Laser Beam Alarm System With Rechargeable Battery for Laser




About: Love cars, especially late 60's and early 70's American and Australian muscle cars!! I like to make things with my hands no matter what it is, and I love pulling things apart just to see how they work. Recen...

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.

Step 2: The Schematics

Now before I let you start soldering your components and making your custom PCB's and stuff I advise that you prototype everything on a Bread board. It took me a very long time to dial in all the components and even longer to get them to work together because I needed to do a lot of self engineering, and also because I am unable to tell you exactly which transistor to use in the LDR and alarm unit. Sorry.

Anyway, this is the first schematic and by far the simplest. The only confusing part is choosing the correct resistor to uses with your LM317T and your chosen rechargeable battery. I will explain how to do this in the next step, its actually pretty easy.

Step 3: Choosing the Correct Resistor to Work With Your LM317T

Now this is important if you are going to use a rechargeable battery and a solar panel, if not you can skip this step but if you are, read carefully.

Ok, a rechargeable battery hooked up to a solar panel will always recharge as long as the solar panel is producing more voltage than what the value of the battery is. For example my 3.6 Volt battery will recharge as long as the voltage is 4 volts and above. My solar panel produced a healthy 10 Volts so that is good; I dont need to worry about not having enough voltage. What I do need to be careful about is current.

Lots of current will charge the battery very quickly but will cause overheating and will kill your battery quickly. Too little current and your battery will charge extremely slow or not at all.

A general rule of thumb is that the optimum flow of current that you should try to maintain is 10% of the batteries current output. For example my battery was 850mA/H (850 milliamps per hour). So, 10% of 850 is...850/10=85. In this case the magic number is 85mA. We want our solar panel to produce an output of no more than 85mA per hour. To do this we need to choose a resistor which will work with the LM317T chip that will give us that control level. To do that we need this table:

Look at the fourth image for the table. You may need to view it at full size to see it clearly.

What you do is find your magic 10% current value and match it up with the closest current value on the table (bottom row) then look at the value above it and that will give you a resistor value. It is this resistor value that will give you the current flow that you need. In my case the closest value on the table that matched up with mine was 83.3mA. Above that is 15 Ohms. That is how I obtained the value for my resistor. You may get the same or you may get a different one, it all depends on the battery that you use. If you need any help with this just message me or leave a comment and I will reply asap.

Step 4: Schematics Part 2, the LDR and Alarm Circuit.

This schematic is much larger and contains a lot more components than the first one. What Im going to do is break it down into two halves and explain how each works. If you have experience with putting together schematics feel free to skip ahead to the image of the final schematic where you can get right to assembling.

For those that want more help continue to the next section where I will explain the first part of the schematic, the LDR part. For those that just want to start assembling, a schematic of the final product is in the image bellow.

Step 5: First Half of the Large Schematic, the LDR Sensor

The first half is the part of the circuit that sense whether or not the laser is on the LDR or not. The sensitivity can be dialed in with the 10K variable resistor. The only advice I can give you is to just play around with the variable resistor because the light levels will vary depending on where you put it.

Set up this half of the circuit on a bread board but leave out the relay, we are going to replace the relay with an LED for now.

TIP: I set mine as sensitive as I could; I then used a tube spray painted black to cover up the LDR to shield it from excess light. This way all I need to do is aim the laser down the tube and I can be sure that no light apart from the laser light will reach the LDR.

Before you throw the relay on, I have shown an LED in my schematic. Using the LED allows you to visually see the LDR working and how sensitive it is. This is how you should dial it in. Play around with the Variable resistor so that the LED lights up in almost complete darkness. When you turn the lights on, the LED should turn off. If you can get it to do this you are heading in the right direction. Next, get a family member, friend, or if you can manage yourself, cup your hand over the LDR, dont completely cover it, and shine the laser on the LDR. You should set it so that the LED is completely off when the laser is on the LED. When you move the Laser off the LDR which is still cupped in your hand, the LED should brightly light up. This means that you have set the correct sensitivity.

For a final test, if you are going to shield your LDR with a tube (I recommend it) put your LDR in it, line up the laser, and you should see that the LED is off. Walk through the laser and the LED should light up.

The next stage is to ditch the LED and replace it with a relay, but not yet!! Its best to understand what is going on in the second half of the circuit which is explained in the next step.

Step 6: Second Half of the Final Schematic, the Alarm.

The main purpose of this half of the schematic is to replace a design floor that I noticed in kipkay's version, no offence dude; I really love your work by the way, awesome!! Anyway, the problem was that when the alarm was triggered in kipkay's it would only stay on for a brief moment after the laser was restored to the LDR. This was because all he had to power it was a capacitor.

I wanted my alarm to stay on even once the laser had been restored to the LDR, and this is what I have done. How it works is the transistor (I dont know what type, I think NPN, pros help me out please) keeps the circuit open. Once contacts one and two (refer to diagram to understand what Im talking about) make contact they trigger the transistor to allow current flow to pass, this flow of current in turn keeps the transistor open, meaning it will not close the circuit (keeping the alarm on) until someone physically flicks a switch to reset/turn it off.

Contacts 1 and 2 are closed using the relay that I was talking about earlier. With the LED from the first circuit replaced with the coils of the relay, when the LDR detects that the laser beam has been broken, current will flow into the coils of the relay. These coils generate a magnetice field which closes the reed switch inside the relay. This reed switch is contacted to contacts 1 and 2, closing them which will turn the alarm on. Now the alarm will stay on because, it has a power supply all of its own.

Very confusing, I dont even know if I fully understand it, but it works, and it works really well!!

Step 7: Now Put It All Together

For those of you that followed the entire process I congratulate you because there is a lot of info which looks overwhelming but it really isn't. I could have cut it really short and not explained stuff but I wanted to because there are a lot of people who make great instructables and put a lot of time into them. This ultimately makes it a much friendlier instructable for people to use. I wanted to follow in the footsteps of theses that helped me with their instructables so I will make an effort to answer all of your questions, suggestions and looking forward to receiving some tips and advice on improvements.

Anyway, I just want to stress that it is important to test this whole system on a bread board first, then you can solder everything in and make custom etched PCB's and what not.

Start with the laser unit and then work on the larger more complex circuit. Once you are done you can make modifications and put them into project boxes to make them all really neat and tidy. I will show you what my final product looks like in the next few steps.

this is what my laser and alarm enclosures looked like once i put it all together:

Step 8: How I Put Together the Laser Unit

This is how I assembled and presented my laser unit. I found that just sticking the laser on the box made it very difficult to aim it into the LDR of the second unit. So I pulled apart an old torch that I had which used a flexi arm so that you can aim the light around corners. I salvaged the flexi arm and ran all the wires to the laser down the flexi tube, hot glued the laser on the end of the arm, covered the laser in shrink wrap to hide the hot glue, and mounted it on the box.

I think it works much better this way and it adds another degree of advancement. I also used a push on/off switch for the laser; some more switches to charge the laser, and used some crimp connector so that I could make my own sockets for the solar panel. This enabled me to remove the solar panel when I didnt need it any more.

Oh and one last note about this laser unit. Because we are making the solar panel charge the battery with 10% of the batteries capacity, it will take 10 hours to charge from dead in full sun. Which is pretty good?

Step 9: How I Put Together the LDR and Alarm Unit

This box is considerably bigger because I had to fit two 9 volt batteries and a pretty big alarm. I removed the LED from the LDR side of the circuit because it is not needed but I kept the LED from the Alarm side because it must be there. I mounted it on the box so that it would light up when the alarm activated. It also acts as an improvised low battery indicator. If the LED light but the alarm doesnt sound, I know that the battery must be weak.

The alarm that I used also had the function to make a pulsing sound instead of a single tone which was cool and it also allows me to have some control over the loudness of the alarm. The alarm I chose is rated at a very loud 120Db at 12 volts, but Im only using a 9 volt battery and only 6 of those volts make it to the alarm, so Im hearing about 60Db which is pretty loud on a full battery.

The switch on the top left turns on the LDR half of the circuit and the one on the far right turns on/ re-sets the alarm.

You can also see what I meant by using a tube as a light shield for the LDR, it works very well and allows the system to be very sensitive.

I cant give you a step by step explanation of how to solder everything because there are just so many possibilities plus I didnt take any photos or video of my soldering all of the components. Anyway take a look at the pictures for a closer look.

Step 10: Possible Improvments and Closing Comments

Well that it. You should have all the info you need to make your own LASER BEAM ALARM SYSTEM by revhead... me!

some possible improvements/modifications that could be made to this are; a battery status indicator could be added to the rechargeable battery that powers the laser; an automatic cut off for the solar panel so that when the battery reaches full charge, the solar panel will automatically stop charging the battery; a green laser is much more reliable, more stable, brighter, and travels greater distances than the cheapo red ones that I used plus they are really cool; a DC voltage converter could power the LDR and Alarm circuitry removing the need for the two 9 volt batteries; and you could rig this up to a microcontroller and some servos that would fire a bb gun/paintball gun all around the area when the laser beam is tripped!!
I have neither the skills, knowledge, nor the equipment to pull that last one off but if someone does it, please let me know.

Anyway, that is my instructable on how to build a LASER BEAM ALARM SYSTEM. I hope I was very clear and thorough in my explanation although I am sure that many people will need to read it twice to understand it because it can be confusing. If you have any questions, suggestions, hints or tips, please dont hesitate to leave a comment or to send a personal message. I will make a solid effort to answer each and every one of them.

Cheers and happy building!!



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152 Discussions


3 years ago

Is there a way to turn this into a door bell type. When you walk in a store and the chime says "Ding - Dong" when you break the laser contact. And the next person walking in the chime will ring again? I notice that you have to install a reset button to have the alarm reset over again once the beam has been broken. I would like to use this as a alarm when a customer walks in and not have to reset it again.



4 years ago

Can some share a video of making this plsss.. All thing is bouncing of my head

flying texan

9 years ago on Step 3

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.

3 replies
flying texanflying texan

Reply 4 years ago on Step 3

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

revheadflying texan

Reply 9 years ago on Step 3

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.

flying texanrevhead

Reply 9 years ago on Step 3

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.


4 years ago on Introduction

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


4 years ago on Introduction

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


4 years 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.


6 years ago on Step 9

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.


6 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.

Charles Mendoza

6 years ago on Step 6

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 ^^


9 years ago on Introduction

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

3 replies

Reply 7 years ago on Introduction

Use Multisim... a very powerful software,,, we use this at school for simulation,,, just try it...=)


Reply 9 years ago on Introduction

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.


7 years ago on Introduction

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


7 years ago on Introduction

Is this relay ok? it's a 12V one. Would it work?

or this one ?