Minor DHT22 Dehumidifier controller update.

Winter's coming again.

Looking at the recent data from the dehumidifier controller (last version update was here.) I'm concerned that it's possible for the dehumidifier to run when it's damn cold up there.

Two reasons why this is daft.

1) The outlet pipe from the dehumidifier may be frozen, resulting in a broken pipe and possible flood
2) There's no point in trying to condense water from the air when it's too cold.

So I've added a simple few lines to prevent the dehumidifier running if it's cold

    if ( Temp <=1 ) { // Frost protection. If it's less than 1 deg - don't run.
      DewFlag = false;
      Timer = 0;
    }

I've also decided to include a maximum value of humidity, set to 65%. I'd noticed some high humidity peaks, and the worry is, as the temperature falls at night, the water will condense, so now the dehumidifier is set to run if this level is exceeded.

  // Dew detect loop. If the dewTrip point is reached, or the maintenance timer is 129600, or the humidity is greater than 65%, start the timer running and set the Dew flag
    if ( Temp <= DewTrip || Timer2 == 129600 || Humidity >= 65 ) {
      DewFlag = true;
      Timer = 1;    
      Timer2 = 0;  // reset maintenance timer 
    } 
    else {
      DewFlag = false;
    }

So the whole code now looks like this:

// Wireless dehumidifier Controller
// 
// Copyright Andy Doswell 24th May 2014 modified Dec 2014. 
// PURPOSE: DHT22 sensor with LCD Display and output for controlling dehumidifier.
// Data send via virtualwire and 433MHZ link to remote display
// Dehumidifier is controlled by a relay attached to pin 7, defined in rlyPin.
// DHT22 sensor is connected to 8, as defined in #define
// LCD is a Hitachi HD44780 or compatible, attached thus:
// LCD RS pin to digital pin 12
// LCD Enable pin to digital pin 11
// LCD D4 pin to digital pin 5
// LCD D5 pin to digital pin 4
// LCD D6 pin to digital pin 3
// LCD D7 pin to digital pin 2
// RW to GND
// V0 Held high (this may or may not work with your LCD, usual practice is to connect this to a 10K pot between +5 and GND)
// TX connected to pin 10
// Modified to transmit every 198 seconds, and to switch dehumidifier on when Dewpoint is within 5 Degrees of actual temperature
// Also brings dehumidifer on every 3 days regardless of temperature or humidity.
// You'll need the wonderful dht library by Rob Tillaart from here : https://github.com/RobTillaart/Arduino/tree/master/libraries/DHTlib 
// and the equally wonderful Virtualwire library from here :https://www.pjrc.com/teensy/td_libs_VirtualWire.html
// 21 Sep 2015 - 65% humidity upper limit added
//             - Frost protection added

// reference: http://en.wikipedia.org/wiki/Dew_point
double dewPointFast(double celsius, double humidity)
{
 double a = 17.271;
 double b = 237.7;
 double temp = (a * celsius) / (b + celsius) + log(humidity*0.01);
 double Td = (b * temp) / (a - temp);
 return Td;
}

#include <VirtualWire.h>
#include <VirtualWire_Config.h>

#include <dht.h>


dht DHT;

#define DHT22_PIN 8

#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

// define constants & global variables
const int rlyPin = 7; //defines pin the relay is connected to. relay is active low
const int TimerLength = 2700;// number of seconds in an 90 mins / 2 - this sets the minimum time the dehumidifier will remain on for.
int Timer = 0; //timer is off to start
boolean DewFlag = false; // This is the dew flag. Off as default.
boolean OutputStatus = false; //Output status is off as default
int chk;
int Temp;
int Dew;
int Humidity;
int DewTrip;
float Timer2 = 0; //maintenance timer - used to run the compressor every ~3 days


// Data structure set up for Transmission

struct TXData
{
  int TX_ID;
  int Temp;
  int Humidity;
  int Dew;
  boolean Output;
};
  

void setup()
{
  lcd.begin(16, 2); // defines the LCD as a 16 x 2
  pinMode (rlyPin, OUTPUT); // sets our relay pin
  digitalWrite (rlyPin, HIGH); // sets the relay off for default condition.
  
  // Virtualwire setup
  vw_set_tx_pin(10); // TX pin set to 10
  vw_set_rx_pin(9); // RX pin set to a spare pin, even though we don't use it. If left as default it interferes with LCD operation.
  vw_setup(300); //sets virtualwire for a tx rate of 300 bits per second
  

}

void loop()
{
  for(int i = 0; i < 100 ; i++) { //transmit only every 99 loops (198 seconds)
    chk = DHT.read22(DHT22_PIN); // these 4 lines get data from the sensor
    Dew = dewPointFast(DHT.temperature, DHT.humidity);
    Temp = (DHT.temperature);
    Humidity = (DHT.humidity);
    DewTrip= Dew + 5; // Brings the dehumidifier on 5 deg C before the dew point. 
  
    // writes information about the system to the LCD
    lcd.clear ();
    lcd.print("Humidity:");
    lcd.print(Humidity);
    lcd.print("%");
    lcd.setCursor(0, 1);
    lcd.print(Temp);
    lcd.print((char)0xDF);
    lcd.print("C");
    lcd.setCursor(6, 1);
    lcd.print("Dew:");
    lcd.print(Dew);
    lcd.print((char)0xDF);
    lcd.print("C");
   
  // Dew detect loop. If the dewTrip point is reached, or the maintenance timer is 129600, or the humidity is greater than 65%, start the timer running and set the Dew flag
    if ( Temp <= DewTrip || Timer2 == 129600 || Humidity >= 65 ) {
      DewFlag = true;
      Timer = 1;    
      Timer2 = 0;  // reset maintenance timer 
    } 
    else {
      DewFlag = false;
    }
    
    if ( Temp <=1 ) { // Frost protection. If it's less than 1 deg - don't run.
      DewFlag = false;
      Timer = 0;
    }
    
  
    if (Timer >= TimerLength and DewFlag == false) { // If the timer has expired and there's no dew, switch the dehumidifier off.
      Timer = 0;
      digitalWrite (rlyPin, HIGH);
   
    }

    if (Timer !=0) {                // If the timer is running, switch the dehumidifier on , and write On to the lcd.
      digitalWrite (rlyPin, LOW);
      lcd.setCursor (13,0);
      lcd.print (" On");
      OutputStatus = true;
      Timer++;
    }
  
    else {
      lcd.setCursor (13,0);
      lcd.print ("off");
      OutputStatus = false;
    }
  
 delay (2000);
 Timer2 ++;
  }
  


  
struct TXData payload; //Loads the Data struct with the payload data
lcd.clear ();
lcd.print ("Transmitting");
  
payload.TX_ID=10;
payload.Temp = Temp;
payload.Humidity = Humidity;
payload.Dew = Dew;
payload.Output = OutputStatus;
vw_send((uint8_t *)&payload, sizeof(payload)); //Transmits the struct
vw_wait_tx(); //waits for the TX to complete

}