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The Intruder Alarm automatically activates a loud alarm whenever any unauthorized entry is attempted. Unlike other alarms, the Intruder Alarm circuitry does not need replacement of a broken loop. The alarm can be turned OFF just by the simple flick of a switch. The unit power on indication by LED is also provided here for better monitoring.
Build yourself a simple intruder alarm circuit to keep the thieves at bay!! No. of ICs used: 2
Things which you will Learn :
*Learn how to use LDR sensor.
*Application of basic gates and flip-flop and hex inverters.
The Intruder Alarm circuit should be typically fitted to the door of your house, to sound an alarm when anyone pushes the door. This way it alarms you against thieves or intruders. The circuit primarily comprises a transmitter and receiver pair. The transmitter is fitted onto the inside of the door frame while the receiver is fitted to the door panel.
The transmitter section consists of a laser diode which is powered by a 9V battery. When the switch (S1) in the transmitter section is closed, the laser diode glows. Closing switch (S2) provides power supply to the receiver section. Light falling from the laser diode on the light-dependent resistor (LDR) in the receiver section provides base current to transistor T1 and it starts conducting. This grounds the base of the transistor (T2), so it doesn’t conduct and the alarm remains off. When somebody pushes the door, light incident on the LDR is interrupted and the transistor (T1) stops conducting since the LDR offers a high resistance in the absence of light. Transistor T2 receives base current and starts conducting. The pulse from the emitter of transistor T2 is connected to the inputs of AND gate N1 (IC1). The high output of AND gate is connected to a JK flip-flop (IC2) that works as a latch. As a result, output pin 12 (Q1) of IC2 goes high to cause conduction of transistor T3 and consequent sounding of the alarm. The alarm can be turned off by switch S2.
Note: While testing at EFY Lab, a laser torch was used in place of the transmitter.