Introduction: Laser Powered Burglar Alarm

Picture of Laser Powered Burglar Alarm

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

Step 1: Materials Required

Picture of 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

Picture of 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

Picture of Solder the Components

Step 5: Make the Stand for the Laser

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of Making It Simpler

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

Step 11: Video

Picture of Video

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

Step 12: The Video Link

Picture of The Video Link

Step 13: All Done

Picture of 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 


swagat rout (author)2015-12-06

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

Abdubuddy (author)2012-10-26

Didnt work for me :(

ElectroPro9460 (author)2012-06-17

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?

ElectroPro9460 (author)2012-06-07

hi can use a 1k resistor instead of the 4.7 resistor?

robot1398 (author)ElectroPro94602012-06-12

Yes you can

ElectroPro9460 (author)2012-06-07

can i use a 2N3904 transistor and can i make your circuit without magnets?

robot1398 (author)ElectroPro94602012-06-12

Yes you can use 1n3904 and you can make the circuit without magnets

whisp3r3r (author)2012-06-11

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?


robot1398 (author)whisp3r3r2012-06-12

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

mthassim (author)2011-08-12

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

robot1398 (author)mthassim2011-08-12

which transistor did you use?????

Raqeeb (author)robot13982011-08-16


salty58 (author)2011-08-07

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

robot1398 (author)salty582011-08-07

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

amruth (author)2011-06-16

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

amruth (author)2011-06-15

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.

robot1398 (author)amruth2011-06-15

i connected all the things with solder at the back of the board

amruth (author)robot13982011-06-16

Can u plz take a pic and show me.

amruth (author)2011-06-07

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.

robot1398 (author)amruth2011-06-07

i will give the circuit by tomorrow as now i have to make it

amruth (author)robot13982011-06-09

its been 2 days when will u give the circuit. :)

robot1398 (author)amruth2011-06-10

i have not made it u should connect the realey from collector of the transistor to the power supply and use only 2 terminals

amruth (author)robot13982011-06-10

K i will try , but still try to make the circuit plz.

amruth (author)2011-06-07

I forgot to ask how to keep the circuit on after the laser is broken and connected again, how to make the buzzer ring even after that. :)

robot1398 (author)amruth2011-06-07

check out my new instructable

robot1398 (author)amruth2011-06-07

a relay i think connect it from the collector of the transistor to the power supply

amruth (author)2011-06-07

Hi robot1398, thx for the advice and the circuit worked but the LDR i use is really big so when i hit a laser the dosent stop but when i hit a LED flashlight it works. I think i need the same high risestive LDR in a smaller size. If u hav any ideas plz tell me. The ldr to the left is the usual size the one i am using is to the right. In the nixt pic i showed it by hitting the laser. Thx for the help.

robot1398 (author)amruth2011-06-07

you should use the smaller LDR because it is almost the size of the laser beam
but the other is too big and the laser beam is too small for such a big LDR

amruth (author)2011-06-05

Sry for bieng such a noob but i wanted to know wat type of ldr and buzzer did u use. Thx for all the help.

robot1398 (author)amruth2011-06-06

i used a simple piezoelectric buzzer and the LDR which my local electronic store gave to me.

amruth (author)2011-06-05

Hey Author i tried ur circuit and when i connect the batteries the buzzer keeps ringing, i checked the laser and the circuit but it is all correct. Can u plz help me and this is a great instructable.

robot1398 (author)amruth2011-06-05

ok now try aming the beam of laser straight onto the ldr or use a led torch if you cant keep your hand stable if want to stop this change the 4.7k resistor to a lower level

amruth (author)2011-06-01

Will this work for sure

robot1398 (author)amruth2011-06-05


Mudbud (author)2011-04-23

C00l *5

robot1398 (author)Mudbud2011-04-23


nfk11 (author)2011-04-20

wow its so complecaded.i think its 5*

robot1398 (author)nfk112011-04-20


robot1398 (author)2011-04-20


jingo69 (author)2011-04-19

I voted you!

robot1398 (author)jingo692011-04-19

whats knex never heard that term

robot1398 (author)jingo692011-04-19

thank you very much

supercapacitor (author)2011-04-19

Looks like this will just make a POP sound. Then silence...

Oh I see, this is one of those buzzers with built in tone generation circuitry.
Nice i'ble.

robot1398 (author)supercapacitor2011-04-19

voted for your both instructables

About This Instructable




Bio: None of your buisness
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