Hello there everybody!
I's my first official time here, but I've been lurking around the forums for almost a year, while trying to build up my custom arduino project...why, it's a clapper switch, of course! 
Let's see...I started with a "simple" SINGLE CLAP ON/OFF LED LIGHT using the code I found on this Arduinonano-Clap-Switch instructables page/.
Code read as follows:
int micpin=10;
int ledpin=12;
void setup() {
pinMode (micpin, INPUT);
pinMode (ledpin, OUTPUT);
}
void loop() {
if (digitalRead (micpin)==HIGH) {
if (digitalRead(ledpin)==LOW) {
digitalWrite (ledpin, HIGH);
} else {
digitalWrite(ledpin,LOW);
}
delay(1000);
}
}
This worked fine, so I went for the next step, turning it into a DOUBLE CLAP ON/OFF LED LIGHT, mixing the original code with a bit I found on a few different pages, such as this HighVoltageCode page and this 14Core page.
This way, I obtained the following code:
int InputSound = 0;
int micpin = 10;
int ledpin = 12;
long DetectSpan = 0;
long SoundDetection = 0;
boolean lightState = false;
void setup() {
 Serial.begin(9600);
 pinMode (micpin, INPUT);
 pinMode (ledpin, OUTPUT);
 Serial.print("Microphone up and running");
}
void loop() {
 int sensorState = digitalRead(micpin);
     Â
  if (sensorState == 0)
  {
   if (InputSound == 0)
   {
    SoundDetection = DetectSpan = millis();
    InputSound++;
   }
   else if (InputSound > 0 && millis()-DetectSpan >= 50)
   {
    DetectSpan = millis();
    InputSound++;
   }
  }
  if (millis() - SoundDetection >= 400)
  {
   if (InputSound == 2)
   {
    if (!lightState)
    {
     lightState = true;
     digitalWrite(ledpin, HIGH);
     Serial.print("Let there be light!");
    }
    else if (lightState)
    { /* altrimenti */
     lightState = false;
     digitalWrite(ledpin, LOW);
     Serial.print("Lights of, please!");
    }
   }
   InputSound = 0;
  }
}
This worked as well...so I went for the big jump and changed the code to use a relay switch to POWER ON/OFF an extension cord. I adapted the code above mixing it with what I found on this Create.Arduino page and again on the HighVoltage page.
Here's the final code:
int InputSound = 0;
int micpin = 10;
int relaypin = 12;
long DetectSpan = 0;
long SoundDetection = 0;
boolean relayState = false;
void setup() {
 Serial.begin(9600);
 pinMode (micpin, INPUT);
 pinMode (relaypin, OUTPUT);
 Serial.print("Microphone up and running");
}
void loop() {
 int sensorState = digitalRead(micpin);
     Â
  if (sensorState == 0)
  {
   if (InputSound == 0)
   {
    SoundDetection = DetectSpan = millis();
    InputSound++;
   }
   else if (InputSound > 0 && millis()-DetectSpan >= 50)
   {
    DetectSpan = millis();
    InputSound++;
   }
  }
  if (millis() - SoundDetection >= 400)
  {
   if (InputSound == 2)
   {
    if (!relayState)
    {
     relayState = true;
     digitalWrite(relaypin, HIGH);
     Serial.print("Let there be light!");
    }
    else if (relayState)
    {
     relayState = false;
     digitalWrite(relaypin, LOW);
     Serial.print("Lights off, please!");
    }
   }
   InputSound = 0;
  }
}
Now...this should work...and yet it does not!
I chopped the power extension, then plugged the + cable (blue) to the C (in) and another piece of blue cable from the NO (out) nack to the plug...yet this does not work!
I tried plugging in the arduino nano to its usb cable, and all of a sudden, the board turned on and seemed to work (nano, microphone and even relay turned on and I could CLAP CLAP ON/OFF the relay),but then, when I turn the relay itself off by double clapping, the electricity still flowed to the plug.
What am I boing wrong? Please help!
Tomorrow I'll provide an image of my setup, to better undersand it.
Many thanks to everyone who can provide a solution.
