Hello everyone,

this tutorial is more focus to programming part of Atmega chip using Arduino programming language.

My goal has been to created railroad signal for my son trains.

Main Goals:

Changing between green and red using button.
Changing between green and red automatically after some period of time.
Build the chassis from LEGO.
Program it on Atmega chip to learn the Arduino language.

Step 1: What We Need...

red LED 3mm
green LED 3mm
resistors - 150 OHM and 140 OHM (depends on LED Voltage and Current)
USB Nano V3.0 ATMEGA328P CH340G 5V 16M Micro-Controller Board For Arduino (or Arduino Nano or compatible)
some wiring
clicking button
soldering station
tape, double side tape
optional 5V battery
optional LEGO pieces

Step 2: Wiring

I decided to use PWM output pins to make the LEDs fading when the LED is on. Of course, you can use standard output to on/off LED. But in my case, I want to change the brightness of led smoothly.

In my case I have 6 PWM ports: D3, D5, D6, D9, D10, D11. I use D5 for red LED and D6 for green LED.

Because I will use INPUT_PULLUP button I don't need resistor on the button ground, because I will use internal one.

It is also good to calculate good resistor for your LED.

My red LED is 2,0 V and 20 mA. Because we connect it to 5 V and LED is 2 V the rest Voltage is 3 V. This is mean that for full brightness of LED we will use 150 Ω resistor because R = U/I = 3/0,02 = 150 Ω.

For my green LED is 2,2 V and 20 mA. We will use 140 Ω resistor because R = U/I = 2,8/0,02 = 140 Ω.

You can use also higher resistor to control a brightness of the LED.

Step 3: Programming

Here is my Arduino code. You can modified the code as you wish. I put comments into the code, so the program should be clear.

I was inspired with these tutorials and combine them together:

semafor.ino
Download

Step 4: Assembly

For assembly the railroad signal I've use good old LEGO. This was also the reason I've used 3mm LEDs. Because these fits into one special LEGO piece, so I can put into it.

For holding a button I have used double side tape. For back side of railroad signal I use tape to cover the back side wiring.

If you want to handle also a power do not forgot to add 5V battery and connect it to VIN and GND. But I've used power from mini-USB port using power bank connected to it.