Knock.ino

int lastVal = 0;
long lastKnock = 0;

void setup() {
  Serial.begin( 9600 );
}

void loop() {
  int readVal = analogRead( A2 );
  if( readVal > lastVal+5 ){
    int knockDiff = abs(readVal)-lastVal;
    long nowTime = millis();
    int diff = nowTime - lastKnock;
    if( lastKnock != 0 && diff > 20 && diff < 2000 ){
      Serial.print(knockDiff);
      Serial.print(":");
      if( diff > 300 ){
        Serial.println( "knock" );
      } else {
        Serial.println( "doubleknock" );
      }
    }
    lastKnock = nowTime;
  }
  lastVal = abs( readVal );
}