Re: [mowbot] Active borders.

Dave Everett (deverett nospam at
Wed, 26 May 1999 07:48:37 +1000

At 09:47 25-05-99 -0500, adamdb nospam at wrote:
>Does anyone have any schematics or references for an "active border"
>system? (You know, kind of like the invisible dog fence, only for my
>mowbot). I am building my mowbot with the intention of using grass
>sensors to try and stay in the uncut grass, but just in case I would also
>like an active border so I don't mow down the wife's garden by accident.
I started playing with this idea a few years back. I got a long piece of
cable and looped it on one side of a 100v line transformer. The other side
was driven by a waveform generator.

ON the robot side I had an AM radio tuned between stations, then I adjusted
the waveform freq until I got peak performance.

The only drawback was that most am radios use a flat bar ferrite for the
antenna so it was very directional. I started building a toroidal antenna
but by this stage I'd decided to dump the wire loop idea and work on an
alternative form of border detection.

Dave Everett.