The frequency generated by the capacitance measuring circuit is between 1 and 10 kHz for the capacitances that we are measuring. At the AP Summer Institute, Patrick Polley suggested driving a speaker with this circuit. I had some time after school yesterday, so I added an NPN transistor to the circuit since the 555 doesn’t provide enough current to drive a speaker. Today at the start of class, I shared the modified circuit with my students. As students pressed on the parallel plate capacitor, the generated tone changed; it became a simple musical instrument!