Picture of Laser powered burglar alarm
i will show you how to make a laser powered burglar alarm
Remove these adsRemove these ads by Signing Up

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
Abdubuddy2 years ago
Didnt work for me :(
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?
hi can use a 1k resistor instead of the 4.7 resistor?
robot1398 (author)  ElectroPro94603 years ago
Yes you can
can i use a 2N3904 transistor and can i make your circuit without magnets?
robot1398 (author)  ElectroPro94603 years ago
Yes you can use 1n3904 and you can make the circuit without magnets
whisp3r3r3 years ago
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)  whisp3r3r3 years ago
I used a piece of tape and stuck it around the laser push button so itis continiously on
mthassim3 years ago
hi i made da circuit it works fine but the transistor gets hot is there anything wrong in my circuit
robot1398 (author)  mthassim3 years ago
which transistor did you use?????
salty583 years ago
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)  salty583 years ago
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
amruth4 years ago
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
amruth4 years ago
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)  amruth4 years ago
i connected all the things with solder at the back of the board
Can u plz take a pic and show me.
amruth4 years ago
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)  amruth4 years ago
i will give the circuit by tomorrow as now i have to make it
its been 2 days when will u give the circuit. :)
robot1398 (author)  amruth4 years ago
i have not made it u should connect the realey from collector of the transistor to the power supply and use only 2 terminals
K i will try , but still try to make the circuit plz.
amruth4 years ago
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)  amruth4 years ago
check out my new instructable
robot1398 (author)  amruth4 years ago
a relay i think connect it from the collector of the transistor to the power supply
amruth4 years ago
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)  amruth4 years ago
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
amruth4 years ago
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)  amruth4 years ago
i used a simple piezoelectric buzzer and the LDR which my local electronic store gave to me.
amruth4 years ago
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)  amruth4 years ago
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
amruth4 years ago
Will this work for sure
robot1398 (author)  amruth4 years ago
Mudbud4 years ago
C00l *5
robot1398 (author)  Mudbud4 years ago
nfk114 years ago
wow its so complecaded.i think its 5*
robot1398 (author)  nfk114 years ago
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)  supercapacitor4 years ago
voted for your both instructables