C I R C U I T I D E A S

ELECTRONICS FOR YOU NOVEMBER 2002

This circuit (Fig. 1), used in conjunc-tion with a thin piezoelectric plate,senses the vibration generated on

knocking a surface (such as a door or atable) to activate the alarm. It uses readily-available, low-cost components and canalso be used to safeguard motor vehicles.The piezoelectric plate is used as the sen-sor. It is the same as used in ordinary

piezobuzzers and is easily available in themarket.

The piezoelectric plate can convert anymechanical vibration into electrical varia-tion. As it doesn’t sense sound from a dis-tance like a microphone, it avoids falsetriggering. The plate can be fixed on a door,cash box, cupboard, etc using adhesive. A 1-1.5m long, shielded wire is connected be-tween the sensor plate and the input of the

KNOCK ALARM

PRADEEP G.

SUNIL KUMAR

circuit. When someone knocks on thedoor, the piezoelectric sensor gener-ates an electrical signal, which is am-plified by transistors T1 through T3.

The amplified signal is rectifiedand filtered to produce a low-levelDC voltage, which is further ampli-fied by the remaining transistors. Thefinal output from the collector of pnptransistor T6 is applied to reset pin 4of 555 (IC1) that is wired as an

astable multivibrator. Whenever the col-lector of transistor T6 goes high, the astablemultivibrator activates to sound an alarmthrough the speaker. The value of resistorR12 is chosen between 220 and 680 ohmssuch that IC1 remains inactive in the ab-sence of any perceptible knock.

When the circuit receives an input sig-nal due to knocking, the alarm gets acti-vated for about 10 seconds. This is the

Fig. 1: The circuit of knock alarm

Fig. 2: Proposed installation of knock alarm

time that capacitor C5 connected betweenthe emitter of transistor T4 and groundtakes to discharge after a knock. The timedelay can be changed by changing thevalue of capacitor C5. After about 10 sec-onds, the alarm is automatically reset.

The circuit operates off a 9V or a 12Vbattery eliminator. The proposed installa-tion of the knock alarm is shown in Fig. 2.

This circuit costs around Rs 75.