Security Guard

Security Guard

Summary
The circuit gives you an audio visual alarm when somebody enters from your door and alerts you. 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. This can also be used in lockers to protect your valuable things.
Description
The circuit comprises of Operational amplifier μA741  is used as a sensitive voltage comparator, LDR, Preset VR1 which provides reference voltage to the non-inverting terminal pin 3 and LDR and  R1 is connected to inverting pin 2 of IC1 and few more components.

Working of circuit is very simple Install the LDR and LED on opposite side of the door so that light from LED constantly fall on LDR as a result device remain off because light is falling on LDR its resistance is high in comparison to R1 and pin 2 of IC1 goes low. Consequently output pin 6 of IC1 remains high and alarm will not sound. This indicates that nobody is present on the door.  

As soon as anyone open the gate or try to enter from the gate light falling on the LDR is obstructed and its resistance go low. As a result pin 2 of IC1 goes high in comparison to pin 3 of IC1 and pin 6 also become low and LED and buzzer connected to it start sounding giving you a indicator that somebody is present on door.

Components used

IC
IC1(LM741)	1
Resistor
R1-R3(470E)	2
R2(82K)	1
VR1(5K)	1
Capacitor
C1(.1uF)	1
Miscellaneous
Buzzer	1
LED	2