Looking for cheap way to detect a signal in a wire buried into the ground  - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: May 20, 2020, 05:45:17 pm
content: I have a robot lawn mower. It use a perimeter wire to detect the garden limits. The cable is about 2.7 mm thick and buried 2 cm in the ground.From time to time, the wire broke somewhere.When this happen, the base station sends a radio signal in the wire that can be detected with a AM radio. I don't know the shape or the frequency but it consists of two distinguable tones.Last time it happened, the radio did not really helped (it was really imprecise) and in the end I had to find the exact place using a multi-meter and a bunch of needles. It was really time consuming.I knowcable locator testersworks quite well for that job. They are made of two parts : one emitter (which has a wire plugged into the ground and the other to one side of the perimeter cable) and one receiver. They are usually not cheap (about 100EUR but maybe there is cheaper models around)I was wondering if there was a way to build a simple circuit that would detect a signal in a wire easily (eg : a LED would progressively light up depending if something is detected or not). I was thinking some Darlington transistor configuration like in this circuit :http://hackaweek.com/hacks/wp-content/uploads/2011/09/Voltage-Detector-Schematic-copy.jpgAFAIK it amplifies whatever electric field is received on the antenna part (because of transistor configuration, with a gain of probably 1 million or more).The antenna could maybe be replaced by some sense coil. Would it work ? Is there other solutions ?EDIT : an op-amp (without any feedback loop) would probably works as well

author: bob91343
author_rank: Super Contributor
timestamp: May 20, 2020, 05:54:39 pm
content: I would imagine that if you excited the wire with a signal then you could probe around to see where it disappears.One approach might be to measure the capacitance. For instance, probe down until you find the wire in about the middle of where it should be and measure capacitance (or resistance, if the wire isn't insulated) from each end and compute the point of the break. A resistance measurement would work regardless of the insulation if you can measure accurately.What I would do is to use my $40 nanoVNA and measure the resonant frequency of the wire, first from one end then from the other. It will tell you the length of each wire and of course that locates the break. You can use a TDR as well (or the nanoVNA in TDR mode).All of this works best if the wire is insulated, so it doesn't confuse readings with ground effects. But even then, resistance measurement to a ground stake should enable calculation of the point of the break.

author: cdev
author_rank: Super Contributor
timestamp: May 20, 2020, 06:32:21 pm
content: The best way to do it if everything was a known quantity, wire buried at a uniform known length, soil all exactly the same, would likely be "time domain reflectometry" measuring the delay between when you send a pulsed signal into the wire and when it returns. A clean break in the wire will cause a ping to return. This can likely be done using a pulse generator. But the previous poster brought up a$40 VNA?But considering it I thought of some possible gotchas. Never having done this, I have to say, I just dont know.Another way might be to use a cable finder, or cable tracker, (rereading your post, yes, its the exact same thing you described as a cable locator tester, seems they now also coe combined with a cat5/6 and phone line tester. You can but one for $27 at amazon.. "Elegant" brand. ) Mine is a velleman brand, probably around $20, it can plug into network cables or just plain wires.The emitter is just a little warbler that is hooked up to wires, and the probe is like a tape head with an audio amplifier basically a (magnetic) probe that allows you to trace them. I have one Ive had for a long time and it is useful for tracing wires of any kind in walls. And they are cheap.. maybe even less than $20. But it does not also have the individual pair testing function, which is very useful.A VNA?However, one thing I wonder, how does the signal get coupled into the wire? My guess is you'd want to connect the VNA up to the wire right where it begins to go into the ground and connect the ground to.. the earth, i.e. ground! A grounding rod?If you dont already have a ground rod right there though.. does it work?(are we sure we need one?) No, we might not. Somebody here likely knows.Then would you need to know thevelocity factorof this system?One thing, I would suspect your velocity factor would change dependng on how wet the ground was. How much?I have no idea.(I think the wire must be insulated, right? Which of the wire is very long (hundreds of feet) is going to be important. Also, what if the velocity factor varies because of different kinds of soil and or different levels of moisture in it.All that said, under ideal conditions you should be able to solve it for where the wire is openhttps://en.wikipedia.org/wiki/Time-domain_reflectometerAll said, I would try the warbler probe first, if you have one around. Unless you really need a VNA for other reasons(like being a ham)If your electric border device already has a signal you can pick up, try using a radio tuned between channels - one with a built in loopstick, tuned between channels to pick it up.I bet you can find a basic cable finder for $10 or less.That may be all you need. I dont think the wire's break is likely to expose the wire to the bare earth enough to make any DC resistance measurement worthwhile. But if you use a bare wire, then the entire length of the wire is bare, That doesnt make a lot of sense to me, but maybe I'm wrong. Dd you bury the wire? (or did you buy the property with this system already in place)Frankly, its every homeowners dream to be able to mow the lawn with a robot. How well does it work?

author: ogden
author_rank: Super Contributor
timestamp: May 20, 2020, 06:49:18 pm
content: How exactly did you search for wire break? Try leaving only one end of the loop connected to the "base station", walk with AM radio, mark place where signal fades away. Then repeat with other end.

author: tigrou
author_rank: Regular Contributor
timestamp: May 20, 2020, 08:19:06 pm
content: Quote from: cdev on May 20, 2020, 06:32:21 pmIt is a good investment if you have a large, flat garden.Expect cable broken from time to time, no issues so far. It's not perfect (there is some small places where the robot cannot go and that you will have to cut yourself) but it save lot of time in general. It's also very efficient in the way that it cut the grass : it follow some clever pattern to do it as fast as possible (it's not random at all)Here is the result of last job :Light gray is what has been cut. Yellow dot is starting point.I have a child swing set and other things in the right bottom area that's why there is some uncut parts. The rest is close to perfect.The robot also will also use weather information and your schedule to find the best moment to do it.The only issue is the price. It's usually around 1000-1500 EUR price range (there might be cheaper models). To be honest it's too much for what it is (it's a niche market). Replacement pieces (eg : new wheels or battery) are overpriced too.

author: bob91343
author_rank: Super Contributor
timestamp: May 20, 2020, 08:23:32 pm
content: Yes the $40 VNA should work. It can be configured as a TDR as well. Connect it to the wire and to a small ground stake and see what the reading is. You don't need to know the velocity factor because you take two readings, one from each end (presuming you have access to both ends) and compare. That should nail it within a foot or so.Anyone interested in electronics could probably find many uses for a VNA. I can use mine to measure components (LCR) for instance.

author: pqass
author_rank: Super Contributor
timestamp: May 20, 2020, 11:04:26 pm
content: If you have a scope and any NAND/NOR/NOT schmitt trigger input logic gate, you can make a TDR.See:I just tried it using my scopes' built-in pulse generator set to 1kHz, 260ns pulse width, 5V output. I was able to measure a 30ft coax (open at far end) within a foot. However, when I tried with a much longer length of spare insulated steel clothes line (open at far end), I got nada; no characteristic step in the waveform! So maybe using TDR in this way only works with coax given that an unshielded wire is just an antenna which represents a load.I don't know much about RF so I would just attach one 9V battery terminal to the buried wire at one end, then walk around with a long wire connected from the other 9V terminal in series with a piezo buzzer then occasionally hunt for the buried wire and poke at it with a pin (in a binary search-like pattern).

author: cdev
author_rank: Super Contributor
timestamp: May 21, 2020, 08:56:18 pm
content: I have always wanted a VNA but couldnt justify itbecause of high prices. Its exactly what I need to take my interest in antennas to a new level. I have to research it a bit but the portable aspect makes it look super useful.

author: wizard69
author_rank: Super Contributor
timestamp: May 21, 2020, 09:15:31 pm
content: You can get items like this:https://www.amazon.com/Circuit-Tester-Automotive-Diagnostic-Tracker/dp/B07KY5LQFP/ref=sr_1_2?dchild=1&keywords=automotive+cable+tester&qid=1590095160&s=hi&sr=1-2that are common in the automotive repair world fairly cheap. They are designed for the realities of the automotive world but should work for a shallowly buried cable.You could do it yourself by throwing a pulse generator on the termination end and building a pulse detector to run along the cable. Probably a good reason to outfit your lab with a pulse generator. Every need like this should be used to justify new lab hardware!!!

author: bob91343
author_rank: Super Contributor
timestamp: May 22, 2020, 12:30:12 am
content: The clothesline experiment probably failed because the antenna happened to be around 50 Ohms. That's why a swept frequency method could be better.But TDR is a powerful tool. I built one and found it accurate but since acquiring the nanoVNA I took it apart. The nano does many things at once, including measuring line length, characteristic impedance, and losses.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 22, 2020, 12:49:37 am
content: Quote from: pqass on May 20, 2020, 11:04:26 pmClothesline wire is normally only a single conductor, not a transmission line, so you would have to use two pieces to provide the second conductor.Or did you use an "earth return"?Even with that, I doubt that you would get a useful result.

author: Qmavam
author_rank: Regular Contributor
timestamp: May 29, 2020, 01:45:25 am
content: Here is a method that I have used and it worked extremely well.You need an AM radio with a ferrite rod antenna. (almost all portable AM radios)Tune it to a fairly strong station, hold the radio so the back is facing the ground. Now rotatethe radio until you null out the station, (you want the station signal to completely fade away).Now hold that angle and run it across the grass over where you think the wire is. The wire will disturb the RF signal and degrade the null, when you get close to the wire suddenly you will hear the radio signal. Be sure to turn off the circuit you are looking for.This helped my find a buried coax 10 years after I buried it.Good Luck, Mikek

author: tigrou
author_rank: Regular Contributor
timestamp: May 29, 2020, 08:25:23 pm
content: I ended up buying some Incutex wire detector on Amazon for 25 EUR.It does not looks like high end product (eg: cheap plastic, to be expected regarding the price) but it does the job very well.As a bonus, it can also can test RJ45 individual wires.