Use this simple Force Sensitive Resistor to measure how much pressure is being applied. This is a small Force Sensitive Resistor with a active sensing area of 0.5" i.e. 13 mm diameter.
The output resistance of the FSR Sensor is inversely proportional to the pressure being applied to the active area. The more the pressure the lesser the resistance. When no pressure is applied to the sensor the output resistance is more than 1 Mohm. And when maximum pressure is applied the resistance is 2.5 Kohm. This FSR can sense applied force anywhere in the range of 100g-10kg.
This is a breadboard friendly sensor i.e. it can be connected directly on the bread board. The output pins of the sensor has 0.1" i.e. 2.5 mm pitch.