Introduction: Laser Beam Alarm System With Rechargeable Battery for Laser

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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. Recentl…

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:
http://www.youtube.com/watch?v=kxvch0Lu3os

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.

http://www.youtube.com/watch?v=kxvch0Lu3os&feature=channel_page

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!!