Project 3 – Traffic Lights
You are now going to create a set of traffic lights that will change from green to red, via amber, and back again, after a set length of time using the four-state UK system. This project could be used to make a set of working traffic lights for a model railway or for a child's toy town. If you're not from the UK, you can modify the code and colors to make them work like the traffic lights in your own country. First, though, make the project as it is and change it once you know how it works.
Parts Required
Breadboard 
Red Diffused LED 
Yellow Diffused LED 
Green Diffused LED 
3 × 150 ohm Resistors* 
Jumper Wires 
*or whatever value you require for your type of LED
Connect It Up
Connect your circuit as shown in Figure 2-6. This time you connect three LEDs with the anode of each one going to Digital Pins 8, 9 and 10 via a 150Ω resistor (or whatever value you require) each.
Take a jumper wire from ground of the Arduino to the ground rail at the top of the breadboard; a ground wire goes from the Cathode leg of each LED to the common ground rail via a resistor—this time connected to the cathode. (For this simple circuit, it doesn't matter if the resistor is connected to the anode or cathode).
Figure 2-6. The circuit for Project 3 – Traffic Lights (see insert for color version)
Enter the Code
Enter the code from Listing 2-3, check it, and upload to your Arduino. The LEDs will now move through four states that simulate the UK traffic light system, as seen in Figure 2-7. If you have followed Projects 1 and 2, both the code and the hardware for Project 3 will be self-explanatory. I shall leave you to examine the code and figure out how it works.
Listing 2-3. Code for Project 3
// Project 3 - Traffic Lights
int ledDelay = 10000; // delay in between changes
int redPin = 10;
int yellowPin = 9;
int greenPin = 8;
void setup() {
pinMode(redPin, OUTPUT);
pinMode(yellowPin, OUTPUT);
pinMode(greenPin, OUTPUT);
}
void loop() {
digitalWrite(redPin, HIGH); // turn the red light on
delay(ledDelay); // wait 5 seconds
digitalWrite(yellowPin, HIGH); // turn on yellow
delay(2000); // wait 2 seconds
digitalWrite(greenPin, HIGH); // turn green on
digitalWrite(redPin, LOW); // turn red off
digitalWrite(yellowPin, LOW); // turn yellow off
delay(ledDelay); // wait ledDelay milliseconds
digitalWrite(yellowPin, HIGH); // turn yellow on
digitalWrite(greenPin, LOW); // turn green off
delay(2000); // wait 2 seconds
digitalWrite(yellowPin, LOW); // turn yellow off
// now our loop repeats
}
Figure 2-7. The four states of the UK traffic light system (image by Alex43223 from WikiMedia) (see insert for color version)