Apparatus need to prepare this:
- IC555
- Transistor PNP &NPN
- Piezzo electric sensor
- Loudspeaker
- Diode
- LED
- Resistor & capacitor
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)
senses the vibration generated on knocking a surface (such as a door or a table)
to activate the alarm. It uses readily available, 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.
When the circuit receives an input signal due to knocking, the alarm gets activated
for about 10 seconds. This is the time that capacitor C5 connected between
the emitter of transistor T4 and ground takes to discharge after a knock. The time
delay can be changed by changing the value of capacitor C5. After about 10 seconds,
the alarm is automatically reset.
The circuit operates off a 9V or a 12V battery eliminator. The proposed installation
of the knock alarm is shown in Fig. 2.
This circuit costs around Rs 75.
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.
When the circuit receives an input signal due to knocking, the alarm gets activated
for about 10 seconds. This is the time that capacitor C5 connected between
the emitter of transistor T4 and ground takes to discharge after a knock. The time
delay can be changed by changing the value of capacitor C5. After about 10 seconds,
the alarm is automatically reset.
The circuit operates off a 9V or a 12V battery eliminator. The proposed installation
of the knock alarm is shown in Fig. 2.
This circuit costs around Rs 75.
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