⚙️ Introducing Interface C — A Modern Motor Speed Controller for LEGO® Power Functions ⚙️

[diegobaca]

I’m excited to share that Interface C is now available.
It’s a modern take on the classic LEGO® 9V Train Controller (4548), designed to give builders precise speed control and versatility — perfect for powering Technic creations, Great Ball Contraptions, and custom motorized builds.


Interface C with Dark Bluish Gray, Light Bluish Gray, and Orange - LEGO® Power Functions colorway.

🔧 What It Does

• Provides six-speed precision control — forward or reverse — with a smooth rotary dial.

• Connects directly to LEGO® Power Functions motors, letting you breathe new life into old builds.

• Remote control via web interface, allowing you to manage motors wirelessly over Wi-Fi.

• Fully compatible with Windows and Mac through the Interface C Utility for firmware updates and configuration.

• USB-C port for firmware flashing and diagnostics.

🛠️ How to Get It

Interface C is available in multiple options depending on how hands-on you want to get:

• Full Kit A — includes the microcontroller.

• Full Kit B — without microcontroller, for those who already own an Arduino Nano ESP32.

• Fully Assembled Unit — ready to use out of the box.

Each kit includes high-quality parts, assembly instructions, and access to digital resources — even STL files to 3D print your own housing.

Learn more and get your kit at 👉 https://interface-c.com


LEGO® 9V Train Controller (4548) compared to the Interface C with Light Bluish Gray, Black, and Yellow colorway.

Edited October 16 by diegobaca

[BatteryPoweredBricks]

This is awesome!  I can't wait to get my hands on one 😁.  I'm still looking through all of the documentation to get a better grasp on things but I did have a couple questions that popped into my mind immediately:

Do you have any plans to release a "dumb" version?  Just a speed control for PF motors without the option for control from the app?  I'm not sure how feasible this would be or how much this would save on cost in the end but thought I would ask.

Do you have any plans to add options for alternate colors for the kits on Tindie?  The black and yellow one definitely appeals to me 😁

 

[msk6003]

I think microcontroller for just speed control is to much. Simple version will be useful.
If you make control center's PF or PU version microcontroller will be essential.

And add some stud or axle/pin hole on dial will be useful too.

Edited October 17 by msk6003

[dr_spock]

The STL files are available.  I suppose one can 3d print the enclosure.  Then hack in a basic PWM board and a direction control SPDT switch for "dumb" speed control.

[Toastie]

16 hours ago, diegobaca said:

Interface C

Whoa!!! That's an announcement! 

So, Interface A had 6 outputs and 2 inputs. Interface B had 8 outputs and 8 inputs. Interface C has 1 output and no input? Seems to me more like a smart power supply rather than an interface? "Smart" as in providing Wi-Fi/BT/BLE/MTQQ/etc. connectivity and control (well, all that an ESP32 provides). This should also allow for control via home-integration/assistants. So yes, it is an interface! And very nicely done - it has a really appealing look. Congratulations on your achievements!!! Bringing such a device to the market is really something. 

With that USB port connected to the ESP32 hybrid controller inside (not only for diagnosis, but power-programming!), along with an X-channel USB switch, you can also orchestrate a nice X-channel Interface. Sensors can be hooked-up wirelessly. It will be a little pricey though - well interfaces A and B were sure as well!

All the best, and thanks for sharing!
Thorsten

[diegobaca]

10 hours ago, BatteryPoweredBricks said:

This is awesome!  I can't wait to get my hands on one 😁.  I'm still looking through all of the documentation to get a better grasp on things but I did have a couple questions that popped into my mind immediately:

Do you have any plans to release a "dumb" version?  Just a speed control for PF motors without the option for control from the app?  I'm not sure how feasible this would be or how much this would save on cost in the end but thought I would ask.

Do you have any plans to add options for alternate colors for the kits on Tindie?  The black and yellow one definitely appeals to me 😁

 

Thank you! I have a few plans in mind that I’m currently working on:

1. Interface C Lite – I’m exploring ways to reduce cost and simplify features. Some of the things I’m considering include:

   a) using a more affordable microcontroller (some are only $2–3 compared to the $20+ one I’m using now),

   b) removing Wi-Fi and web-interface control, and

   c) dropping the OLED screen.

2. Interface C Pro – I’m going all out with this one! I want it to control up to eight motors independently and possibly include programmable functions. I’m still in the research phase for this version as well.

As for color options — I’ve uploaded the 3D models to MakerWorld, so people can print their own cases in any color they like. Depending on interest, I also plan to offer different color options for sale later on.

6 hours ago, msk6003 said:

I think microcontroller for just speed control is to much. Simple version will be useful.
If you make control center's PF or PU version microcontroller will be essential.

And add some stud or axle/pin hole on dial will be useful too.

Yeah, it might be a bit much — but I wanted to add an OLED display to show information, and also allow the controller to be remotely operated through a web interface. So using a microcontroller for this version made sense to me. I’m planning a simpler version that will be more affordable. I’m also researching how to support the new PowerUp system, but that might come later.

I really like your idea of adding studs or axle/pin holes on the dial! I’ll look into that. Thank you for the feedback!

35 minutes ago, Toastie said:

Whoa!!! That's an announcement! 

So, Interface A had 6 outputs and 2 inputs. Interface B had 8 outputs and 8 inputs. Interface C has 1 output and no input? Seems to me more like a smart power supply rather than an interface? "Smart" as in providing Wi-Fi/BT/BLE/MTQQ/etc. connectivity and control (well, all that an ESP32 provides). This should also allow for control via home-integration/assistants. So yes, it is an interface! And very nicely done - it has a really appealing look. Congratulations on your achievements!!! Bringing such a device to the market is really something. 

With that USB port connected to the ESP32 hybrid controller inside (not only for diagnosis, but power-programming!), along with an X-channel USB switch, you can also orchestrate a nice X-channel Interface. Sensors can be hooked-up wirelessly. It will be a little pricey though - well interfaces A and B were sure as well!

All the best, and thanks for sharing!
Thorsten

You’re correct about the reduced outputs and inputs on this version. I’m planning another one that will support eight outputs and add programmability. Since this was my first time creating any kind of hardware or electronics, I wanted to start small — but I hope to keep evolving and adding more functionality to make it even more worthy of the Interface name. 

I love all your ideas for expanding the controller — lots to think about! Thank you for the feedback and kind words.

[Sylvain Maltais]

Hello Diego,

Really interesting, I have a few question for you. What microcontroller are you using?  Is an Arduino, if yes. Is it possible to just transfer the program to after I have paid you for it. I can do the rest at home. 
 

It interest me a lot of what you have done for the controller. The funny part, I just start to collect all the controler LEGO have done.  I cluding the smart brick if I can name them by that name. I whant to contact you and Brian from Battery… about that.  Both of you are them one I saw speaking of LEGO electronics. 

[UltraViolet]

I really like this concept.  It's good to hear there are plans for variations.  When I just checked, the website says no shipping to Canada, so I guess I'll be waiting indefinitely.

[diegobaca]

4 hours ago, Sylvain Maltais said:

Hello Diego,

Really interesting, I have a few question for you. What microcontroller are you using?  Is an Arduino, if yes. Is it possible to just transfer the program to after I have paid you for it. I can do the rest at home. 
 

It interest me a lot of what you have done for the controller. The funny part, I just start to collect all the controler LEGO have done.  I cluding the smart brick if I can name them by that name. I whant to contact you and Brian from Battery… about that.  Both of you are them one I saw speaking of LEGO electronics. 

I’m using an Arduino Nano ESP32. You don’t have to pay me — you can simply visit my website and download the Interface C Utility (for Windows or Mac) to flash the Arduino with the software. That way, you can still build it if you have the parts on a breadboard or an equivalent setup. I also sell a PCB kit on Tindie if you just need some of the basic parts, including the PCB.

2 hours ago, UltraViolet said:

I really like this concept.  It's good to hear there are plans for variations.  When I just checked, the website says no shipping to Canada, so I guess I'll be waiting indefinitely.

Thank you! I’ve just updated my Tindie store to include shipping to Canada.

[UltraViolet]

9 hours ago, diegobaca said:

I’m using an Arduino Nano ESP32. You don’t have to pay me — you can simply visit my website and download the Interface C Utility (for Windows or Mac) to flash the Arduino with the software. That way, you can still build it if you have the parts on a breadboard or an equivalent setup. I also sell a PCB kit on Tindie if you just need some of the basic parts, including the PCB.

Thank you! I’ve just updated my Tindie store to include shipping to Canada.

Awesome.  I am going to order one of these to try it out.  My only observation so far is I wish the power connector was on the back rather than the side so multiple units could sit directly side-by-side.

[M_longer]

What's the advantage over old 9V controller, except wireless/wifi control?

[evank]

My dream would be a version that I can still program in 1980s, line-numbered BASIC. :) #InterfaceAforever

[dr_spock]

1 hour ago, evank said:

My dream would be a version that I can still program in 1980s, line-numbered BASIC. :) #InterfaceAforever

I believe the ESP32 has a built-in BASIC interpreter.  It's basically for debugging if needed.  There are also a number of BASIC interpreters for ESP32 on github. 

You can probably use Leguino to support Power UP hubs too. 

[Toastie]

17 minutes ago, dr_spock said:

I believe the ESP32 has a built-in BASIC interpreter.

It does, here is how to activate it - it is a bit "hidden": https://medium.com/@slovati/esp32-a-look-at-the-built-in-basic-interpreter-4b15725457b

And it is fully in line with @evank's wish to use line numbers: You have to when programming. No line numbers and commands are executed directly - as on any 8 bit wonder ...

Happy POKE'ing!

Best
Thorsten 

[diegobaca]

3 hours ago, evank said:

My dream would be a version that I can still program in 1980s, line-numbered BASIC. :) #InterfaceAforever

I will have to look into something like that :)

1 hour ago, Toastie said:

It does, here is how to activate it - it is a bit "hidden": https://medium.com/@slovati/esp32-a-look-at-the-built-in-basic-interpreter-4b15725457b

And it is fully in line with @evank's wish to use line numbers: You have to when programming. No line numbers and commands are executed directly - as on any 8 bit wonder ...

Happy POKE'ing!

Best
Thorsten 

Nice - did not know about the built in interpreter 👀

4 hours ago, M_longer said:

What's the advantage over old 9V controller, except wireless/wifi control?

There probably aren’t any major advantages, but here are a few things it does differently:

1. It doesn’t need an extension adapter — you can plug a Power Functions motor directly into the device.

2. Wi-Fi / remote control capabilities.

3. It serves as an alternative to the old 9V controller, which is becoming harder to find these days.

4. I’m planning to add some basic programmability — for example, the ability to run simple loops.

[howitzer]

This item does not ship to Finland.

I guess I won't be trying it.

[M_longer]

19 hours ago, diegobaca said:

you can plug a Power Functions motor directly into the device.

Well the extension wire is better, becuase you can place the controller further from the motor.
 

 

19 hours ago, diegobaca said:

which is becoming harder to find these days.

Over 600 units available at Bricklink, not to mention local pages which sell LEGO. And average price is 19$. ANd can work with any 9-12V supply/
With $98 price range it's hard to sell your idea for BT controll for $78

Edited October 19 by M_longer

[diegobaca]

19 minutes ago, M_longer said:

Well the extension wire is better, becuase you can place the controller further from the motor.
 

 

Over 600 units available at Bricklink, not to mention local pages which sell LEGO. And average price is 19$. ANd can work with any 9-12V supply/
With $98 price range it's hard to sell your idea for BT controll for $78

I don’t think it’s better or worse—just different. It simply gives you the option to use the extension cord or not, since it supports both.

Mine also works with any standard 9V power supply—it doesn’t require anything proprietary.

Regarding stock, I live in the U.S., and there are only about 30 available right now. Half of those are priced above $50, so the “build your own kit” option ends up being comparable in cost. I’m not claiming my version is cheap or suggesting everyone should buy one—I’m just sharing a project I made and putting it out there for the community.

There are also some other advantages: it has a screen that displays the speed value, it’s customizable, you can design and print your own case in any color, and it’s easy to modify or hack if you like to tinker.

[UltraViolet]

Need new train feeder wires.  Good ones are scarce.  Still waiting for Fx Bricks to come through.

[Toastie]

41 minutes ago, diegobaca said:

I’m just sharing a project I made and putting it out there for the community.

And I find that absolutely wonderful! And really appreciate this. This is not that common anymore. You put a lot of thoughts into your project - from the electronic design stage all the way through the customizable case!

There are a couple of "things" (let's call them simply "things", not wishes, issues, or dreams; these are naturally diverse, every individual here and elsewhere has individual demands). For one, you are targeting PF and, with a conversion cable, 9V folks. PF and 9V is phased out since long. So folks get their supplies from the BL aftermarket - or from China. The latter is definitely cheaper. Once you take that route, some call The Dark Side (which to me is utter nonsense: The moment I can't get an original part anymore for every single device here in the house, I go China, believe me), you all of a sudden feel "free". At least this happened to me - today I can drill into any LEGO piece if I want to, because TLG did not make what I need, without getting nightmares or any bad feeling. It is ABS I am drilling into.

At that point, the advantages of any newly developed third party device designed for LEGO stuff but targeting the LEGO aftermarket must be compelling. Otherwise, it will be just: Naa, can get this and that here and there for less. See, what I would do, if I required a nicely controllable 9V DC power supply? I'd go with a modern Chinese lab power supply, 0 - 12 V, 0 - 5 or 10 or more amps. The current limitation is good for testing purposes or simply "early motor burn out protection".

9V (train) regulators don't need proprietary power supplies, this is one of the many TLG myths. Or lies. Anyone in the correct range will do. The 9V controller is internally nicely protected, nothing much can happen there.

No, the usual stuff won't work, as Pete Venkman said in Ghostbusters, when he tried to get in touch with hat lady-ghost in the library. Not the PF terminal, not the amperage, not the power supply.

Your approach has this powerful extra, though, a microcontroller. ESPs are crazy. And dead cheap if you skip the Arduino part. With WiFi, BL, BLE, easily connected to an (OLED) display, it has many inputs. Which makes a total difference. The thing is: a) How to use it for b) what purposes. Do GBC folks need that? I don't know. Do train folks require that? Hmm, maybe? Do Technic (machines) need that? I think so. But then you need inputs. You said you want to do loops per programming. Well, there are not that may static/constant loops, there are controlled loops. I have no l clue how to get sensors into your project, but it would make such a difference: A smart >power supply<. Not a smart brick, they are out there: A power supply.

I, for one, like the display. Others will tell you: I can still "read" a dial (which is true, I can as well, but). I like the wireless stuff. Others will tell you: Go to a show and BOOM, you can hear Radio Gaga, but not reliably control your contraption. So what, I don't do shows, I have fun@home. 

Hmm, as said, I love your project. I find it absolutely original, fun, and appealing. See, I make my own stuff, virtually >nobody< here is interested in, except a handful of total nerds (= wonderful people), which share the notion of "just sharing a project I made and putting it out there for the community". These are your words. And THAT is totally cool and what drives me.

I will closely follow what you come up with! Keep on rolling!!!

All the best
Thorsten     

[BatteryPoweredBricks]

I'll go on record and say how awesome I think this project is.  It's not for everyone, but nothing in life is ever for everyone.  Here are some positive things I see:

For GBC builders the higher output current will be great!  I'll do some hands on testing once mine arrives but I would estimate the Interface C could take the place of at least two 9v train controllers and should provide a more steady voltage.  Same goes for train builders.  Need more than two 9v train motors?  You'll need more than one Lego train regulator anyway or some other custom solution.

Having a PF plug output is great!  Official Lego extension cables can get expensive and they won't be needed here at all.  You can buy PF plugs cheaply online along with some silicon insulated wire and make any cable length you desire.  I have yet to see a third party 9v to PF conversion cable and I doubt any company outside of FxBricks will be making aftermarket 9v cables.

I'm still an advocate for a cheaper "dumb" version but I'm sure there are some folks out there that could find creative solutions for the app control or implement their own additions to the code on the micro-controller.

I love the aesthetics of the Interface C, it's very "Lego"!  I could see this inspiring many more projects / devices in the Lego community.  The stl files are freely available so if someone wanted to make their own motor controller they already have a design for an awesome enclosure to put it in.

Lastly... it's fun!  I've been itching for a fun electronics project to build.  At this point I would pay for the building experience even if I never planned on using it 😅

[UltraViolet]

PV-Productions just expanded their custom cable options to include a PF-9V and 9V-9V cable, in addition to a few other adapters that already had a combo 9V/PF connector on one end.  While these only have anti-stud connections, no top studs, it's at least a start, and easily remedied using 9V plates or another cable.  They are quite expensive, but for anyone not willing or able to build their own cables, it's at least a plug-and-play solution.  I'm not expecting Fx Bricks to be cheap either, anyway.

[BatteryPoweredBricks]

Ok my first video about my experience with Diego's Interface C is finally out.  As mentioned in the video I will have two followup videos, one where I got to sit down and talk to Diego about this and many of his other projects and then a full assembly tutorial.  I ended up buying two (with my own money, this isn't any kind of sponsorship) one of which I will keep stock and one modified.  I think it's a great little device and I'm excited to do more mods in the future!  I love to see this kind of thing!

 

 

[MS1]

This is really excellent work, thanks for making this available! Gives me motivation to make some of my own electronics projects available to the community :)  

I would love to see more detailed technical info on the website. Stuff like, how much current can it source, and for how long? Combined with the info from Philo's Motor Tests would help to judge if the Interface C is suitable for a particular build, or if I need more than one to drive a particular creation. I suspect a large portion of the audience for this device would be able to make use of such info.

[Zerobricks]

Such a clean design and I really like how the colors match the Power Functions theme. Well done!