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I have a Flymo 1200R robotic grass lawn mower.

The auto mower detects the boundary position of the edge of the lawn through a low voltage electrical wire that is buried close to the lawn edge which emits some sort of electrical field that the auto mower detects.

This boundary wire runs around the edge of the lawn to contain the mower within the desired grass cutting area.

Boundary wire is about 210 meters long.

Wire breaks in this boundary wire stop the system working.

One way the wire can break is if a rabbit, for example, digs a hole over the boundary wire and then chews through the wire. The mower will then stop working.

Finding the location of a break can be a long, long, task involving lots of digging in different places where the wire is buried.

This boundary wire emits some sort of electrical field or radio signal while the auto mower is mowing. I am unable to ascertain what frequency is emitted after a fairly extensive google search and youtube search.

I have tried moving round the perimiter of the boundary wire, with an am radio, set to a few different frequencies, while one end of the boundary wire is disconnected from the mower docking station, and the other is still connected and transmitting, but this did not pick up any signal, from the wire at any location. This method seems to work for some people as a youtube search can show.

I can disconnect both ends of this boundary wire from the mower docking station and pulse my own signal if need be to aid detection.

Can anyone recommend a low cost signal generator and detector kit, or other method to easily detect the location of the break in the wire.

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    \$\begingroup\$ google cable toner \$\endgroup\$
    – jsotola
    Aug 3, 2020 at 12:17
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    \$\begingroup\$ if there is only one break, then you have to dig only 6 holes to get within 75 cm of the break \$\endgroup\$
    – jsotola
    Aug 3, 2020 at 12:20
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    \$\begingroup\$ if you have access to a big box hardware store, then you may be able to buy a cable tester and try it out ... i think that you would still have to dig holes to get near the wire \$\endgroup\$
    – jsotola
    Aug 3, 2020 at 12:42
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    \$\begingroup\$ From user manual : "Use original boundary wire. It is specially designed to resist dampness from the soil.". If it is broken, you may be better off replacing the full wire. Also, "When the boundary wires to and from the island are laid close together, the robotic lawnmower can drive over the wire.". It is likely detecting magnetic field rather than electric field. \$\endgroup\$
    – AJN
    Aug 3, 2020 at 13:11
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    \$\begingroup\$ Worth a try: Audio amp 10s of Watts + out. Connect one output to ground and other to wire. Drive with ragey audio - higher frequencies may be better. Sensor an insulated metal plate laid along the line of the cable and pulled along. Coupling is capacitive. Detector COULD be as simple as "crystal earpiece" (VERY high impedance) between sensor plate and ground - operator body probably good enough. Or high impedance input amplifier. || \$\endgroup\$
    – Russell McMahon
    Aug 4, 2020 at 0:37

3 Answers 3

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There are dedicated devices for that purpose.

I own a Fluke 2042 :

Fluke 2042

The right device is an RF emitter that you connects to your buried cable.

The left one is a detector that displays the strength of the detected signal.

It needs a little practice, but I am very confident that you could easily follow the buried cable and find out when the signal disappears. As shown in a picture of the user manual :

Fluke 2042 manual

I found mine second hand on a well known auction site.

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  • \$\begingroup\$ I bet that is a fantastic bit of kit. Here in Scotland, Europe, that retails new on Amazon for £659, & used on ebay for £370. And yet it contains simple electronics. Fluke is super reliable. I get that. Yet Fluke devices are just some good quality simple components & an ASIC and LCD screen soldered together & well presented. There must be a way to create, or buy, a very cheap, practical, reliable alternative. Just now, for this job, I have gone for this at a small fraction of the cost. amazon.co.uk/gp/product/B07CSTN4CC/… \$\endgroup\$
    – Kes
    Aug 3, 2020 at 15:43
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Can anyone recommend a low cost signal generator and detector kit,

Well if you want a cheap function generator, you can make your own for very low of a price, since you are in the UK ordering the required components from RS is a good option since they deliever most of their stuff within a day!

There are countless designs online, but try to look into oscillators and find which one suits your application best. Alternatively, you could just buy a cheap microcontroller (like the STM32 blue pill for as low as 2£) to generate your own pwm at your frequency and this does not require that much of a programming knowledge, generating PWM is usually the first example when going thorugh the MCU's documentation. However, if you are familiar with fourier transform, you would know a sqaure wave has different frequency components, but sine is only one peak, and makes for a easier system. For this reason, I think a Phase Shift Oscillator best matches your need since they don't need gain controll.

I must admit I am not aware if there are any standard ways of doing what you mentioned, but what I would personally do if I am done wtih digging, I would make the oscillator as mentioned, and a tuned reciever (This would be your antenna, connected to your notch filter, fixed at your generator's frequency), and walk along the path of the wire, seeing where the picked up signal has the lowest peak.

enter image description here

Alternatevely, you could purchase all the aformentioned devices and do the same thing.

I don't know how fimiliar you are with these stuff, but being able to carry them out does require some level of knwoeldge, but it won't be rocket science.

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  • \$\begingroup\$ thanks for that. I would LOVE to spend time back with blue pills/ STM-32's/ Arduino IDE etc. I certianly have all the stuff for a build. What I don't have is the time. I do know exactly how I would spend that time if I had more of it. That's a great answer BTW. I'm going to think on this, anyway, becasue at some point I'm going to have to get the electronics out to make the automower more sensible by adapting it with RTK 2cm precision and waypoint capability. Far less cutting would be necessary because rather than it's moving randomly, it could be positioned to within 2cm & drive a set pattern \$\endgroup\$
    – Kes
    Aug 3, 2020 at 15:56
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Easy and cheap way: I have used this method a couple of times, use a portable AM radio (an FM-band only radio can not be used)- You will hear a humming, when you are above the cable, listen for variations in the humming.... That is where your cable is broken/cut. no need for digging up the whole lawn..

There is also cheaper alternative to the Fluke instrument, have one for arround 10$.

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  • \$\begingroup\$ I got a cheap £10 AM band radio from the supermarket and could not get this method to work. seems as if it should, but it did'nt. I had to make various bridges across the circuit to find the break. Not sure why it did't work. I will try again in the futire \$\endgroup\$
    – Kes
    May 12, 2021 at 13:58

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