Linkit 7688 Duo DHT22

Both the Linkit 7688 and DHT22 are low cost components and can perfectly be used as a IOT temperature and humidity device. Using the openwrt part of the linkit to upload all the data to your server.

DHT22 specs:

  • 3 to 5V power and I/O
  • 2.5mA max current use during conversion (while requesting data)
  • Good for 0-100% humidity readings with 2-5% accuracy
  • Good for -40 to 125°C temperature readings ±0.5°C accuracy
  • Body size 15.1mm x 25mm x 7.7mm

The DHT22 has has four pins

  • VCC (3 to 5V power)
  • Data out
  • Not connected
  • Ground

You can ignore pin 3, its not used. Please use a pull-up resistor between VCC and the data pin (10k).

Below pictures shows the connection diagram.

Linkit 7688 Duo DHT22
Linkit 7688 Duo DHT22

Now we need to install the DHT library. You can find the latest libraries here: https://github.com/adafruit/DHT-sensor-library

Or download from my website: DHT-sensor-library-master

You can use the following sourcecode written by layada. If you download the libraries you can also get the code from the example menu.

// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain

#include "DHT.h"

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

// Uncomment whatever type you're using!
//#define DHTTYPE DHT11 // DHT 11
#define DHTTYPE DHT22 // DHT 22 (AM2302), AM2321
//#define DHTTYPE DHT21 // DHT 21 (AM2301)

// Connect pin 1 (on the left) of the sensor to +5V
// NOTE: If using a board with 3.3V logic like an Arduino Due connect pin 1
// to 3.3V instead of 5V!
// Connect pin 2 of the sensor to whatever your DHTPIN is
// Connect pin 4 (on the right) of the sensor to GROUND
// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor

// Initialize DHT sensor.
// Note that older versions of this library took an optional third parameter to
// tweak the timings for faster processors. This parameter is no longer needed
// as the current DHT reading algorithm adjusts itself to work on faster procs.
DHT dht(DHTPIN, DHTTYPE);

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

dht.begin();
}

void loop() {
// Wait a few seconds between measurements.
delay(2000);

// 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();
// Read temperature as Celsius (the default)
float t = dht.readTemperature();
// Read temperature as Fahrenheit (isFahrenheit = true)
float f = dht.readTemperature(true);

// Check if any reads failed and exit early (to try again).
if (isnan(h) || isnan(t) || isnan(f)) {
Serial.println("Failed to read from DHT sensor!");
return;
}

// Compute heat index in Fahrenheit (the default)
float hif = dht.computeHeatIndex(f, h);
// Compute heat index in Celsius (isFahreheit = false)
float hic = dht.computeHeatIndex(t, h, false);

Serial.print("Humidity: ");
Serial.print(h);
Serial.print(" %\t");
Serial.print("Temperature: ");
Serial.print(t);
Serial.print(" *C ");
Serial.print(f);
Serial.print(" *F\t");
Serial.print("Heat index: ");
Serial.print(hic);
Serial.print(" *C ");
Serial.print(hif);
Serial.println(" *F");
}

With the arduino ide 1.6.9 you will receive a compile error “magic key not defined”. You can download and use the hourly builds to solve this issue.

The serial output will look like this:

Humidity: 49.40 % Temperature: 25.00 *C 77.00 *F Heat index: 24.85 *C 76.72 *F
Humidity: 49.40 % Temperature: 25.00 *C 77.00 *F Heat index: 24.85 *C 76.72 *F
Humidity: 49.40 % Temperature: 25.00 *C 77.00 *F Heat index: 24.85 *C 76.72 *F
Humidity: 49.40 % Temperature: 24.90 *C 76.82 *F Heat index: 24.74 *C 76.52 *F
Humidity: 49.40 % Temperature: 25.00 *C 77.00 *F Heat index: 24.85 *C 76.72 *F

The fritzing parts you can download here:
LinkIt_Smart_7688_Duo_DHT22_diagrams

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