|Pocket Size alarm using IC 741|
Alarms are of great use in real world. It alerts people when there is a breach also building one will be easy task. You might have come across different types of alarm circuits varying with their complexity and simplicity. Here is a circuit which has a highlighting feature of miniature size so that it can be carried around easily. This anti theft alarm circuit is very simple will not be suitable for complex theft prevention but it will form a great hobby circuit. This circuit comprises of two parts a transmitter and receiver where infra red beam is used in between to detect intrusion by thieves.
WORKING OF ANTI THEFT ALARM CIRCUIT:
The working of the above circuit begins with the transmitter part. An Infra Red LED is powered with a simple battery. A Resistor was used to limit the current flow through the LED and it must be chose depending on the current and voltage consumption of the LED. A high range IR LED must be chosen to employ in this circuit to improve the range obtained.
The receiver forms the next part of the circuit. A LDR was used to detect the infra red beam from the transmitter. As we all know that LDR shows low resistance when light intensity fall on it. It will be high and high resistance when there is no incident light. This circuit uses the above principle to detect any break in the IR beam.
|IC 741 Pin Diagram|
The next stage was formed by Op Amp IC 741 which is employed as a comparator here. Voltage to the inverting and non inverting terminals of the 741 was fed through two potential dividers LDR & R4 and RV1 & R3. When IR beam is incident on LDR it gives low resistance and voltage in the point where R4 and LDR is tied together will be low. Therefore the output of Op Amp will keep the buzzer OFF.
When the IR beam is cut off by any intruder, LDR will provide high resistance resistance and the voltage at R4 and LDR point will be high. This makes the op amp to give high signal output and in turn switches the buzzer using a transistor switch.
SETTING UP THIS CIRCUIT:
You need to adjust the POT RV1 when aligning the Transmitter to the receiver. RV1 should be adjusted in such a way to push the Op Amp to produce low signal output when IR signal is incident on LDR. This also can be used to adjust the sensitivity of the beam detection used in the circuit.
The above circuit can also be modified to guard a particular object placed in between the transmitter and receiver. In this IR beam interrupted by the object to be guarded. In case any intruder picks the object the obstruction will be removed and this trigger the buzzer to sound the alarm. For this you need to exchange the pin connection potential dividers and PNP transistor must be used as switch.
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