DHT11 on Arduino. Send temperature and humidity by serial.

DHT11.ino

#define DHT11_PIN 2

void setup() {
  pinMode(DHT11_PIN, INPUT_PULLUP);
  Serial.begin(115200, SERIAL_8E1);
}

void loop() {
  if (Serial.available() > 0) {
    byte in = Serial.read();
    if (in == 1) {
      byte temp, humi;
      if (!getTempHumi(&temp, &humi)) {
        temp = 255;
        humi = 255;
      }
      Serial.write(temp);
      Serial.write(humi);
  }
}

byte _getbyte() {
  byte recv = 0;
  for (int i=7; i>=0; i--) {
    while (!digitalRead(DHT11_PIN)); 
    delayMicroseconds(50); // 0: 26~28us; 1: 70us high.
    recv |= digitalRead(DHT11_PIN) << i;
    while (digitalRead(DHT11_PIN));
  }
  return recv;
}

boolean getTempHumi(byte *temp, byte *humi) {
  pinMode(DHT11_PIN, OUTPUT);
  digitalWrite(DHT11_PIN, LOW); // Wake IC
  delay(18); // Wait wake
  pinMode(DHT11_PIN, INPUT_PULLUP);
  delayMicroseconds(30); // pullup 20~40us
  
  digitalRead(DHT11_PIN);
  delayMicroseconds(50); // low 80us
  while(!digitalRead(DHT11_PIN));
  delayMicroseconds(50); // high 80us
  while(digitalRead(DHT11_PIN));
  byte b[4];
  byte sum = 0;
  for (int i=0; i<4; i++) {
    b[i] = _getbyte();
    sum += b[i];
  }
  *humi = b[0];
  *temp = b[2];
  return sum == _getbyte();
}