const int MOTION_SENSOR_PIN = 7;   
const int RELAY_PIN         = A5;  
int motionStateCurrent      = LOW;
int motionStatePrevious     = LOW; 
void setup() {
  Serial.begin(9600);                
  pinMode(MOTION_SENSOR_PIN, INPUT); 
  pinMode(RELAY_PIN, OUTPUT);        
}

void loop() {
  motionStatePrevious = motionStateCurrent;            
  motionStateCurrent  = digitalRead(MOTION_SENSOR_PIN); 

  if (motionStatePrevious == LOW && motionStateCurrent == HIGH) { 
    Serial.println("Motion detected!");
    digitalWrite(RELAY_PIN, HIGH); 
  }
  else
  if (motionStatePrevious == HIGH && motionStateCurrent == LOW) { 
    Serial.println("Motion stopped!");
    digitalWrite(RELAY_PIN, LOW); 
  }
}