// sensirion SHT11 humidity/temperature sensor (parallax kit) 

int temperatureCommand  = B00000011;  // command used to read temperature 
int humidityCommand = B00000101;  // command used to read humidity 
 
int clockPin = 0;  // pin used for clock 
int dataPin  = 1;  // pin used for data 
int ack;  // track acknowledgment for errors 
int val;  
float temperature;          
float humidity; 
 
 
 
void setup() { 
  Serial.begin(9600); // open serial at 9600 bps 
} 
 
void loop() { 
  // read the temperature and convert it to centigrades 
  sendCommandSHT(temperatureCommand, dataPin, clockPin); 
  waitForResultSHT(dataPin); 
  val = getData16SHT(dataPin, clockPin); 
  skipCrcSHT(dataPin, clockPin); 
  temperature = (float)val * 0.01 - 40; 
  Serial.print("temperature: "); 
  Serial.print((long)temperature, DEC); 
  
  // read the humidity 
  sendCommandSHT(humidityCommand, dataPin, clockPin); 
  waitForResultSHT(dataPin); 
  val = getData16SHT(dataPin, clockPin); 
  skipCrcSHT(dataPin, clockPin); 
  humidity = -4.0 + 0.0405 * val + -0.0000028 * val * val; 
  Serial.print("humidity: "); 
  Serial.print((long)humidity, DEC); 
 
  delay(300000); // wait for 5 Minutes for next reading 
} 
 
 
// commands for reading/sending data to a SHTx sensor 
 
int shiftIn(int dataPin, int clockPin, int numBits) { 
  int ret = 0; 
 
  for (int i=0; i<numBits; ++i) { 
    digitalWrite(clockPin, HIGH); 
    //delay(10); not needed :) 
    ret = ret*2 + digitalRead(dataPin); 
    digitalWrite(clockPin, LOW); 
  } 
  return(ret); 
} 
 
// send a command to the SHTx sensor 
void sendCommandSHT(int command, int dataPin, int clockPin) { 
  int ack; 
 
  // transmission start 
  pinMode(dataPin, OUTPUT); 
  pinMode(clockPin, OUTPUT); 
  digitalWrite(dataPin, HIGH); 
  digitalWrite(clockPin, HIGH); 
  digitalWrite(dataPin, LOW); 
  digitalWrite(clockPin, LOW); 
  digitalWrite(clockPin, HIGH); 
  digitalWrite(dataPin, HIGH); 
  digitalWrite(clockPin, LOW); 
  
  // shift out the command (the 3 MSB are address and must be 000, the last 5 bits are the command) 
  shiftOut(dataPin, clockPin, MSBFIRST, command); 
  
  // verify we get the right ACK 
  digitalWrite(clockPin, HIGH); 
  pinMode(dataPin, INPUT); 
  ack = digitalRead(dataPin); 
  if (ack != LOW) 
    Serial.println("ACK error 0"); 
  digitalWrite(clockPin, LOW); 
  ack = digitalRead(dataPin); 
  if (ack != HIGH) 
    Serial.println("ACK error 1"); 
} 
 
// wait for the SHTx answer 
void waitForResultSHT(int dataPin) { 
  int ack; 
 
  pinMode(dataPin, INPUT); 
  for(int i=0; i<100; ++i) { 
    delay(10); 
    ack = digitalRead(dataPin); 
    if (ack == LOW) 
      break; 
  } 
  if (ack == HIGH) 
    Serial.println("ACK error 2"); 
} 
 
// get data from the SHTx sensor 
int getData16SHT(int dataPin, int clockPin) { 
  int val; 
 
  // get the MSB (most significant bits) 
  pinMode(dataPin, INPUT); 
  pinMode(clockPin, OUTPUT); 
  val = shiftIn(dataPin, clockPin, 8); 
  val *= 256; // this is equivalent to val << 8; 
  
  // send the required ACK 
  pinMode(dataPin, OUTPUT); 
  digitalWrite(dataPin, HIGH); 
  digitalWrite(dataPin, LOW); 
  digitalWrite(clockPin, HIGH); 
  digitalWrite(clockPin, LOW); 
  
  // get the LSB (less significant bits) 
  pinMode(dataPin, INPUT); 
  val |= shiftIn(dataPin, clockPin, 8); 
  return val; 
} 
 
// skip CRC data from the SHTx sensor 
void skipCrcSHT(int dataPin, int clockPin) { 
  pinMode(dataPin, OUTPUT); 
  pinMode(clockPin, OUTPUT); 
  digitalWrite(dataPin, HIGH); 
  digitalWrite(clockPin, HIGH); 
  digitalWrite(clockPin, LOW); 
}