Click To View Circuit Diagram
Anti-Bag Snatching Alarm
This anti bag snatching alarm builds a circuit that is locked in a plug and socket arrangement in standby mode. The kit can be kept in a bag or suitcase, and simulates a police horn if someone attempts to snatch the bag. Thus, while traveling, this kit enables people to carry cash or valuables without fear. The alarm can be turned off only by the authorized person by using a hidden key.
Number of ICs used: 3
Learning from the project
*Application of the operational amplifier IC CA3140 as a comparator.
*Application of the NE555 timer IC in monostable mode.
*Use of the music generator IC UM3561.
*Use of transistors as signal amplifiers.
Build yourself an awesome anti bag snatching alarm and protect your luggage and travel care-free.
The circuit of the anti bag snatching alarm is designed around the operational amplifier CA3140 (IC1) configured as a comparator. The non-inverting input (pin3) of IC1 is kept at half the supply voltage (around 4.5V) by a potential divider resistor network. 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 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 a coupling capacitor. IC2 is configured in monostable mode. The trigger pin is held high by a resistor of 10 kilo-ohms. Normally, the output of IC2 remains low and the alarm is off. A resistor and a capacitor are connected to the reset pin of IC2 to prevent false triggering.
When there is a bag-snatching attempt, the plug connected to the circuit detaches. At this moment, the voltage at the inverting input of IC1 exceeds the voltage at the non-inverting input and subsequently, the output goes low. This sends a low pulse to trigger pin 2 of IC2 to make its output high. Consequently, the alarm circuit built around IC UM3561 (IC3) gets the supply voltage at pin 5.
Its output is fed to the base of single-stage transistor amplifier BD139 (T1) that amplifies the generated alarm signal. A loudspeaker is connected to the collector of T1 to produce the alarm. The alarm can be put off if the plug is inserted into the socket again.