Perimeter Intrusion Detection

[SETEC_Astronomy]

Anyone have suggestions for a non visible perimeter fence to detect the presence of a human or animal over say 50lbs? I can't use a typical PiR sensor, there are to many obstructions on the property. A LASER would do nicely to follow just the perimeter but it's to dangerous both physically and legally. The area to be covered is about 100ft square. I thought about a Doppler shift motion detector but I've not had luck with those in the past and coverage is a bit an issue, for a hobbyist anyway. Any ideas? Suggestions welcome.

[haklesup]

Doppler shift motion detector

By that do you mean the microwave motion detectors often used in car and home security systems?

Motion detection by way of video camera is also available, is the line of sight too obstructed for that as well? Are you concerned about someone hiding behind a tree or just approaching the target?

For good coverage, you may require an array of sensors. 100SF is not a very big area. About 2 king sized beds worth. Oops, not square feet but foot square, that makes it ~10k SF

How about geophones? Sense vibration. If you're not protecting a bedrock plot or a swamp, it should work. If there is a road nearby, you will need more than just threshold detection but some kind of DSP to process the output and eliminate certain background noise signatures. An array of geophones near the perimiter would allow you to dial down the sensitivity quite a bit so that you could just look for high amplitude vibration.

If you ever watched movies like Mission Impossible, you might conclude just about any perimiter alarm can be defeated if the subject knows how it works.

I suppose you could just get a dog, they make great intrusion detectors. Works in Acoustic, Chemical and Motion detection modes simultaniously and needs no electricity but the duty cycle might be an issue as well as daily maintenance. A small dog detector coupled to a large dog repellant is a good combo.

[SETEC_Astronomy]

I did mean microwave motion detection. Line of sight is to obstructed for cameras, yes. I'm just concerned with someone approaching the target. I'm not too concerned with the system being defeated Mission Impossible style, detection isn't that critical just beneficial.

Sadly a dog was the previous alarm system, she could detect a rabbit at full speed from 1000 meters, worked perfectly.

I plan to use whatever is come up with in a try it once and know better next time capacity. We've been having some trouble with vandals, can't catch them on camera, especially at night. The police aren't of much help without the photographic evidence to back up the claim,

[rolerbe]

If you could mount it high, looking down, say from top floor of house, etc. I.e. get out of the plane of the perimeter which has the obstructions, then how about night vision system to a standard security (time lapse) video recorder? Maybe a rotaing mirror system to get multiple area coverage, or a mirror system coupled to a set of std outdoor IR sensors.

[jwax]

It may take more than one, but PIR detectors have come a long way.
http://www.supercircuits.com/search_res ... 0detectors
80' range is typical, some with more range available, and two or more would cover shadows.
"Trail Cams", in Flash Mode, may just be enough deterrent to trespassers. (Lest they be stolen)
Or, wire up trip wires to fire a flash bulb. They'll think they're being photographed.

[gerty]

A lot of DVRs have motion detection built in to them,when motion is detected on the screen you can get an output on a db9 on the back. You can mask the lower part of the screen to prevent false alarms from animals.The above Supercircuits link is where we get our stuff from for the school.

[SETEC_Astronomy]

Thanks for the suggestions guys but budget constraints knock most of the offers off of the table unfortunately.

What I think I'm going to use for now is a LASER beam-break system and just take my chances. The LASER is a low power $1 store variety that works well enough to go the distance. To reduce the risk of injury by LASER radiation the LASER will be mounted only a few inches from the ground and I'm pulsing the LASER diode at a 1KHz with a .02% duty cycle. At a duty cycle of .02% the LASER is nearly invisible, I can't really tell when it's on in testing. I'm using a missing pulse detector with a photo-transistor to detect breaks which will sound an alert and trigger a flash bulb from a disposable camera (Thanks jwax). Though all of this is less than ideal I think it will serve as a reasonable deterrent and it costs next to nothing to implement.

I know the rules on giving and receiving legal advice so I ask you as voices of reason and common sense does anyone see a problem in going forward with this?

[gerty]

With a small diameter beam width, look for false alarms when leaves start blowing through your yard. We had a similliar problem with a outdoor ir beam set. Worked great untill the fall, then there was a lot of false alarms, because of the leaves.Luckily it was at a factory where a guard would just have to look at a monitor to check it, instead of the sheriff making a bunch of trips for nothing. If you add some timing to allow a leaf to go by, you might have some luck.

[SETEC_Astronomy]

With a small diameter beam width, look for false alarms when leaves start blowing through your yard. We had a similliar problem with a outdoor ir beam set. Worked great untill the fall, then there was a lot of false alarms, because of the leaves.Luckily it was at a factory where a guard would just have to look at a monitor to check it, instead of the sheriff making a bunch of trips for nothing. If you add some timing to allow a leaf to go by, you might have some luck.

Thanks, that's definitely something to pay attention to. The whole system is less than $10 at the moment so with that in mind do you think it would be worth my while to add a second beam spaced 7" or so in from the first to help eliminate false positives? Anyone walking/running onto the property would hit both beams while other things like leaves are far less likely to do so. It would be trivial to add a second beam and compare when the beams were broken to determine it was someone entering the property and not rain or a leaf that happened to break either beam at some point in the day.

[gerty]

I think a second beam would help, you also need to elevate enough so that bowser doesn't set if off. Sound like you can build it cheap enough that if it doesn't work it won't break the bank.

[haklesup]

You probably have a class IIIa laser and operating at 0.02% duty cycle might even shift that into a calss II category. I don't know of any cats that have been blinded playing with a laser pointer.

Here are some basic references on laser safety
http://en.wikipedia.org/wiki/Laser_safety
http://www.osha.gov/SLTC/laserhazards/
http://members.misty.com/don/lasersaf.htm

Lots more just google "Laser Safety"

[SETEC_Astronomy]

You probably have a class IIIa laser and operating at 0.02% duty cycle might even shift that into a calss II category. I don't know of any cats that have been blinded playing with a laser pointer.

Guess I should have included that info in the first place seeing as how it's germane to the conversation. The LASER I'm using is labeled CLASS II with a maximum output < 1mW operating at a wavelength between 600 and 680nm (box says "60-680mm" but that doesn't seem quite right).

I just wanted some opinions on the matter before I hooked everything up and got sued by some young kid who shouldn't be on the property in the first place.

[FOB]

You could always use the good old trip wire with a noisemaker instead of the hand grenades we used in Southeast Asia. Why not an IR laser? If they can't see it they won't know they were detected . Who's gonna look at a knee high laser (that is invisible) anyway? My sympathy is with you ...I think vandals are low-lifes.

[SETEC_Astronomy]

You could always use the good old trip wire with a noisemaker instead of the hand grenades we used in Southeast Asia. Why not an IR laser? If they can't see it they won't know they were detected . Who's gonna look at a knee high laser (that is invisible) anyway? My sympathy is with you ...I think vandals are low-lifes.

An IR LASER was really my first choice but the only IR LASER I have handy is from a CDROM drive. I've been told that LASERs from a CDROM drive are hard to power and are so narrowly focused they're of no use for long range transmission. I could screw with the optics a bit and see where it gets me but I figured it to be a bit of a hassle where failure is the only likely result. If someone knows better or has done this themselves I'd love to hear about it.

[haklesup]

I think you are in a very safe territory. Calss 2 lasers are considered generally safe requiring 10 to 15 minutes of direct unwavering eye exposure to cause limited damage. In my third link, it describes an experiment by a doctor who claims that red laser is considered very safe.

Furthermore, say that laser is 1mW (.001 J/s) and you are on for 200us at a time or 20ms per second cumulative. The exposure would end up being 0.2uW (0.2 J/s) a 10 second exposure would result in about 2 Joules of heat delivered to the eye which is about 0.5 calories which will raise 1g of water by 0.5C. I estimate an eye is 4-8grams (a wild guess) so you could only impart a tiny fraction of a degree of heating to an eye. Okay, so your not heating the whole eye but you get my drift.

With a class 2 laser, you should be on safe legal footing with 100% duty cycle.