The sensor which I am looking at is a Force Sensing Resistor (FSR) from Interlink Electronics,

Original report: Shagun Singh The part number of the model is 402 and it is 0.5" in diameter. The approximate thickness of the sensor is around 0.018". The FSR is a polymer thick film (PTF) device which exhibits a decrease in resistance with an increase in the force applied to the active surface.

Applications

The sensor allows one to detect and measure the change in the applied force and also the rate at which the force is changing. It could detect contact or touch. Identify force thresholds and trigger actions.Its force sensitivity is optimized for use in human touch control of electronic devices.The FSR sensors have wide usage in the commercial and industry arena. The Applications, range from medical, automotive to recreational and Industrial.

Electrical Characteristics

Click for Datasheet,

At the low force end of the force-resistance characteristic, a switch like response is evident. This turn-onthreshold, or break force, that swings the resistance from greater than 100 kohms to about 10 kohms (the beginning of the dynamic range that follows a power-law) is determined by the substrate and overlay thickness and flexibility, size and shape of the actuator, and spacer-adhesive thickness (the gap between the facing conductive elements). Break force increases with increasing substrate and overlay rigidity, actuator size, and spacer adhesive thickness.

At the high force end of the dynamic range, the response deviates from the power-law behavior, and eventually saturates to a point where increases in force yield little or no decrease in resistance. A saturation point is reached.

There are two two solder tabs on the sensor. One goes to the microcontroller and the other goes to +5V, and a fixed resistor to ground. The FSR feeds a varying voltage to the microcontroller. The fixed resistor provides a path to ground. The current will follow the path of least resistance, from the 5V source through the FSR to the microcontroller.

Click here for the user guide and more electrical interfaces with FSR's