Monday, January 7, 2013

Temperature and Humidity Redux

A long while back (in a galaxy very far away), we built a temp and humidity monitor with the SHT-21 chip. It's a very accurate, albeit very expensive, solution. What if good enough, is good enough? If you don't need scientific resolution, the DHT-11 is a very inexpensive solution. Today we built a temp and humidity monitor with this module. I started off with a DHT-11 breakout board from geeetech. I used this module because it has the pull up resistor and a conditioning capacitor onboard. I connected S (signal) to pin 2, - to Gnd, and + to 5vdc.
I downloaded the library from github and followed the tutorial from adafruit. I changed the sketch slightly, as it outputs the temperature in C, and I want to see it in F, so I changed two lines from:

Serial.print(t);
    Serial.println(" *C");

to:

Serial.print(t*1.8+32);
    Serial.println(" *F"); 

Here is the final result:


// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain
// Fahrenheit conversion added by Steve Spence, http://arduinotronics.blogspot.com

#include "DHT.h"

#define DHTPIN 2     // what pin we're connected to

// Uncomment whatever type you're using!
#define DHTTYPE DHT11   // DHT 11 
//#define DHTTYPE DHT22   // DHT 22  (AM2302)
//#define DHTTYPE DHT21   // DHT 21 (AM2301)

// Connect pin + (middle) of the sensor to +5V
// Connect pin S  (on the right) of the sensor to whatever your DHTPIN is
// Connect pin - (on the left) of the sensor to GROUND

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600); 
  Serial.println("DHTxx test!");

  dht.begin();
}

void loop() {
  // Reading temperature or humidity takes about 250 milliseconds!
  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
  float h = dht.readHumidity();
  float t = dht.readTemperature();

  // check if returns are valid, if they are NaN (not a number) then something went wrong!
  if (isnan(t) || isnan(h)) {
    Serial.println("Failed to read from DHT");
  } else {
    Serial.print("Humidity: "); 
    Serial.print(h);
    Serial.print(" %\t");
    Serial.print("Temperature: "); 
    Serial.print(t*1.8+32);
    Serial.println(" *F");
  }
}