Laser Powered Burglar Alarm




About: None of your buisness

i will show you how to make a laser powered burglar alarm

Step 1: Materials Required

1] LDR
2] buzzer
3] coin cell - cr2032
4] one laser 2mv or more (most important =D)
5] pref board or bead board  or you can do it without using both
6] foam (for the stands)
7] 2n2222 transistor
7] resistor 100 ohm and 4.7k ohm
8] Two Neodymium (NdFeB) magnet
9] metal strip
10] tape and double sided tape
11] a long nail or toothpick

Step 2: The Circuit Digram

solder the components according to the circuit digram
the transistors back is facing you.
base-middle pin
collector-left pin
emitter-right pin

Step 3: How the Circuit Works

The 4.7k ohm resistor acts as a sensitivity controller for the LDR 
you can change it between 1k to 100k to change the sensitivity
for this circuit i recommend 4.7 or lower

Step 4: Solder the Components

Step 5: Make the Stand for the Laser

1] cut a small piece of foam
2] then cut a small slit kind of a thing as shown in the pics
3] pierce a toothpick to give stability 

Step 6: Make the Stand for the Circuit

1] cut a small piece of double sided tape
2] stick it on the side of the door or where ever you kept your precious things
3] stick a metal strip on it
4] stick a magnet on the circuit board with the help of double sided tape

Step 7: Now Set Up the Alarm

both the metal strip and the foam piece should be parallel to each other.
now keep the laser on the slit in the foam piece and on it such that the beam falls on the photo transistor 
when you will interrupt the beam a sound goes off

Step 8: Make It With a Phototransistor

you can make this circuit with a phototransistor 
heres how-
you will need
1] buzzer
2] photo transistor [dont have one see next step i will show you how to make one]
3] 2n3904 transistor
4] coin cell

Step 9: Make the Photo Transistor

you will need
1] carbon  tetrachloride
2] AC126 transistor

if you cant get a photo transistor you can make it with a AC126 transistor
first take a AC126 transistor and remove its metallic cap.then wash the inside with carbon tetrachloride till the white paste like substance is completely removed.Now cover the transistor with a transparent glass cap to protect it from moisture and this way your photo transistor is ready.

Step 10: Making It Simpler

if you are new at electronics you can make a simpler alarm.
heres how.

Step 11: Video

see the circuit in action but with a torch as my laser broke when i bought it.

Step 12: The Video Link

Step 13: All Done

you burglar alarm is now ready use it to protect you precious things.
hope you like this instructable.
rate,vote and follow for more instructables 

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    45 Discussions

    swagat rout

    3 years ago

    Heyy plz change the circuit diagram i cant understand a thing plz plz change it or label it

    can i use 1k resistor for 100 ohm resistor?and can i use a small speaker for the buzzer because i cant find buzzers in my area?


    7 years ago on Step 13

    hi.. nicely done!
    i see you used a cheap chinese laser like i did... but (1) how did u keep the button switched on automatically, and (2) my laser fluctuates slightly so the alarm rings continuously... can you help me out on how to deal with this?


    1 reply

    Reply 7 years ago on Introduction

    I used a piece of tape and stuck it around the laser push button so itis continiously on


    7 years ago on Introduction

    hi i made da circuit it works fine but the transistor gets hot is there anything wrong in my circuit

    2 replies

    7 years ago on Step 8

    which pins of the phototransistor are you using ,it seems you are only using 2 ,iam assuming the base is not connected,to anything,is this right

    1 reply

    Reply 7 years ago on Step 8

    the phototransistor that i have has only 2 pins not 3 if u have a 3 pin then use only
    the collector and the base


    8 years ago on Introduction

    Iknow u soldered the components on the board but how did u connect them to each other. I would be nice if showed me a pic


    8 years ago on Introduction

    Hey robot can i know how u connected everything onto the PCB, like using wires or something else, i have probs using wires cause they break after a few days. Can u plz tell me wat method u used.

    2 replies

    8 years ago on Introduction

    Hey robot1398 i didnt actually understand the relay part cause i am not good with relays. It would be easier if u showed it with a circuit. I am using a SPDT relay it has 5 terminals.

    1 reply