Intruder Alarm

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A description with circuit design to build an intruder alarm.

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Project on department : Computer Organization laboratory
Performed by : Abhyuday Chattopadhyay (A – 02)
Debraj Dutta (A – 34)
Information Technology (IT – 1)

INTRODUCTION
As it is understood from the name itself, ‘intruder alarm’ is a security alarm, usually
installed at the door step to alarm the people inside of the fact that someone has crossed the
door. The security thing is maintained as it starts to ring when an intruder, may be a thief, is
at your doorstep.
CIRCUIT DESCRIPTION
This circuit of intruder alarm consists of two parts, one for the transmitter and the other
for the receiver.
The transmitter section is having a laser diode for the transmission issues with a supply
voltage of 18V, given from a DC supply (here two 9V DC batteries are used in series). A
LED, i.e. Light Emitting Diode, is used as an indicator of the circuit being on, connected
directly to the source. Different Resistors are used to control voltage, current as required.
The receiver section is having an LDR, i.e. Light Dependent Resistor, for sensor action. The
principle of LDR is such that: when light is incident upon the LDR, it has a very low
resistance, but if any obstruction appears in the pathway of the light rays from the source to
the LDR, the LDR will have a very high resistance value. The other components used in this
section are a J-K flipflop, three BJTs, i.e. Bipolar Junction Transistors, a Buzzer, two LEDs and
a Push-on switch and a Sliding-on switch. Different Resistors are used to control the
voltage, current as required. The power supply is a 9V DC source.
CIRCUIT DIAGRAM



WORKING PRINCIPLE
As the transmitter circuit power supply is switched on, the LED glows indicating the
circuit is ready for transmission action. The LD (laser diode) emits laser ray which falls
directly on the LDR in our second circuit. Let this circuit is also having its power supply on.
As mentioned earlier, as the light falls on the LDR, it will be having a very low resistance
value and hence the transistor T1 will conduct. The transistor T2 will be at cut-off.
If now the path of the laser is obstructed, our LDR instantaneously will have a very high
resistance value, which can be assumed to be equivalent to an open-circuit condition. This
gives rise to a condition where T1 goes to cut-off and T2 starts conducting. The flipflop will
therefore get a logic 0 at its clock input, giving an enable pulse to it. The J input is at logic 1
and K at logic 0. Therefore with the low clock pulse the Q output of the flipflop will turn
high or logic 1. At this output pin we have our third transistor T3 which is now ready to
conduct. It has the buzzer and a LED connected with it at the collector end. As T3 conducts,
the buzzer starts producing an alarming sound and the LED glows. There is a push-on switch
connected to the reset pin of the flipflop. Pressing it will reset the circuit to its initial
condition.
PRACTICAL APPLICATION
We can have our alarm fixed on two sides of the main entrance of a room in such a way
that whenever the door is pushed the laser beam is obstructed. Initially the door is closed
and the laser beam falls straight on the LDR. Now if someone pushes the door to enter the
room, the laser is obstructed and it does not reach the receiver circuit resulting in an alarm
action.
CONCLUSION
This project was just a model of what can be made in practice as an intruder alarm. But
there is a drawback of this type of circuit. This works at the visible range of light. That is
why if any how there is another light source other than the laser in the transmitter portion,
the LDR would not be interrupted from getting light. If that light is strong enough, it can
even stop the resistance of the LDR being increased to an extent it is required for the circuit
to work properly. For a better result, may be infrared ray would be a better choice for the
transmission action.

Approved by :

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