Touch (knock) sensor with Piezo
If you want to make a simple touch sensor but you dont have the fancy ones around you can also use a piezo element, which come with a lot of basic arduino starter sets.
What you need:
piezo element

breadboard


resistor 1 M ohm

piece of paper tape
piece of paper tape
some male/male jumper wires
This video explains it all. To make the input visual, open the serial Monitor from the Tools menu.