This shock alarm circuit is built around LM358 IC, a dual-channel operational amplifier. When pressure is applied to the piezo sensor, the stored charge is sent into the inverting terminal of the LM358. The LM358 is configured as an inverting Schmitt Trigger. Voltage thresholds are set using the 10K resistors. When the output is high, the threshold voltage is 3.33V. When the output is low, the threshold voltage is 1.66V. The output retains its value as long as the input is between the two levels. This implies a memory effect.
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