16-10-2012, 01:35 PM
KNOCK ALARM
[attachment=36504]
This circuit (Fig. 1), used in conjunction
with a thin piezoelectric plate,
senses the vibration generated on
knocking a surface (such as a door or a
table) to activate the alarm. It uses readilyavailable,
low-cost components and can
also be used to safeguard motor vehicles.
The piezoelectric plate is used as the sensor.
It is the same as used in ordinary
piezobuzzers and is easily available in the
market.
The piezoelectric plate can convert any
mechanical vibration into electrical variation.
As it doesn’t sense sound from a distance
like a microphone, it avoids false
triggering. The plate can be fixed on a door,
cash box, cupboard, etc using adhesive. A 1-
1.5m long, shielded wire is connected between
the sensor plate and the input of the
circuit. When someone knocks on the
door, the piezoelectric sensor generates
an electrical signal, which is amplified
by transistors T1 through T3.
The amplified signal is rectified
and filtered to produce a low-level
DC voltage, which is further amplified
by the remaining transistors. The
final output from the collector of pnp
transistor T6 is applied to reset pin 4
of 555 (IC1) that is wired as an
astable multivibrator. Whenever the collector
of transistor T6 goes high, the astable
multivibrator activates to sound an alarm
through the speaker. The value of resistor
R12 is chosen between 220 and 680 ohms
such that IC1 remains inactive in the absence
of any perceptible knock.
[attachment=36504]
This circuit (Fig. 1), used in conjunction
with a thin piezoelectric plate,
senses the vibration generated on
knocking a surface (such as a door or a
table) to activate the alarm. It uses readilyavailable,
low-cost components and can
also be used to safeguard motor vehicles.
The piezoelectric plate is used as the sensor.
It is the same as used in ordinary
piezobuzzers and is easily available in the
market.
The piezoelectric plate can convert any
mechanical vibration into electrical variation.
As it doesn’t sense sound from a distance
like a microphone, it avoids false
triggering. The plate can be fixed on a door,
cash box, cupboard, etc using adhesive. A 1-
1.5m long, shielded wire is connected between
the sensor plate and the input of the
circuit. When someone knocks on the
door, the piezoelectric sensor generates
an electrical signal, which is amplified
by transistors T1 through T3.
The amplified signal is rectified
and filtered to produce a low-level
DC voltage, which is further amplified
by the remaining transistors. The
final output from the collector of pnp
transistor T6 is applied to reset pin 4
of 555 (IC1) that is wired as an
astable multivibrator. Whenever the collector
of transistor T6 goes high, the astable
multivibrator activates to sound an alarm
through the speaker. The value of resistor
R12 is chosen between 220 and 680 ohms
such that IC1 remains inactive in the absence
of any perceptible knock.