This is the circuit diagram of seismic / vibration sensor. The circuit simulates a seismic sensor to detect vibrations/sounds. It’s really sensitive and may detect vibrations due to the motion of animals or human beings. So it could be utilized to monitor protected locations to restrict entry of undesirable individuals or animals.
The circuit applies readily available electronic parts and the circuit design is simple. A common piezo sensor is applied to detect vibrations/sounds because of pressure variations. The piezo element acts as a small capacitor which has a capacitance of just a few nanofarads. Like a capacitor, it could save charge when a electric power is applied to its terminals. It discharges via VR1, when it happens to be disturbed.
IC TLO71 (IC1) is wired as a differential amplifier with both its inverting and non-inverting inputs tied to the negative rail via a resistive network comprising R1, R2 and R3. Under idle situations (as adjusted by VR1), both of those inputs get nearly equal voltages, which keeps the output low.
TLO71 is a low-noise JFET input op-amp with low input bias and offset current. The BIFET technological innovation brings rapid slew rates. Capacitor C1 is applied in the circuit to maintain the differential input of IC1 for improved performance.
When the piezo element is disturbed (by even a slight motion), it discharges the stored charge. This alters the voltage value on the inputs of IC1 and therefore the output momentarily swings high as indicated by green LED1. This high output is put to use to trigger switching transistor T1, which triggers monostable IC2. The timing period period of IC2 is determined by R7 and C5. Considering the shown values, it will likely be about two minutes. The high output from IC2 activates T2 and then the buzzer begins beeping together with red light indication from LED2.
Assemble the seismic / vibration circuit on a typical PCB and enclose inside a appropriate cabinet. Connect the piezo element to the PCB applying single-core shielded wire. Enclose the piezo element inside a rustproof, compact aluminium box. The piezo element ought to be firmly glued to the enclosure facing the fine side towards the case. Fix the sensor assembly around the back side of a ceramic tile or granite tile with fine adhesive. Fix the tile (or bury it within the earth) close to the entrance considering the sensor assembly facing downwards. Whenever a pressure variation develops close to the sensor, the circuit is going to be activated.
Download the circuit in PDF file here:
» Seismic / Vibration Sensor