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      1 /*
      2  * Author: Jon Trulson <jtrulson (at) ics.com>
      3  * Copyright (c) 2015 Intel Corporation.
      4  *
      5  * Thanks to public domain code by Martin Liddament for some useful clues!
      6  * http://www.veetech.org.uk/CO2_Monitor_Sketch_2_Operation.txt
      7  * and sandbox electronics (http://sandboxelectronics.com/?p=147).
      8  *
      9  * Permission is hereby granted, free of charge, to any person obtaining
     10  * a copy of this software and associated documentation files (the
     11  * "Software"), to deal in the Software without restriction, including
     12  * without limitation the rights to use, copy, modify, merge, publish,
     13  * distribute, sublicense, and/or sell copies of the Software, and to
     14  * permit persons to whom the Software is furnished to do so, subject to
     15  * the following conditions:
     16  *
     17  * The above copyright notice and this permission notice shall be
     18  * included in all copies or substantial portions of the Software.
     19  *
     20  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
     21  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
     22  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
     23  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
     24  * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
     25  * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
     26  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
     27  */
     28 
     29 #include <iostream>
     30 #include <math.h>
     31 #include "mg811.h"
     32 
     33 using namespace std;
     34 using namespace upm;
     35 
     36 // voltage gain of the DC amplifier
     37 static const float dcGain = 8.5;
     38 
     39 MG811::MG811(int pin, int dpin, float aref) :
     40   m_aio(pin), m_gpio(dpin)
     41 {
     42   m_aRes = m_aio.getBit();
     43   m_aref = aref;
     44 
     45   m_gpio.dir(mraa::DIR_IN);
     46 
     47   // these are just placeholder values (coarsely measured during
     48   // development), you should determine the appropriate values (in
     49   // volts) for your environment at the specified concentrations.  Use
     50   // the getReferenceVoltage() method to get these values at 400ppm
     51   // and 1000ppm respectively.  Good luck.
     52   setCalibration(0.5514, 0.370);
     53 }
     54 
     55 MG811::~MG811()
     56 {
     57 }
     58 
     59 float MG811::volts()
     60 {
     61   int val = m_aio.read();
     62 
     63   return(float(val) * (m_aref / float(1 << m_aRes)));
     64 }
     65 
     66 void MG811::setCalibration(float ppm400, float ppm1000)
     67 {
     68   m_zeroPointValue = ppm400;
     69   m_reactionValue = ppm1000;
     70 }
     71 
     72 float MG811::getReferenceVoltage()
     73 {
     74   return (volts() / dcGain);
     75 }
     76 
     77 float MG811::ppm()
     78 {
     79   static const float log400 = log10f(400);
     80   static const float log1000 = log10f(1000);
     81 
     82   float val = volts();
     83 
     84   if ((val / dcGain) >= m_zeroPointValue)
     85     return 0.0;
     86   else
     87     return powf(10.0, ((val/dcGain)-m_zeroPointValue) /
     88                       (m_reactionValue / (log400-log1000))+log400);
     89 }
     90 
     91 bool MG811::thresholdReached()
     92 {
     93   return (m_gpio.read() ? true : false);
     94 }
     95