The door security alarm circuit gives an audio visual alarm when somebody enters from a door. This project can also be used in locker and almirah door. This circuit is based on operational amplifier LM741 and LDR. An op-amp produces an output voltage that is hundreds of thousands times larger than the voltage difference between its input terminals.
LM741 is a 8 pin IC that works as a single operational amplifier circuit, it's pin diagram is shown below:
Fig. 1: Pin Diagram Of LM741 IC
The circuit comprises of Operational amplifier ?A741 which is used as sensitive voltage comparator, LDR and Preset VR1. The preset provides reference voltage to the non-inverting terminal pin 3 and LDR. R2 is connected to inverting pin 2 of IC1. Let’s understand working of the project.
Install the LDR and LED on opposite side of the door so that light from LED constantly fall on LDR . This will make the device remain off as constant light over LDR increases its resistance in comparison to R1 and pin 2 of IC1 goes low. Consequently output pin 6 of IC1 will remain high and alarm will not sound indicating that nobody is present on the door.
As soon as anyone opens the gate or tries to enter from the gate, light falling on the LDR is obstructed and its resistance decreases dramatically. Then, pin 2 of IC1 becomes high in comparison to pin 3 of IC1 making pin 6 also become low. LED and buzzer connected to it start sounding giving an indication that somebody is present on door.
Filed Under: Electronic Projects