The 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 [[wysiwyg_imageupload::]]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. 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. How does the alarm sense someone entering? How is the circuit constructed? What is the role of op-amp in its working? Continue reading to find out how answers to these questions and more. 

LM386 is a low power voltage amplifier IC by National Semiconductors. Specifically designed for low power applications, it provides an internal gain of 20 which [[wysiwyg_imageupload::]]can be easily shoot to 200 using a resistor. This IC has been developed for battery operations and is widely used in circuits such as intercoms, audio systems in televisions, power connectors etc.The circuit described here is based on LM386 which amplifies sound input so that they can suffice enough for interiors of a car or a mobile phone. Fairly compatible, this circuit is widely used commercially. Read on more to find out how the circuit is made and how it works.