Re: [mowbot] Mowbots and Radio Fences

Dave Everett (deverett nospam at
Sat, 25 Apr 1998 09:24:57 +1000

At 01:49 PM 24-04-98 +0000, you wrote:
>Well, spring has sprung in North America and it time to dream about
>mowbot again.
>I've decided that a radio "wire fence" would be the easiest way to keep
>the beast in my yard. At worst I could do a random walk to mow as
>long as it stayed in the yard.
> Wasn't someone on this list experimenting with a radio fence?
Yeah That was me Monta. I used one of those 100watt PA transformers and fed
an AC signal into it from a signal generator.

To receive the signal I tried out various things: AM radios, which work
quite well, in fact I bought a little kit from Dick Smith Electronics that
was a single chip AM radio. My thinking there was that I could buy those
ferrite AM tuner coils pre-built and repeatable so I could have multiple
sensor points on the robot. I wanted the robot to be able to straddle the
wire with 2 sensors so I wouldn't have the uncut boundary problem of the
Solar Mower.

I also tried winding my own coils, I thought I was susccessful, until I
tried to wind more exactly the same. I was using the standard threaded
cores with ferrite slugs so I would be able to peak the tuning, but on the
scope they always seemed to have unmatchable centre frequencies.

If you are simply planning on a single sensor, then the AM radio idea would
work fine as you can connect the 567 somewhere on the speaker circuit.
You'd need to up the freq of your signal, tune the radio to a blank spot
and then retune the signal generator to that region.

This may not be entirely legal in your area, but if you keep the energy low
it's unlikely to ever come to anyones attention.

> Does anyone have any suggestions for building a receiver for
> longwave radio (in the few score of Hz range)?

Radio Electronics had an article on a long wave converter many years ago,
sorry I can't remember the year.

Dave Everett