This Project is based on Home Security system. The main motto of this tutorial to create a laser light security alarm that can be used for home security to avoid the various type of security threat. This alarm contains a light detecting sensor named as LDR( Light Dependent Register ) which detects the presence of light, if the laser light is focused on the LDR then the alarm will remain silent. If any obstacle comes in between the laser diode and the LDR sensor then the intensity of light incident on the LDR will decrease or no light will fall on the LDR then the alarm will start.

The theory behind the project:-

         The main concept behind this is that when light intensity falls on the LDR it allows conducting electricity in it. So Now we have to make a circuit that will make the alarm remain silent when LDR conduct electricity in it and when LDR stops conducting Electricity in it, the circuit should start the alarm.  To Solve this problem we can use PNP transistor.The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity.

     So Transistor will do the job for us... it acts like a valve i.e when the LDR conduct Electricity it will stop the alarm and when LDR stops conducting electricity it will start the alarm.

Parts Required:-

  • Buzzer
  • 9v Battery and Battery cap
  • LDR
  • Red Led
  • 47K PotentioMeter
  • Laser Diode
  • Pencil Battery holder and Pencil Battery
  • Dc switch.
  • Tools and Vero board to build the circuit



  1. First of all we have to make insert two pencil battery in the battery stand and check the polarity i.e Vcc or GND...every Led diode required 3v To here we are connecting two pencil battery(1.5v each) in series to get 3V other wise if you have any 3v Power supply then you can use that.
  2. Connect +ve terminal of the Laser diode to the +ve terminal of the 3v power supply, and connect a switch in between the -ve terminals of the diode and 3v power supply to control the laser diode...
  3.   Now we have to make the circuit...first of all connect the 9v power supply to the Vero board i.e for this instant connect only the 9v battery cap to the Vero Board.
  4. Then insert the BC547 Transistor and connect emitter of the BC547 to Ground of the Power supply
  5. Then connect the red led  -ve terminal to the collector of the BC547 and then connect the buzzer's GND pin to the +ve pin of the red led...and the +ve terminal of the buzzer to the +9v power supply.
  6. Then connect LDR to the emitter and base pin of the BC547...
  7. Then connect the 47k potentiometer to the collector and base pin of the BC547...  
  8. Now its time to connect the 9v battery to the circuit
  9. then adjust the LDR and laser diode position so that laser diode should be focused on the LDR.
  10. then if required adjust the volume of the buzzer using the 47k potentiometer.
Now For making Watch this video....

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