WiFi - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 180

Figure 10-1: The DHT11
This application uses an Arduino Uno and a SainSmart wireless shield.
A DHT11 sensor will be connected to the board, allowing the user to get an
accurate reading. Because I'm terrible with plants, I probably won't check the
reading frequently, so this device must communicate with the outside world
to send alerts. It will monitor the humidity of the dirt and send e-mails when
the humidity level drops below a certain level. For this, it must be connected
to the Internet. Because I don't have a wired access point nearby, I'll be using
a wireless network.
This project requires a certain number of services to be put in place. First, it
requires a DHCP server on the current network. Most Internet modems have their
own DHCP server, so it should be compatible with most wireless access points.
Secondly, it requires access to an SMTP server, a server used to send e-mail.
Most Internet providers give you access to an e-mail server, but they may refuse
e-mail that does not come from their network. Your Internet provider or e-mail
service provider can give you information on how to access its mail servers.
The DHT11 is an interesting component in that it uses only one wire for
communication. The Arduino is able to switch between input and output, so
that isn't a problem.