Seismic Sensor Circuit

This circuit is a very sensitive seismic sensor that detects vibrations/sounds. It can detect seismic vibration or sounds caused by animals or humans. So it can be used to monitor protected areas to restrict entry of unwanted persons or animals.

Circuit Diagram Seismic Sensorof : Click  to enlarge

This circuit is composed of readily available components and the design is very simple. A standard piezo sensor is used to detect vibrations/sounds due to pressure changes. The piezo element acts as a small capacitor having a capacitance of a few nanofarads. Like a capacitor, it can store charge when a potential is applied to its terminals. It discharges through VR1, when it is disturbed. In the circuit, IC TLO71 (IC1) is wired as a differential amplifier with both its inverting and non-inverting inputs tied to the negative rail through a resistive network comprising R1, R2 and R3.

At normal conditions, the difference in input voltages is zero so that the output is low. TLO71 is a low-noise JFET input op-amp with low input bias and offset current. TLO71 is a low-noise JFET input op-amp with low input bias and offset current. The BIFET technology provides fast slew rates. Capacitor C1 is provided in the circuit to keep the differential input of IC1 for better performance.

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 IC1 and the output momentarily swings high as indicated by green LED1. This high output is used to trigger switching transistor T1, which triggers monostable IC  NE555. The timing period of IC2 is determined by R7 and C5. With the shown values, it will be around two minutes. The high output from IC2 activates T2 and the buzzer starts beeping along with red light indication from LED2.

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