The circuit comprises of Operational amplifier. ?A741 is used as a sensitive voltage comparator, NE555 wired in mono-stable mode. In mono-stable mode IC is once trigged, it will hold the output for a specific interval of time as defined by the timing component before returning to its original state and we are utilizing this property in our circuit. LDR, Preset VR1 which provides reference voltage to the non-inverting terminal pin 3 and LDR and R1 and R2 is connected to inverting pin 2 of IC1 and few more components.
Working of circuit
In this circuit we have made a two potential divider circuit one at inverting terminal (pin2) and another at non inverting terminal (pin 3) of IC1. We have used resistor R1 and R2 to provide half supply voltage at inverting terminal. LDR and variable resistor VR1 is used to provide variable voltage input at non inverting terminal pin 3.
Initially when nobody is present in front of sensor (LDR), output pin 6 of IC1 is at high voltage level which makes the base of transistor T1 also high and its emitter is also high that's why at this time transistor T1 will not conduct. Because if this IC2 will not oscillate and alarm will remain off.Now as person enters its shadow will fall on LDR because of change in light intensity on LDR voltage at non inverting input pin 3 will become low. The output pin 6 of IC1 become low causing the transistor T1 to conduct. This will trigger the IC2 and the alarm or buzzer connected to it will start sounding. It is recommended that keep LDR inside black tube or pipe to increase the sensitivity of LDR. You can also increase the sensitivity by the variable resistor VR1.