Burglar alarm a burglar alarm system is designed to detect an unauthorized entry into a house or area. Burglar alarm systems can be used in residential buildings, commercial buildings, offices, industries and even in military locations. How it works: as we block the light source, ldr will be under darkness and buzzer beeps. The source of light and ldr is so adjusted with a reflector that light will directly fall on the ldr, but when robber enters inside then it will block the beam of light and ldr will be under darkness. We have made a potential divider circuit with ldr and resistor connected in series. Voltage is directly proportional to conductance so more voltage we will get by this divider when ldr is getting light and low voltage in darkness. Sensitiveness can be adjusted by variable resistance. Divided voltage is given to pin 2nd of 555. As soon as ldr gets dark the voltage of the pin 2 drops 1/3 Of the supply voltage and pin 3 gets high and buzzer beeps.
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