Picture of Laser Beam Alarm System with Rechargeable Battery for Laser
Hi Everybody... I'm Revhead, and this is my first instructable so please feel free to give me advise and point out areas in which to improve.
The inspiration for this project came from Kipkay who posted a similar version (PROTECT YOUR HOME WITH LASER BEAMS) After looking at the comments from his instructable, I found that many people where having trouble getting it to work and thought there were some limitations to it, so here I am, posting my version of the laser beam alarm system which I built for my year 12 final in Systems Engineering. (Which made it to the short listings for the TOP DESIGNS EXEBITION.)

Once your finished looking, please give it an honest rating, thank you!

My version is different in the following ways; I have a solar panel to recharge the battery which powers the laser, a current regulator to control current flow to the battery, a different LDR (Light Dependent Resistor) circuit, and a relay circuit so that the alarm stays on once the laser beam is broken.

Step 1: Parts That You Will Need

Below you will find a list of materials and components that you will need to construct this instructable, A Laser Beam Alarm System!

Laser and rechargeable battery unit:- Solar Cell capable of anywhere between 6-12 Volts
- A Laser pointer which you can pull apart (I used a cheapo red one but it would be really cool if you had the money for a green one)
- LM317T current regulator chip
- Appropriate resistor for LM317T (will be explained later)
- A 3 Volt rechargeable Battery (I got mine from an old cordless phone)(the battery doesnt need to be three volts, that just what my laser needed, choose a battery that is appropriate to your laser)
- Some switches
- Soldering equipment
- Adjustable flexi arm for aiming the laser (optional but worth it)
- Hot Glue
- Shrink Wrap
- Small Project Box
- Crimp Connector

LDR and Alarm unit:- LDR
- 10K (10,000 Ohms), Variable Resistor
- 10K (10,000 Ohms), resistor
- NPN transistor (I used a 2N3904 type but any should work)
- LED (I used Green)
- 510 Ohm resistor
- A Small Reed Relay (I used a 5 Volt DC one)
- 2K2 (2,200 Ohm) resistor
- 120 Ohm resistor
- Buzzer 6-12 Volts will work
- A Second Transistor ( thank you to collard41 who clarified that this is infact a NPN transistor)
- Some Switches
- Two 9 volt batteries

It looks like a lot and seems hard but it really isnt, I will guide you step by step and as best I can.
JonathanA12 made it!16 days ago

Nice post. The latest one with advanced technology is always appreciable by us. All of us like to spend a secure life. Security is the first and foremost thing to spend a relax and comfortable life. We use different security system to prevent the upcoming problems and make us alert from the upcoming problems. The need of security systems are gradually increasing with respect to time. http://security.chubb.co.nz/products/looking-for-a-safe

ShivanshS11 month ago
Can some share a video of making this plsss.. All thing is bouncing of my head
Why is it that the chart shows that you need higher resistance for lower current? to me it seems backwards, it makes more sense to have more resistance to dampen down more current.

face palm... i stand to correct myself. (big DUH to me) higher resistance = less resultant voltage and therefore less current.

revhead (author)  flying texan5 years ago
im not to sure, i pulled that info directly from a web site. i dont hav a very good understandin of the chip that i used, havnt used one since then. i think to properly understand how to choose resistors you would need to understand the inner workings of the chip.
I bet the chip is just a high tolerance anolog switching transistor rather than the most common digital ones that are on/off-only when you apply voltage to it's gate. this one is probably a transistor that controls current flow depending on how much voltage is applied to the gate, and I say high tolerance because it's able to be bolted to a heat sink. To me it sounds like it's synonymous to a variable resistor, but instead of calibrating a knob you calibrate voltage.
mfakhar9 months ago

PLEASE tell me that, how can i make my buzzer to sound only for a few seconds, when the laser is disconnected, please tell me the schematic or what should i change in this circuit, plz tell me ASAP

mfakhar9 months ago

PLEASE tell me that, how can i make my buzzer to sound only for a few seconds, when the laser is disconnected, please tell me the schematic or what should i change in this circuit, plz tell me ASAP

dsr980110 months ago
working fine...i connected a doorbell in alarm part..going to try connecting a handset that will send me a message on my phone!
I like the circuit, going to make a crack at building one soon. I'll likely modify it with a microcontroller, keypad, and LCD, Making it to where the system can be armed and disarmed with a keycode.
scastillo42 years ago
revhead, or anyone please help me. The LED that will be replaced with a relay later on turns of when the LDR is in complete darkness. The LDR need a little light in order for the LED to work. If I point the laser to the LDR when there is little light, it works normally. But when I point the laser to the LDR with total darkness, nothing happens. Please help me.
VK2WAR2 years ago
G'day people. Well done Rev. Excellent work and well presented. The issue many I think were having was using a PNP transistor and hence quite correctly Rev used an NPN transistor. One thing I would suggest, even as a Military Electronics Tech in Communications we were told point blankly at Category School, never screw around with regulating power from an AC source whether it be 115VAC (As is on all US military ships as it is in US Households) or 240VAC as is the case in Australian households. You do not want to start a fire or corona your regulator. You need to keep the smokey stuff inside your circuit. When the Smokey stuff comes out of your components the result is never good. Therefore, what I suggest is a trip to Jaycar, or Walmart depending what continent you live on. Buy a 12 Volt Transformer. Or better still, what I would do, is get a decent 12V Sealed Lead Acid Battery. I would use a diode (as someone already stated) or a solar charger unit to provide a slightly better regulation performance. You can now get at least from Jaycar, a variable DC-DC voltage transformer with multiple outputs. You can add IC voltage regulators if you want to have your battery power more than just your laser or a simple voltage divider circuit can help you out if you don't want to worry about the expense.

Over all I reckon you've done an excellent job. But as I said earlier, unless you either have no alternative, or you are a whiz on creating regulated power step down transformers, then DO NOT screw with your household AC circuit by building a perfectly good fire source. It's just not worth it, and your insurance company will probably throw a fit. SLA's even small level Amp Hour batteries will do the job and if your solar panel provides enough power to charge the battery to full capacity during day light, and the battery has a high enough rating to cope with running the laser constantly then you will have accomplished the job. One thing you could do is utilise some fast pulse code modulation on the laser, and providing a delay circuit on the receiving LDR so you save an amount of battery power but do not trip the circuit. During the off cycle that the PCM creates. Food for thought, but a brilliant first crack, and I am very envious of your attempt. I made a similar circuit back around 1985 when I programmed my ATARI 800XL to do the very same job utilising some BASIC programming and a cannibalised Infra Red remote control.
Greetings, that's a really great project, but the alarm circuit is troubling me... does it work? is it really possible to connect the base of the transistor to the positive terminal of the battery through a 120 Ohm resistor? According to my understanding in transistors, it only turns on a load (siren or LED) when the Collector is connected to the positive terminal and the emitter being connected to the ground while the base being connected on the positive terminal. But when you remove the connection from the base to the positive terminal of the battery, the transistor turns OFF... and same as it is with the siren. I hope this helps ^^
pjsiwa3 years ago
akinich5 years ago
hey its me again
i know its been a long time
i have finally completed the project
its so cool
ok my question is
did you use a special software to design the circuits
if you have please tell me about it
jjayag akinich3 years ago
Use Multisim... a very powerful software,,, we use this at school for simulation,,, just try it...=)
akinich jjayag3 years ago
hey thanks
will do!
revhead (author)  akinich5 years ago
hey, good to hear that you finished it!! did you have fun building it? i used a program called "DIA" its basically like Microsoft paint but it has a lot of electrical/electronic symbles which you can drag onto your picture. just Google it and you should be able to fid it, its free.
about the transistors, they should be the same both are NPN. if you google seach the code on the transistor (e.g 2N3904) you will be able to find a circuit diagram of that type of transistor. a NPN will have an arrow that points out (picture of circuit diagram) this is also a good way to find out wich pin is the emmitor. make sure you have the transistors in the right way. on most the middle pin is the BASE, the pin with the arrow on it (mentioned above) is the EMMITOR, and the remaining one is the COLLECTOR.
jjayag3 years ago
could you please specify the part number of the reed relay??. I really need it for my project.. I've already completed the circuit and is working fine,,, but the alarm wont remain "on" once the path is covered already,,, Actually the reed relay is the one that makes this purpose.,, But according to your design the reed relay must have 6 pins (2 for the input, 2 for the output and 2 for the reset)... Is this the one that you used?
Pls reply as ASAP,,, tnx in advance
Is this relay ok? it's a 12V one. Would it work?


or this one ?
revhead (author)  mayur.phadte3 years ago
not to sure if those larger mechanical relay will work? you need to keep in mind that my design uses at 9v battery and a 3v battery, not a 12v one. the voltage rating on my relay was 5v.
you can easily adapt my design to run off 12v mains power (using an adaptor) and just change the resistor values
I am getting the exact opposite in step 2.
My Led Goes on when there is light,and off when dark.
I will try to get it right,but do u have any suggestions so that I can modify something else and live with this shortcoming of mine ?
revhead (author)  mayur.phadte4 years ago
i cant remember off the top of my head, it was a long time ago. but from memory i think if you switch the positions of the LDR and the variable resistor you can change the way the ciruit opperates. example you can change it so that the relay is active in the presence of light (how yours is) or when there is no light.

i think i put a note about this near the section with the diagram of the LDR circuit.
So its just like when ur relay is connected mine isnt and when urs isnt mine is.
I am not able to think of anything that will invert this effect.
What if I use any Hex Inverter Gate like maybe the 7404 and inverter the 1-0 ?
Any help?
cknudstrup4 years ago
One thing you may want to include is a Diode in the Charging circuit from the Solar panel so it does not rob power from the battery at night unless your solr panel already has a diode built into it. Also you really only need one switch from the solar panel tothe circuit on either the positive or negative circuit as either one will cut the power from the solar panel to the rest of the circuit. Its not wrong the way you have it just overly redundant.

Good job as a first major project though.

BILALOMARI4 years ago
jcatuling4 years ago
wow! awesome.. hahahah I wanna try it even though i'm a newbie in electronics..(I learned electronics on our physics course and having fun with it lol),

-Jenriel (philippines)

ynakamura4 years ago
wow i really appreciate it. can i ask some advice . can you help me make a simple low cost laser alarm? step by step . or can you make some video's ? i hope you can help me . thanks bro. goodluck . im begging you :)
revhead (author)  ynakamura4 years ago
no worries. well this is about as step by step as i can get it. i did make this in 2008 so it has been a while since i hav even looked at it haha. if you are able to read the wiring diagrams you shouldnt have any problems but if u do feel free to ask.
can u name the relay please..!
My shop vendor says he cannot sell components without proper name,the police has prevented them from doing so.
revhead (author)  mayur.phadte4 years ago
um ok haha. from memory mine was called a "5 Volt Reed Relay"
dats it?
revhead (author)  mayur.phadte4 years ago
yeah, thats what the relay is called. well, thats what its called in australia.
Digy4 years ago
Here is my schematic of what I used for all those people who unable to keep their alarm going after its been tripped, also does anybody have any ideas on how to wire this up to an ac adapter, cuz it uses a crapload of power, probably because my design uses two relays...

Also I have included pictures of my final design...
I also used a separate sensor unit for added ease of laser positioning.
Alarm Unit Diagram.jpg2010-12-04 22-14-10.438.jpg2010-12-04 22-14-29.792.jpg2010-12-04 22-14-46.484.jpg2010-12-04 22-15-19.022.jpg2010-12-04 22-15-31.781.jpg2010-12-04 22-15-58.524.jpgAlarm Unit Diagram.jpg
revhead (author)  Digy4 years ago
wow nice work mate! looks really good and thanks for modifying the schematic and posting it aswell, will help out alot of people, thank you. nice design and looks really clean. i would also like to see some pics of the insides and everything that makes it work because im interested in seeing different ways of laying out the components? i was thinking of re-doing mine so i will give your design a try. i am also thinking of making my own pcb using some copper board and etching solution, have you tried this before because this is new to me?
thanks for the comment and feedback,
Digy revhead4 years ago
Cool, glad you like it! I'll post pics of the insides after I get some parts to get a separate battery box with closed circuit alarm because the inside is so cramped with two nine volts, My box is actually a lot smaller than yours, at least it looks like that to me. But, ill give you a quick runnup of how I laid out the components, I basically took a 1" by 2" pcb board that I could break in half if iwanted from radio shak, and put one main circuit on one side, the other main on the other side, and the reed relay in the middle. Let me know if you have anymore questions.

Also, do you think If i took a regular 9 volt ac adapter and hooked up to the battery clips (both at the same time, in a parallels circuit, that it would work, b/c my batteries always runnout after a couple of hours of running, or couple minute of alarm activation.
revhead (author)  Digy4 years ago
form the sounds of it you live in america? well i live in australia and our mains power supply here is 240V AC. if i wanted to convert my circuit so that it runs off mains power i would buy/make a 9-12V dc adaptor that plugs into the wall to convert 240V AC into 9-12V DC. you could then plug the converted voltage/current into this circuit and it would work fine. just keep in mind though that this is a DC circuit, so it wont run on AC power.
Digy revhead4 years ago
Cool, I'll try that, I think I'll try to make some battery backup box too, and yea I do live in america, but I'm from europe.
Digy Digy4 years ago
Also, i am working on a system the that will prevent the power from being cut if it is hooked up to an ac power source, and the ability to add an external alarm and trigger, (expandibility)

I will post it up when finished.
tomx634 years ago
Dude, I've just started studying electronics and I have to say that this is quite impressive and very inspiring. Really awesome work! Thanks!
revhead (author)  tomx634 years ago
thanks mate. yeah this was my first real electronics project that i put together. i made it for my year 12 systems class.
u can tell its my first large-ish project cos the circuit board is just a mess of wires haha. if i were to make it again i would make it alot neater and clean looking. thanks for the positive comment!! ifyou end up making this and need a hand just shoot me a message and il do my best to help.
tomx63 revhead4 years ago
Yeah man. Thanks for the offer. I'll almost certainly take you up on that. I want to use this idea to turn on a light for some pre-determined amount of time. Originally I thought PIR motion detector, but I live close to a wooded area and I don't want the light coming on every time some critter gets too close. So the laser is an awesome idea.
Just curious... had you given any thought to also making the alarm unit work off of a renewable energy source?
revhead (author)  tomx634 years ago
that sounds like a cool idea! just make sure you buy yourself a good laser that can deal with being on for very long periods of time. yeah i did think about it but i was on a tight dead line because i had to finish the project asap so i could work on the folio that accompanied it for my year 12 subject. another idea i was going to integrate was to have a 5v DC converter option on the laser and or alarm unit so you can decide whether to run the system off battery, solar, or mains power?? never got around to it. anyways good luck with designing and building your project! id be happy to help if you run into any problems.
        My hearty congratulations for you. It is a simple but effective piece of work. I really appreciate that. But, Does it really works...? I mean, Is it possible to connect the BASE of the transistor directly to 9V battery supply through just a 120 ohm resister....?
        And more over, how can a transistor be in ON position, if its collector terminals are OPEN...? You said that the LED keeps the transistor ON. But I really couldn't get the logic in it... Please can you explain me that.....? I am eagerly waiting for your reply....! Please.........
revhead (author)  Dhinesh kumar5 years ago
well my project is very dusty know and is sitting right in front of me and it work perfectly, apart from the fact that my sister dropped the laser housing and broke the laser diode!! i am not very experienced with electronics, i dont study it, it is a fun hobby of mine and was my year 12 systems engineering project, all i had to do was present a basic understanding of the circuitry and modify existing circuits to combine them into one project. from your comment you really seem to understand what you’re talking about, so if you have time i would appreciate it if you could explain how it "actually works" and what you would change?? as to your question if it works...yes, it does exactly what i intended it to do, laser shines on LDR, when laser is broken from LDR it activates relay, relay closes secondary circuit which activates alarm, when laser is returned to LDR relay opens secondary circuit but alarm stays on thanks to transistor which keeps a segment of the secondary circuit closed to power alarm! that’s pretty much my knowledge, if you could expand on that it would be great.
benzy4iva5 years ago
hi rev,its me again,i have now got the ldr and alarm system(that is i have fixed the relay problem) to be working as it is but my problem again is that the alarm doesnt stay on when the laser is intruded and to make the alarm to stay on you only need to stand in the way of the laser for the number of time you want it to stay on...which i believe thats not how it supposed to work..please help me out.asap
revhead (author)  benzy4iva5 years ago
ok, the alarm turns on so that means your relay is working that means the problem is within the alarm circuit. the problem is probably the transistor that you are using, are you using the right type (NPN, PNP) i think i specified that it was a NPN, can’t remember off the top of my head. try switch it around or buying a new 1. the main problem with the alarm circuit is that i made that small portion in a year 7 technology class which was 6 years before i used/modified it to make my year 12 project! the thing is i dont know exactly what transistor it is??? i can’t really help because i don’t have a very good knowledge on transistors. the other problem may be that you are using the wrong resistors, make sure that they are the right ones with a multimeter, but its most probably the transistor.
also remember that the relay is what touches the two contacts together from the alarm circuit, so when the laser is removed the relay turns off which opens the contacts but this should still leave the alarm on, but it doesn’t so there is a problem with the alarm circuit. Instead of fiddling around with shining the laser on the LDR to test it, put the circuit back to the breadboard and test it by just getting two pieces of wire for the contacts and touch them together, when you touch the contacts together and the alarm turns on thats good, but when you open the contacts and the alarm stays on then it works perfectly and you can plug in the relay and the rest of the circuit.
if u need any more help im happy to help!
benzy4iva5 years ago
hi rev,i need some help here,is the led in the ldr unit suppose to go off when i ditch in the coils of the relay...and also am getting the alarm to stay on even when i have not placed the relay yet..pls help asap..
revhead (author)  benzy4iva5 years ago
with the alarm circuit, it is good if you get the alarm to stay on when the two contacts touch!!! the point of the relay is two link the LDR circuit with the Alram circuit. basically the relay acts like a switch.
not too sure about whaat your saying with the relay "ditch in the coils..."??
you have a very big problem if the alarm is going off even if you haven’t touched the two contacts together, the alarm isnt meant to sound until the contacts are touched, if this is your problem it has something to do with the transistor, try switching it around or getting a new one!
benzy4iva5 years ago
i have got the led on the alarm part to light when i touch the two contacts....my only difficulty now is the choice of relay since icant get that small amp one here....also i wanna know how you connected the relay....
benzy4iva5 years ago
thanks rev ..i got the ldr sorted today..am now lookin forward to sort the relay and alrm unit..thanx once more
revhead (author)  benzy4iva5 years ago
ok cool, good work! so you alright to get started on the alarm part yourself?? one tip, do not add the alarm yet to the alarm circuit, start out with just the LED and get that to turn on and stay on when the two contacts touch and are removed (very inportant that the LED stays on once the contacts have been closed, then opened!!!) once you get that working, then you can add the alarm to the circuit in parallel with the LED.
let me know if you need any help,
benzy4iva5 years ago
yes am using a breadboard,my project supervisor asked me to do the same thing,(that is switching the position of the ldr and the variable resistor) and i did but what i had was the led light stays on whether i cup the ldr or shine light on it...so pls help me out here..also can u give me the exact value and specs of the reed relay you used and also am in Ghana ..west africa...since my supervisor has no time for me i have chosen you as my supervisor...
revhead (author)  benzy4iva5 years ago
ok, cool. have you tried adjusting the variable resister? the variable resister is used to adjust the amount of light needed to turn the LED on. what you are aiming for is the LED to be off in bright light, and to be on in dark light. shine a torch on the LDR or go outside in the sun so the LDR is in a bright area and start turning the varaible resister until the LED goes off.
if the LED still stays lit then you have a problem with the transiter you are useing, make sure it is NPN, try turning it around, it might be broken so replace it...
if this doesnt work, take a picture of the bread board with all the connections on it and il see if its wiered up correctly.
also what power supply are you using, is it a 9v battery? also try useing the optional second 10K resister (normal one, non-variable) i didnt use it and it worked fine but you may find that you might need to.
oh and the reed relay that i used is a 5v DC low amp type, it looks like a small cylender with thin metal legs sticking out, you can see it in this link, mine was green: http://www.instructables.com/file/FN5A2OSFQ6DX8IY/
you can also see my 10K variable resister in the above link (small blue and whight box with a white screw on top, does yours look like mine??
benzy4iva5 years ago
hi rev ,thanks for the clarification,but check this out ,after i have wired the ldr unit(only) wit the 2n3904 transistor i had no outcome so i change it wit an old transistor i have( which i dont know its an npn or pnp)..the results i got was i had the led lit when there is light,and goes off when i cap the ldr which i suppose its vice versa to how to set the ldr...i need ur help pls to complete my school project...thanx..help me finish it pls
revhead (author)  benzy4iva5 years ago
ok, this circuit is reversible meaning that you can chose if u want the LED to turn on when there is light or turn on when there is no light. you do this by switching the position on the variable resistor and the LED! so...in the place where you have the LDR, put the variable resistor there, and where the variable resistor was, put the LDR there, hope you understand this, if not let me know, one question are you using a bread/prototyping board?? Because if you are this make testing sooooooo much easier than twisting wires together!!! you can get small one pretty cheap at electronics store. if you live in Australia (like me) check out JayCar or Dicksmith electronics!!
benzy4iva5 years ago
 rev pls i wanna kno...after u wire only the alarm ccct without the ldr unit connected to it,is the alarm suppose to come on? ...the reason am asking is that i wired it and only the led lit but the alarm did not come on....i need ur help pls asap.. secondly i wanna kno if its advisorble to use the  same npn transistor for the alarm unit...pls help
revhead (author)  benzy4iva5 years ago
Ok, your power supply should be good, when choosing the resister go for the higher value one just to be safe...
I’m not sure about your relay, did a quick Google search and it needs at least 5v and 1A to trigger, if you r getting these values and above in your circuit then it will work, large mechanical relays like your needs more power compared to small reed relays...
with the alarm circuit on its own, so...when you touched the two contacts the LED came on??? If it did then well done, that circuit works!!! (Make sure that the LED stays on after you open the contacts, this is the whole point of this circuit!) with the alarm not turning on, make sure that it is the right polarity (+,-), make sure it is connected in parallel with the LED (very important) what type of alarm is it, it might be too powerful for the circuit/broken/if it’s a piezo buzzer it might not work/make sure it is a DC buzzer not an AC buzzer.

I hope this info helps you out, if you are still having problems feel free to message me, I’m happy to help,
benzy4iva5 years ago
i got my self a 10ADC6V...JZC-20F....relay..i wanna know if its gonna do the job...pls asap 
benzy4iva5 years ago
i have got my self 10ADC6V... JZC-20F relay.....can i know if its gonna do the job
benzy4iva5 years ago
hi rev...wit the 10% of the current rating of the batt,,i had mine to be 250.... am wondering whether to use the 266....or the.....223 column..which one should i use...
benzy4iva5 years ago
rev thanks for ur good works..i wanna kno if i can use any relay not being a reed relay...the reason is i cant find the reed one in my side of town
revhead (author)  benzy4iva5 years ago
you dont have to use a reed relay, i just chose to use it becasue of its simplicity and ability to work with very low current/voltage. you can try to use a standard relay (not sure if it will work in this circuit?) but buy a small one that is designed to work with small current and voltage.
let me know how u go with it!
Digy5 years ago

People of Instructubles and Revhead! Please Help!

I'm building your circuit as my science project and am having some trouble.

I am able to get the ldr part of the circuit to work, and the buzzer ciircuit if I move the transistor (NPN the same one as in the ldr) up one so that the alarm activates only if the two conductors are connected together, becuase if I wire to your schematic, the alarm won't turn off. but when I connect the relay, the circuit wont work.

I really need your help becuase I need to be done by friday (4/23/10), so I need help asap!

I am using 2% tollerance resistors at 1/4 watt.

The relay is a reed rlay rated at .5a at 125vac with a coil resistance of 250 ohms and a nominal current of 20mA.

Please help ASAP!

Digy Digy5 years ago

I am acutually using a SPST - 12VDC Reed Relay Rated at .5 A at 125VAC with a coil resistance of 1050 ohms, and a nominal current of 11 mA!
Digy Digy5 years ago
Second update:

I fixed the relay issue but am unable to get the alarm to stay on when someone crosses the beam.

I have made one modification to the schematic that revhead gave me, i wired the npn transistor differently because the alarm wouldn't turn off when i built just the alarm portion.

PLEASE HELP!!!!!!!!!
revhead (author)  Digy5 years ago
hey very sorry I couldn’t reply to your comments earlier, just got wireless at home and my internet has been down for a couple of days thanks to a faulty router!
the only thing that I discovered from experience from messing around with this circuit is to leave the LED's in the circuit, DO NOT remove them because they act as diodes keeping the current flow in 1 direction.
The other problem is probably the transistor (faulty/damaged I went through 3 trying to get mine to work) or your power supply is two low/high for the components you are using
very sorry if I couldn’t help in time, let me know how your science project goes,
Digy revhead5 years ago
 Hey revhead,

thanks for the reply and for the advice, but i already got it to work cuz i just added another relay and changed the transistor position, there wasn't any issue with the transistor, just that your schematics for the buzzer part of the alarm system don't really make sense, because when i hooked it up according to your schematics, the buzzer wouldn't shut off because it was a complete circuit, i will later post the schematic i made for my "redesign" that works for me.

but thanks anyway.
revhead (author)  Digy5 years ago
The idea for my circuit was that once the laser was tripped the alarm would stay on until manually shut off, if this is what is happening then that is what my circuit was designed for. if you had the problem of the alarm turning on immediately after you added power to the circuit then the problem would definitely be the transistor, I probably made a mistake while drawing my schematic? 
I agree totally with your project and congratulate you for an astounding work piece. I find it interesting glancing through your project, but if you could convince me on you voltage and current ratings of your components, I think it would be much formal and complete a project.
revhead (author)  frank buxton5 years ago
thanks for your feedback, where do you suggest specifically that i need to improve my description?
handyman295 years ago
Where are the 4 fixed resistors on you IC board. I only see the pot. and one other resistor beside the 6 year old transistor?
handyman295 years ago
I have a question. Can I hook up a siren circuit (two 555 timers, a few resistors and capacitors) that I made where the "BUZZER" goes? It makes a sound similar to a police siren. If so then maybe I can give you the schematic.
luudvig5 years ago

Yes, by putting a electrolytic capacitor in paralell with the buzzer

ramirzeus5 years ago
hi bro!
can you please tell us how to connect the 2 schematics you draw 1 is with LRD and 1 is with buzzer or alarm on this part i'm so verry confused. thank's

you connect the contacts of the alarm circuit to the reed in the reed relay
and the relay pins parallel to the led in the lrd circuit
How much was the total cost of this system?

akinich5 years ago
sorry to bother again
another question
can the transistor in the alarm circuit be the same one as the one in the LDR  circuit because i got another one of that? and it is not working properly

                                                                                       thanks akinich

always put female on live coponents (solar panel)
and male conectors on power needing coponents (charger)

but everyone make fail in their life   me too i did some fail XD
revhead (author)  garrys newman5 years ago
i think that commenting with "fail!!!!!!!!!!!" is a little harsh and kind of rude, its good to point out peoples mistakes because it helps us to learn from them but don't tell them they "failed" just say that they made a mistake or suggest an improvement. i hope you don't go around to other peoples instructables and tell them all that they "failed" in doing something.
Kryptonite5 years ago
Would it be possible to make it so that the buzzer bit only goes for around a second?
Going to hook it up to a nerf dart shooter for an Indiana Jones style walk way.
revhead (author)  Kryptonite5 years ago
That sounds cool!
what I suggest is you do is scrap everything after the relay from my circuit and use the relay as your switch, just replace the nerf dart gun switch with the relay. If your nerf gun doesn’t have a power supply then you would need to connect a power supply in series with the relay and the nerf gun. Hope this helps. If you need any help or get confused just message me and I’ll try to help. 
minaedwar5 years ago
revhead (author)  minaedwar5 years ago
Yours is good, I tried not to use an IC just to make it simpler. The way you have it set out, does the buzzer stay on even when the laser beam is no longer obstructed (does it just ring once then turn off)? Because mine may be a little more complex but the buzzer stays on continuously even if the laser beam is returned to the LDR after it has been broken and will only turn off with a master switch. I think this feature is cool because the intruder that set the alarm off can’t turn it off even when they move out of the laser beam.
No , for my circuit the buzzer is non latching.It is actually designed to be a photocell , which can count the number of times the beam is cut (calculate the number of people coming into a room , or the number of boxes over a production line). THX For reply. Any way i found a solution for that problem , it's included in the comment.
LDR LASER tripped wire.bmp
mobhacker5 years ago
Thanks again I found one in the end from from a site called maplin.co.uk I some how dont think this will be the last time you hear from me never done anything like this before and im useless with a soldering iron so it should be fun.
revhead (author)  mobhacker5 years ago
good work on finding it, i would recomend that you buy a second one, just for back up, becuase if your not so good with a soldering iron, there is always a risk of damaging components. if you didnt want to use a soldering iron, you could always buy a small prototyping/bread board. these are great because they are just plug and play and they can be used over and over again for other projects. cheers!
mobhacker5 years ago
Thanks for that I will start ordering the bits tonight I think.
revhead (author)  mobhacker5 years ago
oh and about you asking about not being able to get the 5v reed relay, in australia i brought my components from a store called "JayCar" google search their store to find the home page and you will find it there. i know they do post their stuff but im not sure if they post to the UK. do some digging around on their site to see what countries they ship to. let me know how you go. CHEERS!
thanks mate I understand now. If it weren't for you I'd fail, my electronics teacher is useless.
revhead (author)  III ScYThE III5 years ago
haha, no worries happy to help. CHEERS!
mobhacker5 years ago
This is a nice bit of kit I'm going to have a crack at making it but even though I want it to charge I want to be able to charge it up using to phone's plug. Is it as simple as pulling out the charging unit in the phone and using that instead?
revhead (author)  mobhacker5 years ago
if you had an old phone that you dont use any more and still have the charger for it, yeah it should work no problems. what i suggest u do is keep the phone circuit bord in one piece, and just solder two wire leads in parallel to the battery. doing this is actually very good because it iliminates the need for the charging ciruit that i used which im a bit iffy about how well it works. nice idea, let me know if u get it to work. if u have any problems and need to troubleshoot just message me, im happy to help out. CHEERS!
I'm doing this for a school project in electronics and i really don't understand the reed relay part and the variable resister part (it has 3 leads and u only seem to use two) could u please show me somehow how these two things work?
revhead (author)  III ScYThE III5 years ago
ok first of all, the variable resistor. With most variable resistors there are three leads. For example a 10k ohm variable resistor has two leads on the end and one in the middle. if u don’t want it to be variable you connect the two outside leads to your circuit, but if you want it to be variable you need to hook up the middle lead and one of the end leads, so basically you need to hook up the middle lead to one side of the circuit and any one of the outside leads to the other side (only 1 of the outside leads, not both, u will end up with one lead on the outside not being used) The reed relay is built into a nice small package but is actual two parts, a little glass tube with bits of wire inside (like a tiny switch) and wrapped around the glass tube is alot of wire. The relay has four leads, one on one side and three on the other. The one on the end is one side of the glass tube and the other side of the glass tube is in the middle of the three leads. So, on the alarm side of the circuit you connect the one lead on its own and the middle lead of the three leads. On the LDR side you need to connect the two outside leads left over. The reed relay you buy will probably be different so u need to check the code on the side of it. Google this code and you will surly find a web site that has a picture of what each lead does. This is a lot of info and will probably be confusing. Just let me know if you need any help. Happy to help you out, cheers
kk khrab5 years ago
Could You plz send me a message as soon as possible i am only interested in charging the batteries. i got heaps of batteries but no knowledge of charging it with solar panel or cell i use i blocking diode which is not as good.
revhead (author)  kk khrab5 years ago
Sorry it took me so long to reply, I have been getting lazy with checking my emails. I can help, but I need to know what battery you want to charge? Is it a small rechargeable battery or a larger battery like 12 volts?? I need to know how many "volts" the battery is rated at and how many "mA/h" it is rated at. The mA/h rating it usually found printed on the side of the battery. Oh, I’m sure you know this but I will mention it any way, you can only recharge batteries which say that they are declarable, you can’t recharge all batteries. Let me know the details of the battery and I will help as best I can. :) CHEERS!
kk khrab5 years ago
Could you plz help me with finding the correct resistor value i have a solar cell which produces i.3 V.
imakethings6 years ago
you have made it much difficult (sorry for that) heres my version with capacitors instead of realy
bamphy6 years ago
I like the Instructable, but I would like to see it incorporated with a webcam. but that is another story and another Instructable
fvbno6 years ago
where did you get your pieces ???????????????????
revhead (author)  fvbno6 years ago
well I don't know what country you live in so I can't be precise as to tell you which stores to go to. I live in Australia and I found all the components that I needed at "DickSmith Electronics" and "JayCar. " you shouldn't have any problem looking for the components at these stores. if you live near one of these stores let me know and I can give you the exact product codes for each item! Cheers!!
pyroten revhead6 years ago
haha. i recognised the "green chrome bevel led thingi" from jaycar? yea i live in sydney. so if possible, could u post up the codes so i don't hav to go searching? thx and btw what dicksmith did u use. cuz the one at the mall is preety bad. thanks
if you live in the u.s., you should be able to buy all the parts at radioshack.
akinich6 years ago
hey how do you know when the photocell is at the most sensitive position?
revhead (author)  akinich6 years ago
good question. it is a bit tricky but easy when you know what to do. firstly I will ask, have you built this half of the circuit completely? you can only test the LDR when the circuit is completely built. secondly I just want to make sure that you understand: in the big circuit diagram on the LDR half you see a LED and part of the relay is connected in parallel to the LED, well that is the LED that we are going to use to test our LDR. when you have correctly set the LDR, you don't need this LED anymore, you replace it with the coils of the relay. OK now, how to test the LDR. what you need to do is build the circuit. once you build it connect the battery, and turn it on. if you have lots of light in the room, point your LDR toward the light, you will see that the LED will not light. if you now cover up the LDR with your finger, you should see that the LED will light up now. if this happens to you then well done, you have correctly built the first half of the circuit! now to make it really sensitive you need to do a second test, for this you will need the laser that you are going to use (it doesn't need to be pulled apart, it can still be a proper laser pointer). now, in a bright room with alot of light cover up the LDR with your hand to make a tunnel. you should cover up the LDR in a way so that your hand is blocking all the light from outside but leave a gap so that you can shine your laser done through your hand and onto the LDR. get someone to turn the battery on, and when the laser is on the LDR the LED should be off. now, while the Laser is still on the LDR get someone to turn the variable resistor a direction so that you can start to see the LED come on. now turn the variable resistor in the opposite direction until the LED just turns off. now test it. shine the laser onto the LDR that is covered up with your hand, turn the battery on, and quickly get a friend to swipe their hand through the laser beam, the LED should flash on and then quickly flash off again when the laser is back on the LDR. if this works for you then well done, you have correctly set the LDR. now be careful not to accidently move the variable resistor. now you can start to build the second half of the circuit. if you found this confusing or you are not sure of what to do at a particular stage, just let me know and I will happily help out. Cheers!!
akinich revhead6 years ago
hey thanks please put up the circuit digram i beg you cheers
revhead (author)  akinich6 years ago
no problem. what circuit diagram do you want. they are all there. if you dont want to follow the large one, i have also provided the two halves that make it up seperatly. all you need to do is build the two circuits (the alarm one and the LDR one) and join them together with the relay.
akinich6 years ago
hey what happened to all he comments i sent you?
revhead (author)  akinich6 years ago
i just checked instructables and they are all there? i answered them all and nothing seems to be missing. try opening it again, make sure you are loged in correctly, refresh the page. hope this helped
akinich revhead6 years ago
hey can you paste a picture of the circuit with both sides facing properly so i cam show it to my teacher (no problem know?) by today it is 7 30 am in chennai
revhead (author)  akinich6 years ago
sorry, i just went to the personal message thing and found out that you cant attach pictures. personal message me your email and i will send them to you that way. i dont know if you will get them by the time that you asked for. i only just got back from school and saw your comment. it is 5:07 in australia, 25th feb.
akinich revhead6 years ago
no i want the alarm circuit diagram not the pictures of the circuit because the digram is not proper half of it is upside down
revhead (author)  akinich6 years ago
oh sorry. the circuit diagram that is up there now is fine, just the writing is up side down, all of the components are in the right place. you could flip the alarm circuit if you wanted and it wouldn't matter because all that connects the alarm circuit to the LDR circuit is the relay. the relay does not carry any electricity, it is only a type of switch. all you need to do is make sure that the coils of the relay (looks like a spiral spring in the diagram)are connected to the LDR circuit, and the reed switch (looks like a normal switch in side a tube on the diagram) of the relay are connected to contacts 1 and 2 of the alarm circuit. I hope this cleared things up. I know this sounds confusing and it does. my advise is this. build the first half of the circuit (LDR) on a separate board. then build the second half of the circuit (ALARM) on a second board. then get a relay and connect them together. this would be much easier than trying to put it all on one board. I hope I helped clear this up, feel free to keep asking questions, I am happy to answer them, I will try to guide you through the circuit step by step when you start the build it.
akinich6 years ago
hey dude this rules to the core but it is a little more complicated than kipkays and i am only 12 can i use a one Milli Wat laser pointer can i take apart kipkays and make this or do i have to buy new things thanks akinich
revhead (author)  akinich6 years ago
It’s great to hear that you like it a lot, thanks! You can rate it at the top right of the instructable, you will see a bunch of stars and you just high light your rating. Yes, you can use a 1 mW laser but the range will be greatly limited, it might be a little tricky to get the LDR to work with it (cover the LDR with a tube like I did, this will help) My version is more complex than kip kays, but ultimately it has more functionality. I needed to make it more complicated because it was for my year 12 final last year. You will need to buy more components like different transistor, resistors, and a relay. If you live in Australia like me, you can go to DickSmith or JayCar and they will have everything you need there. If you live in America or another country I can’t be too much of a help there, sorry but you will still be able to find the parts at electronics outlets, hardware stores… Age shouldn’t be a problem here, its experience that counts, if you have messed around with electronics before, and know how to follow schematics, you should be fine. If you decide to go ahead and built it, let me know and I will be happy to help. i will be putting up a video of it working soon so check it out! Happy building CHEERS!!
akinich revhead6 years ago
hey thanks for the advice i live in Chennai which is in India . i have asked some one else to do the circuit for me . i rated this as 5 stars hey can you make the buzzer part of the circuit normal it is currently up side down
revhead (author)  akinich6 years ago
Thanks for the 5 star rating I appreciate it!! About the alarm circuit being upside down, I can’t fix it because I designed them separately on the computer, but didn’t realize till I stuck them down in my folio that they were upside down, oh well...

I think most 1 mW red lasers run on 3-3.5 volts. If you added more voltage you could make the beam brighter but it will probably break the laser very quickly. You can however adjust the focus of the laser to make the dot smaller, or more spread out. If you look at the front of the laser (were the beam comes out) there will be a small groove in it, so that you can put a small screw driver in there. When you twist it, it will adjust the beam. This should allow you to have some control over the laser dot.

You could use any laser you like, the more powerful/brighter the better. In Australia the highest laser rating that is legal is 5mW. This restriction also applies to green lasers which seem to be very popular. If you are interested in online buying you can visit this web site:


This is where I brought a cheap green laser. It isn’t the best quality but it is a huge improvement to the red laser pointer which you and I used.

Hope this helps. If you have any questions I’m happy to help you out.
akinich revhead6 years ago
can you enhance the beam from the 1 Milli Wat laser? before i took it apart it could but not after.which laser should i use help please
akinich6 years ago
hey how do you rate this thing
frankenboom6 years ago
I'm sorry to keep bugging you, but i still can't get it to work. I used a 2N3904 NPN transistor, which is the one i used in the LDR circuit. Thank you for any help!
revhead (author)  frankenboom6 years ago
dont worry, your not bugging me at all, im happy to help. the first thing i can say is that i am not entirley sure if that transistor works with that circuit becuase i used some kind of unkown type, sorry about that. also, are you trying to put the whole alarm and LDR circuit together with the relay on the one bread board at one time? if you are i would advise to you that you sholud break it up into two segments like i did, the LDR schematic, and the alarm schematic (forget about the realy for now). you said that you got the LDR circuit running which is great. know we just need to tinker around with the alarm circuit to get it to work. try the following; try different types of transistor (NPN), try a different alarm buzzer, make sure resistor values are correct, tripple check all connentions, try different LED's and try swaping the LED around. (LED behaves as a diode, you need it therer) i hope this helps you out, oh and when i was messing around with transistors i found that they are not very reliable (two of the three that i brought for the LDR circut where faulty!!) so buy a few of each type, there not very expensive and its always good to have them! and the relay can be abit of a problem, so get both circuit up and running, then worry about putting them together with the rely! CHEERS!!
solace1086 years ago
I congratulate you for your good efforts.
revhead (author)  solace1086 years ago
thank you very much, I appreciate it!! don't forget to rate, CHEERS!!
frankenboom6 years ago
That's really cool. but I can't figure out one part of the buzzer circuit. are the buzzer and led supposed to be hooked up to the base of the transistor, or the collector? Please help, because when i made it on a bredboard, i couldn't get it to work. But at least the detector part works when i build it, thats the closest any of these circuits have come so far! thank you, and i love this!
revhead (author)  frankenboom6 years ago
happy to help! the buzzer and the LED are connected in parallel with each other. they then connect to the base of the transistor. if this doesn't work it will most likely be the transistor that you are using. I used a really old transistor that I used in a year 7 project, so I can't give you the exact properties of it, sorry. if it doesn't work try using the transistor that you used in the LDR circuit. Oh and make sure that the transistors that you use are NPN, and not PNP. I hope this helps you, if not just message me or leave a comment and I will try to trouble shoot with you. happy building, dont forget to rate, CHEERS!!
frankenboom6 years ago
That's really cool. but I can't figure out one part of the buzzer circuit. are the buzzer and led supposed to be hooked up to the base of the transistor, or the collector? please help! thank you, and i love this!
solace1086 years ago
Thanks for your beautiful site. I hope in the future I can give service on business alarm systems - http://securityco-op.com
ReCreate6 years ago
revhead (author)  ReCreate6 years ago
if that "WOW" is saying that you really liked the Instructable, then I greatly appreciate your comment. thank you and don't forget to rate!
hey i will make this...but in a much simpler way
revhead (author)  ReCreate6 years ago
cool, its great to hear that you will be building your own version, thanks for the rating. if you run into any roblems during your build, PM me and i will try my best to help out! when you make it I would love to see some pictures of the finished product, and I would also like to know how you changed/modified it to be simpler. like I said, I look forward to tips and hints, and other peoples incarnations of my design. happy building, cheers!!
yeah but connected to a computer through the serial port so that it will record every time the beam is broken probably make it take a picture from the webcam every time the beam is broken!
revhead (author)  ReCreate6 years ago
nice idea. that would be like a professional security system if you got it working! I neither have the skill or knowledge as to how to go about doing this but if you get yours to work, I would like to see some pictures or a video. Cheers!!
actually i dont think it is that complicated...the only part that may be a bit hard is the photo(transistor?) is to get the computer to test for continuity from the photo(transistor?)is that what it is called? in fact i think i could replace the switch of a mouse(right click? ) with the photo transistor and make a program that takes a picture every time the switch is off(maybe use a keyboard instead?)
yeah i do like it and i forgot to rate ill rate it know
DonQuijote6 years ago
what a coincidence, i built one of these just yesterday. of course mine will not be solar-charged, but that's subject to retrofitting. here's an idea for you: when someone triggers the lase, the buzer only sounds while the laser is interrupted, which can be for a fraction of a second only. i am no electronics guru meself, but a little internet research on the 555 timer chip revealed that, in the proper configuration with a capacitor and a resistor, when triggered, even for a brief moment, the chip can turn on a buzzer for a longer time, depending on the values of the capacitor and resistor.
revhead (author)  DonQuijote6 years ago
when the laser beam is broken on mine the buzzer always stays on even if the laser is returned to the LDR. I have set mine up to be so sensitive that even for the briefest of moments, if the laser beam is interrupted the alarm will sound, and it will stay on until someone flicks the re-set switch. I will put a video up soon showing this function in action. I can't right know because I think my laser diode died while it was over at the TOP DESIGNS judging. that sucks for me :( thanks for your comment mate. dont forget to rate!
in that case, excuse my misunderstanding of electronics. i did not do that to my circuit as well, because it is supposed to be installed at a gate, where it should only warn that someone came in, nt really as an alarm system. but by carefully choosing capacitor, and resistor values for the 555 timer circuit, one can prolong that alarm-on period for quite some time... ....
revhead (author)  DonQuijote6 years ago
my original circuit was like your but with no 555 timer, I just used a large cap. I wasn't happy with this so I found the LDR circuit and adapted it to a relay circuit. the LDR acts as a trigger for the relay which completes a separate circuit, which sounds the alarm until some flicks a switch, or the battery runs dry. you could easily adapt my relay circuit with your gate alarm so that it could behave as an intruder alert system, keeping the alarm on to startle the would-be intruder, and alert the nieghbours. CHEERS!!
jezznar6 years ago
hmmm I'm wondering if you can change the 3rd module(alarm/buzzer part) into something like a motor circuit. I wanted to try and interface it into a door lock.
revhead (author)  jezznar6 years ago
That’s a sweet idea!! I think if you were to choose the motor wisely, perhaps hook it up to a gear box for greater torque it would be achievable. This would be so cool say on a bedroom door where all you need to do to open it was to walk past the beam or place your hand through the beam to disengage the lock. You could also make a spring mechanism to push open the door when it was unlocked. I like your thinking, very smart. Thanks for the comment, don’t forget to rate it!!
jezznar revhead6 years ago
Yeah that's a nice idea! Thanks! well I'm really new to all of these stuff. I'm still exploring how things work on motor circuits and currently studying them. Thanks for the appreciation! And I love your instructable! <3
revhead (author)  jezznar6 years ago
thank you for the rating!

if you are interested in motor control you should check out his Instructable for motor speed control:


I am currently making a Laser Spirograph using three of these PMW (Pulse Width Modulation) motor control circuits. it is very easy to follow and works great. I'm new with motors as well and making this PWM circuit has taught me alot, and I'm sure you will also learn alot.

jezznar revhead6 years ago
cheers! =) thanks =)
collard416 years ago
i see from the alarm schematic that you have the buzzer and led in parallel and then a resistor in series with both. correct me if im wrong, but that is probably a bad idea because the buzzer already have its own resistance (impedance) and therefore the current is limited across both the buzzer and the LED. and therefore the speaker may not function properly and the led might be getting to much current
revhead (author)  collard416 years ago
thank you very much for that tip, much appreciated. my unit now seems to be working fine, I just turned it on for a couple of minutes and felt the LED to see if it was over heating, and it felt fine, it was only a little bit warm to the touch. you seem to know what your talking about so if you wouldn't mind could you please give me some advise on how to correctly set up that section of the schematic, oh and thank you for pointing out what that unknown transistor was. cheers.
collard416 years ago
you are using NPN transistors throughout the circuit.
Very Nice Instructable! Well documented! Joe