Simple Knock Alarm With Piezo Sensor

This circuit uses a thin
piezoelectric sensor to sense the vibrations generated by knocking on a
surface; eg, a door or table. Basically, it amplifies and processes the
signal from the sensor and sounds an alarm for a preset period. In
operation, the piezoelectric sensor converts mechanical vibration into
an electrical signal. This sensor can be attached to a door, a cash box,
cupboard, etc using adhesive. A 1-1.5m long shielded cable can then be
connected between the sensor plate and the input of the circuit. The
signal generated by the sensor is amplified by transistors Q1-Q3 which
are wired as common-emitter amplifiers.

Simple Knock Alarm Circuit

Simple Knock Alarm Circuit Diagram

The signal is then rectified by diode D1 and amplified by
transistors Q4-Q6. As shown, the output from Q6’s collector is fed to
pin 4 (reset) of 555 timer IC1. This is wired as an astable
multivibrator. Each time Q6 turns on, its collector goes high and IC1
activates and produces an alarm tone in the speaker. The alarm
automatically turns off 10s after knocking ceases – ie, the time taken
for the 22µF capacitor on Q4’s emitter to discharge. Finally, note that
it may be necessary to adjust the 470O resistor in Q6’s collector
circuit to ensure that IC1 remains off in the absence of any perceptible
knock. A value somewhere between 220O and 680O should be suitable.

Author: Raj. K. Gorkhali – Copyright: Silicon Chip Electronics

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