#include <Wire.h> // Include the networking library.
int ledprobe = 5;
void setup() 
{
  Serial.begin(115200); // Open the serial port
  Wire.begin(); // It is not necessary to declare an address for the main board.
  pinMode(4, OUTPUT); // This is the main board output.
  pinMode(ledprobe, OUTPUT);
}

void loop() 
{
  digitalWrite(ledprobe, HIGH);
  while (Serial.available()) 
  {
    char c = Serial.read(); // Read the character written through the serial monitor.
    if (c == 'A') 
    { 
      Wire.beginTransmission(2); // This is a secondary board with address 2.
      Wire.write('A');
      Wire.endTransmission();
      Serial.println("Start");
    }
    if (c == 'S') 
    {
      Wire.beginTransmission(2); // This is a secondary board with address 2.
      Wire.write('S');
      Wire.endTransmission();
      Serial.println("Stop");
    }
  }
}