Published August 18, 2025 © MIT

Duplo Train Blinky

My grandson recently got a used Duplo train set as a present. He wanted blinking lights on the train. How hard would it be to add an LED?

BeginnerShowcase (no instructions)4 hours44

Things used in this project

Hardware components

M5Stack M5StickC PLUS ESP32-PICO Mini IoT Development Kit

RGB Diffused Common Cathode

Software apps and online services

Arduino IDE

OpenSCAD

Hand tools and fabrication machines

Anycubic Kobra 2 3D Printer

Story

Introduction

My grandson has been interested in trains since he started with a non-powered Mega Bloks train when he was a toddler. He has since graduated to a Lego Duplo Steam train at home that has a battery powered engine and also can be controlled by a phone app. A friend recently bought him an older used Duplo train set that came with two battery powered engines, a lot of track, and other accessories. We use that at our house when we watch him. Not as full featured as his Steam train but it does have sound and a timed shutdown. He found a fake light bar among the accessories and he asked me why it didn't work. I explained to him that it wasn't a real light but it got me to wondering how difficult it would be to add some LEDs on the old train engine,

Implementation

I didn't want to do anything that required modifying the existing train engine, so I thought that I would add a standalone module that would could plug onto the existing Duplo brick pattern. The module would have its own battery and power switch.

The simplest implementation would would just use a single color self blinking LED that commonly flashes at about 1.5Hz. That would probably satisfy my grandson for now, but I thought it would be neat build him something that would be extensible in the future.

I've used M5Stack M5StickC modules to interface and control sensors remotely and they have neat features like a small LCD for display, a power button, a couple of user buttons, and a small LiPo battery with charging circuit. The latest version the Plus2 has a 200 mAh battery which should be fine if I'm just operating LEDs. I have the previous version, the Plus, which will be fine for proof of concept.

I decided to use a single 5mm RGB diffused common cathode LED and just control the red and green LEDs with the M5StickC Plus. I 3D printed a housing for the StickC and the RGB LED that fits onto the 2x4 brick pattern on top of the engine cab. I am driving the individual LEDs using 2 GPIOs on the female HAT header interface. The components are shown below.

Test video

Demonstrates operation of the program which is written using the Arduino IDE. The RGB LED blinking cycles through Red to Green to Off with successive presses of btnA on the front of the M5StickC Plus.

Engine assembly

Mounted on the Duplo train engine.

Operational video

A quick spin around a short track.

Custom parts and enclosures

Sketchfab still processing.

Schematics

Code

M5StickC_Duplo_LCD_Test.ino

#include <M5StickCPlus.h>
#include "time.h"  // Required for time functions

#define RED_PIN   25
#define GREEN_PIN 26

// Button A is GPIO 39 on M5StickC Plus
#define BTN_A     39

int colorState = 0;         // 0 = Red, 1 = Green
bool ledOn = false;
unsigned long previousMillis = 0;
const long interval = 500;  // blink interval in ms

void setup() {
  M5.begin();
  M5.Lcd.setRotation(3);
  M5.Lcd.fillScreen(BLACK);
  M5.Lcd.setCursor(10, 10, 2);
  M5.Lcd.setTextSize(2);
  M5.Lcd.printf("Duplo Train");
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);

  // Start with both LEDs off
  digitalWrite(RED_PIN, LOW);
  digitalWrite(GREEN_PIN, LOW);
 }

void loop() {
  M5.update(); // update button states

  // Check for button press to toggle color
  if (M5.BtnA.wasPressed()) {
    colorState++;
    if (colorState > 2) colorState = 0;
    ledOn = false;               // reset blink
    previousMillis = millis();
    // ensure both off immediately
    digitalWrite(RED_PIN, LOW);
    digitalWrite(GREEN_PIN, LOW);
  }

  // Non-blocking blink
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    ledOn = !ledOn;  // toggle blink

    // Only toggle the selected color
    if (colorState == 1) {        // Red
      digitalWrite(RED_PIN, ledOn ? LOW : HIGH);
      digitalWrite(GREEN_PIN, LOW); // green always off
    } else if (colorState == 2) {   // Green
      digitalWrite(GREEN_PIN, ledOn ? LOW : HIGH);
      digitalWrite(RED_PIN, LOW);   // red always off
    } else {                       // Off
      digitalWrite(GREEN_PIN, LOW); // green always off
      digitalWrite(RED_PIN, LOW);   // red always off
    }
  }


}

Credits

Ralph Yamamoto

14 projects • 26 followers

Duplo Train Blinky

Things used in this project

Hardware components

Software apps and online services

Hand tools and fabrication machines

Story

Introduction

Implementation

Test video

Engine assembly

Operational video

Custom parts and enclosures

Duplo M5StickC Plus RGB LED Housing

M5StickC Plus Power Button

RGB LED Clip

Schematics

Duplo Train LED

Code

M5StickC_Duplo_LCD_Test.ino

Credits

Ralph Yamamoto

Comments

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Duplo Train Blinky

Duplo Train Blinky

Things used in this project

Hardware components

Software apps and online services

Hand tools and fabrication machines

Story

Introduction

Implementation

Test video

Engine assembly

Operational video

Custom parts and enclosures

Duplo M5StickC Plus RGB LED Housing

M5StickC Plus Power Button

RGB LED Clip

Schematics

Duplo Train LED

Code

M5StickC_Duplo_LCD_Test.ino

Credits

Ralph Yamamoto

Comments

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