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/*
  Measuring temperature with Arduino

  Alan Wendt, PhD
  
  A thermistor is a resistor whose resistance decreases
  as the temperature increases.  With some care, you can
  use Arduino to read accurate temperatures.  My project
  is a decent evaporative cooler controller.  In Arizona, you
  can usually run the fan motor on high or low, and you can
  also turn the water on and off.  If you measure the temperature
  in the house, outside in the shade, and the air right after the
  evaporative pads, you can calculate the relative humidity,
  and also program the unit to shut down the pump if the outside
  air is already cool enough.

  To measure the current resistance of the thermistor,
  wire a thermistor and a 10K resistor up in serial.
  Connect the open end of the thermistor to +5V, and the
  open end of the resistor to ground.  Now, there's a 5V drop
  across the pair, and the drop across the thermistor is
  proportional to its resistance as a fraction of the
  sum of both resistances.  For example, if the voltage at the
  junction of resistor and thermistor is 4, that means that the
  thermistor dropped 1 volt, and the fixed resistor dropped 4, so the
  thermistor's current resistance is 1/4th of the fixed.

  Once you have the resistance, the temperature of the thermistor is     approximated with the formula:

  T = 1 / (A + B log(R) + C log(R) ^ 3)

  where A, B, and C are experimentally-determined constants.
  (The Steinhart-Hart approximation).  You need to collect
  4-5 few data points, preferably with a big temperature spread.

  I put the board into a refrigerator along with a little
  Radio Shack remote thermometer sensor, and snaked out
  the USB cable and the remote sensor cable.  Once the program's
  downloaded, enter the "serial monitor" in the Arduino
  SDK to see the output.

  Google "steinhart hart moshier"  for further discussion of calibration,    and for software to calculate the constants.

 */

int firstSensor = 5;    // first analog sensor

void setup()
{
  // start serial port at 9600 bps:
  Serial.begin(9600);
}

void loop()
{
    // read the analog input
    float a5 = analogRead(5);
    Serial.print("analogread = ");
    Serial.print(a5);
    Serial.print("\n");

    // calculate voltage
    float voltage = a5 / 1024 * 5.0;
    Serial.print("voltage = ");
    Serial.print(voltage);
    Serial.print("\n");

    // calculate resistance
    float resistance = (10000 * voltage) / (5.0 - voltage);
    Serial.print("resistance = ");
    Serial.print(resistance);
    Serial.print("\n");

    // calcuate temperature.  Use these values for A, B, and C till you
    // get everything working, and then do some measurements to calibrate
    // your thermistor in circuit.
    float logcubed = log(resistance);
    logcubed = logcubed * logcubed * logcubed;
    float kelvin = 1.0 / (-7.5e-4 + 6.23e-4 * log(resistance) - 1.73e-6 * (logcubed));

    // Convert to Fahrenheit
    float f = (kelvin - 273.15) * 9.0/5.0 + 32.0;
    Serial.print("temp = ");
    Serial.print(f);
    Serial.print("\n");

    // delay 1s to let the ADC recover:
    delay(1000);
}

[Reply #1 on:]

Thanks for posting this - good intro to something I was about to start looking into!
-SA

[Reply #2 on:]

Some thermistors (especially tiny ones) can self-heat with quite small amounts of current.  Check the specs.  You may have to switch on the 5V just briefly while making the measurement.

[Reply #3 on:]

Regarding jdoll's comment about heat generation, would the solution be to wire the +5 of the circuit to a digital output and set it high shortly before taking a reading, then setting it low immediately after?  If so, any suggestions on how long before taking the reading to set the output high to get the most accurate reading?

Also, is anyone else working with the thermistor that comes as part of the makershed advanced arduino kit?  I'm trying to find out the specs for this part so that I can plug in the appropriate constants to the program as the default calculations don't appear to be correct for this part.