 2 100kW 6 10kW 1 1kW 1 100W The DCC Decoder 4 10W shown at actual size. 1 0.68W ¼W The main role of decoding the incoming DCC signal is performed by an interrupt service routine (ISR) that is triggered every 22μs. DCC uses pulse widths of 58μs (nominal) to encode a binary ‘1’ bit, with widths over 100μs encoding a ‘0’ bit. By counting the time between DCC polarity changes, the bits can be decoded. The Decoder then assembles the bits into packets, looking for a long string of 1s (‘preamble’) as the start marker. Since each byte is preceded by a zero bit, the firmware can easily determine its position within a packet. Each packet also includes a checksum; if this doesn’t match what the Decoder calculates, the packet is rejected as the data has been corrupted. Finally, the packet address (encoded as the first one or two bytes) is used to determine how the packet should be treated. siliconchip.com.au Australia's electronics magazine DCC Decoder Kit (SC7524, $25): includes everything in the parts list December 2025  73 Parts List – DCC Decoder DCC Glossary Accessory Decoder: A DCC decoder that is designed to be used in a fixed location; for example, to control points or signals, or fixed lights like streetlights. They operate within a separate address space to the decoders used in locomotives. Analog: Describes the traditional mode of model railway operation, where a voltage is applied to the track that directly drives the locomotive’s motor via wheel pickups. The analog controller sets the voltage and polarity to determine the speed and direction. Base Station: A device that drives tracks with a DCC signal and has the means to accept user input to control the decoders. Many also incorporate programming features as well. Booster: A device that can drive tracks with a DCC signal but has no user input. It might take its signal from a separate Base Station or other system. A booster is often used to increase the amount of power that can be delivered to the track. Consist: In full-scale railway parlance, a consist is a grouping of rolling stock that operates together and might include locomotives, passenger cars and freight cars. In DCC, it describes a grouping of mobile decoders with a common consist address, allowing them to be operated as though they were a single unit. CV (Configuration Variable): A non-volatile variable (typically stored in EEPROM) that determines a decoder’s operating behaviour. This includes the address (fleet number) that it responds to and how its motors and functions react to user input. DC: See Analog. Decoder: A device that receives power and data from a DCC signal and drives lights, motors etc. Mobile decoders can be fitted to rolling stock, while accessory decoders can control trackside equipment. Keep-Alive: A means of providing power to a mobile decoder if the track contact is poor or intermittent. It usually consists of a capacitor and some circuitry to keep the capacitor charged, feeding power back to the decoder as needed. Locomotive: For DCC, this typically includes any item of rolling stock that is fitted with a motor and mobile decoder, including passenger railcars and motorised tenders. Long Address: A 14-bit address with valid values from 0 to 10239. This range includes all four-digit numbers. Mobile Decoder or Multifunction Decoder: Also known as a locomotive decoder, it is a decoder designed to operate the motor and lights of a mobile item of rolling stock, such as a locomotive, although it could be fitted to a passenger railcar or other powered unit. Operations Mode: A form of CV programming that can occur on an operating track. The programming packets are addressed and can target a specific decoder. This is the only form of programming that can occur while the locomotive is active. Programmer: A device that can perform CV programming. This might include a Base Station that includes programming features or a standalone device like our Arduino-based Programmer from October 2018 (siliconchip.au/Article/11261). Service Mode: Address-only, Physical, Paged and Direct modes do not program decoders by address, so they must occur on a segregated track in service mode to ensure that only the target decoder receives the programming commands. Short Address: A 7-bit address in CV1. It is the only CV that is mandatory. Addresses up to 127 are valid, but values above 111 conflict with the addresses used for service mode programming packets and are best avoided. As well as commands addressed to specific locomotives, there could also be broadcast packets on address 0. These allow all trains to be stopped quickly. Some packets are used to program configuration variables. The 22μs timer is also divided down into 7ms intervals to manage the acceleration and deceleration of the motor. This is a feature of DCC that allows the locomotive speed to be ramped up or down at a rate to simulate inertia. A separate 4Hz timer is used to allow the function outputs to be modulated in various flashing patterns. An 74 Silicon Chip eight-bit CV is used to set whether or not a function output should be switched on, so flash rates from ½Hz to 2Hz are possible, as well as other sequences. Using these timers, the Decoder also monitors how long it has been since it has received a valid packet addressed to it. This can be used to stop the loco if a packet has not been received within a certain period. If the firmware sees a correctly addressed packet, it is decoded and the internal state is updated. This could involve changing the speed or Australia's electronics magazine direction, or switching some of the function outputs on or off. IC1’s internal analog-to-digital converter is used to measure the voltage on the 12V rail. This can be used to compensate the motor and function outputs, to even out variations on the 12V rail. This can happen with dirty track or if the Decoder needs to source current from the keep-alive capacitor. The motor driver is provided with a PWM (pulse-width modulated) signal to vary the motor speed. This raw speed value received is altered by the acceleration, deceleration and compensation features and can also be adjusted by means of CVs that can be used to alter the speed mapping profile. There are also CVs that can be used to remap the function commands (as received in the DCC packets) to specific function outputs, so this is another processing step that occurs, as well as applying any flashing patterns that have been set. CV programming CV programming involves modifying the configuration variables (CVs) to change the Decoder’s behaviour. The CVs are kept in non-volatile EEPROM, so are not lost during a power cycle. The list of CVs supported by our Decoder is shown in Table 1. These operate as described in the DCC standards, except for those set aside for manufacturer-specific features. For example, CV1 is used to hold a 7-bit value that is known as the short address. Thus, changing CV1 allows the Decoder to respond to a different address. This is key to how DCC allows multiple locomotives to operate on the same tracks. Typically, the address will correspond to the fleet number painted on the locomotive or item of rolling stock. The firmware also looks for the specific sequences needed for CV programming and updates the EEPROM and all live operating parameters. Since CV programming can make permanent changes, extra checks are made to validate these commands. One check that is performed (and required by the standard) is to only make changes when two consecutive, identical packets are received. There are several different programming modes, but our Decoder fully supports Address-only, Physical, Paged and Direct modes. These fall siliconchip.com.au Table 1: configuration variables supported CV Notes Default 1 A 7-bit value for the short address (the eighth bit is ignored). Values above 99 are not recommended because some of them conflict with the addresses used for service mode programming. Many systems restrict this to the range 0-99 and use the long address (CV17 and CV18) for all 3- and 4-digit addresses. The default address of 3 is standard. 3 2 Start voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the lowest speed setting. 0 3 Acceleration. A value of 0 means acceleration is disabled (the speed is updated instantly). Different systems are in use, but this Decoder applies a speed step every (7ms × value of CV3). Higher values mean slower acceleration, so a setting of 3 will result in one speed step every 21ms. 0 4 Deceleration. Works the same as Acceleration, but allows a different rate to be applied for deceleration. 0 5 High voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the highest speed setting. If it is 0 or 1, it is treated the same as 255, ie, full voltage. 0 6 Mid voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the middle speed setting. If it is 0 or 1, it is ignored (only the start and high voltages are used in calculations). 0 7 Manufacturer version number (read only). 0x5C (‘SC’) 8 Manufacturer identification number (read only). The value of 13 has been allocated by the NMRA for the identification of DIY decoders. 13 11 Packet timeout. This value dictates how many seconds a decoder will maintain speed without receiving a validly addressed packet. When set to 0, the decoder will not time out and will continue to maintain speed. 0 17 Most significant bits of the long address. The long address is nominally 14 bits and the top two bits of CV17 are set. The range of valid values for CV17 is restricted to 192-231, allowing the meaningful bits to represent the values 0-39. This gives an address range from 0-10239, which includes all four-digit decimal numbers. 192 18 The least significant bits of the long address. 0 19 Consist address and direction. Consisting allows multiple locomotives to be allocated to a train under the control of a single address, the consist address. The lower seven bits form a short address that can be used for consist addressing. When these seven bits are set to 0, consist mode is off; otherwise, the Decoder will respond to its consist address instead of its short or long address. When the upper bit is set, the locomotive operates in the opposite of its normal direction (eg, for running back-to-back with another locomotive). 0 29 Basic configuration. The bits in this CV each correspond to a basic configuration setting. Our Decoder implements: Bit 0: When set, the locomotive reverses its direction of travel. Bit 1: When set, the Decoder interprets baseline speed packets as 28-step; when clear, as 14-step. The Decoder will also respond to 128-step packets at all times. Bit 5: When set, the Decoder responds to its long address; otherwise, it uses its short address. 2 The default values for CVs 33-37 are 1, 2, 4, 8 and 0 respectively, meaning the functions are mapped across each row. It is possible to configure multiple commands to control a single output or for a command to control multiple outputs. 33 Headlight forward command → F0F output (RA5) 1 34 Headlight reverse command → F0R output (RA4) 2 35 F1 command → F1 output (RC3) 4 36 F2 command → F2 output (RC2) 8 37 F3 command → Mapped to none 0 CVs in the range 47-64 are free for custom use by designers. We have implemented them as follows. 47 Voltage compensation. When enabled, the Decoder adjusts the PWM duty cycle to counteract changes in the supply voltage. Bits 0-3 (values 1-15): when set, the Decoder sets the function outputs to match the voltage to the setting. For example, if the Decoder’s supply voltage (measured at the bridge output) is 14V and bits 0-3 are set to 7 (0111), then the functions will be driven with a 50% duty cycle output. Bit 4: when set, the decoder compensates the motor PWM to make the full speed value correspond to 16V. When either value is set to 0, the corresponding compensation is disabled. 0 For CVs 49-52 each bit corresponds to a time slice of ¼ second over a two-second period, with the MSB occurring first. A set bit means the function output is on and thus different values will give different flash patterns. Where two different values are provided, they will operate out of phase. All values 0-255 are valid, see the right-hand column for what some example values will output. 255: solid on 85 or 170: 2Hz 51 or 204: 1Hz 15 or 240: ½Hz 49 F0F output (RA5) → Flash pattern 255 50 F0R output (RA4) → Flash pattern 255 51 F1 output (RC3) → Flash pattern siliconchip.com.au 52 F2 output (RC2) → Flash pattern Australia's electronics magazine 255 December 255 2025  75 Figs.3 & 4: there is little room on the PCB for silkscreen markings, so you will have to pay close attention to these overlay diagrams, shown at ~400% scale. The Decoder will typically be wired into a locomotive as shown here. If using lamps, the polarity doesn’t matter & series resistors are not needed. into the category of what is known as service mode programming. These modes do not use the Decoder address, so are typically done on a segregated programming track to target a single Decoder. They would most often be used during initial setup of the locomotive after the Decoder is installed, before it is placed on the mainline track. Our Decoder also supports the long form of operations mode programming, which is intended to be used on a mainline track. It thus includes an address parameter, to ensure that the commands are directed to the correct decoder amongst the many that might be on the main track. This mode could be used to tweak 76 Silicon Chip the speed or acceleration profile and then perform testing without having to shuffle the locomotive to and from the programming track. As such, this mode does not permit changes to critical CVs like addresses. Decoder assembly The Decoder is built on a double-­ sided PCB coded 09111241 that measures just 14 × 23mm. It has been deliberately kept as small as possible, to fit in tight spaces (it’s even thinner than a normal PCB at 0.8mm), so there isn’t much room for designators on the silkscreen. You will have to closely follow the overlay diagrams (Figs.3 & 4) to ensure all parts are fitted correctly. Australia's electronics magazine The passive components are all M2012/0805 size (2.0 × 1.2mm), while the rest are in SOT-23 or SOIC packages. None are too difficult to solder, but it would be ideal for prospective constructors to have some SMD experience. We recommend that you have on hand flux paste (ideally in a syringe), solder-wicking braid, a magnifier, finetipped tweezers and the usual SMD gear. We found it easier to start assembly with the top side of the PCB; this has the track and motor connections (M and T) and the SOIC-8 chip, IC2. A PCB holder or some Blu-Tack will be handy to secure the board and prevent it from moving around. IC2 has an exposed pad on its underside. We have placed a large hole under that pad so that you can flow solder through from the other side, or melt solder paste via that hole. To ensure that solder will flow through this hole from the other side of the board, add plenty of flux paste to the pad under the IC and inside that hole. That should help to draw solder through from the other side of the board. Solder this first so that you can check that this joint is secure before starting on any other pins. This could also be soldered with a reflow oven, if you have one. We secured the chip flat against the PCB with polyimide (Kapton) tape and then flipped the board over to feed solder through the pad hole on the other side. Flow solder into the hole and apply heat for a few seconds. If the chip is properly soldered, after removing the tape, the chip remains in place. Then solder the remaining leads. After that, fit REG1 as shown. The single diode, D1, has its cathode towards the bridge rectifier and track connections. The two capacitors on this side are both 10μF parts. Once they are in place, solder the bridge rectifier, being sure to orientate it correctly. Follow with Q4 and the resistors on this side, being careful to match their values. Flip the PCB over and solder Mosfets Q1-Q3 and IC1. Note that both ICs have their pin 1s towards the centre of the PCB. The sole 100nF capacitor is in one corner of the PCB. The seven remaining resistors can then be fitted. Clean off the excess flux with an appropriate solvent and allow the PCB to dry, inspect it carefully and rectify any siliconchip.com.au We have wired up this compact N-scale motorised chassis (Kato 11-107) using our Decoder, allowing us to run tests on a short length of track. We removed two folded metal strips from inside the chassis to break the electrical connection between the track pickups and the motor. For clarity, we have left off the heatshrink, but we recommend you fit it to prevent short circuits. For views inside the mechanism, see www.hookstonemodels.co.uk/dcc-conversion/kato/11-107-chassis solder bridges or dry joints before powering up the Decoder. Programming IC1 If you have purchased a kit or programmed IC, IC1 will be programmed and programming will not be needed. Otherwise, you can use a Snap, PICkit BASIC, PICkit 4 or PICkit 5 to program the 0911124A.HEX file onto the chip. We were able to program one of our prototypes by fitting a 5-way pin header strip to a PICkit 5’s header socket and then pressing that against the Decoder’s ICSP header pads. We used the PICkit 5 to provide 3V power. If that doesn’t work, then you may need to temporarily solder a header strip to the Decoder’s ICSP header pads. The pad spacings are slightly narrower than 0.1in (2.54mm), but a standard 0.1in header can be made to fit without too much trouble. Testing If you would like to run some tests on the Decoder, you can apply power from a 9V battery to the ‘T’ pads that feed into the bridge rectifier. Because of the bridge, the polarity does not matter. This should power up the Decoder and you should see about 7.5V across the keep-alive pads. The output of the 3.3V regulator should be available on pins 2 and 3 of the ICSP header. You can see these connections in Figs. 3 & 4. If these voltages are present and nothing is getting hot, all is as well as can be expected. Connections The connections to the Decoder are shown in Figs.3 & 4, with the wire colours chosen to match those set out in the DCC standards. The other siliconchip.com.au connections are not part of the standard, but could be useful. If you have previously installed a decoder in a locomotive, these colours should be familiar. There is a lot of detail in fitting a Decoder to a locomotive. We can only provide some general advice, but anyone with a reasonable understanding of electronics should be able to figure it out. It’s a good idea to check the voltage and current demands of the locomotive and any additions before making any changes. We expect that LEDs and most small, modern motors will satisfy the requirements, but incandescent globes and older motors may draw more current than the Decoder can provide. One of the most important things is to ensure that all existing connections between the wheel pickups (to the track) and the motor or lights are broken. An existing connection may conflict with a connection made by the Decoder, causing a short circuit. In Fig.1(a), the red lines show the wires that might typically need to be cut; note that the diodes will probably be discarded in this case, as the Decoder provides rectified DC to the function outputs. Check the polarity of any LEDs carefully as well, since they may need to be reversed. In practice, the red and black (track) wires will go to the wheel pickups on the locomotive. They are effectively interchangeable, but the standards specify that the red wire should connect to the right-hand rail when the locomotive is travelling in the forward direction. The grey and orange wires go to the motor and, again, they are interchangeable in practice. The standards say that it should be wired up so that the locomotive moves forward when the orange wire has the more positive voltage. We suggest you connect these wires in whatever way is easiest. If it turns out that the locomotive drives the wrong way, that can be fixed by simply changing a configuration variable. The blue wire effectively provides a positive supply, with the various function outputs connecting to negative when active. Thus, LEDs should have their anodes connected to the blue wire; common-anode LEDs are typical. There is usually enough voltage to allow two or maybe three LEDs to be connected in series with a single ballast resistor. The default configuration makes the white wire active when the locomotive is going forward and the yellow Fig.5: wire up a Pico 2 and a 9V battery like this to use the Arduino sketch to test the Decoder. The resistors are simply provided for protection against damage to the Pico 2 or a connected computer. You should also connect some lights or a motor as per this diagram to see the outputs working. Australia's electronics magazine December 2025  77 Silicon Chip PDFs on USB wire active when the locomotive is in reverse. Again, it might make sense to use whatever wire is convenient and change the function mapping using the configuration variables. Remember to thread heatshrink tubing over the wires before soldering them, then shrink it into place once everything is tested and operational to prevent accidental short circuits. If you don’t plan on wiring an external capacitor to the keep-alive pads, you might be able to fit another surface-­ mounting capacitor (similar to the 10μF parts already on the PCB) directly across these pads. That will give the Decoder a small extra power reserve. Arduino test sketch The USB also comes with its own case ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). Conclusion THE FIRST SIX BLOCKS COST $100 OR PAY $650 FOR ALL SEVEN (+ POST) NOVEMBER 1987 – DECEMBER 1994 JANUARY 2005 – DECEMBER 2009 JANUARY 1995 – DECEMBER 1999 JANUARY 2010 – DECEMBER 2014 JANUARY 2000 – DECEMBER 2004 JANUARY 2015 – DECEMBER 2019 OUR NEWEST BLOCK COSTS $150 → JANUARY 2020 – DECEMBER 2024 WWW.SILICONCHIP.COM.AU/SHOP/DIGITAL_PDFS 78 Silicon Chip We are providing an Arduino test sketch (DCC_ADD3_TEST) that can be used if you have a locomotive (or motors and lamps) and Decoder, but not a base station. It is written for a Pico or Pico 2 using the arduino-pico board profile. It is based on the code we have developed for the upcoming Base Station. The sketch produces a logic-level DCC signal between GP0 & GP1 pins and expects power to be provided by other means. We can use the keep-alive connections for this purpose; something like a 9V battery would work and provide a limited current. Note that the polarity is important when connecting to the keep-alive pads. Fig.5 shows the wiring needed to run a test with the Arduino sketch. You will also need to connect a motor or lights to see any effect. The sketch produces commands with the default address of 3, and drives the motor outputs in both the forwards and reverse directions. The function outputs are activated in turn, with the actions shown on the serial monitor. Australia's electronics magazine Readers who already have a DCC system will quickly integrate DCC Decoders into their layout and have their own ideas about how to set them up. A DCC Decoder is not much fun on its own, so we will be following up with the DCC Base Station next month. We will also provide more background with a feature article for getting started with DCC in case you have not worked with a DCC system SC before. siliconchip.com.au