Arduino - HTF3223 Humidity Sensor

Over the past six months or so I have been helping my daughter with a remote mounted sensors project to supplement the info from her weather station on her blog. Light and temperature sensing via the analogue ports has presented no problems (LDR and TMP36), but when it comes down to humidity sensing its a bit more complex.

Although you can buy some inexpensive resistive humidity sensors, these are not suitable for DC reading and have to be read using an AC waveform at a few KHz to avoid eventual damage to the devices.

There are ready made humidity sensor system which use SPI/I2C bus communication which give very good results, but looking at them,they tend to be a little pricey and the required source code is a bit beyond the level of Arduino coding Emily is happy with.

After a LOT of digging I came up with the Humirel HTF3223 Sensor.

Unlike other sensors, it provides an analogue value out which is linear with the relative humidity, but rather than a voltage it outputs a variable frequency square wave at between 8 and 10KHz.

So - how are we going to convert frequency to something useful??

Well there is a frequency counter lib available, but for a top frequency of around 10KHz its a bit like using a sledgehammer to crack a nut.

After a bit of googling I found several examples of using the pulsein function. The following code gives a very quick example of reading the frequency from the humidity module, and using a simple conversion formula, convert it to relative humidity.

/*

Humidity sensor test for HTF3223 humidity sensor

By G7NBP - Chris Williams

V0.0.1 28th - May 2011

The inexpensive HTF3223 sensor (ebay etc) provides good accuracy relative humidity
sensor readings (typically +/- 5%) and has a simple linear output.

Its output however is in the form of a variable frequency proportionate to the humidity.

The formula for convertion is RH = (9740/Freq)/18

Although it is possible to use the frequency library to collect the frequency, this
is somewhat overkill as the freq is not expected to be above 10KHz.

Instead the pulseIn function is used and frequency averaged over 4096 counts.



connect H pin on  HTF3223 to digital pin 7

*/

const int hpin = 7;  // the sensor pin

 
void setup() {
  Serial.begin(9600);
  pinMode(hpin, INPUT);

}

void loop() {
  long sensorValue = getFrequency(hpin);    // get the raw sensor frequency
  long humidity = convertToRH(sensorValue); // convert it to RH%
  Serial.print("S:");
  Serial.print(sensorValue, DEC);
  Serial.print(" H:");
  Serial.print(humidity, DEC);
  Serial.println("%");
  delay(3000);
 
}

long getFrequency(int pin){
  #define SAMPLES 4096
  long freq = 0;
  for(unsigned int j=0; j<SAMPLES; j++) freq+= 500000/pulseIn(pin, HIGH, 250000);
  return freq / SAMPLES;
}


long convertToRH(long sensorValue){
  long rh = (9740-sensorValue)/18;
  return rh;
}