int tono = 10000; 

void setup (){
     pinMode (8, OUTPUT);
     for (tono=10000; tono>=100; tono=tono-10){
         digitalWrite (8, HIGH);
         delayMicroseconds (tono/2);
         digitalWrite (8, LOW);
         delayMicroseconds (tono/2);
     }  
}

void loop (){}



