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

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Abdubuddy1 year 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)  ElectroPro94602 years ago
Yes you can
can i use a 2N3904 transistor and can i make your circuit without magnets?
robot1398 (author)  ElectroPro94602 years ago
Yes you can use 1n3904 and you can make the circuit without magnets
whisp3r3r2 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?

thanks!
robot1398 (author)  whisp3r3r2 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?????
2N2222
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
amruth3 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
amruth3 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)  amruth3 years ago
i connected all the things with solder at the back of the board
Can u plz take a pic and show me.
amruth3 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)  amruth3 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)  amruth3 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.
amruth3 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)  amruth3 years ago
check out my new instructable
http://www.instructables.com/id/LED-Wheel-Effect-Without-a-Microcontroller-/
robot1398 (author)  amruth3 years ago
a relay i think connect it from the collector of the transistor to the power supply
amruth3 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.
DSC03091.JPGDSC03094.JPGDSC03095.JPG
robot1398 (author)  amruth3 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
amruth3 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)  amruth3 years ago
i used a simple piezoelectric buzzer and the LDR which my local electronic store gave to me.
amruth3 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)  amruth3 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
amruth3 years ago
Will this work for sure
robot1398 (author)  amruth3 years ago
yes
Mudbud3 years ago
C00l *5
robot1398 (author)  Mudbud3 years ago
thanks
nfk113 years ago
wow its so complecaded.i think its 5*
robot1398 (author)  nfk113 years ago
thanks
robot1398 (author) 3 years ago
(removed by author or community request)
MROHM robot13983 years ago
(removed by author or community request)
robot1398 (author)  MROHM3 years ago
thanks
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