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.