Day 6: Build a Traffic Light System

Hi Anish! Today is your first real project. No new concepts — we’re going to combine everything from Days 1-5 to build a traffic light. Red, yellow, green, with realistic timing. And if you feel brave, we’ll add a pedestrian button that interrupts the cycle when someone wants to cross.

This is the kind of project a working engineer might actually build (with fancier parts). You’re about to ship one with an Arduino, 3 LEDs, and 20 lines of code.

What you need today

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• Arduino Uno + USB cable
• Breadboard
• 3 LEDs — red, yellow, and green
• 3 × 220Ω resistors
• 1 push button (for the pedestrian button — optional)
• 1 10kΩ resistor (pull-down for the button — optional)
• 6-8 jumper wires

The circuit

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Same 3-LED setup as Day 5, plus an optional button wired like Day 3.

graph LR
    PIN8["Pin 8"] --> R1["220Ω"] --> RED["Red LED"] --> G1["GND"]
    PIN9["Pin 9"] --> R2["220Ω"] --> YEL["Yellow LED"] --> G2["GND"]
    PIN10["Pin 10"] --> R3["220Ω"] --> GRN["Green LED"] --> G3["GND"]
    V5["5V"] --> BTNa["Button"] -.->|pressed| BTNb["Button"]
    BTNb --> PIN2["Pin 2"]
    BTNb --> R10k["10kΩ pull-down"] --> G4["GND"]

Pin assignments:

• Pin 8 — Red LED
• Pin 9 — Yellow LED
• Pin 10 — Green LED
• Pin 2 — Pedestrian button (input, with 10kΩ pull-down)

Wire the 3 LEDs exactly like Day 5. Wire the button exactly like Day 3 — but this time plug it into pin 2 instead of pin 7. (Why pin 2? Because later in the series we’ll use pins 2 and 3 for a special feature called “interrupts” — good habit to start now.)

Part 1: Basic traffic light (no button)

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How does a real traffic light work?

• Green stays on for a long time (cars flow freely)
• Yellow flashes briefly as a warning (“get ready to stop!”)
• Red stays on for a long time (stopped)
• Then back to green. Repeat forever.

In code:

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void setup() {
  pinMode(8, OUTPUT);   // Red
  pinMode(9, OUTPUT);   // Yellow
  pinMode(10, OUTPUT);  // Green
}

void loop() {
  // GREEN — cars go
  digitalWrite(10, HIGH);
  delay(3000);            // 3 seconds
  digitalWrite(10, LOW);

  // YELLOW — warning
  digitalWrite(9, HIGH);
  delay(1000);            // 1 second
  digitalWrite(9, LOW);

  // RED — cars stop
  digitalWrite(8, HIGH);
  delay(3000);            // 3 seconds
  digitalWrite(8, LOW);
}

Upload. You should see: green for 3 seconds → yellow for 1 second → red for 3 seconds → back to green, forever.

Nothing new in this code — you already know pinMode, digitalWrite, delay. We just chained them together in a real-world pattern.

Try tweaking the timings. Real-life traffic lights use about 25 seconds for green/red and 4 seconds for yellow. The ratio is what matters: green and red are roughly equal, yellow is short.

Part 2: Adding the pedestrian button

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Now the fun part. When someone presses the button, the light should switch to red immediately so the pedestrian can cross.

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void setup() {
  pinMode(8, OUTPUT);   // Red
  pinMode(9, OUTPUT);   // Yellow
  pinMode(10, OUTPUT);  // Green
  pinMode(2, INPUT);    // Button
}

void loop() {
  // GREEN — unless someone presses the button
  digitalWrite(10, HIGH);
  for (int t = 0; t < 30; t++) {      // 30 × 100ms = 3 seconds
    if (digitalRead(2) == HIGH) {
      break;                           // ← button pressed, jump out of for-loop
    }
    delay(100);
  }
  digitalWrite(10, LOW);

  // YELLOW — warning (always 1 second)
  digitalWrite(9, HIGH);
  delay(1000);
  digitalWrite(9, LOW);

  // RED — let people cross
  digitalWrite(8, HIGH);
  delay(3000);
  digitalWrite(8, LOW);
}

Upload. Now during the green phase, press the button. Within a fraction of a second, the light jumps to yellow, then red. Pedestrians cross safely.

Wait — what is break;?

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New keyword today! break means “stop this loop right now and jump out.” It only works inside a for or while loop. When Arduino hits break;, it exits the loop immediately and runs the next instruction after it.

Here, we’re using it to say: “Normally loop 30 times (waiting 100ms each = 3 seconds total), BUT if at any point the button is pressed, bail out early.”

Why not just use delay(3000)?

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Because delay(3000) is blocking. While Arduino is counting 3 seconds, it can’t do anything else — including checking the button. The whole board just sits there, deaf to the world.

The trick is: instead of one long delay(3000), we do 30 short delay(100)s in a for loop, and check the button between each one. We’re still waiting 3 seconds in total, but now we peek at the button 30 times along the way.

This is the first taste of a really important idea in Arduino: if you want to do multiple things at once, don’t use long delays. Break them into small chunks with work in between. On Day 25 we’ll learn a fancier way using millis(), but the “break a big delay into small delays” trick is a solid starting point.

Try this

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1. Make yellow blink before going red. Flash yellow on/off 3 times during the warning phase. (Hint: a for loop inside the yellow section.)
2. Add a blinking red “walk” signal — after showing red, blink a 4th LED (on pin 11) to simulate a “WALK” signal for 2 seconds before going back to green.
3. Change the timings to look realistic. Use int greenTime = 5000; and int redTime = 5000; at the top of the file. Variables make tweaking easier.
4. Two-way traffic. Wire a second set of 3 LEDs on pins 5, 6, 7. When one direction is green, the other is red. Like a real intersection.

What you learned today

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• How to combine multiple LEDs with realistic timing
• How to read a button in the middle of a timing loop (check between small delays)
• break; — exit a loop early
• Why long delay() calls are “blocking” and limit what Arduino can do
• Your first real project: a traffic light with pedestrian override

What is next

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Day 7 — we revisit the button, but with a much cleaner way to wire it. You’ll learn about INPUT_PULLUP, which saves you an entire resistor and simplifies the circuit. Same idea as Day 3, but grown up.

Brilliant work, Anish. You just built a real thing.