RelayControl.ino

int channel_pin[] = {2, 4};
static int relayStateA = 0;
static int relayStateB = 0;

void setup() {
  // initialize serial communication:
  Serial.begin(9600); 

  for (int i = 0; i < sizeof(channel_pin); ++i)
  {
    pinMode(channel_pin[i], OUTPUT);
  }
}

void loop() {
  if (Serial.available() > 0) {
    char inByte = Serial.read();
    
    switch (inByte){
      case 'a':
      {
          relayStateA ^= 1;
          Serial.println(inByte);
          break;
      }
      case 'b':
      {
          relayStateB ^= 1;
          Serial.println(inByte);
          break;
      } 
      default:
      {
         Serial.println("Caractere 'a' para Relay 1 / Caractere 'b' para Relay 2");
      }
    }
    
    if (relayStateA){
      digitalWrite(channel_pin[0], HIGH);
    }
    else{
      digitalWrite(channel_pin[0], LOW);
    }
    
    if (relayStateB){
      digitalWrite(channel_pin[1], HIGH);
    }
    else{
      digitalWrite(channel_pin[1], LOW);
    }
  }
}