# 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.

## Components required

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

## Working

• 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.

## Video Demonstration of Home security Alarm

Here is the video from Circuits Gallery video lab

## 79 thoughts on “Home security alarm system circuit diagram”

1. sumaia says:

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???

• Hi sumaia,
Please check the configuration of your buzzer. Confirm whether is is 6V or some thing other. You must use 6V buzzer

2. jayram says:

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….

• You can use BC548 transistor for this application. It is a substitute for BC547

3. can you elaborate the point 4 of working

4. Anonymous says:

Can we use a 6v relay replacing the Buzzer??

• Use your name while commenting for better response instead of being anonymous. You can use 6V relay but better use BC548 instead of BC547.

5. Anonymous says:

i made the circuit bt it goes of as soon as i connect to battery. the laser light dsnt affect it…pls help

• 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.

6. Anonymous says:

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)

7. Simran says:

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.

• Simran says:

The fault would be when the transistor is connected wrong.

8. Makhdoom says:

thanks

9. Harris says:

What if i use 12v battery instead of 6v? because i have only 12v battery. thanks

10. Harris says:

sorry it is 9V, not 12V
thanks

• Yaseen says:

Yes you can use 9 V battery.

11. akrash says:

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

• Jaseem vp says:

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

12. Akshay says:

Can I use a 12v (active buzzer / Piezo Buzzer) because i am not getting a 6v buzzer.

• Jaseem vp says:

Hi Akshay,
Yes, It is possible, Use 12V buzzer along with 12V powersupply (Vcc)

13. akrash says:

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

• Yaseen says:

Hi Akrash
Don’t worry, connect positive on VCC and negetive on ground symbol on the circuit diagram.

14. TINUTOM says:

i need the simle diagram

15. marwan saleem says:

sir, i want to make the project of combined laser alam -shadow alarm or laser alam- heat alarm so please if it is posible send me the circuit diagram on my email i.e marwansaleem1995@gmail.com

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………..

17. Parth says:

Hi I would like to know if there is any video telling how to make it

18. sharath says:

can we use a LED instead of the buzzer ,if yes what would be the best suitable LED

19. SINAN says:

CAN I USE 4V BATTERY?

• MOHD SAIF FAROOQUI says:

ONLY 6V SUPPLY USE IN CIRCUIT BECAUSE AT LOW VOLTAGE CURRENT IS LESS FLOW

20. DINU K JAMES says:

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

21. Junaid says:

22. Junaid says:

23. Gretel says:

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

this circuit is very simple can u please give me a some complex circuit to made !!

25. anmol says:

How to make alarm keep on ringin wen laser is interrupted ???

26. praveen says:

sir i want comerence product projects urjent

27. jatin says:

Sir will this project will actually work as I have my last chance to submit this as a project

28. abhishek bhatia says:

how to increase timings of alarm when it rings

29. Navdeep says:

sir i wanna ask that can u tell me the sinple diagram for this laser security

30. sonal says:

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, ﻿

31. RUSHI says:

ya I made it its working but as soon as laser is again on buzzer goes off

we need to latch it
plz help

32. babu m says:

pls tell how to do this project

33. TEJESH says:

CAN WE USE ANY OTHER LIGHT SOURCE OR WE HAVE TO USE ONLY LASER LIGHT??

34. David says:

I really need a project to work on…….Am to submit a project proposal by next week wednesday

35. Aniruddha says:

Can I use a simple torch light insead of a lasor light ????

36. shoaib says:

Hellow sir,
I tryed to make this laser alarm.
But sir, its not working.
Plz tell me what i do.

37. kanchan says:

sir, can u plz send me the applications of this security system argently maximum 5-7

38. arun Kumar says:

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

• Jaseem vp says:

Hi arun Kumar,
Cover the LDR in a small box with a small opening through which Laser beam can pass

-Jaseem

39. fairul says:

what the suitable value lux for laser light?

40. Aditya Gawali says:

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?

41. Ramesh Ayyala says:

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

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.

43. SUBHAJIT says:

can i use any louder speaker instead of buzzer?

44. dj says:

Not working with a 5v mobile charger supply. Is it necessary to use 6v?

45. I need extra large sound buzzer
I want to put on roof of my home with light so what can I do

46. how to differentiate the family members and other members

I need good mini projects . New and innovative one . Please help me out. It would be a great help. Thank you.

48. Shashwat Verma says:

How can we use potentiometer in it ?

49. tushar says:

i cant understand how to make this circuit could any one would help mee

50. krishna says:

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

51. saim says:

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

52. richu says:

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

53. Pragya says:

hello i want such a model which is select for national science exhibition .

54. dhanush says:

When I connect the above circuit I heard the beep sound continuously. What can I do for that?

• Yaseen says:

Hi Dhanish
Use 100K ohm POT instead of 150K ohm fixed resistor.

55. yash says:

my circuit is not working
my buzzer starts beeping still in light

• Yaseen says:

Hi Yash
Use 100K ohm POT instead of 150K resistor and vary it.

56. Akshay says:

Thanks Yaseen

57. Guru says:

PLZZ… help.. 100k potentiometer has 3 points how to connect it to that circuit??????

58. Guru says:

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..

• Yaseen says:

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).

59. Guru says:

THnq yaseen bro..bt i cant understand what o u mean by short cicuited…

• Yaseen says:

Hi Guru
Use Center terminal and any one of the remaining two.

60. Guru says:

Plz..expln it another time…

61. Aditya darji says:

can i use 9v battery and 9v buzer?

• Yaseen says: