Turn on a LED when an other LED turn off

Question

I'm a beginner on arduino.

I have an arduino mega with two LED (like the image with only 2 less and without the buzzer). A red LED and a green LED. When the arduino receive the code 200 I want the red LED turn on and the green LED turn off. But when I test that but any led turn off or turn on..

void setup()
{
  pinMode(LR, OUTPUT);
  pinMode(LV, OUTPUT);
  Serial.begin(9600);
}

void readSerial(){
  while(Serial.available() > 0){
    car = Serial.read();

    if(car != '+'){
      buff[i] = car;
      buff[i+1] = '\0';
      i++;
    } 
    else{
      sscanf(buff, "%d", &received_code);
    }
  }
}

void loop()
{     
  readSerial();
  if (received_code == 200){
    LRstate = HIGH;
    digitalWrite(LR, LRstate); // turn the LED on (HIGH is the voltage level)
    digitalWrite(LR,LOW);
    delay(100);
    digitalWrite(LV,LOW); 
  } 
}

How can I solve my problem?

Thanks a lot.

Answer 1

First, make sure the LEDs are connected correctly: I presume the red LED is connected to whichever pin LR is, and the green LED to LV.

According to your diagram, your LEDs will be on if the output is HIGH, and off if the output is LOW. Your code sets LR to high, but then immediately to LOW again.

Here's a fixed (I hope) verson of your loop():

void loop()
{     
  readSerial();
  if (received_code == 200){
    // Turn the red LED on
    digitalWrite(LR,HIGH);

    // Turn the green LED off
    digitalWrite(LV,LOW); 
  } 
}

---

Unrelated to the problem you're having, but worth a mention anyway:

There's nothing in your code to stop you overrunning buf if you get a longer input than you're expecting. If this happens you'll get unpredictable results as you trash over whatever happens to be in that part of memory.

Answer 2

In addition to the code changes noted by @MarkSmith, the LEDs are not connected properly. The straight LED pin on the Fritzing diagram is its cathode and must be connected to grounded, and the bent pin is its anode and must be at +V (+5v, in this case), for the LED to light. To turn the LED on, the anode must be at +V and the cathode at ground. To turn the LED off bring both pins to the same voltage, usually by grounding the anode (logic 0 on its connected pin).

To identify the cathode on a real LED, look inside its plastic blob (that's a technical term :) and you'll see two bits of metal, each on a wire, like two flags. The larger flag is the cathode. Usually, the cathode also has a shorter external lead and the plastic blob has a flat edge on that side, but the larger flag is definitively the cathode: