I was trying to do a LED display that turns on a LED at pin 13 that goes on then turns off after a delay then the next pin (which is pin 12 counting down) will turn on and then turn off.

I also put in a potentiometer that can change the delay to make it faster or slower.

Here's the code:

float pinOn = 13;
int outMin = 0;
int outMax = 13;
float delayNum = 1000;

void setup () {
  for (int i=outMin; i<=outMax; i++) {
  pinMode(i, OUTPUT); }

void loop () {
  // Reads Potentiometer -- sets delay relative to the Potentiometer input
  int sensorValue = analogRead(A0);
  delayNum = sensorValue;
  // Turns on the Pin designated by 'pinOn'
  digitalWrite(pinOn, HIGH);
  digitalWrite(pinOn, LOW);
  // Checks if 'pinOn' is 0 which then resets it back to 13 if not 0 then counts down
  if (pinOn = 0) {
    pinOn + 13;
  } else {
    pinOn - 1;

All it does is turn on pin 13 for a moment then permanently turns on pin 1 and 0 and it gets stuck like that.

  • Bugs like this are why everyone should learn to use a debugger, and enable all warnings on your compiler -- every modern compiler will catch if (a = b) and warn you about it if you enable all warnings and, ideally, make it treat warnings as errors. I also think this counts as a "general coding" problem; if you replaced the contents of the blocks with puts("foo") or other non-Arduino C statements, the answer would be exactly the same.
    – anon
    Nov 29, 2017 at 18:24

1 Answer 1


if (pinOn = 0) is doing an assignment (single =) not a comparison (double ==), so pinOn is begin set to 0 each time through the loop. The result of an assignment is the value being assigned. So it's the same as if ( 0 ), which is false.

Now, on to the two branches. pinOn + 13 is a statement and the result is not being assigned to anything. You need an equals sign somewhere:

if (pinOn == 0) { // == means "is equal to" 
  pinOn += 13; // same as pinOn = pinOn + 13
else {
  pinOn -= 1;
  • Thanks so much! I forgot that the comparator equals is supposed to be '==' not '=' :P it works now thank you! May 18, 2017 at 2:39

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