In this schematic, a piezo is the sensor. Piezos generate voltage when physically bent or deformed, the the foltage is in the millivolt range. The direction that the piezo is deformed determines the polarity: bend it one way, get a positive voltage. Bend it the other way, get a negative voltage.
In this circuit, the piezo is put through a full-wave rectifier bridge (the four diodes) to make its voltage always positive. The output of the bridge is sent into one of the LM358's amplifiers that's configured as a voltage summing amp. The output of that amp is then fed into the other amp on the LM358 that's configured as a DC voltage gain amp. The output from the second amp is approximately 0.2 - 3.0 V DC.
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