Archive | March, 2015

Arduino Weather Station

21 Mar

After not doing any Arduino fiddling for quite a while I have finally started a small project again. I have made a small weather station for measuring temperature and humidity levels in my home.

IMAG0092_2

The setup is based on an Arduino Uno with the standard Ethernet Shield (because it has an SD card reader) and a DHT-22 sensor that is connected as follows (following the docs):

  • Leg 1 (from the left) –> +5V
  • Leg 2 –> pin 2 (data input)
  • Leg 3 –> not connected
  • Leg 4 –> GND
  • 10k OHM resistor between +5V and data pin 2.

The code is pretty standard, a mixture of the SD library and DHT library code examples …  not really interesting. However, I have added a bit of code to calculate the dew point based on formulas I found on the web, which may be worth sharing here:

// Compute dew point (formula taken from here: http://www.wettermail.de/wetter/feuchte.html
// You need to include the math.h library

float temperature = 20.0; // Set to current temperature in *C (>= 0).
float humidity = 50.0; // Set to current % of relative humidity.

float a = 7.5; // For temperatures >= 0 *C.
float b = 237.3; // For temperatures >= 0 *C.

float sdd = 6.1078 * pow(10, (a * temperature) / (b + temperature));
float dd = humidity / 100 * sdd;
float v = log10(dd / 6.1078);

float dewpoint = b * v / (a – v);

So far the setup seems to work well, but the sensor readings regarding the temperature look a bit high (maybe influenced by the Arduino which gets warm over time … its probably better to place the sensor  away from the board, and not right on top of it).

Edit: In case you are interested, you can find the code of the project here: https://bitbucket.org/snippets/boden/xr746 (without the dewpoint computation).

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