Strain Gauge Bending Sensor Module (BF350-3AA/1-.5AA)
 
Its key parts are a BF350 foil resistance strain gauge, an LM358 op-amp, and a TL431 precision shunt regulator. The BF350 strain gauge has a nominal resistance of 350Ω which varies when a force is applied. The module operates on 5VDC power supply and provides an analog output in 0 to 3.5V range.
The module is combination of a small foil resistance linear strain gauge (BF350), an excitation/reference voltage generator (TL431), and a strain gauge bridge amplifier (LM358). 
 
Strain gauge is an analog sensor whose resistance varies with applied force. The change in resistance is directly proportional to how much strain the sensor is experiencing due to the force applied. To measure the variation in resistance, the strain gauge is placed in a bridge configuration. This design uses a single-supply strain gauge bridge amplifier circuitry to handle the differential signal created by the change in resistance of the strain gauge.