Home security alarm system circuit diagram

Have you ever thought about implementing your own home security alarm systems? It’s one of the simplest and interesting circuits for electronic beginners. Our new home security equipment uses a LDR (Light Depended Resistor) to detect security problems. Theft attempt and other security threats can be controlled by using this simple circuit to improve your security systems. This is also best suited for school students for their high school science projects.

To implement this alarm system for home, you have to provide an optical path (with LASER beams) around your home. The LASER path is made possible with one LASER torch and 3 mirror arrangements which encloses the whole area. Please refer the project arrangement section below to make this optical (light) path. We have explained about home security system in detail with animation/ simulation.

Circuit diagram of Home security alarm

Home security alarm circuit

Components required

  1. Battery 6V
  2. Resistors ¼ watt (150kΩ)
  3. LDR (Light Depended Resistor)
  4. Transistor BC547
  5. 6V Buzzer


  • This circuit is based on LDR (Light Depended Resistor), a variable resistor in which the resistance varies according to the light intensity falling on it.
  • The LDR and resistor R1 forms a potential divider network, which is the main part of our security alarm circuit.
  • The voltage drop across the LDR is used to drive the transistor switch. When the voltage drop is above cut in voltage (0.6V), the transistor is turned ON.
  • LDR has low resistance (mΩ range) in the presence of light and high resistance (MΩ range) in the absence of light.
  • In our security alarm, a LASER light is allowed to fall on the LDR continuously using 3 mirrors (see the project arrangement figure).
  • Light from other sources should not be allowed to fall on the LDR, so place the LDR in a box with a single hole to pass LASER.
  • In this situation, the resistance offered by LDR is too low, since the LASER light is continuously allowed to fall on the LDR  surface.

  • Thus the voltage drop across the LDR is also low [V=IR (Ohm’s law)] which is insufficient to turn ON the transistor, so the transistor remains in OFF state.
  • When a person (eg: thief) makes a block to the continuous flow of LASER beam, then the light falling on the LDR gets blocked. Thus its resistance increases to a high value in the order of MΩ range (According to Ohm’s law V=IR).

  • While resistance increases the voltage drop also increases, when this voltage drop exceeds the cut in voltage of the silicon NPN transistor (0.6V; BC547), it will turn ON.
  • Then current from Vcc starts flowing to ground via the buzzer and transistor, which makes the beep sound.
  • The beep sound from the security alarm gives the indication of some security failures.

Simulation (animation) of the project

To realize the concept we are providing animation/ simulation of home security alarm circuit below. For animated demonstration, LED is used instead of buzzer.

Project arrangements

The arrangement of LASER path is shown in the above image.

Also Read: How to make a burglar alarm circuit with IR and photodiode?

BC547 pinout

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132 thoughts on “Home security alarm system circuit diagram

  1. I made this circuit, its working
    I have a concern my buzzer rings so slowly with a low volume. How to make it rings fast???

  2. hi.. i just wanna ask if what transistor i can substitute on the above bc547… i made this circuit using 2n222 but it ddnt,, work.. help me pls….

    • Do the follwing:

      1) Use a 100K potentiometer instead of 150K resistor and set the intensity of light.
      2)Enclose the LDR with a black hollow cylinder such that only LASER beam passes in.

  3. sir, can i replace the 547 transistor using a darling-ton pair so that this schematic can operate a relay instead of a buzzer??
    i will be thankful for your help….my email address is (dnkwenti @ gmail.com)

  4. I made this project.
    The best thing about projects is the knowledge you gain while making it.
    I would like to suggest something- if circuit gets activated when LDR is illuminated, then please swap the (150kΩ) resistor and the LDR.

  5. hello, i’m a beginner…I dont know anything about resistors,transistors,etc..so..can you tell me where to give +ve and -ve voltages

    • Hi Akrash,
      We are following standard circuit diagrams and drawing style. For your information only Vcc=+Ve volatage; Ground= -Ve voltage

  6. hello, i’m a beginner…I dont know anything about resistors,transistors,etc..so..can you tell me where to give +ve and -ve voltages

  7. Sir I made a fire alarm cct bt it function only few meters so I wanna to at least 20 to 30 meter circle as long………can you explain how does it works………..

  8. How can a connect a simple D flip flop to this circuit , so that buzzer sounds continuously when laser is cut once..

  9. hello admin.i just want to ask if the PIC16F84A can also load a burglar alarm? i have this project and when i test this in Proteus, the code that i made is working but when i assemble all the wirings following all the circuits it doesnt work. please help me to fix.thannk you and God Bless

  10. sir,I am a student of class 12th. I made a working model of laser security system for my physics project. For this I used a laser(1mw) ,a transistor (2N7000) , two resistors (100 ohm , 6.8 k ), LED ,LDR , a battery (9V) and a buzzer of 3 V. I want to know the principle on which this system is based and also the explanation of its working, 

  11. Hi I want one help . it work but without laser light . my hand is near my the circuit. Is work but laser is pass throughit it not work

  12. i just have one question ..since it is a burglar alarm the buzzer should beep continuously but in this case as light again falls on LDR the buzzer stops. I think that is its biggest drawback .. how can we correct this drawback?

  13. sir I made the project. the laser light how created…where r laser light the contineusly flow laser light. u

  14. I’m a Nigerian and an Electronics hobbist.As long as I would have gone in Electronics new discoveries and inventions,I have been constraint with lack of working tools like some electronics components accessibbility in my country have been so difficult for me like Transistors,diodes,capacitors, the chips,and so on.I shall be grateful, if those can be shipped to me in Nigeria.This is my phone number incase of any conversation 08066644074.Thanks.

  15. Hey guys.. What is vcc?..and can this connection be made on bread board?… Please help… And what are the terminals that are to be connected to ground

  16. how to connect the BC547 it has 3 end we have to use only two from where i should connect centre & left , centre & right or left & right

  17. pls show how to make this on breadboard.I connected it but buzzer rings even if there is no laser light i made a box but still it dont work

  18. can i get the circuit diagram… I am just a beginner plz..help..how to connect that potentiometer in the place of 150 k resistor..someone plz help..i need ur help quickly..

    • Hi Guru
      150K resistor is a fixed resistor and it has two terminal, POT has three terminal. If you want to replace it with a variable resistor (POT) of 100K, short circuit center terminal and any one of the remaining two terminal, then you will get two terminal POT(1.Short circuited 2. Remaining one).

  19. hii,
    i have used a LED in the circuit along with a 100ohm resistor and the other resistor is 6.8k and transistor is 2N7000…..so can you please give me the working with those components.

  20. hai i am making your LDR home security system .i dont know how to make it can you give me your ph no by e mail or call me at 9895598627.please

  21. Hello Sir,
    We need a circuit diagram of Burglar Alarm which consists of
    a) 2 Resistors with values
    b) 2 Transistors(pnp and npn) with values
    c) 1 Capacitor
    d) 1 LDR
    e) Buzzer
    f) 0 PCB
    g) 6V Battery

  22. Sir I done it but it is not working if we connect wire to battery it
    Will ring if we touch the laser it will not ring pls tell because tomorrow
    Is my science exhibition

  23. hi i’m Valentina and i m just 12 and i am not understanding how to make this project i have the materials but i have no clue how to make this can you explain in easy words PLEASE and as quick as possible

  24. Hi , I am Aute Sanjay . I succesfully done this project .its really working thanks to giving information .
    Again thanks …..

  25. Im an bangladeshi i need all things of circuit i dont know ehere i will get if you could write its description it would be better
    And how it works what occurs i need answer

  26. I am making your project but i don’t know how I use the transistor in the given circuit please help me na

  27. I am new in this . I made the circuit ,buzzer is also working but when I put the lazer on LDR it doesn’t stop

  28. Thanks for this model I got the first place in my schools science exhibition for making this circuit project thanks a lot tat. .. …….

  29. Sir ,I am making your project but i don’t know how I
    use the transistor in the given circuit please help

  30. i create this circuit but where do iconnect batteries.help me some one i am an hurry on my competition.
    the items i have are ldr.resistor 150k.transistor bc547.6v bttery and buzzer.

  31. Hi. I would like to use piezoelectric buzzer for louder sound instead of 6 volts magnetic buzzer. Please give me rating of that piezoelectric buzzer, transistor, resistor, battery

  32. Hey there,I made the mentioned circuit with a 9v battery.The problem is the buzzer makes sound all time,no matter what type of light falls on ldr.
    What may be the reason?
    Reply ASAP

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