Need a sensor to detect weight/force/pressure, Force Sensitive Resistor aka FSR is the component you are looking for. This FSR is model 402 by Interlink Electronics with the sensing region diameter 14.7mm.
FSR's are basically a resistor that changes its resistive value (in ohms Ω) depending on how much it's pressed. These sensors are fairly low cost, and easy to use but they're rarely accurate. They also vary some from sensor to sensor perhaps 10%. So basically when you use FSR's you should only expect to get ranges of response. While FSRs can detect weight, they're a bad choice for detecting exactly how many pounds of weight are on them.The model 402 FSR is a single-zone Force Sensing Resistor optimized for use in human touch control of electronic devices such as automotive electronics, medical systems, and in industrial and robotics applications. FSRs are two-wire devices. They are robust polymer thick film (PTF) sensors that exhibit a decrease in resistance with increase in force applied to the surface of the sensor. It has a 14.7mm diameter active area and is available in 4 connection options.