float r1=10000,r2,v=0,e;
void setup(){
  pinMode(A0,INPUT);
  pinMode(8,OUTPUT);
  Serial.begin(9600);
}
void loop(){
  v=analogRead(A0);
  r2=(r1*(1023.0/v)-1.0);
e=pow(10,3.81-2.0*log10(r2/1000));
  Serial.println(e);
  if(e>50)
  digitalWrite(8,HIGH);
  else
  digitalWrite(8,LOW);
  delay(1000);
}