Overview - Force Sensitive Resistor (FSR)
FSRs are sensors that allow you to detect physical pressure, squeezing and weight. They are simple to use and low cost. This is a photo of an FSR, specifically the Interlink 402 model. The 1/2" diameter round part is the sensitive bit.
The resistance of an FSR varies as the force on the sensor increases or decreases. When no pressure is being applied to the FSR, its resistance will be larger than 1MΩ. The harder you press on the sensor’s head, the lower the resistance between the two terminals drops. By combining the FSR with a static resistor to create a voltage divider, you can produce a variable voltage that can be read by a microcontroller’s analog-to-digital converter.
Simple demonstration of use