You can create sound with a gliding frequency that you control using the 3 touchpads (thumbtacks)
It is a very simple circuit with quite some limitations but you can build it fast and have some fun using it. Apart from the Arduino you only need a breadboard, 3 thumbtacks, a 10nF capacitor, a 470 ohm resistor and a loudspeaker.
Operation:
The middle thumbtack is the touchsensor and is connected to the capacitor, the voltage on the capacitor is measured with an analog input pin on the Arduino. The left thumbtack is connected to ground, the right thumbtack is connected to +5V. If you put one finger on the middle pad you can charge or discharge the capacitor by touching the right or left pad. In the software the resulting voltage is translated in a tone.
