Shadow detector

                         
                                            This is the circuit of a simple burglar alarm that produces a loud beep when somebody crosses a protected area or door. The circuit is highly sensitive and can detect the shadow of the moving person from a distance of 1 meter.We can adjust the sensitivity of the circuit by the help of the variable resistor VR1.

                                         

Working Principle: Op Amp IC UA 741 is used as a voltage comparator. Its inverting input pin2 receives half supply voltage through the potential resistors R1 and R2. The non inverting input pin3 gets a variable voltage through LDR and VR. Normally when the LDR gets light, its resistance will be low and it conducts and provide a high voltage to the non inverting input of IC. This makes the output of IC high. The high output from IC is given to the base of T1 through a current limiting resistor R3. T1 is PNP transistor and it conducts only when its base becomes negative. Here normally the base of T1 will be high due to the high output from IC. So T1 remains off and Buzzer and LED connected to its emitter remains off. When a person passes in front of LDR, the shadow of the person reduces the resistance of LDR and the voltage at the non inverting input of IC decreases. This makes the output of IC low. Immediately T1 conducts activating Buzzer and LED indicating the entry of a person.

Applications :- Burglar alarm ,Light senser etc..