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Old 09-25-2012, 12:18 AM #1
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Default Laser Obstacle Course

In fear of being the "noob" asking a stupid question (or a series of stupid questions) I have tried to do as much research on this topic as possible, but seeing as how I am new to the world of lasers and optics, I wanted to engage a savvy and informative group of individuals to make sure I did not overlook something simple or dangerous.

I recently saw an episode of the Big Bang Theory where they did a laser obstacle course (YouTube: "secret agent laser obstacle chess"), and I thought, wouldn't that be an amazing game for a youth event at my local church. Of course my first concern is for their safety, the second is how do I make this look awesome and have the kids talking about it for years to come. Who knows, it may inspire some future laser enthusiasts. I noticed two threads on this already, but they seem to have died off very quickly. If this idea is at all feasible I intend to make sure this thread is completed with schematics and pictures of the event.

The first thing I did is make sure I could design cheap receivers to detect if the beam has been broken. I have an electrical engineering background, so this was fairly easy to accomplish.

Next I checked on the safety of lasers, eyes, and crazy junior highers. My conclusion from what I have read so far is that for 5mW or below, our natural blink reflex will protect us from permanent damage. With the exception of green lasers, which may have a dangerous infrared component (if not properly filtered, as may be the case with many Chinese, cheap lasers) that will not trigger said reflex. Red and violet lasers appear to be generated directly, and therefore are safer as far as invisible components are concerned. So this leads me to my first question: how do I tell if my laser pointer is really <5mW? I have read up on DIY laser power meters, etc, but I was wondering if there are any ideas for how to do this at under 100 dollars. I would hate for this safety check to cost 10 times that of the rest of the project.

Well, that is probably a good starting point for this thread. At this point I am sure some of you have suggestions, questions, answers, or are screaming inside your heads (or out loud) "What is this idiot doing?" Any ways, let's discuss!


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Old 09-25-2012, 12:48 AM #2
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Default Re: Laser Obstacle Course

Help - Niece wants to set up a laser obstacle course
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Old 09-25-2012, 01:21 AM #3
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Default Re: Laser Obstacle Course

If you want the kids to actually be able to see the beam, while still being power powered enough to not cause any damage, then you're doing to want a green laser. They are by far the brightest for the power because of the eye's response.

I would not worry too much about IR, as it's usually an exaggeration anything more than 30cm or so from the laser. If you're talking about a 5mW green laser, the IR leakage is going to be minimal. If you are worried, either buy ones with IR filters (Which can be distinguished by usually a slightly blue reflective optic on the front), or buy some IR filters and install them yourself. Alternatively, place the lasers far enough away from the pathway that they can't stick their eyes directly onto it.

You will of course need a fog machine/hazer and a very dim room to get a good effect, as 5mW will be barely visible at best.

How can you tell if your laser is under 5mW, well, simply, you can't without a power meter. I'd first try holding a lens of some sort in front of the laser (regular magnifying glass will do), and get some thin black plastic. Put the plastic in the focal point and hold it for a few seconds, and see if it burns. If it does, then either your laser is over 30mW or so (very roughly), or it's putting out IR (In which case, try a bit further away from the laser itself).

If no matter what you do, you can't get the plastic to even melt in the slightest, you'll know it's probably sub 20mW. Below that the difference in power is basically discernible.

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Old 09-25-2012, 04:51 AM #4
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Default Re: Laser Obstacle Course

Thanks, Things, that was some useful information. I was aware that green would give me the most bang per milliwatt, but I was scared about the infrared by the report at laserpointersafety.com

That magnifying lens test sounds very practical. I read definitively that 5mW is safe as long you don't stare into the laser. Will the blink reflex still protect the eye at up to 20mW?

I ordered an assortment of parts already, I got the first of the red lasers in today. The green and violet lasers as well as the large box of resisters, MOSFETs, and photoresistors are still on there way. I am starting to get really excited for this.
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Old 09-25-2012, 04:56 AM #5
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Default Re: Laser Obstacle Course

Quote:
Originally Posted by T_Warne View Post
Thanks, I read that thread, but it ended up somewhere I didn't want to go with it. I didn't realize it was so recent, otherwise I would have started the discussion back up again. Do old HDD platters really work well as reflectors? If the cheap mirrors I bought don't do the trick, I may decommission some old drives
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Old 09-25-2012, 01:08 PM #6
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Default Re: Laser Obstacle Course

HDD platters are great mirrors, they're just a pain to cut (or break). Just don't get any fingerprints on it
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Old 09-25-2012, 04:00 PM #7
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Default Re: Laser Obstacle Course

^^Good advice about the power meter. But since power meters cost a lot for someone just trying to make a fun toy, you can instead use a photodiode to measure power. You are only working with one color here, so if you use a green laser calibrate a voltage seen on the photodiode, you can make the regression that shows linear response to intensity. You just need a green laser of known power, a photodiode, and a simple op-amp circuit to measure the relative power of your laser modules.

I think everyone here is correct about IR filtering and keeping things < 5mws.

To compensate for power and to save some money, buy a $40 fog machine and let it rip!
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Old 09-25-2012, 09:16 PM #8
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Default Re: Laser Obstacle Course

Expensive part is the needed haze machine to make a reasonably safe 5 mW unit decent.

Somebody is going to chime in and say, why a hazer? Well.....
Water based fog machines would work, but are messy. Not to mention a direct blast of fog in a kid's lungs is not a good idea.

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Old 09-25-2012, 10:53 PM #9
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Default Re: Laser Obstacle Course

Fine. We can put a few fan in front of the fog machine's nozzle to quickly disperse the cloud.

But hey, anything other than open air going directly into someone's lungs is not a good idea. Second hand smoking is probably the less harmful of the other compounds I can think of.
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Old 09-26-2012, 06:43 PM #10
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Default Re: Laser Obstacle Course

The youth group already has a fog machine, so I don't need to worry about acquiring one of those. I tested some red laser pointers I got in the mail with just a mister, and they appear to be quite visible. I am trying to borrow the fog machine right now to try that out.

I read somewhere that red laser pointers are rarely over 5mw, is this true?

I had a moment of brilliance regarding the safety of the whole setup: turn off the lasers when the beam is broken. I think my circuit can respond in 10ms, ten times faster than the blink reflex. Also, it is difficult (if not impossible) to break the beam with your eye, without first blocking it with some other part of the body. The end result: very little if any chance of prolonged exposure (>10ms) to the eye. I am planning on testing this with a 60fps camera if I can borrow one from a friend. That will give me 20ms resolution to ensure it works.
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Old 09-26-2012, 07:14 PM #11
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Default Re: Laser Obstacle Course

So I was reading up on all the safety lens posts that we have here. I am interested in using safety lenses for this setup, just to practice good habits and show the kids that safety is important. But obviously at 5mW, I need some low OD glasses if I still want to see the laser dot in the slightest. Do you know of any cheap glasses that fit this build, or a post/thread related to this? I want to use them during the aligning of the mirrors, since this is the greatest chance of getting a beam in the eye. After that, I will engage the auto-off and the rig should be safe for people even without the glasses.
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Old 09-26-2012, 09:56 PM #12
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Default Re: Laser Obstacle Course

Maybe dial down the laser diode current to where you can start with 1mw and increment from there?
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