17-10-2017, 11:49 AM
Here is a simple alarm circuit to thwart the theft of your valuables while traveling. The circuit stored in your purse or suitcase sounds a great 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 with his hands in the dough. In standby mode, the circuit is blocked by a plug-and-socket arrangement (a mono plug with short-circuited cables plugged into the unit's mono-jack). When the thief tries to snatch the bag, the plug separates from the base of the unit to activate the alarm.
![[Image: 2D6_feb_23_efy.png]](http://electronicsforu.com/wp-contents/uploads/2016/05/2D6_feb_23_efy.png)
The circuit is designed around the CA3140 IC amplifier option (IC1), which is configured as a comparator. The non-inverting input (pin 3) of IC1 is maintained at the middle of the supply voltage (about 4.5V) by the potential divider comprising resistors R2 and R3 of 100 kilohms each. The inverter 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 IC 1 is high.
The output of the pin 6 of IC1 is fed to the trip pin 2 of IC NE555 (IC2) through the coupling capacitor C1 (0.0047 μF). IC2 is configured as a monostable. Its trigger pin 2 is held high by the R4 resistor (10 kilo-ohms). Normally, the IC2 output remains low and the alarm is off. The resistor R6, together with the capacitor C3 connected to reset pin 4 of IC2, prevents any false activation. The R5 resistor (10 megohms), the preset VR (10 megohms) and the capacitor C2 (4.7 mF, 16V) are timing components. With these values, the output at pin 3 of IC2 is approximately one minute, which can be increased by increasing the value of the capacitor C2 or the predetermined VR.
When there is an attempt to snatch, the plug connected to the circuit is separated. At that time, the voltage at the inverting input of IC1 exceeds the voltage at the non-inverting input and subsequently its low output. This sends a low pulse to activate pin 2 of IC2 to make its output pin 3 high. Consequently, the alarm circuit built around IC UM3561 (IC3) obtains the supply voltage at its pin 5. IC UM3561 is a complex ROM with a built-in oscillator. The resistor R8 forms the oscillator 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 T1 collector to produce the alarm. The alarm can be delayed if the plug is inserted into the socket again. Transistor T1 requires a heat sink.
The resistance R7 (330 ohms) limits the current to IC3 and the zener diode ZD1 limits the supply voltage to IC3 to a safe level of 3.3 volts. Resistance R9 limits the current to the base of T1.
The circuit can be easily built into a Vero board or general purpose PCB. Use a small box to accommodate the circuit and 9V battery. The speaker should be small so the gadget is useful. Connect a thin plastic cable to the plug and secure it by hand or tie it aside so that when the plug is pulled, the plug is easily detached from the plug.
It can be understood in the following video:
