Chris writes in:
I have been experimenting with my Arduino and piezo buzzers as simple speakers – noisy and fun. But one question has been bugging me, what on earth is the third blue lead for on some piezos? If I leave the blue lead disconnected, the piezo seems to behave identically to its two-lead cousins. Am I missing out on some noise making opportunities?
Aha, good question. The short answer is no- the third lead is most likely used for feedback in an oscillator circuit, so leaving it disconnected shouldn’t affect your circuit. The long answer is, well, maybe, if want to make your piezo into a buzzer.
There are two kinds of piezoelectric devices that are commonly sold as piezos: buzzers and transducers. Though they both use the same kind of ceramic disc to make noise, the difference is in how they are controlled. A piezo buzzer already contains some circuitry to create a buzzing noise, so all you have to do to make it work is connect it to a power source. The buzz can range from a tolerable alert to signal that your clothes are dry, to the ear-splitting noise of a fire alarm. A piezo transducer works more like a speaker, where you have to feed it an audio signal to get it to make noise. This is what you are using if you are generating your own frequencies with your Arduino.
So what does this have to do with your question? Well, it turns out that a really simple way to make a piezo buzzer is to use the feedback electrode that you were talking about to make a Harley oscillator circuit. As an example, I found this schematic in Murata’s (a piezo manufacturer) Piezo Electric Sound Components Applications Manual:
The circuit is a little advanced, however the basic idea is that a small amount of the energy fed into the piezo device is fed into the input of the transistor, which amplifies the signal and feeds it back into the piezo. If the component values are chosen correctly, the resonation can be very efficient and loud, perfect for that fire alarm!
[title photo by Flickr user Josh Kopel]