This circuit simulates a Piezoelectric sensor to detect vibrations/sounds. It is very sensitive and can detect vibrations caused by the movement of animals or human beings. This Seismic Sensor can be used to monitor protected areas to restrict the entry of unwanted persons or animals. This high output is used to trigger the switching transistor, which triggers mono-stable IC 555. The timing period of IC 555 is determined by resistance and capacitance value.

Seismic Sensor Circuit IC TLO71 is wired as a differential amplifier with both its inverting and non-inverting inputs tied to the negative rail through a resistive network comprising both the inputs receive almost equal voltages, which keeps the output low. When the piezo element is disturbed (by even a slight movement), it discharges the stored charge. This alters the voltage level at the inputs of IC and the output momentarily swings high as indicated by a green LED.