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By Tim Blythman μDCC Decoder Accessory Decoder I2C Controller 2 I C Controller Destination Display for DCC Accessory Decoders Programming accessory decoders can be tricky, even though the standards provide for it, since not all base stations provide the capability. This small board makes it easy to program our DCC Accessory Decoders. It only requires power and two signal lines on an I2C bus, so it could be a handy device for other projects that need a simple user interface. A s we noted in the DCC Accessory Decoders article, it can be tricky to program accessory decoders. They are usually permanently wired to the main track circuit, so they can only be programmed in ‘operations mode’. There are means for bidirectional communication in operations mode (to read back and verify programmed values), but that requires extra hardware, both on the decoder and on the base station. Providing a display and user controls for each Accessory Decoder would make this much easier, but that would be a waste as it would sit idle most of the time and you’d need one per decoder. It would also make the Decoder larger, harder to build and more expensive. This I2C Controller is a simple removable device that provides a display and some buttons. One Controller can be used to set up several Accessory Decoders, one at a time, then be put aside for future use. We also think that the I2C Controller might come in handy for other projects that require a similar interface. In this article, we’ll describe the I2C Controller and its construction. We’ll also detail how it can be used with the DCC Accessory Decoders described elsewhere in this issue, including programming their CVs and monitoring their operation. Circuit details The circuit of the I2C Controller is shown in Fig.1. CON1 is a four-way header that has the SCL and SDA I2C signals, plus ground and power. This connects to a matching header on the DCC Accessory Decoder (or other device that needs a control interface). The Decoder provides power and acts as an I2C master. LED2 is simply a power indicator LED with a 1kW series resistor. The I2C Controller provides several I2C slave interfaces that can be controlled by the master. The first of these is an OLED module, MOD1. It contains a display controller IC, a 3.3V regulator (for the display controller) and pullups from the SDA and SCL lines to 3.3V. IC1 is a PCF8574 (or PCF8574A) I2C IO expander (IC1); this also connects to the I2C bus at its pins 14 and 15. IC1 is powered from the supply at pins 8 and 16. These are bypassed by a 100nF capacitor. Three pins (1, 2 and 3) are pulled low to set the I2C sub-address of DCC PROJECT KITS Snap-type Accessory Decoder (SC7685, $40) includes the PCB and all onboard parts, including the electrolytic capacitor Servo-type Accessory Decoder (SC7686, $40) includes the PCB and all required onboard parts I2C Controller (SC7690, $30) includes all the parts in the parts list overleaf 80 Silicon Chip Australia's electronics magazine IC1 to zero. This means that a PCF8574 will appear on address 0x20 and a PCF8574A on address 0x38. Eight of the remaining pins are designated as quasi-bidirectional I/O pins. In practice, these are open-drain outputs with weak internal pullups. The chip also implements a pullup accelerator that briefly applies a stronger pullup as the open-drain output is switched off. Pin 13 (INT) is an output that is triggered on an I/O state change and is not used here. The open-drain arrangement means that the pins can be safely pulled to ground without causing a conflict. IC1 has commands to read its I/O pin state and to control its open-drain outputs. Switches S1-S4 are connected between the I/O pins and ground, so a switch closure is detected as a low level by IC1. LED1 is a two-pin bicolour LED, with each lead connected to another of IC1’s pins. They are also pulled up to the supply by 1kW resistors. When one of LED1’s leads is taken low by IC1, current flows into the resistor on the other lead and out via the grounded pin. Current is also wasted on the other resistor, which is now directly connected across the supply. Still, by taking one or the other pins low, either the red or green element in LED1 can be lit up. Taking both pins low means that they are at the same potential and no current flows through the LED, holding it off. Helpfully, the wiring of CON1 is the same as the I2C OLED modules that we use, so the two can be interchanged to a degree. Be careful, though; we have seen some I2C OLED modules with siliconchip.com.au Fig.1: this board does little more than break out an I/O expander IC connected to some pushbuttons and a bicolour LED along with an OLED module. swapped power and ground connections! For example, an OLED module can be plugged in where an I2C Controller is expected, and the display will work as intended. The default display is a status information screen that can be viewed without the pushbuttons needing to be pressed. Since the pinouts are identical, other projects or devices that use an I2C OLED module could be reprogrammed to access the I/O expander chip to add extra inputs (S1-S4) to an existing interface. Having said that, note that the PCF8574 and PCF8574A parts have different I2C timing requirements to the OLED module’s display controller. So the I2C Controller may not work in place of a bare OLED module unless the firmware takes account of this. Options We have marked the PCB to suit a polarised header so that the I2C Controller and Accessory Decoders can be connected by flying leads without a risk of reversed connections. However, you might prefer a different plug and socket arrangement. For example, it might be just as easy to hard-wire a flying lead (with plug) directly to the Controller, since there will be little need to detach the lead from the Controller. Our early prototypes used a simple header plug and socket arrangement. pin 1 indicator lines up with the mark on the board. Tack one lead and check that the other leads are on their pads, then solder the remaining leads and after that, refresh the first lead. Next, solder the three resistors and one capacitor; none of these are polarised. Clean off the flux residue and allow the board to dry. Now fit the four switches, making sure their bodies are flat against the PCB, then mount LED2 with its cathode to the left as shown. LED1 has the red element’s cathode as marked. Use a multimeter on diode mode to check the orientation before installing it. When this LED lights up red on a tester, the negative lead (usually black) is connected to the cathode of the red element. When trimming the leads of the LEDs, put the offcuts aside. Attach the pin header to the OLED module if necessary, then solder it in place, being sure to leave a small space between it and the parts below. A piece of card could be used as a temporary spacer. Finally, use the offcuts to secure the lower mounting pads in the OLED module to the PCB below using solder. You can test the I2C Controller by applying 3.3-5V between the G (negative) and V (positive) connections. If LED2 lights up, everything is working as well as can be tested without an external microcontroller driving the I2C bus. Wiring harness The photo overleaf shows how the harness is assembled; we recommend it be no more than 10cm in length. Remember that I2C is short for inter-­ integrated circuit and is designed to cover short distances within a PCB. Both ends are wired the same, and the colour code we have used matches black for GND and red for V, so there are also some visual cues to ensure it is not wired up incorrectly. Fig.2: both the decoders and I2C Controller are marked with their pin layouts, but we’ve chosen a polarised cable to ensure that the boards are always connected correctly. The OLED module is fitted last and sits over the other components. We designed this board to interface with the DCC Accessory Decoders (elsewhere in this issue), but it could be a handy addition to any project that needs a simple user interface. Construction Referring to Fig.2, the overlay diagram, apply flux paste to the pads for the SMD parts. Place IC1, ensuring its siliconchip.com.au Australia's electronics magazine July 2026  81 Use the headers and lead offcuts to space the OLED module off the PCB and clear of the parts below. We used the colour coding shown here (matching our prototype) but it isn’t critical since the plugs are polarised. The two ends can be interchanged without any problems. How to use it As shown in Fig.2, connect the I2C Controller to CON7 of the DCC Accessory Decoder (based on a PCB coded 09111254 or 09111255) and power up your DCC system. Both LEDs and the OLED screen should light up. LED1 should be green after a second or two, and the OLED will display something like Screen 1; it will be slightly different on the Servo-­ type Decoder. In general, buttons S3 and S4 (left and right) cycle through the screens, while S1 and S2 (down and up) edit the values on the screen. Pressing S3 and S4 together resets the OLED display controller; try this if the display is corrupted. Screen 1 is a status screen, useful for monitoring the Accessory Decoder’s operation. As mentioned before, this will appear if you just plug an OLED screen in too. The second line is only present on the Snap-type Decoder and shows the voltages on the motor supply before and after the 100W resistor. The screen will show VM OK and a green LED1 if the 4700μF capacitor is fully charged, or “VM --” and a red LED1 if it is charging. The servo motor supply rail voltage is shown by the Servo-­ type Decoder. The DCC text shows if DCC packets are being received, while the last line shows the address of the most recent accessory packet, or dashes if none have been seen in the last five seconds. These should allow you to check that your DCC base station is sending packets to the expected addresses. Pressing S4 will cycle to Screen 2, showing the main mode. This screen only affects the Snap-type Decoder since the Servo-type Decoder is fixed at four outputs. When two outputs are selected, they are full-bridge types, while the four outputs are open-drain types. S1 or S2 can be used to change this setting. Screens 3-8 are repeated for each output (1-4) and the output number is shown at top left. These mostly allow direct editing of the CVs (configuration variables). On these screens, S1 GPS-Synchronised Analog Clock with long battery life ➡ Convert an ordinary wall clock into a highlyaccurate time keeping device (within seconds). ➡ Nearly eight years of battery life with a pair of C cells! ➡ Automatically adjusts for daylight saving time. ➡ Track time with a VK2828U7G5LF GPS or D1 Mini WiFi module (select one as an option with the kit; D1 Mini requires programming). ➡ Learn how to build it from the article in the September 2022 issue of Silicon Chip (siliconchip. au/Article/15466). Check out the article in the November 2022 issue for how to use the D1 Mini WiFi module with the Driver (siliconchip.au/Article/15550). Complete kit available from $55 + postage (batteries & clock not included) siliconchip.com.au/Shop/20/6472 – Catalog SC6472 82 Silicon Chip Australia's electronics magazine siliconchip.com.au and S2 increment by one, unless the other button (S2 or S1) is held down; this will increment by ten instead, allowing you to reach the desired setting faster. Screen 3 is the address, which can be set from 1 to 2048. Multiple outputs can be set to the same address so that they respond to the same commands, although this will not work with CV programming packets. Screen 4 sets the output runtime in multiples of 10ms, so the top-right value is the raw CV3 value, while the lower value is the actual runtime. A value of zero means the output runs indefinitely, at least until its counterpart is activated. Screens 5 and 6 set the pulse width that is applied to the servo motors in the Servo-type Decoder. If an output is active and its CV3 is set to zero, the servo position will update the output in real-time to allow it to be easily fine-tuned. Note that these screens are not present on the Snap-type Decoder since they would serve no purpose there. Screen 7 allows the manual operation of an output. This means that it’s entirely possible to operate the DCC Accessory Decoders using just the I2C Controller, without a DCC base station. Screen 8 is used to reset the selected output’s CVs back to the default values. The Accessory Decoders have been programmed to recognise when the I2C Controller is connected, but we recommend that it is only connected or disconnected while the power is switched off to the decoders. This should help to avoid damage due to stray voltages while the connection is being made or broken. Screen 1: the first screen shows status information, meaning that even a bare OLED module can be used to check and monitor an Accessory Decoder. Screen 2: the Snap-type Decoder can be configured to have four open-drain outputs on this page. All changes are made by pressing S1 or S2. Screen 3: like the other pages, the values can be quickly changed by holding down S1 while pressing S2 (or vice versa). Screen 4: the runtime applies to all decoders. When it is set to zero, the output is on until the other output is activated. Screen 5: this screen sets the servo pulse width for the THROW output; values between 1000μs and 2000μs (1ms and 2ms) are typical. Screen 6: the servo pulse width setting for the CLOSE output. The defaults should give about 90° of movement, but most constructors will need to trim the values to suit their installation. Screen 7: this screen allows manual operation, giving yet another option (besides the onboard pushbuttons) for using the Decoders without a DCC system. Screen 8: each output can be set to its defaults by pushing S1 or S2; this includes the address, run time and servo pulse widths. Parts List – I2C Controller The I2C Controller is little more than a module with a few I2C devices on it, so it should be easy to connect to microcontrollers like Arduinos and Micromites. As we mentioned, the PCF8574(A) has different timing requirements to the OLED modules, so you might find you need to tweak your bus speed to suit. Using an I2C scanning program is a good way to test whether the bus is working correctly. The PCF8574(A) should be seen on 7-bit addresses of either 0x20 or 0x38 (32 or 56), while the OLED modules are typically set SC to 0x3C (60). 1 57 × 40mm double-sided PCB coded 09111256 1 4-way 2.54mm/0.1-inch pitch polarised header or similar (CON1) [Jaycar HM3414, Altronics P5494] 1 1.3-inch OLED display module (MOD1) [Silicon Chip SC5026, SC6511] 4 6 × 6mm through-hole tactile switches (S1-S4) Semiconductors 1 PCF8574 or PCF8574A I/O expander IC, wide SOIC-16 (IC1) 1 red/green 3mm bicolour LED (LED1) 1 green 3mm LED (LED2) Capacitors/resistors 1 100nF 50V X7R M3216/1206-size SMD MLCC capacitor 3 1kW ¼W M3216/1206-size SMD resistors Cable 2 4-way 2.54mm/0.1-inch pitch polarised header plugs with crimp pins [Jaycar HM3404, Altronics P5474 + P5470A] 1 10cm length of 4-way ribbon cable OR 1 40cm length of light-duty hookup wire siliconchip.com.au Australia's electronics magazine Using it with other projects July 2026  83