// ARDUINO NANO V3 CODE FÜR SENDEEINHEIT
const byte piezopin = 3;        // Timer 2 "B" output: OC2B
const long frequency = 40000L;  // PWM Frequenz in Hz
const byte buttonPin = 9;        // PIN mit angeschlossenem Taster
const byte led_builtin = 13;
int buttonState = 0;  

void setup() 
{
  pinMode (piezopin, OUTPUT); // Code von Nick Gammon (http://www.gammon.com.au/forum/?id=11504&reply=6#reply6)

  TCCR2A = (1<<WGM20) | (1<<WGM21) | (1<<COM2B1); // fast PWM, OC2B löschen bei Vergleich
  TCCR2B = (1<<WGM22) | (1<<CS21);                 // fast PWM, Prescaler auf 8
  OCR2A =  ((F_CPU / 8) / frequency) - 1;            // Relative Null setzen  
  OCR2B = ((OCR2A + 1) / 2) - 1;                     // 50% Tastgrad

  pinMode(buttonPin, INPUT_PULLUP);

  pinMode(led_builtin, OUTPUT);
  } 

void loop() {
  buttonState = digitalRead(buttonPin); // Status buttonpin einlesen
  
if (buttonState == LOW) {            // wenn Taster gedrückt
    digitalWrite(LED_BUILTIN, HIGH);   // LED an 
    TCCR2A = (1<<WGM20) | (1<<WGM21) | (1<<COM2B1); 
    TCCR2B = (1<<WGM22) | (1<<CS21);
    }
else {
    digitalWrite(LED_BUILTIN, LOW);    // LED aus 
    TCCR2A = (1<<WGM20) | (1<<WGM21) | (0<<COM2B1); 
    TCCR2B = (1<<WGM22) | (1<<CS21);
    }
  }