Piezo Ceramic Disc Elements - Various Sizes - 10 Pack
Now available from PMD Way are piezo ceramic disc elements in various sizes. Use the options menu to select your required diameter. Sold in packs of ten.
You can use them to create sound - just give them 5V and a PWM signal. Or they can be used as a knock sensor.
Either way, lots of fun on the cheap. Just solder a wire to the outside ring and one to the inner section - you can see how this is done in the following video that shows how they can generate tiny amounts of energy: