Laser Perimeter Alarm

Picture of Laser Perimeter Alarm
Learn how to fortify your fortress no matter what the size with this ingenious myriad of a customizable laser grid. Once someone steps through and breaks the laser signal, then off goes a quite noticeable, piercing alarm siren. Guard your room, office or workshop from pesky invaders and use it to safeguard your most prized possessions from high-profile robotic creation to the last jelly filled donut!

For more similar projects, kits for this and other gadgets, and much more just go to Ocalon Electronics. If your having any problems with getting the circuit to work, or just general Q&A questions feel free to leave them here.
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Step 1: Materials/Supplies

Picture of Materials/Supplies
Parts List Includes

1. A single 1000uF Capacitor
2. A 5K Trimpot (larger values will work)
3. CdS Photocell (Cadmium Sulfide Cell)
4. Some Perforated Board
5. A 9v battery and clip
6. The 2N3904 Transistor
7. Several Small mirrors
8. About 5-12VDC Piezo Siren (102dB)
9. Any General Laser (650nm 5mw)


8. An L7805 5v Regulator
9. The Project Case
10. A 5 - 9 volt Adapter

Step 2: How Does it Work?

Picture of How Does it Work?
The heart of the system is the sensor. Without it we would not be able to do what we want, which is to sense a break in the beam. The cadmium sulphide photocell works by changing its resistance depending on the amount of light striking its surface.

We'll be using this large resistance change (about 10k ohm in daylight to about 1M ohm in pitch dark) to switch on/off a transistor for detecting the dark (when the beam is broken).

They capacitor is used to run the buzzer for several seconds (depending on the size of the cap) even in the beam is broken for only a fraction of a second. To get the siren to go off longer, just use a bigger capacitor (1000 uF) or simply put more capacitors in series with the current one.
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could this be use in ac power?
mmo41 year ago
the left arm for cd cell where to connect
and the other on of 2N3904 Transistor (the down ones) where to connect too ?!
please help
usmm2 years ago
i made this project but how i connect the thing in broad band i can not understand from the diagram is it in parallel or series plz reply
DrBondo4 years ago
 Works great, but I have to use a massive array of caps to get it to run for any acceptable length of time.  One 1000uf cap sends it fading out very quickly, so I wired about 10 of them in a series, and now it buzzes for about 4-5 seconds before fading.  All these caps, however, caused a delay in the way the transistor switches, so if someone walks quickly through the laser, it may not trip.

I think I may try using just one really big cap, instead of an array.  Maybe that will eliminate the delay.
Digy DrBondo3 years ago
Just use my schematic, the alarm will stay on forever, as long as there is power and the relays don't fail, even though it is much more complex it is worth to use it.

Alarm Unit Diagram.jpg
arulchris Digy3 years ago
can u post a clear picture pls.
thank u remyzero7 4 the pic.
i'll try this too..
amruth3 years ago
I have connected everything according to the circuit, the first time the alarm kept ringing even if the laser was on, so i tried again and this time the alarm wont go off, not even i the darkness. Weezoh or anyone help plz i am new to these stuff.
amruth3 years ago
HI, tried soldering the circuit but the alarm dosent seem to go off in the dark. I tried checking if i had done something wrong but everything seems to be alright. Can someone help me plz.
litcrit3 years ago
How weak of a laser beam can you use and still get this to work? Or is there any way to make the system more sensitive? Would a bright LED work if it was placed extremely close to to the photocell?
timmi4 years ago
 Could this be done with an inferred lazer and inferred light detector ? so they don't see the beam at night? And would the laser wear out ater a while?
kewltek timmi3 years ago
Yes. I have an '83 book that explains how to do something like this, but I am looking for a normally open LASCR in place of the photocell.
schumy005 years ago
where I can put a relay to make an alarm ringing continuu?
Digy schumy003 years ago
look up at my comment above.
kewltek Digy3 years ago
Post a higher res pic pls
I was wondering how the capacitor keeps the alarm circuit to stay on because wont the base stop that cuurent once the beam is on the photoresistor and the current flows through the photoresistor again? If so, does the capacitor keep the transistor to stay on or something?
Mxous5 years ago
Link doesn't work.
yea it does. try using a different browser
fringe4 years ago
looking at the transistor as a switch, the collector has 9v coming in, and the base has that but a photocell and resistor inline with it.  when the base gets >5v it should let the collector's 9v flow go through to the emitter yes? that seems to be the case/idea..

given this, i would see the photocell needing to add large amounts of resistance whenever it has light in order to bring down the voltage (so the alarm doesn't trigger)  and have less resistance when things go black (in order to trigger and set off the alarm).  this is a fair assumption yes?

my photocells gain resistance w/out light and have little resistance with light.. which is the inverse of what we want in order for this to work is it not?  am i reading this wrong?  did i buy the wrong types of photocells? .. i'm very very confused.. somewhere this logic has gone inverse on me.  :)

Rememer electricity follows the path of least resistance.

When the photocell has light on it (low resistance), the the flow wants to go through it  to ground instead of through the transistor to ground.

When the light is off, the photocell has high resistance and it's easier for the flow to get to ground through the transistor, which then switches it on and sounds the buzzer. :)
fringe fringe4 years ago
Looked like I needed a pnp for this.. not a npn transistor as noted..
fringe fringe4 years ago
and that's not helping either.. i think i'm not meant to create this thing.  if a NPN is meant to be used.. can you explain how it's triggering?  sorry for being such a n00b..
iliasbill4 years ago
hi,what is the one left down the photocell --- and the one down the transistor ---
                                                                           --                                                             --
                                                                            -                                                              -
hi there my buzzer is very low can anyone help me please pete
cdousley4 years ago
Could i replace the photocell with a photo transistor. Heres a pic of the photo transistor

rushsraf4 years ago
plss send u me full schematic diagram of this so that i can start my project...plzz reply asap

92600744 years ago
specification of photcell plssss??
mates4 years ago
Great circuit!  Can you replace the alarm with another type of noise device?  I would like to use this circuit for another purpose and connect it to a train noise from a Hallmark music card.  I tried to make the connection myself and had little success.  However, I mananged not the fry my train noise circuit.  I would also like to use the 3 volt battery that comes with the card.  If you can help or make suggestions, I would appreciate it.
Colonel884 years ago
Hey this was taken from Graham McGowan's 101 Spy Gadgets for the Evil Genius, so give him credit. The schematic is the same and the thing is the same and the idea is the same so give the man his credit.
ipwnuall24 years ago
hey weezoh, its me again... I have one more question, with the 10k ohm 15 turn potentiometer are the pins the pins the same? does it matter which ones i connect? do i conect the 2 closer pins or the middle pin closer to the wheel to the far end one?
weezoh (author)  ipwnuall24 years ago
Hey! Sorry for the delay, but yeah your trimpot should connect the same. You'll notice there's a middle pin between the two outer pins (the two outer one are on opposite ends). And so its just like any other trimpot - middle pin to one end pin to +9v and the other end pin to the base of the transistor and to one end pin of the CdS cell.
also which way do you turn the trimpot?  when the pins are like this like this -->   |---|--|*        *  = wheel
                           |  = Pin
                           -  = space between pins
Hey, i found out what was wrong about my first one, (transistor) i accidently put it in backwards.  But I still need an anwser because i'm not sure if the transistor was the only thing wrong.
please reply.... i really don't want to start over again.
pook4 years ago
That schematic is exactly the same one on page 212 of 101 spy Gadgets for the evil Genius. I still can't get it to work! Has anyone actually gotten this to work?
weezoh (author)  pook4 years ago
Actually, there's a small difference. The base of the transistor connects in between the trimpot and photocell up above whereas in the Spy Gadgets book it connects at the + 9 volt junction. which is why it keeps going off continuously. hope that helps!
the_lark weezoh4 years ago
9v to the base??? wouldn't that kill the transistor?
pook weezoh4 years ago
Well thank you for repling and for correcting the schematic! I have a basic knowledge of electronics but I bought the 3 pin trim pots like the one shown in the book. There is one closest to the trim screw, a middle and the end pin. Can you tell me which pin should connect to the cell? Whcih two should be soldered together? The transistor I have has the collector on the left and the emmiter on the right. The emitter should connect ot the cell, or the collector? Thank you for being there!
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