boolean sendData = false;

void setup()
{
Serial.begin(9600);
pinMode(A0,INPUT);
Serial.print("Press 'START' to start the ADC");
Serial.println("Press 'STOP' to stop the ADC");
}


void loop() 

{
if(Serial.available() > 0)

{
String intruction = Serial.readStringUntil('\n');
intruction.trim();
intruction.toUpperCase();
if(intruction == 'START')
{
Serial.print("You have pressed : ");
Serial.println(intruction);
sendData = true;
}
else if(intruction == 'STOP')
{
Serial.print("You have pressed : ");
Serial.println(intruction);
sendData = false;
}
}
if(sendData == true);
{
int analog = analogRead(A0);
float voltage = analog * (3.3 / 255.0);
Serial.print("Analog Value : ");
Serial.print(analog);
Serial.print("\tVoltage: ");
Serial.println(voltage);
}
delay(500);
}