Re: [mowbot] Buried Boundary

Dave Everett (deverett nospam at
Thu, 31 Jul 1997 15:08:53 +1000

At 09:40 AM 7/29/97 -0600, you wrote:
> Some random thoughts with respect to boundary marking using an
> electromagnetic signal.
>Have you looked at the "radio" fences for pets? They seem to
>have detection up to about 0.6 meters above the cable. One may
>be able to use one of these almost intact, or reverse engineer

Now that is a genuine gold-plated idea! I immediately tracked down a few
systems on the internet and the operation sounds ideal.
>Does Tandy carry a "buried treasure finder", if they do, one
>nice thing about Tandy is that you can get the service manuals
>and schematics for just about all the items they sell.

Yes they sell those things here, as a matter of fact I found a schematic on
the internet for one.

>Be persistent, they are available no matter what the clerk at the
>local store may tell you. I've bought several service manuals
>for things I never owned, just to see how they did it.

Hehe, yep I've been down that path myself :-)

>I liked your idea of using the buried cable for more than
>boundary marking. You could bury several different coded cables,
>1) absolute boundary, 2) path to charger, 3) path to back yard
>over the driveway, 4) mow on even days, 5) mow on odd days, etc.,
>you get the idea. They could be differentiated by frequency, or

Yes I can see how that could be done by imposing a modulation on the
carrier frequency.

>For testing various frequencies, waveforms, transformers, or
>drivers you may want to use a coil of wire attached to a scope
>probe as the detector.

That's how I did my test with the Tandy inductive pickups. I set my signal
generator for sine wave at about 17khz and full amplitude driving a 5w line
transformer. The output windings were run though short pieces of wire to an
8 ohm resistor completeing the loop. I connected the pickup (which had the
output loop wrapped around it) to a CRO and played around with cap values
to try to tune the signal. In technical terms, the signal was 'pissy'.

Dave Everett.