Anti-Bag Snatching Alarm

The Anti-Bag Snatching Alarm kit employs a circuit that is locked by a plug and socket arrangement in the standby mode. The kit can be kept in a bag or suitcase and simulates a police horn if someone attempts to snatch the same. Thus, while travelling, this kit entitles the person to carry cash or valuables without fear. The alarm can be turned off only by the authorised person by using a hidden key. Number of ICs used: 3 The circuit of the Ant-Bag Snatching Alarm is designed around the operational amplifier CA3140 (IC1)which is 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 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 a coupling capacitor. IC2 is configured as a monostable. Its trigger pin 2 is held high by a resistor of10 kilo-ohms. Normally, the output of IC2 remains low and the alarm is off. A resistor along with a capacitor is connected to reset pin 4 of IC2 in order to prevent false triggering. When there is a bag-snatching attempt, 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. 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. Learning: Application of the operational amplifier IC CA3140 as a comparator. Application of the the NE555 timer IC in the monostable mode. Use of the melody generator IC UM3561. Use of transistors as signal amplifiers. Brand: Kits'n'spares, Marketed by: Kits'n'spares