Inductive Loops

I have been looking for a circuit to build an inductive loop, that is easily installed like the 'electronic dog fence' things, around my
yard. I also need a corresponding sensor to keep my 'bot in the yard.
Any idea where I can find such a circuit?
I know that the Friendly Robotics mowers use this technology.
I also remember YEARS (like in the 1960's) an article in Mechanics Illustrated or some such to put a coil around your living room then have wireless headphones to listed to your TV or whatever.
Thanks ... JC
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jc wrote:

I cant say I'v come across what your talking about (though I think I get the idea) however I can tell you that I'v found that 48V telco relay coils make great pickup coils.
dan
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rue_mohr wrote:

I recall seeing this done once using just a surplus audio amplifier and a cheap audio signal generator. They'd used a run of 3 core mains cable around the perimeter with the cores crossed to make a 3 turn loop. I can't remember if they even bothered with any impedance matching - might have just been a resistor in series to just save the amplifier from shorts on the loop.
IIRC The pick ups were 200 to 300 turns of fine wire on a small ferrite rod. It might have been in Practical Electronics (UK mag) some years ago (20 or so).
Might be something to play with. But watch you don't destroy a valuable amplifier, they aren't really designed for driving long lengths of wire!
Jon.
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I have found articles on the internet, about making inductive coils for the hearing impared.
Let's say you have a hearing aid and want to hear your stereo in the living room. If your stereo is set up for two sets of speakers, set one of them to 'mono', and determine what impedance of speakers it likes to drive (typically 8 ohm, but there are others). You want to wrap your room 2 to 5 times (no more, due to distortion), determine how much wire it takes to do this. Now get enough wire to do that plus make connections where its resistance is about (plus or minus half an ohm of what the amplifier wants for its speakers) and connect the wires from your loop up as the speaker.
Get your hearing impared person to set their hearing aids to T or T&M (for telecoil), and slowly crank up the volume on the 'coil' you just installed till they can here it nicely.
... Not to do the same for my 'bot!
I don't know what frequency that I should use (60Hz would be easiest for me in the US), and see if I can find a circuit to detect that frequency and so I can read it easily into my controller (TTL levels of course).
Suggestions?
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jc wrote:

the higher the frequency the easier it is to pick up
go for 60Khz
dan
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I think that's going to set up a magnetic field that is of fairly constant strength inside the coil and drops off outside the coil (not all that quickly). That works well for allowing the receiver to move around inside the room without creating large signal level variations. I'm not sure if that's going to be as easy for your bot to detect because it might have to move a good distance outside the coil before the signal drops enough to be sure you are actually outside the coil. And at the same time, for a large yard with odd shapes, the different signal strengths inside the coil might really confuse it. (not being able to tell if it just hit a low signal spot, or if it has left the yard).
Not to mention the problems your electric motors might create in terms of creating magnetic fields that could confuse a purely inductive circuit trying to detect a very low level magnetic field.
My guess is that you don't want to use an inductive loop but instead, use a simple radio transmitter and receiver. But I could easily be wrong!!!
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Remember the old drive in movies? Underground cable acted like a transmitting antenna to your car radio to hear the movie. Ditmico may have history on this. Also, a past client of mine, Cart-Tronics, uses brined cable around a Safeway or other grocery store that will trigger a mechanical lock on a shopping cart to prevent theft. So the technology is old and should be available for gratis.
Wayne

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I could be wrong, but I suspect those systems are nothing but standard radio transmitters and receivers. They are just tuned to activate only when the signal is strong enough - which only happens when you are within a foot or two of the transmitting wire. Signal strength goes up exponentially as you get closer to the transmitter so it's simple to pick a level that will allow you to detect distance to the transmitter when you are close to the antenna.
You can probably make it work by using nearly any radio transmitter and receiver set with the correct power levels. I don't think you would need any type of fancy antenna or anything. The simplest would just be a constant wave transmitter, and a receiver that outputs an analog voltage relative to the power level received - a simple L/C tuned circuit with a diode and filter capacitor might be all you need (no power needed for the receiver - like a crystal radio - you won't even need an antenna). Run that to an A/D input on your robot and calibrate it for the effect you want. Something along that level of complexity I would guess should work fine to keep the bot in the yard. Plus, with the A/D, you get a signal strength reading that lets you know how close you are to the wire and whether you are getting closer to it or moving away. That might help you write code to correctly stay away from the wire.
If you just search for science fair circuits for a simple radio transmitter and receiver you can probably find something that work.
I can't say I've tried this, so it might not work as well I would suspect, but it wouldn't cost much to build a very cheap circuit and try it.
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jc wrote:

That's the sortof thing Robo-Lawnmowers use (some anyway). I would imagine that putting 100Khz through a single wire around your yard would give you a good field. (Don't ask me any specifics because I'm pretty new to radio =D). To pick it up, using a ferrite rod coupled with an op-amp or Darlington pair should work. The drop off distance for the single wire should be very short and you can tune that by varying the power you put through it.
Hope this helps a little
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Brendan Gillatt wrote:

iirc my tests found it worked better without the ferrite rod, also I learned interesting, but obvoius polarity things
robot platform ___________________ O ----------------------wire------------------------------
with the wire running right to left, you want the spool orientated flat, wound on around the 'O' front of the robot to the left, back to teh right
this was using 48V relay coils for pickup with no core, that combination yielded by far the best signal, iirc it dropped off quite quickly as you moved off the transmitt wire
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Dont forget about lightning strikes, you need to protect your circuitry that is generating the signal because a lightning strike a mile away could cause some good voltage spikes on that wire loop. -howy
jc wrote:

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