int pinsensor =A0;
int pinled = 2;
int nilaisensor;

void setup() {
Serial.begin(9600);
pinMode(pinsensor, INPUT);
pinMode(pinled, OUTPUT);
}

void loop() {
nilaisensor = analogRead(pinsensor);
 float voltage = nilaisensor / 1024. * 5;
Serial.println(voltage);

if(voltage >= 4)
{
    digitalWrite(pinled, HIGH);
}
   else
    digitalWrite(pinled, LOW);
}