Simple Digital Security System

You can use this simple and reliable
security system as a watchdog
by installing the sensing
loops around your building. You have
to stretch the loop wires two feet above
the ground to sense the unauthorised
entry into your premises.
Wire loops 1, 2 and 4 are connected
to the A, B and C inputs of 7-segment
decoder 4511 (IC1), respectively, while
the D input of IC1 is grounded permanently.
The loops are also connected to
a dual 3-input NOR gate and inverter
CD4000 (IC2) to activate the alarm.
Fig. 1 shows the circuit of the digital
security system, while Fig. 2 shows
the proposed wiring diagram for the
loops around the premises. Before using
this security system, make sure
that loops shown in Fig. 2 are con-
�� PHERDAUS ISLAM nected as shown in Fig. 1. If you don’t
want to use a buzzer, switch it off by
opening switch S2.
The circuit works off a 9V regulated
power supply. However, battery
back-up is recommended. A commoncathode,
7-segment display (LTS543)
is used for displaying whether the
loops are intact or not.
If loop 1 is broken, the display will
show ‘1’. If two or all the three loops
are broken, the display will show the
sum of the respective broken loop