I was testing piezo electric speakers for use with a product at work. I have researched touch probe technologies in the past and was vaguely aware of how sensitive they are. As I had an oscilloscope to hand so I decided to see how sensitive they were and was amazingly surprised by how ridiculous even the cheapest of piezo speakers are. The fundamental physical principle of piezo electric crystals is that a voltage is generated whenever the crystal experiences shear forces. The reason that they can be used as speakers is that this can also work in reverse, such that an applied voltage will change the shape of the actuator which emits an audible sound wave.
This is a trace of the voltage measured at the piezo by my Rigol DS1054Z. This is a typical trace for a direct tap on the piezo but its difficult to make repeatable. Note the vertical scale of 2V per division which shows the piezo is capable of huge output voltage which bodes well for this project. The horizontal scale is 2ms per division which means the response is fast and very low power.
Its starting to look like we might be able to make this work. Now we need to move on to looking at electronic detection methods.