I was looking through my parts bin and stumbled upon the piezo. The book says that it is “A small element that vibrates when it receives electricity. When it moves, it displaces air around it, creating sound waves.”
This sounds a lot like a speaker to me, but I heard in lab that a piezo can be used as an input sensor as well as for output (which I think a speaker can do as well, however most speaker cones are probably too heavy to be useful as a microphone).
Just for fun I decided to build the Arduino Light Theremin project. It was fairly straightforward and required a photoresistor as a light sensor to control the piezo. Here is the completed circuit:
Next, I followed the book’s code example and setup my Arduino code. The program will first calibrate the light sensor during the first 5 seconds of plugging it in. Then, it uses these calibrated values to control the piezo’s output on digital pin 8.