Ethernet Shield ($45)
The official shield from the makers of Arduino, the Ethernet shield is a great way to make your Arduino project work independently of the computer while still maintaining network contact to communicate with the world. Another interesting addition to this board is a microSD card slot; so if your project needs large data files like videos or mp3s, or just templates for your led cube, you can store them there.
You can even host a website that you can use to present information about your project’s sensors to the world.
Before buying, be careful: Ethernet shields depend on versions . I bought the v3 only to find it didn’t fit on my Uno v2 board — two extra pins were added to the v3 Uno. Since the Ethernet Shield is actually more expensive than the Uno, if you find yourself in this situation then the easiest solution is to just buy a new Uno to use with the Ethernet and then keep the old board for other projects.
Or you can just check the version number on the back of the board before you buy, but where’s the fun in that?
Shield with 4 relays ($20)
Now you can actually turn on the kettle over the Internet!
Relays are a key component in many Arduino home automation projects, simply because they allow higher voltage circuits, such as mains-powered devices, to be turned on and off—electronically isolating one circuit from another. You’ve probably already played with one relay in your starter kit, and you’ll know that you need a few other components, such as a transistor or an optocoupler and a diode. If you need more than one relay, your breadboard will load very quickly.
This 4 relay board solves by neatly packing 4 sets of relays and necessary bits on one board. Using 4 IO pins, you simply pull the HIGH pin to switch the corresponding relay. Each relay will carry up to 3 amps, although you can of course use a low power relay as a replacement for the switch. This particular shield can also be connected to an RFBee instead of an Arduino for radio control.