BURGLAR ALARM

circuit diagram &theory

Item = BURGLAR ALARM
Rs=150.00 (+ 80 Shipping)

Cost including Shipping:

India = Rs:230/ $5.11 USD
Other Country = $34.00 USD

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       The circuit illustrated below is used as an Burglar alarm. LDR is kept at such a place that when thief enters our house then a shadow will fall on the LDR. A small beam of light source is also needed to supply continuous signal to LDR. For best Light source we can use Laser diode which will work for few KMs. For home use Infra Red LED's will be good and will be tricky to thief and works with same efficiency at night.
  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 level. So small change in the voltage of pin 2 is enough to change the output of pin 3 from 1 to 0 and 0 to 1. The output has only two states high and low and can not remain in any intermediate stage. It is power by 9V battery for portable use. The circuit is economic 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 2-5 k 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.


LED= Light Emitting Diode   LDR= Light Dependent Resistance   IC= Integrated Circuit

Project Circuits
deals with CBSE and other School Physics Project for student, Hobbyist. Please also contact for science exhibition project material, Components and ready made project as shown in the fig.

Part List of Burglar Alarm :-
1. LED 2 pcs
2. LDR 1pc
3. Resistance 470 Ohms 2pcs
4. Variable resistance/preset 100 K
5. IC 555
6. Battery 9V with Snap
7 Switch
8 Buzzer

* For Demo we have used simple LED
for LED1 may be Red or White Colour