// sensor MQ7.  alternate heater 5V/1.4V  60sec/90sec
// read sensor @ 88sec(of 90-period)
// Kn.Ny '16
// This program to be used as CO-alarm in tents wintertime during night. (gasoline or firewood burners)
// ****************************************************************************************************
#define a_pin A3
#define powerpin 3
#define lotime 90000UL  // 90 sek lav
#define hitime 60000UL  // 60 sek høy

int start_val, end_val; // read  analog value
unsigned long next_time,read_time;
boolean low_now,done_reading; // remember status

void setup()
{
  Serial.begin(9600);
  pinMode(powerpin, OUTPUT);
  pinMode(13, OUTPUT); // may be omitted
}

void loop()
{
  if (millis() >= next_time) // switch voltage 5V <-> 1.4V
  {
    if (low_now) // then switch to high
    {
      next_time += hitime; // set next change to 90 sec
      digitalWrite(powerpin, HIGH); // => 5V
      digitalWrite(13, HIGH); // LED may be omitted
      low_now = false;
    }
    else
    {
      next_time += lotime;
      read_time=next_time-2000; // read 2 sek ahead of end low period
      done_reading=false; // note: ready for new reading @ end of this period
      digitalWrite(powerpin,LOW); //  => 1.4V
      digitalWrite(13, LOW); // LED - optional
      low_now = true;
    }
  }
  if (!done_reading & (millis()>read_time))  // then do a reading now
  {
    int co_eqv=1023-analogRead(a_pin); // with this code: alarm @ level 235 (ca. 100 ppm CO)
    Serial.println(co_eqv); // should be range 15..25 after 30 min runtime
    done_reading=true;
    // check for alarm.  In MY setup 100ppm CO will give reading 235+-
    if (co_eqv > 235) ; // take action - sound alarm
  }
}