const int trig = 2; const int echo = 3; const int relay = 4; long duration, distanceCm; int limitCm = 15; void setup() { Serial.begin(9600); pinMode(relay, OUTPUT); } void loop() { pinMode(trig, OUTPUT); digitalWrite(trig, LOW); delayMicroseconds(2); digitalWrite(trig, HIGH); delayMicroseconds(5); digitalWrite(trig, LOW); pinMode(echo,INPUT); duration = pulseIn(echo ,HIGH); distanceCm = microsecondsToCm(duration); Serial.print(distanceCm); Serial.print("CM"); Serial.println(); void checkLimit(); delay(100); } void checkLimit(){ if(distanceCm < limitCm){ digitalWrite(relay,HIGH); }else{ digitalWrite(relay,LOW); } } long microsecondsToCm(long microseconds) { return microseconds / 29 / 2; }