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+diagram Home security alarm system circuit diagram

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.

Volatage+drop+%282%29 Home security alarm system circuit diagram

  • 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).

Volatage+drop+%281%29 Home security alarm system circuit diagram

  • 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.
Home+security+alarm+simulation animation Home security alarm system circuit diagram

Project arrangements

Home+security+systems Home security alarm system circuit diagram
The arrangement of LASER path is shown in the above image.

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

Video Demonstration of Home security Alarm

Here is the video from Circuits Gallery video lab

BC547 pinout

Similar circuits

48 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, 

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