The Pi is a fine little computer board, though not nearly as good as the Arduino when it comes to I/O capabilities. The beautifully-engineered Gertboard is a plug-in add-on that neatly overcomes this deficiency by giving the Pi access to an ATMega328, but it's a very complex and expensive solution. An alternative would be to interface an Arduino running at 3.3 volts to the Pi, though this is easier said than done. But, we won't let that stop us, will we? You can buy a prebuilt commercial version of something similar to this project. It's called an AlaMode, and it goes for something like $50. Our version will cost about a third of that to build. As a first step, we will build an Arduino plug-in board for the Pi. It is customary to call Pi boards "plates," but indulge me and permit me to name this particular board a hoody. Thank you. We will be using generic stripboard to build our project. Small (3-3/4" x 2-1/16") boards can be obtained on eBay for a bit over a dollar apiece. It is also possible to use something like a Radio Shack 276-168 protoboard ($3.49). The main component, though, is an 3.3-volt Arduino Pro Mini. This will permit connecting directly to the Raspberry Pi ports and other 3.3-volt devices without having to do level shifting.