Wireless Dehumidifier Controller Completion.

The new transmitter and receiver pair duly arrived, fitted and tested. Great range now. I haven't fully tested it's range, but it works beautifully from my attic to my workshop on the ground floor. During testing I became aware of other transmitters sending data on the same channel every now and again (remember my doorbell and weather station?) Perhaps sending data at such a slow datarate every 2 seconds is a bit anti-social, likely to clutter up the band a bit, and is unnecessary, so I've altered the sketch to send the data every 18 seconds.

Finished receiver in a Hammond clear blue case. Posh eh?

Powered from a redundant mobile phone charger (see note below).

Display on the receiver.

A note on aerials. The aerial (or antenna if you must) is simply 173mm of copper wire. Keep it as straight as possible and vertical. 

For the revised transmission rate, only the transmitter sketch changes. The receiver remains the same.

// 
// FILE:  wireless_dehumid.pde
// Written by Andy Doswell 24th May 2014
// PURPOSE: DHT11 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.
// DHT11 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
// delta max = 0.6544 wrt dewPoint()
// 6.9 x faster than dewPoint()
// 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 DHT11_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 = 1800;// number of seconds in an hour / 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;


// 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 < 10 ; i++) { //transmit only every 9 loops (18 seconds)
    chk = DHT.read11(DHT11_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, start the timer running and set the Dew flag
    if ( Temp <= DewTrip ) {
      DewFlag = true;
      Timer = 1;       
    } 
    else {
      DewFlag = false;
    }

  
    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);
  }
  
struct TXData payload; //Loads the Data struct with the payload data
  
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

}  
  
//
// END OF FILE
//

The receiver is powered from a redundant mobile phone charger, rated at 5V at 800mA. I was quite surprised to find it's off load voltage to be just under 7 volts. I expected it to be regulated. I fitted a 7805 voltage regulator inside the case. I was worried if the 7805 had enough headroom, but it seems to function well enough.