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FrozenGate by Avery

Laser Safety Perimeter Fence, experts please help! Rookie here!

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Gravinus

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Hello Everyone,

I've done a bit of searching and I'm not finding too much info on exactly what I need for my project.

I have a pond behind my house and 3 small daughter (2 of them 2 year old twins). Basically I want to build a system that would alarm if they cross the beam and I would arm it when we are outside. I have a shed with power in the middle of the area I need to "fence off" to use as a starting point. I basically need to beam a laser 200' on both sides of the shed to mirrors and then I'm assuming back to the shed to a receiver of some type.

I've got a home automation system and a PLC that I can handle the activation and event triggering with, but I'm at a total loss on the feasibility and hardware needed to actually build this.

Does anyone have any suggestions on what would be the most appropriate path to make something like this work and be effective?

Any ideas and insight would be GREATLY appreciated!

Thanks!
 





Basically you want to build a garage door sensor thingy. Not sure what type of distance they can travel but something like that will already be set up.

The issue you may run into is that power of a laser required to go 200' may be outside of the "eye safe" range. And with the children closer to the beam they will see (or not see if its IR though the danger is still there) more reflected light coming from it.

Can it be done? Yes

Safely? I'm not sure, maybe someone else can chime in.
 
Thanks for the reply!

Yep, that is pretty much what I need to build, except the laser and the receiver would both be on the same side. So in all actuality I'm probably close to 400' in distance for each.

I was hoping to use an IR system so as not to attract them to it. As far as safety goes. I was hoping that when the receiver triggers a breakage in the beam, it would immediately deactivate the lasers power and it would require a reset to reactivate it. Similar to an estop circuit in design.

Thanks!
Ben
 
You're looking at some aligning issue if you win to bounce it back. Plus with the mirror out in the weatherit would most likely have to be cleaned frequently to not scatter the beam to much. As far as the laser cut off and reset I have no idea. It would be some circuitry I'm unfamiliar with but I'm sure it.can easily be done.

Just thinking.out loud here, if your receiving sensor was inside a tube of some sort that blocked out the abient lighting that was aimed at the reflected laser it should help you out. I think you would be able to do it with a much lower powered laser then. Still unsure if an"eye safe" laser can go that far and still be recognized.
 
Thanks for the replies and info guys!

Firelaser, thanks for looking that up! That is what I'm hoping to build, but something with additional range. That system is rated for 70' outdoors and I have to go about 200'. I could make that work if I strung 3 of them together and buried a bunch of low voltage wire. My hopes were to have the emitter/receiver in the same spot so I didn't have to run all that wire, but maybe that isn't going to be realistic.

As far as safety goes, I really don't have any basis of knowledge regarding laser/eye safety. But my thought was that with a safety relay I could disable the laser within 25 milliseconds of beam interruption. At that speed, I wouldn't even think it would be possible to orient an eye directly into its path, but I could be off on that assumption.

Thanks Again,
Ben
 
I can easily be shut off before someones eye is directly in line with the beam. The issues with high powered lasers is it reflecting off anything. It only takes a split second to do any damage.

I think the best bet would be to build it with a 5mW red laser(very cheap on ebay) and see if its even possible to get the reflection back and aim all set up with a visible beam. Then build a small light detecting circuit, put the detector on the inside of a pvc pipe with an endcap painted black all around as to not reflect any unwanted light into the sensor and then turn the beam on. See how your resistance across the photoresitor changes and build you're switching circuit off of that. When all is said and don't you can switch it over to a similar power laser in the IR range and at dusk use either a video camera or cell phone camera too see the beam and align it. I recomend the mirror also be enclosed protecting it from the weather. Also would have to be on a sturdy fixture so the wind or something wont knock it out of alignment.

Help that helps some
Speedy
 
Thanks for the info guys, that do it yourself project is exactly what I need for the receiver and I think mounting it in the black PVC as Speedy suggested is the only way it might work.

I tested a 5mw laser today and the beam held pretty tight out to about 150' and would probably even ready @ 200' without any issues, but the return trip and the collision with the mirror turned it into a muddled mess! I'm not so certain my sensor will get a solid reading on it, but I haven't had a chance to test it yet. I'll let you guys know how it works out!

Thanks!
Ben
 
Thats what I feared. Are you using a front surface mirror? A standard mirror has the reflective material behind glass. A front surface, as you might suspect, is on the front with no glass. You will get more of the beam reflected and a clearer reflection with a FS mirror.
 
This was just a standard mirror and you could see the beam being absorbed in the glass and causing it to glow slightly. That is a good point, I will have to see if I can order a front surface mirror, I think that could be a dramatic improvement.

Thanks!
 
Beam divergence will be a major issue. You may want to try placing a beam expander (lowers divergence) in the laser's path right after the collimation lens.

As far as eye safety goes, you should be fine if you stick to <25mW and ensure that the laser turns off after the beam has been broken.

I would suggest you not try to bounce the beam back to the point of transmission, as alignment will be hellish and mirrors get dirty VERY easily. Instead separate the receiver from the transmitter and run the low voltage power lines as needed. One beam path greatly reduces alignment headaches and reduces the output power needed by the laser due to removal of losses. With a beam expander and single beam path you could probably use 5mW without too much trouble. If the beam is too diverged at the shed you could recollimate it with a pair of lenses and double your effective range.
 
I didnt read the last 3 or 4 posts.

1- you can use an IR laser any time of the day. Im still not sure of the distance you need to cover. by 400' i assumed 400 feet (121m). A really well collimated 808 or 700nm beam (which you would need to know how to collimate, as you cannot see it) would still be in good shape for a photodiode. Either way, a 3-5mW red laser is easier to use.

2-You should collimate it as best as you can. As laser light is coherent and is able to keep its "tightness" for quite a long range, even at that distance a photodiode (few cents) can pick up a difference between no laser light, to 1mW to 3 mW to 5 mW. Id recommend though reverse voltaging around 18V for a nice signal (and a capacitor to kill some unwanted ripples). If u are still struggling with signal though, you can always add a simple lens before the diode and focus the beam on the photodiode...this way for sure you have a clean signal (personal experience)

3- As i think you wont have it on during rainy days, as the girls wouldnt be playing outside you dont need to worry about light scattering due to rain.

4-You HAVE to use a front surface mirror, and just some plastic tubing around it to protect from dirt and debris that can fall near it. Secondly, you will have to find one that also has a protection coating over the metal. A metalic coated only mirror wouldnt survive for a long time due to atmospheric humidity. I guess you can point it back to a place near your source, and then just displace the detector to wherever the beam is coming back.

5-As you said, that you can have it turn off as soon as something breaks the beam, you should not worry. A 3mW red beam, if shut in ms, will mimic our eye lid shutting, and you wont have problems. Plus humans, naturally shut their eyes to such things

6-My other concern is a bird passing in front of it or something...it would set off the alarm xD

*EDIT*
I read now the post above.
I think the divergence wont be a big issue. If you have a focuseable red laser, you can try to minimize it as much as possible, and then use the said lens before the photodiode. If the beam is big, you just focus it on the detector with a easily available lens ^^
 
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Thanks for all the info guys! I guess I'll try it with the mirror first, it would make things a lot easier. I was going to mount the mirror inside a small section of PVC pipe to protect it and spray a bunch of bug repellent all over it to keep the spiders and other bugs out.

So is there a benefit to going to a 10mw or 20mw laser? Also, does anyone have a quality laser with a good lens that they would recommend?

Thanks for all the excellent info guys!
 
The benefit is more power. So if the laser has diverged to far or isn't as clean on the return path there would be more of it to still activate the sensor. I would recommend shipping on ebay for something you can power with a 12VDC source. Thats a pretty typical power output. It depends on your source though.
 
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