UCLA BMES Build Team

Winter Break Problem Set

Problem 1 - Traffic Light

Task:

• You are setting up a new traffic light for the De Neve crosswalk. You want to give both the pedestrians and the drivers the power to change the light at the press of a button. Let’s assume that people don’t press the button unless they need to (so a pedestrian doesn’t stop to push the button if he or she can already cross).

• Set up a circuit with a green, yellow, and red LED like a standard traffic light. Initially the red LED will be on. When the button is pressed the first time (we assume by a driver), the light will turn green and stay green until the button is pressed again. When the button is pressed next (this time by a pedestrian), transition the LEDs from green to red. This involves turning the yellow LED on for 5 seconds before the red LED is turned on. Further button presses will continue this pattern. Construct a circuit and Arduino code that will accomplish this task.

  • Note: Assume that the button will only be pressed when it is not in a transition state (i.e. when the yellow LED is on). This means you can use delay().

Circuit:

Code:

int red = 13;
int yellow = 7;
int green = 2;
int buttonPin = 8;
int previousState = LOW;                 // LOW = red, HIGH = green

void setup()
{
  pinMode(red, OUTPUT);
  pinMode(yellow, OUTPUT);
  pinMode(green, OUTPUT);
  pinMode(buttonPin, INPUT);
  digitalWrite(red, HIGH);
}

void loop()
{
  if (digitalRead(buttonPin) == HIGH)
  {
    if (previousState == LOW)            // if light is currently red
    {
      digitalWrite(red, LOW);
      digitalWrite(green, HIGH);
      previousState = HIGH;              // light is now green
    }
    else                                 // if light is currently green
    {
      digitalWrite(green, LOW);
      digitalWrite(yellow, HIGH);
      delay(5000);
      digitalWrite(yellow, LOW);
      digitalWrite(red, HIGH);
      previousState = LOW;               // light is now red
    }
    while (digitalRead(buttonPin) == HIGH)
    {
      // do nothing while button is still pressed so that code only runs once
    }
  }
}

UCLA BMES Build Team