25-08-2012, 04:20 PM
Door knock alarm with timer
General purpose alarm.docx (Size: 396.49 KB / Downloads: 83)
ABSTRACT:-
The alarm may be used in many ways, such as a sudden fear or distressing suspense due to awareness of danger. It is an automatic device that serves to warn of danger, as fire, to arouse someone from sleep, or to call attention to a particular thing.
This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface. The piezoelectric element plate is fixed at the centre of the door wing by using a cello tape. Apply a small quantity of adhesive at the edges between the plate and the door. Extend wires about 1-1.5 metres from the piezoelectric to the circuit. IC NE555 (IC1) is configured in monostable mode. When it gets an input pulse its output goes high for a period set by VR1, resistor R5 and capacitor C3. IC UM66 (IC2) is used as a melody generator. When the door is knocked at, the piezo plate generates an input pulse, which is amplified by transistor T1.
Introduction:-
This circuit generates an alarm sensing the vibration generated on knocking a surface. In some apartment buildings and class rooms that are not equipped with a doorbell, it may be hard to hear someone knocking on the front door. This circuit provides a means to activate a sound inside, whenever someone knocks on the door from outside. It uses readily available, low cost components whereas almost all commercial devices sold use a more expensive and power consuming circuit.
Circuit operation:
Here a piezoelectric plate is used as the sensor. It can convert any mechanical vibration into electrical signal. The piezoelectric sensor plate is fixed at the centre of the door wing by using a cello tape. A small quantity of adhesive is applied at the edges between the plate and the door. Wires are extended from piezoelectric to the circuit.
NE 555 IC is configured in monostable mode. When it gets an input pulse, its output goes high for a period set by VR1, resistor R5 and capacitor C. When door is knocked at, the piezoelectric plate generates an input pulse, which is amplified by transistor T1. The amplified signal triggers the IC 555 and its output pin 3 goes high to enable the buzzer. After the set time period, the alarm stops. The circuit works at 5v-12v dc supply. The alarm time can be adjusted through VR1 by changing the RC time constant of the timer.
Conclusion:-
Using a melody generator IC, like UM66, at the output of timer we can hear a melodious music when someone knocks the door. This circuit uses a piezoelectric plate to sense any mechanical vibration. Hence it avoids false triggering. The plate can be fixed on a door, cash box, cupboard etc using adhesive. With some small modifications this can also be used as burglar alarms and to safeguard motor vehicles. If a burglar touches the outside door knob this circuit will instantly emit a loud alarm to scare him away and alert you of the attempted entry.