Arduino LCD DHT11 Temperature – Humidity Sensor

As a first useful project on Arduino, a DHT11 sensor is a great place to start.

The DHT11 is a simple digital temperature/humidity sensor that easily connects to any Arduino. Most sample programs just take the reading and output it to the serial port. This example code will let you connect a LCD screen and track the min and max temp/humidity readings.

PARTS

• Arduino Uno R3 ($24)
• SainSmart 1602 LCD Shield ($13)
• DHT11 ($2-$5)
• Breadboard Jumper Wires ($6)

It doesn’t take much to put this all together. Place the SainSmart LCD Shield over the Arduino and connect the headers together. I then used 3 male to female breadboard wires to connect the sensor into the A5 (Analog 5) port.  Then write/copy the software and you are ready to go.

Simplest DHT11 Sensor Code

If you just want the quickest way to get your sensor reading, copy and paste the 3 boxes below:

#include <Wire.h> // builtin library
#include <LiquidCrystal.h> // builtin library
LiquidCrystal lcd(8, 9, 4, 5, 6, 7); // sainsmart 1602 lcd

#include <dht11.h> // download this from http://playground.arduino.cc/main/DHT11Lib
dht11 DHT;

void setup(){
  lcd.begin(16, 2);
}

void loop() {
  DHT.read(A5);
  lcd.print((DHT.temperature * (9.0/5)) + 32); // Converts C to F
  delay(1000); // DHT11 can't poll faster than 1x a second
}

More Complex Code with Min/Max and Humidity

#include <Wire.h>
#include <LiquidCrystal.h>
LiquidCrystal lcd(8, 9, 4, 5, 6, 7); // sainsmart 1602 lcd

#include "dht11.h"
#define dht_dpin A5 // Set to channel sensor is on
dht11 DHT;
int i;
int tmin=0;
int tmax=0;
int temp=0;
int hmin=0;
int hmax=0;
int humid=0;

void setup(){
 lcd.begin(16, 2);
 lcd.print("starting");

 // Get the initial values for min/max values
 DHT.read(dht_dpin);
 tmin=int((DHT.temperature * (9.0/5.0)) + 32.0);
 tmax=tmin;
 hmin=int(DHT.humidity+22);
 hmax=hmin;
}

/* LCD SCREEN -- to show where the cursors correlates on the LCD
   00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15
0:  T  E  M  P     L  6  7     6 8      H  6  9
1: 
*/

void loop(){
  DHT.read(dht_dpin);
  //Process Temperature ------------------------------- 
  temp=int((DHT.temperature * (9.0/5.0)) + 32.0); // F to C

  lcd.setCursor (0,0);
  lcd.print("TEMP L");
  lcd.print(tmin);
  lcd.setCursor(9,0);
  lcd.print(temp);
  lcd.setCursor(12,0);
  lcd.print("H");
  lcd.print(tmax);

  if(temp > tmax){ tmax=temp; }
  if(temp < tmin){ tmin=temp; }

  // Process Humidity ---------------------------------
  humid=int(DHT.humidity+22); // includes calibration constant

  lcd.setCursor(0,1);
  lcd.print("HUM  L");
  lcd.print(hmin);
  lcd.setCursor(9,1);
  lcd.print(humid);
  lcd.setCursor(12,1);
  lcd.print("H");
  lcd.print(hmax);

  if(humid > hmax){ hmax=humid; }
  if(humid < hmin){ hmin=humid; }

  // This blinks a period at the top right LCD to show it is running
  lcd.setCursor(15,0);
  if(i==1){
    lcd.print(" ");
    i=0;
  }else{
    lcd.print(".");
    i=1;
  }

  delay(800); // DHT11 can't poll faster than 1x a second
}

Results

After building this I have one complaint, is the data from the DHT11 even realistic. The temperature result is an easy one to check but the relative humidity can seem more like a predictable random number. In the project, you will see various fudge factors to align the DHT11 to a more realistic number. The question raises, does the DHT11 need a constant calibration factor or does it need varying amounts of correction at different temperatures and humidity.

The DHT11 has an accuracy of 1 degree in Celsius, which after conversion gives you about 2 degree accuracy in Fahrenheit. For most applications, it’s probably not a big deal, but it leaves you wanting more resolution. The reason for decimal points in the 9.0/5.0 (C to F conversion) is to force the compiler to treat 9/5 as a floating point [1.88] instead of an integer [1].

The RH (relative humidity) is a whole other issue. I have 3 different RH meters and none of them seem to agree. And unlike the temperature where you can use a space heater to quickly heat a small room to different temperatures, RH is a hard one to change for testing.

Feel free to leave comments and questions below.