Anti-Bag Snatching Alarm

Anti-Bag Snatching Alarm
Product Code: HEUK5
Reward Points: 0
Availability: In Stock
Price: INR 250.00
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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.

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