HOUSE SECURITY SYSTEM LASER USED

Here is a low-cost, invisible laser
circuit to protect your house
from thieves or trespassers. A
laser pointer torch, which is easily available
in the market, can be used to operate
this device.
The block diagram of the unit shown
in Fig. 1 depicts the overall arrangement
for providing security to a house.
A laser torch powered by 3V powersupply
is used for generating a laser
beam. A combination of plain mirrors
M1 through M6 is used to direct the
laser beam around the house to form a
net. The laser beam is directed to finally
fall on an LDR that forms part of
the receiver unit as shown in Fig. 2.
Any interruption of the beam by a thief/
trespasser will result into energisation
of the alarm. The 3V power-supply circuit
is a conventional full-wave rectifier-
filter circuit. Any alarm unit that
operates on 230V AC can be connected
at the output.
The receiver
unit comprises
two identical
step-down transformers (X1 and X2), two
6V relays (RL1 and RL2), an LDR, a
transistor, and a few other passive components.
When switches S1 and S2 are
activated, transformer X1, followed by a
full-wave rectifier and smoothing capacitor
C1, drives relay RL1 through the
laser switch.
The laser beam should be aimed continuously
on LDR. As long as the laser
beam falls on LDR, transistor T1 remains
forward biased and relay RL1 is
thus in de-energised condition. When a
person crosses the line of laser beam,
relay RL1 turns on and transformer X2
gets power supply and RL2
energises. In this condition,
the laser beam will have no
effect on LDR and the alarm
will continue to operate as long
as switch S2 is on.
When the torch is switched
on, the pointed laser beam is
reflected from a definite point/
place on the periphery of the
house. Making use of a set of
properly oriented mirrors
one can form an invisible net
of laser rays as shown in the
block diagram. The final ray
should fall on LDR of the
circuit.
Note. LDR should be kept
in a long pipe to protect it from
other sources of light, and
its total distance from the
source may be kept limited to
500 metres.