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By Tim Blythman Remote Controller DCC Booster Stepper Motor Driver μDCC Decoder microDCC Decoder μDCC The DCC Decoder design in the December 2025 issue is very small, but sometimes not small enough. The μDCC Decoder is designed to be a bare minimum decoder to take up less space, but we’ve still managed to squeeze in a couple of handy features that make it very useful beyond just Image source: https://unsplash.com/photos/black-model-train-moving-through-a-garden-hc9xarcmpM8 being smaller than its predecessor. W e designed the DCC Decoder, from the December 2025 issue, as a simple, inexpensive but complete unit that can add DCC capabilities to small model railway locomotives in the HO and N scales. As I started adding them to my fleet of models, I realised that I could make a couple of changes that would improve their usefulness. I’m not saying that this design is better or worse than the original Decoder, but it is smaller, and I have added some features that I think might be of interest. I recently made the jump to N scale after previously working with HO scale. With the help of a 3D printer, I started scratch-building some model trams, which are even smaller than trains! I thought that the original Decoder would be a good size for what I wanted to model, but those who have done any work at this scale will know that anything that can save space will Features & Specifications be helpful. So I looked at the earlier design to see what I could take out to make it even smaller. First, I didn’t think that I really needed four function outputs, so I discarded two of them. This removes four resistors and two transistors from the board. Next, I removed the circuitry to sense the incoming supply voltage; two more resistors removed. This means that the μDCC Decoder has only two function outputs and does not have the ability to compensate for supply voltage changes. I also figured I could do without the 100nF capacitor on the microcontroller since the micro would be close enough to the existing 10μF regulator output filter capacitor. Hardware-wise, these are pretty much the only differences between the original Decoder and the μDCC Decoder. The newer board is only 12 × 18mm, down from 13 × 28mm; only In model railways, smaller is generally better. The μDCC Decoder is only 12mm × 18mm with two function outputs and even has a basic sound function. 🛤 Size: 18 × 12 × 4mm 🛤 Two 100mA function outputs 🛤 Sound output 🛤 Standard DCC features like the December 2025 DCC Decoder 84 Silicon Chip Australia's electronics magazine 60% of the area! Fig.1 shows the circuit diagram, and you can see that it really is just a cut-down version of the earlier design. It looks like there are some unused pins that are wasted, but I have redeployed I/O pin 11 to supply the 3.3V reference that came directly from the 3.3V regulator in the earlier design; the firmware simply holds this at a high level (3.3V) at all times. This avoids an awkward trace that would otherwise have had to cut across the board. Having a few unused pins made the PCB trace routing easier and more compact, so it actually ended up being a good compromise. I have made some extra signals available on the RA0/PGD and RA1/PGC pins; they have been chosen mainly because they already have external connections available at the ICSP (in-circuit serial programming) header. Just like in the earlier design, track power is rectified by diode bridge BR1. REG1 provides 3.3V to power the microcontroller. The DCC signal polarity is sensed via the two 100kW resistors, and the micro drives the outputs on pins 2, 3, 5 and 6 to control transistors Q1 and Q2 and motor driver IC2. These would be connected siliconchip.com.au Fig.1: the μDCC Decoder circuit is very similar to the December 2025 DCC Decoder, with a few components removed. The 3.3V reference for the motor driver IC comes from a pin on IC1 to simplify the PCB routing. to accessories (such as lights) and the locomotive motor, respectively. The 100W resistor and series diode D1 allow a capacitor to be fitted to provide ‘keep-alive’ power that can help compensate for intermittent contact due to dirty track. In other respects, operation is the same as the earlier design. Bonus features The PIC16F181xx family of chips has an 8-bit DAC (digital-to-analog converter) that has reasonable drive strength. It isn’t specified what current it can deliver, but tests indicated that it would be possible to source and sink up to 20mA. After removing excess features from the earlier Decoder firmware, the PIC16F18126 has around 12kB of unused flash memory, which is enough to hold a fraction of a second of 8-bit sampled audio data. So I investigated driving a small piezo transducer with the DAC to reproduce audio. The DAC output is directed to pin 13, since this is broken out amongst the ICSP pins. It has a ground pin next to it on the ICSP header, so it’s fairly easy to make the necessary connections to the transducer. An electromagnetic siliconchip.com.au speaker will likely have an impedance that is too low to work; you must use a high-impedance device like a piezo transducer. The piezo I tested measures 9mm square and 2mm thick. Its model code is in the parts list; I’ve managed to squeeze this device into several N-scale models. The piezo transducer has a peak response around 4kHz, which is quite high, and I quickly found that high-pitched sounds were reproduced much better than lower-­ pitched sounds. This means that a high sampling rate is needed; fortunately, the DCC firmware already includes a 22μs timer interrupt, which (at 45.4kHz) is fairly close to the 44.1kHz sample rate used in audio from sources like CDs. This made it easy to experiment with existing samples. So, onboard audio production is possible, but the result is not hifi! Still, I was able to recreate some recognisable sounds for a model railway. The best sound I could recreate was a tram bell. This could also pass for the level crossing bell used on some diesel locomotives. The μDCC Decoder would also work well as a stationary decoder for a level crossing’s lights & bells. I figured that a steam locomotive whistle might also be sufficiently highpitched to work, so I’ve synthesised a sample that emulates this. We’ve made a recording of these sounds being played by the μDCC Decoder, so that you can hear for yourself. It’s an MP3 audio recording from siliconchip.au/ Shop/6/3587 The 8-bit microcontroller has modest processing power and would struggle to mix the two sounds, so we have DCC PROJECT KITS DCC Base Station, January 2026 (SC7539, $90) DCC Remote Controller, February 2026 (SC7552, $35) DCC Booster, March 2026 (SC7579, $45) DCC Stepper Motor Driver & Decoder, April 2026 (SC7601, $30) microDCC (μDCC) Decoder, May 2026 (SC7617, $25) includes all the parts and the optional piezo (wire not included). Specify if May 2026  85 Australia's electronics magazine you want a bell or whistle sound programmed into the microcontroller. Pay close attention to the resistor values and component polarities. Fortunately, the two capacitors are of the same value. The regulator and transistors are all in SOT-23 packages, so be sure not to mix them up. Screen 1: It’s incredible what is possible with model trains; tiny LCD modules like these add another element of realism. The CV48 serial data feature is intended to control features that don’t map well to traditional DCC function outputs. Source: https://youtu.be/tC_t22RfQ0c created two firmware files: one for the bell sounds and one for the whistle sound. If combined, the samples would also have to be shorter. Sound is controlled by a function output. The bell sound will repeat as long as the function is active, and a cheery “ding-ding” will be heard if the function is held for about half a second. The whistle sound will ramp up and keep playing until the function is switched off, after which it quickly decays to silence. We’ve also added another output to the μDCC Decoder. It is intended to allow communication with another microcontroller that could implement other features. One application that came to mind is a form of headboard or destination display, such as a second microcontroller driving a small OLED module or LCD. When it receives a byte over the serial link, it can update the display. This would only happen occasionally, so would be easy to control with the Base Station’s CV programming page. The YouTuber diorama111 has implemented this type of display in HO scale models, although it is controlled through an infrared remote control. Screen 1 shows a still from the video at https://youtu.be/tC_t22RfQ0c The output is a UART (serial data) signal that is available on RA1/PGC, the other I/O pin that is free on the ICSP header. It operates at 3.3V, 9600 baud with eight data bits. This protocol EEPROM location Stepper Driver μDCC Decoder Extra output Decimal Hex 86 DCC Decoder CV Default Hex CV Default Hex CV Default Hex 0 0x00 29 2 0x02 29 2 0x02 29 2 0x02 1 0x01 1 3 0x03 1 3 0x03 1 3 0x03 2 0x02 19 0 0x00 19 0 0x00 19 0 0x00 3 0x03 18 0 0x00 18 0 0x00 18 0 0x00 4 0x04 17 192 0xC0 17 192 0xC0 17 192 0xC0 5 0x05 2 0 0x00 3 0 0x00 2 0 0x00 6 0x06 3 0 0x00 4 0 0x00 3 0 0x00 7 0x07 4 0 0x00 5 64 0x40 4 0 0x00 8 0x08 5 0 0x00 33 1 0x01 5 0 0x00 9 0x09 6 0 0x00 34 2 0x02 6 0 0x00 10 0x0A 33 1 0x01 35 0 0x00 33 1 0x01 11 0x0B 34 2 0x02 36 0 0x00 34 2 0x02 12 0x0C 35 4 0x04 37 0 0x00 35 4 0x04 13 0x0D 36 8 0x08 49 255 0xFF 36 0 0x00 14 0x0E 37 0 0x00 50 255 0xFF 37 0 0x00 15 0x0F 49 255 0xFF 11 0 0x00 49 255 0xFF 16 0x10 50 255 0xFF 50 255 0xFF 17 0x11 51 255 0xFF 11 0 0x00 18 0x12 52 255 0xFF 19 0x13 11 0 0x00 20 0x14 Chip47 0 Silicon 0x00 TableAustralia's 1: CV toelectronics EEPROMmagazine mapping is simple and common enough that any microcontroller should be able to receive it and provide some custom functions. It is controlled through a virtual configuration variable (CV), CV48. Operations mode programming allows this CV to be programmed ‘on the mainline’. Any time the μDCC Decoder receives a write command to program CV48, it sends the corresponding data byte over the serial output. That’s all there is to it. These pins are shown on the overlay/wiring diagrams later in the article. If you don’t want or need these two features, you can just leave these pins disconnected. Construction Like the earlier Decoders, this is a small design using surface-­mounting parts, so you’ll need the gear and expertise to handle that. Many of the comments from the DCC Decoder also apply here. For example, you can increase the value of the 0.68W resistor to reduce the motor current limit, although you should not decrease it below 0.68W. The μDCC Decoder is built on a double-­ sided PCB coded 09111247 that measures 12 × 18mm and is 0.8mm thick. Work through the overlay diagrams, Figs.2 & 3. Start with the side that has IC2 and BR1. Solder these first, noting their polarity. Follow with D1, making sure its cathode stripe is nearest the pads marked T. Also on this side is one of the 10μF capacitors, right next to the bridge rectifier outputs. The two 100kW resistors, the 100W resistor and the 0.68W resistor are also on this side of the PCB. Flip the board over and fit REG1 (near IC1) and Q1 and Q2 (near the edge of the board). Solder IC1 in place with its pin 1 marker nearest to REG1. The resistors on this side are the 10kW and 10W siliconchip.com.au Programming a DCC Decoder without a DCC Programmer We’ve presented a thorough series of DCC system components over recent issues, including the Base Station hardware, which has comprehensive DCC programming capabilities. But it occurred to us that many of our readers will probably have hardware at their disposal that will allow programming our Decoders (from this series) without a dedicated DCC programmer. Our Decoders are all based on PIC microcontrollers, which are easily programmed with devices like the various PICkit programmers or even the Snap programmer (which we now carry in the Silicon Chip Online Shop at siliconchip.au/Shop/7/7588). The configuration variables (CVs) that are involved in Decoder programming are simply locations in EEPROM and thus they can be changed with the appropriate PIC programming hardware. So this guide explains how to program the CVs in our Decoders using a PIC programmer. Table 1 shows which CVs correspond to which EEPROM address on each Decoder. Below we explain how to modify the EEPROM values for programming. We’ll assume you’ve used a programmer like this before, and know how to make the necessary wiring connections to program a PIC microcontroller. It’s also assumed that you understand the CVs that you want to program. Read-only locations like CV7 and CV8 are not implemented in EEPROM, so cannot be modified. Of course, we have provided the source code for all three projects, so you can modify the source code and recompile the project (using MPLAB X IDE) to make those or any changes you like. The default values for all the CVs are set near the start of the dcc.h file. If you are simply looking to adjust some of the CVs, we recommend just using the MPLAB IPE (integrated programming environment) software. Screen 2 shows the IPE with the PIC16F18126 selected; as you would need for any of the Decoder projects. Select the appropriate HEX file by using the Browse button and then open the EEPROM view from Window → Target Memory Views → EE Data Memory. From here, you can edit the EEPROM values directly. The hexadecimal values in Screen 2 correspond to the original DCC Decoder from December 2025. Editing the EE Data Memory window will not directly change the HEX file, but you can export the edited file from the File menu as a new HEX file. The exported HEX file can be reloaded later using the Browse button noted above. When you have made the necessary edits, hook up your programmer to the Decoder, press Connect and then press Program to change the values stored on the chip. Remember that you should not have anything else connected to the ICSP pins during programming. You can also download the contents of the PIC’s non-volatile memory (including flash memory, configuration bits and EEPROM) with the Read button. You can then edit the EEPROM values and program the new values back into the chip. While this is a slightly convoluted method of CV programming, you can also use it to save and restore program memory images and CV settings of the decoders for safekeeping. We had a detailed guide to CV programming in the Getting Started with DCC guide in the January 2026 issue (siliconchip.au/Article/19560). ◀ Screen 2: The MPLAB IPE can be downloaded as part of the MPLAB X IDE and provides an interface for programming PIC microcontrollers (and other Microchip parts). The EEPROM entries at the bottom match the DCC Decoder, with other locations left blank (0xFF). parts, so take care not to mix them up. Don’t forget the other 10μF capacitor. Clean the board of any excess flux, inspect the board and allow it to dry. If necessary, you can program the chip at this point. Note that you cannot use a PIC16F18124 or PIC16F18125 for this project, since the larger flash memory of the PIC16F18126 is needed to store the audio samples. You shouldn’t need to program the chip if you have purchased it from the Silicon Chip Online Shop. Also be sure not to connect the piezo transducer or any other circuitry to the ICSP pins (except a programmer) siliconchip.com.au during programming, since this will interfere with the programming process. The remaining steps for testing and wiring the μDCC Decoder to a locomotive are much the same as the DCC Decoder. Operation The μDCC Decoder operates in much the same fashion as the DCC Decoder from December; the implemented CVs all work the same. We’ve given the μDCC Decoder a model ID (CV7) of 0x5E (94 in decimal) to differentiate it from the other two Decoders. Australia's electronics magazine The other main differences (compared to the DCC Decoder) are that it lacks CV47, CV51 and CV52. CV47 is for voltage compensation, which the μDCC Decoder can’t do. CV51 and CV52 are not needed, since the corresponding function outputs have been deleted. The EEPROM Mapping panel in the December issue has more information about the CVs, see the panel on programming with a PIC programmer. The audio output is equivalent to the green wire function output (F1) in other decoders. There are no effects that can be applied, but it’s possible to May 2026  87 Figs.2 & 3: the external connections to the μDCC Decoder are via bare solder pads as shown here. We have been able to keep the main DCC connections on the same side of the PCB, with the audio output using some of the ICSP pads on the reverse. Parts List – microDCC (μDCC) Decoder 1 double-sided 12 × 18mm PCB coded 09111247, 0.8mm thick 1 PIC16F18126-I/SL 8-bit microcontroller programmed with 0911124G.HEX (bell sound) or 0911124W.HEX (whistle sound), SOIC-14 (IC1) 1 DRV8231DDAR motor driver IC, SOIC-8 (IC2) 1 MCP1703A-3302 3.3V LDO linear regulator, SOT-23 (REG1) 2 2N7002 N-channel Mosfets, SOT-23 (Q1, Q2) 1 MBS4 or CD-MMBL110S 1A SMD bridge rectifier (BR1) 1 1N5819WS 40V 1A schottky diode, SOD-323 (D1) 1 2cm length of 20mm diameter heatshrink tubing (to insulate Decoder) 2 10μF 25V X5R SMD M2012/0805 size MLCC capacitors 1 Same Sky CPT-9019A-SMT-TR piezo transducer (optional) various lengths of wire as needed Resistors (all SMD ±1%, M2012/0805 size, ⅛W unless noted) 2 100kW 3 10kW 1 100W 2 10W 1 0.68W ¼W reduce the volume by adding a resistor in series with the transducer. The default setting maps the audio to the F1 function output, so you can use the F1 control on the Base Station to test the sound. The mapping is due to the value of 4 appearing in CV35. You can use other function outputs to control the audio by ORing 88 Silicon Chip CV33-CV37 with a value of 4. For example, to use F2 to control it, program CV36 with the value 4. CV48 is not mapped into EEPROM, so it can’t be read back. It will respond to writes in all programming modes, but we expect it will be most useful in operations mode on the main track. Base Stations will typically send Australia's electronics magazine repeated programming packets, so the μDCC Decoder may deliver multiple serial bytes in response to this. Custom sounds It’s possible to change sounds, but you will need to recompile the project files to do this. The audio samples and config are in audio.c and audio.h. The maximum sample size is around 13kB, which corresponds to around 300ms at 44.1kHz. Be sure to select compiler optimisation level 2, which is available even with a free license. The samples are effectively 8-bit unsigned values, but they should start and end with a zero value (by ramping up from zero and down to zero) so that the DAC idles at 0V when not playing. This will prevent power supply noise from being produced at these times. There are options to play the audio either as a one-shot or as a loop. Note that the one-shot will repeat if the function stays on. Use the bell sound as a template for one-shot sounds and the whistle as your guide for looping sounds. For looping, you’ll need to set the AUDIO_ LOOP_START and AUDIO_LOOP_ END points. During playback of a looping sound, the sound will play up to the loop end point and jump back to the loop start to maintain a continuous sound. Our process to generate the samples is to use Audacity (free software) to create an 8-bit, 44.1kHz mono WAV file. We then use the HxD hex editor program to strip out the 44-byte WAV header and export the file contents as a C byte array that can be pasted into the assignment for the audioData variable in audio.c. The code can automatically work out the data size to stop playback when the end of the data is reached. The available sample space will be slightly smaller for looping sounds, since there is extra code needed to handle the looping that will use up some of the flash memory allocation. Summary While I had intended this design to allow me to add DCC to some of my smaller models, I’m quite proud of being able to cram some simple sound effects and other features into a tiny 8-bit microcontroller. I’ve built a few of these μDCC Decoders and now all my N scale models are soundSC equipped! siliconchip.com.au