Introduction: 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.

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.

Step 3: Proto-Breadboard Walkthrough - If Needed!

Picture of Proto-Breadboard Walkthrough - If Needed!

Here is a step-by-step walkthrough of building the circuit on a breadboard to test and see that it is indeed working. The point of doing this is to swap out any faulty parts that might become a hassle to replace once soldered together in the circuit. You can either follow each step below or skip ahead to begin soldering it all together.

I wasn't able to transfer it here, but if you need it just click here.

Step 4: Begin With the Board

Picture of Begin With the Board

For easy, one page viewing of all the steps needed to create the board click here.

First begin with a piece of perfboard as seen in the second image, with about the dimensions of about 1" inch by 1.5" inches. There will be excess board left over but you can just cut it away to your satisfaction. Then insert the CdS (Cadmium Sulfide) cell.

Bend the leads back 90 degrees so that they are parallel with the board and make sure the sensor is positioned up at an angle to suit your design. Next insert the 5k ohm trimming potentiometer next to the CdS cell and bend the leads at a 90 degree angle and flush with the board. Then solder the end pin of the trimpot to one of the leads of the CdS cell.

Step 5: Add the Trimpot and Transistor

Picture of Add the Trimpot and Transistor

Now just simply bend together the remaining two leads of the trimpot (the other end and the center leads) together and solder them both together (as seen in the last two pictures of the row above). The next step will be to bend the second lead of the CdS cell off to the side. As you can see below, we chose to weave it through the perfboard holes for a more sophisticated feel.

Then you mount the transistor (in the 3rd picture above the flat end is facing the transistor) and bend the middle pin (the transistor's middle pin is referred to as the 'base'). Now make sure to solder the middle pin of the transistor to the two trimpot pins that are connected to each other (3rd/4th picture above). Now bend the transistor's right pin (the pins are based on a front-side view, i.e. looking at the flat portion of the casing that has its information printed on it) to meet with the remaining pin of the CdS cell, and solder them together as seen in the 4th image.

Step 6: In Goes the Capacitor and Buzzer

Picture of In Goes the Capacitor and Buzzer

Now you need to put in the capacitor. The negative end (marked with the black stripe with a minus sign on it) will connect to the transistor's emitter pin (leftmost pin when looking at its front) and the positive lead will connect to the joint - middle transistor pin and the two trimpot pins (images 2 and 3 above).

Once that is completed do the exact same thing with the buzzer. The negative lead (the black wire) will connect to the negative lead of the capacitor and the positive lead (red wire) will connect to the other pin on the capacitor.

Step 7: Power Up!

Picture of Power Up!

Now comes the last step, solder the positive lead of your 9 volt battery connector (again positive will always be red) to the positive capacitor and buzzer pins. Then solder the negative battery connector lead to the CdS cell pin that's only connected to the right transistor pin.

And there you have it, a fully functioning, laser perimeter alarm! As you can see, The very last photo is with a laser beam fixated on the sensor so that the alarm doesn't go off!

If you have any problems with getting the circuit to work, or just general Q&A questions feel free to leave them here. For more similar projects, kits for this and other gadgets, and much more just go to Ocalon Electronics


ElectricGS12 (author)2013-07-02

could this be use in ac power?

mmo4 (author)2012-08-08

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

usmm (author)2012-03-19

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

DrBondo (author)2010-02-20

 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 (author)DrBondo2010-12-04

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.

arulchris (author)Digy2011-01-01

can u post a clear picture pls.

remyzero7 (author)arulchris2011-06-09

arulchris (author)remyzero72011-06-12

thank u remyzero7 4 the pic.
i'll try this too..

amruth (author)2011-06-01

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.

amruth (author)2011-06-01

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.

litcrit (author)2011-05-17

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?

timmi (author)2009-11-01

 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 (author)timmi2011-01-15

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.

schumy00 (author)2009-03-17

where I can put a relay to make an alarm ringing continuu?

Digy (author)schumy002010-12-04

look up at my comment above.

kewltek (author)Digy2011-01-15

Post a higher res pic pls

randomness72.5 (author)2010-12-25

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?

Mxous (author)2009-03-02

Link doesn't work.

charchar97 (author)Mxous2010-08-04

yea it does. try using a different browser

fringe (author)2010-04-02

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.  :)

capjbadger (author)fringe2010-04-28

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 (author)fringe2010-04-02

Looked like I needed a pnp for this.. not a npn transistor as noted..

fringe (author)fringe2010-04-02

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..

iliasbill (author)2010-02-20

hi,what is the one left down the photocell --- and the one down the transistor ---
                                                                           --                                                             --
                                                                            -                                                              -

psmithplumbing (author)2010-01-16

hi there my buzzer is very low can anyone help me please pete

rushsraf (author)2009-12-07

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

9260074 (author)2009-11-22

specification of photcell plssss??

mates (author)2009-11-05

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.

Colonel88 (author)2009-10-16

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.

ipwnuall2 (author)2009-10-08

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)ipwnuall22009-10-09

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.

ipwnuall2 (author)weezoh2009-10-10

also which way do you turn the trimpot?  when the pins are like this like this -->   |---|--|*        *  = wheel
                           |  = Pin
                           -  = space between pins

ipwnuall2 (author)ipwnuall22009-10-08

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.

ipwnuall2 (author)ipwnuall22009-10-09

please reply.... i really don't want to start over again.

pook (author)2009-08-22

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)pook2009-08-23

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 (author)weezoh2009-09-22

9v to the base??? wouldn't that kill the transistor?

pook (author)weezoh2009-08-25

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!

weezoh (author)pook2009-08-25

The outer pins of the trim pot are interchangable, there's no difference between them they're the exact same. You can use either one as whichever. The way the trimming potentiometer works is that its a variable resistor. between the two outer pins is the full resistance of the trimpot say, 10k ohms or 5k ohms depending on the one you have. The middle pin divides up that resistance on both sides. So if its centered, then 50% of the resistance is between the middle pin and each of the outer pins. Or it can be 60%-40% etc. The way we're using it, we have it shorted so that one of the sides is always bypassed (where the connection between the middle and outer pin connected to the + 9 volts is). So its a one-sided variable resistance. Now, one of those outer pins should connect to the base of the transistor and to the photocell. The other outer pin should connect back to the middle pin of the trim pot and also to the +9 v. The emitter is the pin in the diagram with the outerward facing arrow and connects to ground or the negative lead of your battery. The collector connects to the negative leads of the capacitor and buzzer duo. Let me know if its still a little unclear.

pook (author)weezoh2009-08-26

Thanks for your help. I am still confused, in step 5 it says "Now make sure to solder the middle pin of the transistor to the two trimpot pins that are connected to each other" but you wrote: "Now, one of those outer pins should connect to the base of the transistor and to the photocell. The other outer pin should connect back to the middle pin of the trim pot and also to the +9 v. " So I am not sure what to do. Sorry for all the questions

weezoh (author)pook2009-08-28

Whoops, yeah the directions in step 5 are outdated I need to change them. Right, the connection of the transistor base to the two connected trimpot pins is outdated. Its based off of the 101 Spy Gadgets circuit which would ideally work off of current draws but doesn't work consistantly. This version of the circuit uses a voltage division between the trimpot and the CdS cell to regulate the voltage to the base of the transistor. Just go off of the directions i mentioned to you.

pook (author)weezoh2009-08-28

That is understandable..What would really solve this would be a wiring diagram. I drew images of all the components with . indicating "connects to": Collector pin>Negative capacitor>buzzer Base>one end of trim pot>one pin on photo cell Two trim pot pins,center and other end connected>+9v Emitter>-9v Can you makes a simple wiring diagram? I would never get that from the schematic Thanks Now what connects to the other photo cell pin, -9v? And does the buzzer/capacitor + connect to(>) ?

pook (author)pook2009-08-28

I meant > indicates that it connects to....

pook (author)2009-08-23

I did all this and when I clip on the battery,the alarm sounds. I put the lazer on the cell,but the alarm just keeps sounding. Trim pot doesn't do much. What have I missed? thanks

bigcape (author)2009-08-05

There has to be something I am missing. With a 5k pot there is not enough resistance to cut the voltage. Going all the way up to a 100k pot I can get adustment but is real faint buzzer Are you sure this doesnt take a lower voltage than 9v?

weezoh (author)bigcape2009-08-05

Try a 10k ohm pot to see if that helps, if not it might be because off the cell not giving that large enough resistance swing in response to the light. Are you using a laser pointer on it at all?

bigcape (author)weezoh2009-08-08

The trick is to get the Cds cell to match with the right pot!!! In my case (WE) had to use a 50k pot trimmed out to 32k at working room lighting. I don't know the math but using a 5k pot lets to much current pass and will fry the transistor. Hope this helps others trying to get it to work! Cause it WILL!

I am having the same problem with my circuit. I am using the 10k trim pot and it is cranked all the way up and I still have about 2-3 volts powering the buzzer. Any suggestions? Also I have a 9v power adapter that I would like to use to power my alarm. I would have a 9v batter as a backup in case someone unplugged it. Would this be possible or would I run into conflicts? I could also make it so there is only 5 volts coming from the power adapter. That way if someone unplugs it there wouldn't be enough resistance even with the laser on so the alarm would sound. That would be an added safety feature.

I hooked up a 6V power supply, and adjusted the resistance, but how would I go about installing a battery backup? I'm fairly new to electronics, and don't quite understand how to hook up the battery backup. If I hook it to the same wires as the adapter, I'm afraid it will try to charge the battery, until it over charges it, or will the circuit take the current off of the battery? So, basically I would like to know that, and how to hook up a battery for backup. Any help is appreciated!

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