Traffic Light

Costs:

• Arduino Nano                                                              ea $5.33

• Pushbutton                                                                   ea. $0.50

• Red, Yellow, Green LEDs (6)                                        $1.00                              

• OLED Display                                                                       $3.00

• 220 ohm Resistors                                                             $1.00

• Half-Breadboard                                                         ea $1.26

• AMS-1117 5V Regulator                                         ea $0.50

• 9V Battery and Clip                                                    ea $3.00

• Barrel-Jack Adaptor                                                   ea $0.79

                                                                                                 $16.38

Students can build a real intersection with cardboard and other makerspace building materials. Especially encourage them to keep building in more complexity, and take it as far as they possibly can.

https://projecthub.arduino.cc/krishna_agarwal/traffic-light-using-arduino-a-beginner-project-35f8c6

Phase 1: Traffic lights turn green, yellow, then red in a repeating cycle. Make this sequence into a function.

Phase 2: Add Pedestrian Crossing Switch

Students can add a push-button to act as a pedestrian crossing switch, forcing the light sequence to change when pressed. This introduces the use of digital inputs,conditional logic, and hardware interrupts.

Create a new function for the Walk sequence.

To ensure the system catches the button press immediately—even if it happens while the Arduino is stuck in a long delay() waiting for a light to change—we use a hardware interrupt. The interrupt calls an interrupt service routine(ISR), which sets a flag variable.This allows the Arduino to run its normal cycle, instantly flag a button press whenever it occurs, and process that request when it reaches the Red light phase.

Phase 3: Add a walk/don't walk sign using an OLED or dot matrix display.

Wiring Guide

• Button: Connect one leg to Pin 2 and the other leg to GND. (The code uses the internal pull-up resistor, so no external resistor is needed).

• Car Lights: Red to Pin 4, Yellow to Pin 5, Green to Pin 6 (with 220Ω or 330Ω resistors).

• Pedestrian Lights: Red (Don't Walk) to Pin 7, Green (Walk) to Pin 8 (with resistors).

How It Works:

1. The Interrupt (attachInterrupt): Standard delay() functions pause the Arduino entirely. If someone pressed the button during a 5-second green light, the Arduino normally wouldn't notice. The interrupt bypasses this by constantly watching Pin 2 in the background.

volatile bool pedestrianWaiting = false;

pinMode(buttonPin, INPUT_PULLUP); // Attach the interrupt to Pin 2.

   attachInterrupt(digitalPinToInterrupt(buttonPin), buttonPressISR, FALLING);

    // 'FALLING' triggers the function when the signal goes from HIGH to LOW (button pressed)

// --- Interrupt Service Routine (ISR) This function runs instantly the moment the button is pressed 

void buttonPressISR() { 

pedestrianWaiting = true;  //sets flag variable

}

2. volatile bool pedestrianWaiting: When the button is pressed, the interrupt function (buttonPressISR) instantly sets this variable to true. The volatile keyword is critical—it tells the Arduino's memory that this variable can change outside the normal flow of the loop().
   
   

3. The Cycle Execution: When the main loop() hits the Red Light phase, it checks the pedestrianWaiting variable. If it's true, it executes the 5-second pedestrian crossing sequence and resets the variable. If false, it just waits through a standard red light cycle.

How It Works

1. The Interrupt (attachInterrupt): Standard delay() functions pause the Arduino entirely. If someone pressed the button during a 5-second green light, the Arduino normally wouldn't notice. The interrupt bypasses this by constantly watching Pin 2 in the background.

2. volatile bool pedestrianWaiting: When the button is pressed, the interrupt function (buttonPressISR) instantly sets this variable to true. The volatile keyword is critical—it tells the Arduino's memory that this variable can change outside the normal flow of the loop().

3. The Cycle Execution: When the main loop() hits the Red Light phase, it checks the pedestrianWaiting variable. If it's true, it executes the 5-second pedestrian crossing sequence and resets the variable. If false, it just waits through a standard red light cycle.

// --- Pin Definitions ---

/*Note: Pin 2 is used because it supports external interrupts

on the Arduino Uno and Arduino Nano. The Nano also supports interrupts on Pin 3*/

const int buttonPin = 2;

const int carRed = 4;

const int carYellow = 5;

const int carGreen = 6;

const int pedRed = 7;

const int pedGreen = 8;

// --- System Variables ---

// 'volatile' tells the compiler this variable can change at any time (via the interrupt)

volatile bool pedestrianWaiting = false;

void setup() {

 // Set light pins as outputs

 pinMode(carRed, OUTPUT);

 pinMode(carYellow, OUTPUT);

 pinMode(carGreen, OUTPUT);

 pinMode(pedRed, OUTPUT);

 pinMode(pedGreen, OUTPUT);

 // Set button pin as input with internal pull-up resistor

 // The pin will read HIGH normally, and LOW when pressed

 pinMode(buttonPin, INPUT_PULLUP);

 // Attach the interrupt to Pin 2.

 // 'FALLING' triggers the function when the signal goes from HIGH to LOW (button pressed)

 attachInterrupt(digitalPinToInterrupt(buttonPin), buttonPressISR, FALLING);

 // Initialize starting state: Cars go, Pedestrians stop

 digitalWrite(carGreen, HIGH);

 digitalWrite(pedRed, HIGH);

}

void loop() {

 // 1. CAR GREEN PHASE

 digitalWrite(carRed, LOW);

 digitalWrite(carGreen, HIGH);

 delay(5000); // Cars have a green light for 5 seconds

 // 2. CAR YELLOW PHASE

 digitalWrite(carGreen, LOW);

 digitalWrite(carYellow, HIGH);

 delay(2000); // Yellow light for 2 seconds

 // 3. CAR RED PHASE

 digitalWrite(carYellow, LOW);

 digitalWrite(carRed, HIGH);

 delay(1000); // 1-second safety pause before pedestrians or standard red

 // 4. PEDESTRIAN LOGIC

 if (pedestrianWaiting) {

   // Turn on the Walk sign

   digitalWrite(pedRed, LOW);

   digitalWrite(pedGreen, HIGH);

   delay(5000); // Walk sign stays on for 5 seconds

   // Turn off Walk sign, turn on Don't Walk

   digitalWrite(pedGreen, LOW);

   digitalWrite(pedRed, HIGH);

   delay(1000); // 1-second safety pause before cars get green again

   // Reset the flag so the walk sign doesn't trigger again until pressed

   pedestrianWaiting = false;

 } else {

   // If no button was pressed, just wait a normal red light duration

   delay(3000);

 }

}

// --- Interrupt Service Routine (ISR) ---

// This function runs instantly the moment the button is pressed

void buttonPressISR() {

 pedestrianWaiting = true;

}