I have a school project, where I'm going to make a traffic light system on a pls. I want to try it out on an Arduino first, just to get the hang of it.

To make it a little bit more advanced than I have found on the internet, I want to make the lights go in "emergency mode" if the green lights in both directions are on at the same time. It will then go into a yellow blink.

My idea is something like this:

// Yellow blinking if fault in the system or operator activates it
int killState = digitalRead(Kill);
int resState = digitalRead(Res);
if (killState == HIGH /*|| greenlights in both directions */) {
 while (resState == LOW;) { // Yellow blink as long as system is not reset

I guess I need a variable or two that tells what state the lights are in, but I'm stuck. Any good solutions out there?

Another thing is the reset button. I haven't put much effort in it, but I want it to go into the starting point.

The link to the complete script can be found here.


For starters, take a close look at this line:

while (resState == LOW;)

That is not the correct syntax. But the real issue is how is this while loop terminated? The value is never set again?


  • I guessed so. As I said, I haven't put much effort in it yet. I need to take one step at a time. Thanks for the quick reply, though! The idea is that the state will not change until the reset button is activated. And the ' ; ' was a type-o. I am new to 'while' and 'for' loops, but i think I'm starting to get the hang on 'if' and 'else' criterias. Feb 12 '16 at 10:54

You could do something simple to test if both red and green are on, eg.

if (digitalRead (RED_PIN) == HIGH && digitalRead (GREEN_PIN) == HIGH)
  digitalWrite (RED_PIN, LOW);  // turn off red
  digitalWrite (GREEN_PIN, LOW);  // turn off green
  while (true)
    digitalWrite (YELLOW_PIN, HIGH);
    delay (500);
    digitalWrite (YELLOW_PIN, LOW);
    delay (500);
    }  // end of while
  }  // end of it

If you have other questions please post your code and not a snippet - however I suggest you try to get further on your own. This is obviously an exercise, and the more you try to work it out yourself the more you will learn.

  • Thanks! That was a good idea. I will add this to the script. I will add the full code next time. I added it as a link, though. Feb 13 '16 at 10:34
  • What is it that is true in the while (true) part? Feb 13 '16 at 11:59
  • true is true. That particular loop loops indefinitely. In other words, if there is some failure in the traffic lights, it will flash yellow ... forever. (Kind of a scary thought, eh?)
    – Nick Gammon
    Feb 14 '16 at 5:59
  • I could have written while (5 == 5) - it just means to do something forever.
    – Nick Gammon
    Feb 14 '16 at 6:00

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