06-02-2013, 03:53 PM
Anti-Bag-Snatching Alarm
1Anti-Bag.pdf (Size: 41.81 KB / Downloads: 32)
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
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
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
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.