Raspberry Pi and Force Sensitive Resistor
I had an idea that involved a Force Sensitive Resistor (FSR).
Fortunately for me other people have used a FSR with Raspberry Pi and have provided really good information about getting it done.
In this article the person has a link to his code, BUT, the new version doesn't work. As the code is on GitHub you can access the first version - which is basic and does what you need.