New to Arduino, getting into String variables and reading them from serial monitor. Basically what I'm trying to do is have the user input either red, green, yellow. Whichever String they enter the LED comes on.

What is happening seems to be that the if statements are not true and are never entered thus not LEDs come on. I do have a serial.Println(myColor) at the end which displays myColor as the user has inputted into serial monitor. I just don't understand why the if statements are not firing. its really weird.

String msg="What LED do you want to turn on?";
String myColor;
int redPin = 12;
int greenPin = 13;
int yellowPin = 8;

void setup(){
pinMode(redPin, OUTPUT);
pinMode(greenPin, OUTPUT);
pinMode(yellowPin, OUTPUT);

void loop(){
    //LED ColorDEmo

    Serial.println(msg);                //print the question to serial monitor
    while(Serial.available() == 0){     //while we wait do nothing till data is available
    myColor = Serial.readString();      // set myColor to serial value

    if (myColor == "red"){              // if the value is red
        Serial.println("In red");           
        turnOffLEDS();                  // turn off all LEDS
        digitalWrite(redPin, HIGH);     // turn on red lED

    if (myColor == "green"){            // if the value is green
        Serial.println("In green");
        turnOffLEDS();                  // turn off all LEDS
        digitalWrite(greenPin, HIGH);   // turn on green LED

    if (myColor == "yellow"){           // if the value is yellow
        Serial.println("In yellow");
        turnOffLEDS();                  // turn off all LEDS
        digitalWrite(yellowPin, HIGH);  // turn on yellow LED
    Serial.println(myColor);            // print myColor

// turns all LEDS off
void turnOffLEDS(){
    digitalWrite(yellowPin, LOW);
    digitalWrite(greenPin, LOW);
    digitalWrite(redPin, LOW);
  • debug your code by printing data to the serial console ... print the received message and print the length of the message
    – jsotola
    Jul 20, 2022 at 2:58
  • myColor = Serial.readString(); Serial.println(myColor.length()); Serial.println(myColor) results 4 red So it seems it is attaching an additional character tot he end of the String variable. SHould i be using readStringUntil() instead
    – Moose
    Jul 20, 2022 at 3:17
  • Just a little side-point: It is a good to get used to using char arrays rather then strings. I know the current way arduino moves is to keep users on lover level, (buing more shields) but once you get to more advanced tasks and have to deal with memory footprint, then habit of using char array makes your life easier latter.
    – Tomas
    Jul 20, 2022 at 3:51
  • Thanks @Tomas, that's a really good point. String variables are memory hogs from what I've heard from others as well.
    – Moose
    Jul 20, 2022 at 4:08
  • Also, what is the program to do if the user enters "RED" instead of "red", "RE" only, "R" only, etc. That is why single character responses are generally better and less error-prone.
    – starship15
    Jul 26, 2022 at 17:53

2 Answers 2


Avoiding String objects (back to the basics) often makes life easier. Simply check if the answer contains one of the characters 'd' 'g' 'y'

Or (and) simplify your protocol to only send one character.

BTW:You might notice a delay in readString, waiting for a multi-character input to finish, because Serial.available() is true already after the first character of an input, and readString terminates only if it times out

  • thats a good idea, simplify into single characters.
    – Moose
    Jul 21, 2022 at 17:38

thank you @jsotola

throught he debugging you mentioned i was able to learn that the '\n' was added to the end of the string. Instead of using Serial.readString() I used Serial.readStringUntil('\n');

this would check for a delimiter character '\n' then stop. Thus, my myColor variable would be red instead of red'\n'

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