IR sensor is directly connected to LM311 IC. It is a voltage comparator IC, here it is we used for reference voltage.Whenever the IR signal is interrupted, its output in receiver feed to LM311. and IC2 is triggered at pin 2 through transistor LM311. IC2 NE555 work as in the monostable multivibrator mode. We can adjust the time period through R7 & C2. The output of monostable directly connected to clock of the D-flipflop. In D-flipflop when input data is high then output is also high. Here we input pin of the D-flip flop connected to +5v(Always high). As a result when clock signal appear, a high voltage(+5v) is feed to the base of the  transistor BC547 & LED. Buzzer is connected to transistor collector and VCC. When Transistor ON Buzzer will produce sound. Finally a thief interrupt the IR signal simultaneously a buzzer sound will produce.

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