Metal Earthquake and Vibration Alarm, Shape : Rectangular

Earthquake/Vibration Alarm converts mechanical vibration into electrical vibration and gives alarm. The Piezo electric sensor is used to sense any kind of vibration /jerk. It produces small ac signal when activated by a jerk or vibration. Since it is very sensitive to any kind of vibration it can serve as earthquake alarm also. A piezo electrical sensor is available in market in the form of a round disc. It has two terminals for output. When we want to use it, it can be placed on the surface or can be adhered to the surface with the help of cello tape or a general purpose chemical adhesive. When the surface encounters any vibrations, these are transferred to this piezo sensor and it produces a signal.
About The Circuit :
A simple circuit is built around a 555 timer IC. The output from sensor is fed to a transistor T1. Capacitor C1 and Resistor R1 act as fixed bias to the transistor T1. T1 amplifies the signal and delivers amplified output at the collector. This output is again fed to another transistor through ac coupling in order to further amplify the signal. Since T1 is weekly biased, it automatically rectifies the ac signal and only pulsating dc waveform is amplified and delivered by T2. Output of this T2 is fed to trigger input of IC555 through capacitor C3. Capacitor C3 differentiates the rising and falling edge of the signal produced at the collector of T2. The positive going pulse will not have any effect on trigger input. However negative going pulse will trigger the timer and produce HI state at its output. This HI state will glow LED D1. Also buzzer starts sounding when the sensor signal is detected. The 555 configuration is a monostable flipflop and the signal goes on retriggering this monostable. Therefore output is continuously available as long as signal is present. VR1 sets the sensitivity of the circuit and VR2 sets the timing of the monostable. If the trigger pulses which triggers the mono are  generated at delayed intervals, the output may not be continuous. TO make it continuous at even the lowest frequency of signal, VR2 should be set. The circuit can be powered either from 9 v battery or from mains through eliminator.