This circuit uses a popular timer I.C which is 555. I.C 555 is connected as comparator with pin 6 connected with positive supply, the output goes high-1 when the trigger pin 2 is at lower than 1/3 level of the supply voltage, conversely the output goes low-0 when it is above 1/3. So small change in voltage of pin 2 is enough to change the output state of pin 3 from 1 to 0 and 0 to 1. The output has only two states high and low and cannot remain in any intermediate stage. It is power by 9V battery for portable use. The circuit is economical in power consumption. Pin 4,6& 8 is connected to the positive supply and pin 1 is grounded.

    To detect the present robber we have used LDR and a source of light. LDR is a special type of resistance whose value depends on the brightness of the light, which is falling on it. It has a resistance of about 1 megaohms when in total darkness, but a resistance of only about 2-5k ohms when brightly illuminated. It responds to a large part of the light spectrum.

    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 100 K variable resistance 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.

In this Demo we have used ordinary LED.