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