perimeter fence detection

There seems to be very little information about the robomow perimeter fence, apart from the obvious that an alternating current is passed round the perimeter wire producing a magnetic field.
To detect the magnetic field I first attempted to reuse the sensors built I to the robomow on the assumption that they were induction coils with a lm386 amplifier, but I got no signal from them.
I got hold of some 100mH induction coils and wired them up to the lm386 amplifier and discovered the signal!!!
It appears to be a square wave of about 4.5kHz with roughly a duty cycle of 25%, the following image is the data captured by the Arduino.
I could clean up the signal, either with some extra electronics, or via a software algorithm, but as I’m only interested in the amplitude of the signal (it will be larger then the inductor is over the wire) I don’t think I’ll bother …
… just as an aside the inductor goes absolutely mental when it’s near my induction hob!!! (as you’d expect)

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