This is a function I wrote to convert the value from an analogRead call of a pin with a thermistor connected to it to a temperature. Unlike most other programs that use a look-up table, this function utilizes the Steinhart-Hart Thermistor Equation to convert "Thermistor Resistance" to "Temperature in Degrees Kelvin." I found the equation here, but it can also be found at Wikipedia.
The more simple program shows how little code is necessary to do the conversion, and how much memory and program space can be saved when compared to a large look-up table.
The more elaborate program shows the power of the function. It displays lots of data including the ADC (from 0 to 1023), the voltage on the analog pin (note: ideally, the arduino would be running at 5.0 volts, but my power supply gives it 4.86 volts, so I divide by 4.86. Look at the code to change that to whatever your power supply is giving), the resistance of the Thermistor, and the temperature in Celsius and Fahrenheit. It uses the printDouble routine which I found in a forum post here.
I hope this comes in handy for everyone. Like most of my code, it's freely usable and modifiable, and no credit is needed. Free, like arduino should be.
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(Ground) ---- (10k-Resister) -------|------- (Thermistor) ---- (+5v)
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Analog Pin 0
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#include <math.h>
double Thermister(int RawADC) {
double Temp;
Temp = log(((10240000/RawADC) - 10000));
Temp = 1 / (0.001129148 + (0.000234125 * Temp) + (0.0000000876741 * Temp * Temp * Temp));
Temp = Temp - 273.15; // Convert Kelvin to Celcius
Temp = (Temp * 9.0)/ 5.0 + 32.0; // Convert Celcius to Fahrenheit
return Temp;
}
void setup() {
Serial.begin(115200);
}
void loop() {
Serial.println(int(Thermister(analogRead(0)))); // display Fahrenheit
delay(100);
}
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#include <math.h>
//Schematic:
// [Ground] ---- [10k-Resister] -------|------- [Thermistor] ---- [+5v]
// |
// Analog Pin 0
double Thermistor(int RawADC) {
// Inputs ADC Value from Thermistor and outputs Temperature in Celsius
// requires: include <math.h>
// Utilizes the Steinhart-Hart Thermistor Equation:
// Temperature in Kelvin = 1 / {A + B[ln(R)] + C[ln(R)]^3}
// where A = 0.001129148, B = 0.000234125 and C = 8.76741E-08
long Resistance; double Temp; // Dual-Purpose variable to save space.
Resistance=((10240000/RawADC) - 10000); // Assuming a 10k Thermistor. Calculation is actually: Resistance = (1024/ADC)
Temp = log(Resistance); // Saving the Log(resistance) so not to calculate it 4 times later. // "Temp" means "Temporary" on this line.
Temp = 1 / (0.001129148 + (0.000234125 * Temp) + (0.0000000876741 * Temp * Temp * Temp)); // Now it means both "Temporary" and "Temperature"
Temp = Temp - 273.15; // Convert Kelvin to Celsius // Now it only means "Temperature"
// BEGIN- Remove these lines for the function not to display anything
Serial.print("ADC: "); Serial.print(RawADC); Serial.print("/1024"); // Print out RAW ADC Number
Serial.print(", Volts: "); printDouble(((RawADC*4.860)/1024.0),3); // 4.860 volts is what my USB Port outputs.
Serial.print(", Resistance: "); Serial.print(Resistance); Serial.print("ohms");
// END- Remove these lines for the function not to display anything
// Uncomment this line for the function to return Fahrenheit instead.
//Temp = (Temp * 9.0)/ 5.0 + 32.0; // Convert to Fahrenheit
return Temp; // Return the Temperature
}
void printDouble(double val, byte precision) {
// prints val with number of decimal places determine by precision
// precision is a number from 0 to 6 indicating the desired decimal places
// example: printDouble(3.1415, 2); // prints 3.14 (two decimal places)
Serial.print (int(val)); //prints the int part
if( precision > 0) {
Serial.print("."); // print the decimal point
unsigned long frac, mult = 1;
byte padding = precision -1;
while(precision--) mult *=10;
if(val >= 0) frac = (val - int(val)) * mult; else frac = (int(val) - val) * mult;
unsigned long frac1 = frac;
while(frac1 /= 10) padding--;
while(padding--) Serial.print("0");
Serial.print(frac,DEC) ;
}
}
void setup() {
Serial.begin(115200);
}
#define ThermistorPIN 0 // Analog Pin 0
double temp;
void loop() {
temp=Thermistor(analogRead(ThermistorPIN)); // read ADC and convert it to Celsius
Serial.print(", Celsius: "); printDouble(temp,3); // display Celsius
temp = (temp * 9.0)/ 5.0 + 32.0; // converts to Fahrenheit
Serial.print(", Fahrenheit: "); printDouble(temp,3); // display Fahrenheit
Serial.println(""); // End of Line
delay(100); // Delay a bit... for fun, and to not Serial.print faster than the serial connection can output
}