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KNOCK ALARM

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					 CIRCUIT IDEAS




KNOCK ALARM                                                       SUN
                                                                        IL KU
                                                                                MAR




PRADEEP G.                                       circuit. When someone knocks on the
                                                 door, the piezoelectric sensor gener-


T
        his circuit (Fig. 1), used in conjunc-   ates an electrical signal, which is am-
       tion with a thin piezoelectric plate,     plified by transistors T1 through T3.
       senses the vibration generated on              The amplified signal is rectified
knocking a surface (such as a door or a          and filtered to produce a low-level
table) to activate the alarm. It uses readily-   DC voltage, which is further ampli-
available, low-cost components and can           fied by the remaining transistors. The
also be used to safeguard motor vehicles.        final output from the collector of pnp
The piezoelectric plate is used as the sen-      transistor T6 is applied to reset pin 4
sor. It is the same as used in ordinary          of 555 (IC1) that is wired as an Fig. 2: Proposed installation of knock alarm




Fig. 1: The circuit of knock alarm
piezobuzzers and is easily available in the      astable multivibrator. Whenever the col-         time that capacitor C5 connected between
market.                                          lector of transistor T6 goes high, the astable   the emitter of transistor T4 and ground
    The piezoelectric plate can convert any      multivibrator activates to sound an alarm        takes to discharge after a knock. The time
mechanical vibration into electrical varia-      through the speaker. The value of resistor       delay can be changed by changing the
tion. As it doesn’t sense sound from a dis-      R12 is chosen between 220 and 680 ohms           value of capacitor C5. After about 10 sec-
tance like a microphone, it avoids false         such that IC1 remains inactive in the ab-        onds, the alarm is automatically reset.
triggering. The plate can be fixed on a door,    sence of any perceptible knock.                      The circuit operates off a 9V or a 12V
cash box, cupboard, etc using adhesive. A 1-         When the circuit receives an input sig-      battery eliminator. The proposed installa-
1.5m long, shielded wire is connected be-        nal due to knocking, the alarm gets acti-        tion of the knock alarm is shown in Fig. 2.
tween the sensor plate and the input of the      vated for about 10 seconds. This is the              This circuit costs around Rs 75.




      ELECTRONICS FOR YOU   NOVEMBER 2002

				
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Description: mini project for engineering students