int current_reading;
int past_reading;
const int RadarPin = 17;
void setup() {
  Serial.begin(115200);      // initialize serial communications
  pinMode(RadarPin, INPUT);  //digital input pin
}

void loop() {
  current_reading = digitalRead(RadarPin);  // read the value from the sensor (0 or 1)
  if (current_reading == 1 && past_reading == 0) {
    past_reading = 1;
    Serial.println("GOTCHA!");
  }
  if (current_reading == 0 && past_reading == 1) {
    past_reading = 0;
    Serial.println("WHERE ARE YOU?");
  }
}