Picture of Arduino for Garden, Greenhouse or Growbox: Humiditity and temperature
Fo rmeasuring of the humidity and temperature I used a cheap DHT11 module, It comes in various forms but though the bare module has 4 legs, it usually comes on a small PCB with only 3 pins, or 4 pins in which one is not connected. There are various libraries for the DHT11 to be used by the arduino. A good library however is the one from LadyAda.
Usually it is pretty well indicated what the supply lines and the signal lines are.
As some libraries presume the signal line is connected to arduino digital pin 2, that is what I have followed in this design
Note: after using it for a while, i found the DHT11 rather unreliable. The temperature can fluctuate 4 degrees between two subsequent reaadings and has a tendency to be too high. The DHT22 is a more accurate Sensor (so I am told),but it is more expensive.
i do not know how accurate the DHT11 is with regard to the humidity... but you could opt to keep the DHT11 but add a BMP180 for more accurate temperature. That gives you atmospheric pressure as well. It is an I2C sensor so it is easy to add.