Here is a simple shock sensitive alarm circuit that has many applications from home to automobiles. The main application of this circuit is to use it as an anti-theft alarm in automobiles.
A piezo electric sensor is used as the shock sensor which has to mounted on the door which you have to protect.
IC1 = LM358
R1 = 15K
VR2 = 22K
R3 = 1M
S1 = SPST Switch
D1 = 1N4001
K1 = Piezo buzzer
B1 = Battery 3-6volt
Here the IC1 LM3558 is wired as an inverting Schmitt Trigger. The POT R1 sets the threshold voltage of the circuit. R1 is used as a feedback resistor.
When not activated the output from the piezo sensor will be low and so do the output of the IC. When the piezo sensor is activated its output voltage goes high and triggers the Schmitt trigger.
This results the beeping of the buzzer. The buzzer remains beeping for some time even if the vibration is removed. This is because the increase in the inverting input has little effect when the IC is triggered and the state can’t be easily reversed.