20-04-2012, 12:46 PM
ANTI-BAG-SNATCHING ALARM
CI-02-anti-bag.pdf (Size: 67.36 KB / Downloads: 39)
Here is a simple alarm circuit to
thwart snatching of your valuables
while travelling. The circuit kept
in your bag or suitcase sounds a loud
alarm, simulating a police horn, if someone
attempts to snatch your bag or suitcase.
This will draw the attention of other
passengers and the burglar can be caught
red handed.
In the standby mode, the circuit is
locked by a plug and socket arrangement
(a mono plug with shorted leads plugged
into the mono-jack socket of the unit).
When the burglar tries to snatch the bag,
the plug detaches from the unit’s socket
to activate the alarm.
The circuit is designed around op-amp
IC CA3140 (IC1), which is configured as
a comparator. The non-inverting input
(pin 3) of IC1 is kept at half the supply
voltage (around 4.5V) by the potential
divider comprising resistors R2 and R3 of
100 kilo-ohms each. The inverting input
(pin 2) of IC1 is kept low through the
shorted plug at the socket. As a result,
the voltage at the non-inverting input is
higher than at the inverting input and the
output of IC1 is high.
The output from pin 6 of IC1 is fed to
trigger pin 2 of IC NE555 (IC2) via coupling
capacitor C1 (0.0047 μF). IC2 is configured
as a monostable. Its trigger pin 2 is held
high by resistor R4 (10 kilo-ohms). Normally,
the output of IC2 remains low and
the alarm is off. Resistor R6, along with
capacitor C3 connected to reset pin 4 of
IC2, prevents any false triggering. Resistor
R5 (10 mega-ohms), preset VR (10 megaohms)
and capacitor C2 (4.7 μF, 16V) are
timing components. With these values, the
output at pin 3 of IC2 is about one minute,
which can be increased by increasing either
the value of capacitor C2 or preset VR.
When there is an attempt at snatching,
the plug connected to the circuit detaches.
At that moment, the voltage at
the inverting input of IC1 exceeds the
voltage at the non-inverting input and subsequently
its output goes low. This sends
a low pulse to trigger pin 2 of IC2 to
make its output pin 3 high. Consequently,
the alarm circuit built around IC UM3561
(IC3) gets the supply voltage at its pin 5.
IC UM3561 is a complex ROM with
an inbuilt oscillator. Resistor R8 forms
the oscillator component. Its output is fed
to the base of single-stage transistor amplifier
BD139 (T1) through resistor R9 (1
kilo-ohm).