int redled = 11;
int trigpin = 10;
int echopin = 12;
float distance;
int duration;


void setup() {
pinMode(redled, OUTPUT);
pinMode(trigpin, OUTPUT);
pinMode(echopin, INPUT);
Serial.begin(9600);
}

void loop() {
digitalWrite(trigpin, LOW);
delay(1000);
digitalWrite(trigpin, HIGH);
delay(1000);
digitalWrite(trigpin, LOW);
duration = pulseIn(echopin, HIGH);
distance = duration * 0.034 / 2;
delay(500);
Serial.println(" ");
Serial.println("Distance = ");
Serial.println(distance);
Serial.println(" CM");
Serial.println("");

if (distance<=40) {
digitalWrite(redled, HIGH);
} else {
  digitalWrite(redled, LOW);
}
}