You can use this laser-based
intruder alarm conveniently
at the entrance of a hall or any
other similar location to protect the
same from unauthorised access as and
 |
| Fig 1 |
when desired. Its range can be extended
further to protect the perimeter of a
building from unauthorised intrusion
by using a cascade arrangement of
multiple alarms.
The circuit consists of a transmitter
and a receiver.
The transmitter circuit (shown in
Fig. 1) is nothing but a laser diode
(LD1) driven by a 9V PP3 battery. The
output of IC1 is regulated 5V as long as
its input remains equal to or more than
7.5V, thus ensuring a constant drive
current for the laser diode. The battery
(Batt1) is connected to the circuit
through switch S1.
The laser diode (LD1) can be
replaced with a laser pointer (torch)
 |
| Fig 2 |
pointer itself can be used as a transmitter.
The pointer has in-built series
resistance, on/off switch and battery.
The receiver circuit (shown in Fig.
2) is basically a current-to-voltage converter
built around IC LM356 (IC3).
The output of IC3 is fed to the monostable
built around 555 timer (IC4).
The high output of the monostable
drives the piezobuzzer to sound an audio alarm
The receiver section operates off 5V
DC generated from another 9V battery
and voltage regulator IC 7805 (IC2).
The battery (Batt.2) is connected to the
circuit through switch S2.
When the laser light transmitted
through LD1 falls on phototransistor
T1, the output of the op-amp (IC3) at
its pin 6 remains high. In this condition,
the output of IC4 remains low
and the buzzer does not sound.
When the laser beam falling on
phototransistor T1 is interrupted by
someone, the output of op-amp IC3
goes low and IC3 produces a pulse.
This pulse triggers monostable IC4
and its output goes high to sound
the alarm for a time period of about
R8xC8.
Assemble the transmitter and
receiver circuits on separate generalpurpose
PCBs and enclose in suitable
cabinets. Mount the transmitter and
receiver units on opposite pillars of the
entrance, aligning the two such that
the laser beam from the transmitter
directly falls on the phototransistor.
Block the laser beam with your hand
and measure the op-amp output. It
should not be low. At pin 3 of IC4, you
should get a positive-going pulse of
one-second duration beginning with
high-to-low edge of the trigger pulse
appearing at pin 2 of IC4 or collector
of transistor T2.
super
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