Video
Circuit Diagram
Source Code
1// ─────────────────────────────────────────
2// BlueGrays · bluegrays.com
3// Traffic Light
4// ─────────────────────────────────────────
5// Traffic light cycle with red, yellow and green LEDs: red 2 s, yellow 0.3
6// s, green 2 s, yellow 0.3 s, then it starts over.
7//
8// Pins:
9// D2 -> Red LED
10// D3 -> Yellow LED
11// D4 -> Green LED
12// Each LED with a 330 ohm resistor
13
14void setup() {
15 pinMode(2,OUTPUT); // Red
16 pinMode(3,OUTPUT); // Yellow
17 pinMode(4,OUTPUT); // Green
18}
19
20void loop() {
21 // Red on, wait 2 s
22 digitalWrite(2,HIGH);
23 digitalWrite(3,LOW);
24 digitalWrite(4,LOW);
25 delay(2000);
26 // Yellow on, wait 0.3 s
27 digitalWrite(2,LOW);
28 digitalWrite(3,HIGH);
29 digitalWrite(4,LOW);
30 delay(300);
31 // Green on, wait 2 s
32 digitalWrite(2,LOW);
33 digitalWrite(3,LOW);
34 digitalWrite(4,HIGH);
35 delay(2000);
36 // Yellow on, then back to red
37 digitalWrite(2,LOW);
38 digitalWrite(3,HIGH);
39 digitalWrite(4,LOW);
40 delay(300);
41}
42FAQ
How long should the yellow light at a traffic light stay on?▾
In real traffic lights, the yellow light usually stays on for 3–4 seconds. In the project, this duration is set using the `delay()` function; if the red and green lights are kept on longer and the yellow light is kept on shorter, a cycle that closely resembles reality is achieved.
Why is a 330 Ω resistor used instead of a 220 Ω one?▾
A 330 Ω resistor reduces the current slightly (to about 9–10 mA); the LEDs are still clearly visible, and less total current is drawn from the pins. It also works with a 220 Ω resistor; the LEDs are just a little brighter.
Can I add a pedestrian button?▾
Yes. You can add a push button and a 10 kΩ pull-down resistor and read it using `digitalRead`. When the button is pressed, you can switch the loop to red and turn on a separate green LED for pedestrians; using `millis()` instead of `delay` ensures that the button is detected immediately.
The order of the LEDs is mixed up—what should I do?▾
Compare the pin definitions in the code (red, yellow, green) with the wires on the circuit one by one. The easiest way is to light up each LED using a separate test code and verify which pin it is connected to.