|Pocket Size alarm using IC 741|
Alarms are of great use in real world as it alerts people when come across a threat and 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 where a 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 where a LDR was used to detect the infra red beam from the transmitter.As we all know that the LDR shows high resistance when light in incident on it and low 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 the LDR it gives high resistance thereby the output of Op Amp cannot switch the buzzer ON.
When the IR beam is cut off by any intruder, LDR will provide less resistance and the voltage obtained at the pin 3 will be high than the pin 2. This makes the op amp to give high signal output and in turn switches the buzzer using a transistor switch.
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. 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 and 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.