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I
n this circuit, the alarm will be switched on under the following
four different conditions: 1. When light falls on LDR1 (at the
entry to the premises). 2. When light falling on LDR2 is
obstructed. 3. When door switches are opened or a wire is broken.
4. When a handle is touched. The light dependent resistor LDR1
should be placed in darkness near the door lock or handle etc. If
an intruder flashes his torch, its light will fall on LDR1,
reducing the voltage drop across it and so also the voltage
applied to trigger 1 (pin 6) of IC1. Thus transistor T2 will get
forward biased and relay RL1 energise and operate the alarm.
Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2
may be placed on one side of a corridor such that the beam of
light from a light source always falls on it. When an intruder
passes through the corridor, his shadow falls on LDR2. As a result
voltage drop across LDR2 increases and pin 8 of IC1 goes low while
output pin 9 of IC1 goes high. Transistor T2 gets switched on and
the relay operates to set the alarm. The sensitivity of LDR2 can
be adjusted by varying potentiometer VR2. A long but very thin
wire may be connected between the points A and B or C and D across
a window or a door. This long wire may even be used to lock or tie
something. If anyone cuts or breaks this wire, the alarm will be
switched on as pin 8 or 6 will go low. In place of the wire
between points A and B or C and D door switches can be connected.
These switches should be fixed on the door in such a way that when
the door is closed the switch gets closed and when the door is
open the switch remains open. If the switches or wire, are not
used between these points, the points should be shorted. With the
help of a wire, connect the touch point (P) with the handle of a
door or some other suitable object made of conducting material.
When one touches this handle or the other connected object, pin 6
of IC1 goes ‘low’. So the alarm and the relay gets switched on.
Remember that the object connected to this touch point should be
well insulated from ground. For good touch action, potentiometer
VR3 should be properly adjusted. If potentiometer VR3 tapping is
held more towards ground, the alarm will get switched on even
without touching. In such a situation, the tapping should be
raised. But the tapping point should not be raised too much as the
touch action would then vanish. When you vary potentiometer VR1,
re-adjust the sensitivity of the touch point with the help of
potentiometer VR3 properly. If the alarm has a voltage rating of
other than 6V (more than 6V), or if it draws a high current (more
than 150 mA), connect it through the relay points as shown by the
dotted lines. As a burglar alarm, battery backup is necessary for
this circuit. Note: Electric sparking in the vicinity of this
circuit may cause false triggering of the circuit. To avoid this
adjust potentiometer VR3 properly.
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