When building Arduino projects for beginners, you'll usually need both input and output sensors. However, you can come across some simple projects like the Blink example that only require an LED ...

You can also just jam wire into the connector, but that’s not always robust. The controllers use 3.3V which isn’t unusual these days. There’s an available library that makes reading them easy.

The AtoD has a high input impedance/low capacitabce, but reading the AtoD should use up some of the charge on the wire. By my calculations the AtoD on an Arduino maxes out at about 400,000 samples ...