Thermistor.cpp

00001  /*
00002  * Thermistor Temperature library
00003  * Licensed under the Apache License, Version 2.0 (the "License");
00004  * you may not use this file except in compliance with the License.
00005  * You may obtain a copy of the License at
00006  *
00007  *     http://www.apache.org/licenses/LICENSE-2.0
00008  *
00009  * Unless required by applicable law or agreed to in writing, software
00010  * distributed under the License is distributed on an "AS IS" BASIS,
00011  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00012  * See the License for the specific language governing permissions and
00013  * limitations under the License.
00014  */
00015 #include "mbed.h"
00016 #include "Thermistor.h"
00017 
00018 
00019     Thermistor::Thermistor(PinName pin) : _pin(pin) {  // _pin(pin) means pass pin to the AnalogIn constructor
00020         init();
00021     }
00022  
00023     void Thermistor::init() {
00024         ThermistorNominal = THERMISTORNOMINAL;
00025         TemperatureNominal = TEMPERATURENOMINAL;
00026         BCoefficient = BCOEFFICIENT;
00027         SeriesResistor = SERIESRESISTOR;
00028     }
00029  
00030      // This is the workhorse routine that calculates the temperature
00031     // using the Steinhart-Hart equation for thermistors
00032     // https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
00033     float Thermistor::get_temperature()
00034     {
00035         double temperature =0, resistance =0;
00036         double steinhart =0;
00037         double a=0;
00038         int smooth = 5;             // Number of samples to smooth
00039         
00040         for(int i=0;i<smooth;i++) {
00041             a += _pin.read_u16();       // Read 16bit Analog value
00042         }
00043         a = a/smooth;                   // Get average of samples    
00044  //       pc.printf("Raw Analog Value for Thermistor = %d\r\n",a);
00045       
00046         /* Calculate the resistance of the thermistor from analog votage read. */
00047         resistance = (float) SeriesResistor / ((65536.0 / a) - 1);
00048  //       pc.printf("Resistance for Thermistor = %f\r\n",resistance);
00049        
00050         steinhart = resistance / ThermistorNominal;         // (R/Ro)
00051         steinhart = log(steinhart);                         // ln(R/Ro)
00052         steinhart /= BCoefficient;                          // 1/B * ln(R/Ro)
00053         steinhart += 1.0 / (TemperatureNominal + 273.15);   // + (1/To)
00054         steinhart = 1.0 / steinhart;                        // Invert
00055         temperature = steinhart - 273.15;                   // convert to C
00056     
00057         return temperature;
00058     }
00059      
00060     void Thermistor::set_ThermistorNominal(float thermnom) {
00061         ThermistorNominal = thermnom;
00062     }    
00063     void Thermistor::set_TemperatureNominal(float tempnom){
00064         TemperatureNominal = tempnom;
00065     }    
00066     void Thermistor::set_BCoefficient(float bcoefficient){
00067         BCoefficient = bcoefficient;
00068     }    
00069     void Thermistor::set_SeriesResistor(float resistor){
00070         SeriesResistor = resistor;
00071     }    
00072     
00073 
00074 
00075