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advantages of anti bag snatching alarm
Post: #1

Please give me advantage and disadvantages anti bag snatching
Post: #2
Here is a simple alarm circuit against theft of bags to prevent theft of your valuables while traveling. The circuit stored in your bag or suitcase sounds a loud alarm, simulating a police horn, if someone tries to snatch your bag or suitcase. This will attract the attention of other passengers and the thief can be caught red-handed. In the standby mode, the circuit is blocked by a plug and socket arrangement (a mono plug with short taps connected to the monaural socket of the unit). When the thief tries to snatch the bag, the plug is disconnected from the plug of the unit to activate the alarm.

Anti-theft alarm circuit

[Image: 2D6_feb_23_efy.png]

The circuit is designed around the operational amplifier IC CA3140 (IC1), which is configured as a comparator. The non-inverting input (pin 3) of IC1 is maintained at half the supply voltage (around 4.5 V) by the potential splitter comprising the resistors R2 and R3 of 100 kilohms each. The inversion input (pin 2) of IC1 is kept low through the short-circuited plug in the socket. As a result, the voltage at the non-inverting input is higher than at the inverting input and the output from IC1 is high.

The output of pin 6 of IC1 is fed to pin 2 of activation of IC NE555 (IC2) through coupling capacitor C1 (0.0047 μF). IC2 is configured as monostable. Its trigger pin 2 is held high with resistance R4 (10 kilo-ohms). Normally, the output of IC2 remains low and the alarm is off. The resistor R6, together with the capacitor C3 connected to the restart pin 4 of IC2, prevents any false triggering. The resistor R5 (10 mega-ohms), the predefined VR (10 mega-ohms) and the capacitor C2 (4.7 μF, 16V) are timing components. With these values, the output on pin 3 of IC2 is around one minute, which can be increased by increasing the value of capacitor C2 or the preset VR.

Circuit in action

When there is an attempt to snatch, the plug connected to the circuit is disconnected. At that time, the voltage at the inverting input of IC1 exceeds the voltage at the non-inverting input and, subsequently, its output is low. This sends a low pulse to activate pin 2 of IC2 to make its output pin 3 high. Consequently, the alarm circuit built around the IC UM3561 (IC3) obtains the supply voltage on its pin 5. IC UM3561 is a complex ROM with a built-in oscillator. The resistance R8 forms the oscillating component. Its output is fed to the base of the single-stage transistor amplifier BD139 (T1) through resistor R9 (1 kilo-ohm).

The alarm tone generated by IC3 is amplified by transistor T1. A loudspeaker is connected to the collector of T1 to produce the alarm. The alarm can be postponed if the plug is inserted back into the socket. Transistor T1 requires a heat sink.

Resistor R7 (330 ohms) limits the current to IC3 and Zener diode ZD1 limits the supply voltage to IC3 to a safe level of 3.3 volts. Resistor R9 limits the current to the base of T1.


The circuit can be easily constructed on a printed circuit board or general purpose PCB. Use a small box to house the circuit and the 9V battery. The speaker must be small so that the device is at hand. Connect a thin plastic cable to the plug and secure it in your hand or tie it in another place so that when you pull the bag, the plug will easily detach from the plug.

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