An alarm is activated when when the barbed wire around my house is cut. - Page 1
Forum section: electronics / beginners

author: da1944
author_rank: Newbie
timestamp: September 04, 2014, 07:34:19 am
content: I built a 50 ft. barbed wire fence at the back of my house. I would like to connect the barbed wire to a simple alarm system that emits sound when the barbed wire is cut. Please help.

author: Kjelt
author_rank: Super Contributor
timestamp: September 04, 2014, 07:38:39 am
content: If you use a too simple alarm system then the smart Seal team that is coming to get you will just clip a wire on one side to the other side bridging the cut their going to make. Do you want it to detect this too?

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 04, 2014, 07:47:46 am
content: Ooh , sounds like my first paid electronics design project.This was an upgrade to a circa 1960 building alarm , it ran tripwires across the windows, a telecom type relay was continually powered by the loop to keep the alarm contact open. Problem was this needed battery replacement every month. I added a BC108 transistor with a 100k pullup on the base, and the alarm loop was connected across b-e of the BC108 (to keep transistor off when loop was intact).

author: da1944
author_rank: Newbie
timestamp: September 04, 2014, 08:06:39 am
content: Thank you for the reply. There is no smart Seal team in my neighborhood. This alarm is just to prevent school kids from jumping into my yard. Therefore a simple system is enough for me. I am not good at electronics. A simple circuit using a 12V relay and an alarm (like smoke detector alarm) would do the job. I do not know where to buy the components and how to connect them.Thanks

author: da1944
author_rank: Newbie
timestamp: September 04, 2014, 08:37:55 am
content: Is there any such alarm system available at an electronics store or at hobby store? While searching the answer through Google, I sawthe following reply to someone's similar question:"All you need is a 12 V relay which drops out when the loop is cut. Connect the siren through the back contacts".I could not find more details. Please help.

author: tautech
author_rank: Super Contributor
timestamp: September 04, 2014, 08:38:25 am
content: Quote from: da1944 on September 04, 2014, 08:06:39 amWithout a flag in your profile we can't suggest a local supplier

author: Kjelt
author_rank: Super Contributor
timestamp: September 04, 2014, 09:27:16 am
content: Ok well it is basically a beginners question which should be in beginners but if you have no seal team on your doorstep and just annoing kids to counter this should work.You need a normally closed 12V DC relay (so it opens if you power it up), a 12V DC isolated powersupply or battery (but since the relay continuously drains the battery you might need to replace it each month or so, or you take a rechargeable battery), a 12V piezo loud enough to alarm you, silent enough to protect your heart and ears.Connect as in the schematic and you should be fine.As a source for components we don't know where you are located, but if you have a radio shack, or electronic surplus store in the vicinity that would be your first choice.Because all are 12V components a car shop might also be a good start.

author: Psi
author_rank: Super Contributor
timestamp: September 04, 2014, 10:09:13 am
content: A typical SLA battery will run flat in a week or two when holding a relay in.You could use a reed relay to drive a large relay that powers the siren.Or have a 13.8V float charger across the battery all the time.A transistor and relay combination would be best, but that is getting more advanced.

author: SL4P
author_rank: Super Contributor
timestamp: September 04, 2014, 02:56:58 pm
content: Quote from: Psi on September 04, 2014, 10:09:13 amA big dog would be better.

author: G7PSK
author_rank: Super Contributor
timestamp: September 04, 2014, 03:22:59 pm
content: Do school kids come armed with wire cutters these days? It is obvious that your barbed wire is not tight enough if it was you would know when it had been cut due to the ensuing screams as the kid was wound up onto the next post by the wire.Not my circuit but the first one I found on the net, just try googleing broken wire alarm circuit it will bring up loads of them

author: Stonent
author_rank: Super Contributor
timestamp: September 04, 2014, 03:27:04 pm
content: Yeah just a relay that closes when the circuit is broken. You don't even have to use the barbed wire as the conductor, just wrap a few turns per foot of stainless steel electric fence wire around the barbed wire. They'll have to cut both to get the barbed wire out of the way, but you don't have to worry about the electric fence wire rusting away.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 04, 2014, 03:37:36 pm
content: For that matter, a mechanical switch. The tension on the barbed wire holds the switch open. If the wire is cut, the switch closes. No electrons were used for this example.

author: Stonent
author_rank: Super Contributor
timestamp: September 04, 2014, 03:48:31 pm
content: I suppose you could also just use a constant current power supply set just low enough to make the siren loud and make the barbed wire a big short, but run a siren in parallel with the wire so if the wire gets cut, the siren becomes the path of least resistance. You could even have several sirens so the one closest to the break would go off.

author: Kjelt
author_rank: Super Contributor
timestamp: September 04, 2014, 04:08:49 pm
content: Quote from: Richard Crowley on September 04, 2014, 03:37:36 pmI don,t know how they do it in the us but here barbed wire is me hanically fastened on a lot of poles in between, thats a lot of switches and wiring needed

author: Anks
author_rank: Frequent Contributor
timestamp: September 04, 2014, 04:16:07 pm
content: You could just use two lines of barbed wire next to each other and if the touch trigger a latching relay that sounds the alarm.Also a fine trip line made of fishing line could be used in the example above

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 04, 2014, 05:05:40 pm
content: Quote from: Kjelt on September 04, 2014, 04:08:49 pmA 50-foot span doesn't require "mechanical fastening on a lot of poles". It can be a simple stretched length that lets go completely if cut or disconnected.

author: eas
author_rank: Frequent Contributor
timestamp: September 04, 2014, 05:43:01 pm
content: Quote from: SL4P on September 04, 2014, 02:56:58 pmThen you have problem #2, and a robot that cleans up after a dog is a much more involved project.

author: 22swg
author_rank: Frequent Contributor
timestamp: September 04, 2014, 06:37:16 pm
content: The wire if taught would act as like a guitar string , connect a coil pick-up / amp to it and listen to the 'ping' when the wire cut .

author: SeanB
author_rank: Super Contributor
timestamp: September 04, 2014, 06:41:42 pm
content: Simplest is an outdoor IR beam to detect that there is a break in the beam as somebody goes over, or just run a wire in the middle of the coil of barbed wire and detect that it gets shorted to the barbed wire, or is cut.

author: Macbeth
author_rank: Super Contributor
timestamp: September 04, 2014, 06:54:47 pm
content: In my experience any form of electronic burglar alarm is useless. Everyone ignores them. The police only turn up much later, if ever to tell you your alarm is causing a nuisance, etc...Just get yourself some dogs. They are far far better than any electronic stuff and much cheaper in the long run (as in since my house has had dogs I haven't had a single attempt at a break in for 25 years, before that - plenty. I live in an inner city area. No need to pay professional alarm monitoring companies fees either).Dogs are much more fun too compared to a rubbish burglar alarm panel. I can leave my backdoor open at night and no smackhead dares his opportune burglary when faced with a pack of rabid wild dogs (they are cuties really, but scumbag doesn't know that!)

author: os40la
author_rank: Regular Contributor
timestamp: September 04, 2014, 07:33:56 pm
content: Place High-Voltage signs on the fence.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2014, 02:48:59 am
content: Quote from: os40la on September 04, 2014, 07:33:56 pmAnd apply high voltage.http://www.gallagher.co.nz/energizers.aspx

author: Dooces
author_rank: Newbie
timestamp: September 05, 2014, 03:25:46 pm
content: Quote from: SL4P on September 04, 2014, 02:56:58 pmQuote from: tautech on September 05, 2014, 02:48:59 amMight not want to do both.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 05, 2014, 03:32:57 pm
content: Quote from: Dooces on September 05, 2014, 03:25:46 pm100s of millions of animals are contained within electric fences around the planet.Fence chargers have been used for over 100 years.There have been some electric fences that used continuous, lethal charges (the Nazis were notorious for this).

author: da1944
author_rank: Newbie
timestamp: September 06, 2014, 06:11:24 am
content: What a wonderful group at eev ! I have never been in such a helpful and friendly group. Thank you.I will try the salbayeng (reply #2) circuit. It seems this is the easiest one. I will also work on kjelt's circuit (reply #6). Please help kjelt. Where do I find the normally closed 12v D.C. relay? Radio shack and ebay have several 12v relays. How do I find which one is the normally closed relay? Is there some specific name to it? Once I get this circuit working, I will work on transistor-relay combination as suggested by Psi (reply #7).I will also build the circuit suggested by G7PSK. By the way...G7PSK. Wire cutters are carried only by 1st and 2nd graders. A 3rd grader would be embarrassed if he is caught ONLY with a wire cutter.

author: IanB
author_rank: Super Contributor
timestamp: September 06, 2014, 07:07:00 am
content: Quote from: da1944 on September 06, 2014, 06:11:24 amNormally closed will usually be marked as "NC". However, many relays will have three terminals and can be wired as normally open or normally closed depending on which pair of terminals you choose.

author: Kjelt
author_rank: Super Contributor
timestamp: September 07, 2014, 08:08:52 pm
content: @da1944 see IanB reply. Can you use a multimeter and do you have one? Would be a great help for you to test some things if you have problems, you don't need a very expensive one.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 08, 2014, 02:00:04 am
content: Quote from: da1944 on September 04, 2014, 07:34:19 amDo you live in that big greystone house with the towers?

author: AlfBaz
author_rank: Super Contributor
timestamp: September 08, 2014, 02:17:18 am
content: Quote from: vk6zgo on September 08, 2014, 02:00:04 amhttp://youtu.be/JaLjwSpZ6Cs

author: vk6zgo
author_rank: Super Contributor
timestamp: September 08, 2014, 04:54:15 am
content: I was thinking more along the lines of this place:http://images.smh.com.au/2011/09/01/2595564/pentridge-729-420x0.jpg

author: G7PSK
author_rank: Super Contributor
timestamp: September 08, 2014, 09:01:45 am
content: If 1st and 2nd graders use wire cutters what do 3rd graders use, Bangalore torpedoes. Where is this school sounds more like a reform centre.

author: da1944
author_rank: Newbie
timestamp: September 08, 2014, 09:45:31 am
content: The place is United States of America. They start bringing guns.

author: da1944
author_rank: Newbie
timestamp: September 08, 2014, 10:25:52 am
content: Thanks Kjelt. I do have a multimeter. I can measure voltages and resistances. Ian B's message is very useful. It should help me find a normally closed relay. I have placed some orders at eBay to buy the components.I do not feel comfortable in my ability to build an electronic circuit. Is there somewhere I can buy a commercially made circuit. I would be glad to pay someone who is able to make such a circuit for me. I have several ac to dc convertors for power supply.Thank you for your help.

author: Kjelt
author_rank: Super Contributor
timestamp: September 08, 2014, 10:59:43 am
content: Are you sure you don't want to start with this hobby, it can be very addictiveIf you don't want to start and need help, better find someone more experienced closer in your vicinity, an free add in your local supermarket or contact the local radio ham amateurs is an idea perhaps?

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2014, 11:09:35 am
content: Quote from: da1944 on September 08, 2014, 10:25:52 amHow many strands of barb wire are there in the fence?If one is cut will both ends catch on another barb wire and still complete the circuit?You may be better with plain smooth wires?If you choose to connect an electric stock fence unit, you would be well advised to replace the barbs with plain wires so nobody could get tangled and possibly electrocuted with the pulses.Yes, it can happen and I have seen it with livestock....then deadstock.

author: da1944
author_rank: Newbie
timestamp: September 09, 2014, 12:40:58 am
content: Thanks tautech. Great point. There are 2 parallel wires which are connected in series. I will now use only one line. High voltage lines are too scary for me.Thanks again, Kjelt. I am already getting addicted. I have ordered parts for 3 different circuits, but do not feel confident. Therefore, it would be nice to have a commercially available back-up device. I live in Brownsville, Texas....a small town located at the southeast tip of Texas. It is not easy to find an electronics professional.Regards