[mowbot] Mowbots and Radio Fences

Monta Elkins (Monta nospam at bev.net)
Fri, 24 Apr 1998 13:49:12 +0000

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.

Radio Fence
So I've been looking at longwave radio transmitters and burying a wire
around the perimeter of my house and yard. unfortunamte I know
very little about RF but heres some of what I've go so far.

I built a "tranmitter" - its a 555 timer at about 42Khz, it passes
through a jk flip-flop to give it exactly a 50% duty cycle (at 21Khz)
and then uses a couple of 2n222 transisters to switch current through
an 8ohm audio transformer. (Hey 21Khz IS almost audio).

I can pick up the signal from a few inches with a coil of wire and my
'scope, but I'm having trouble building an amp for it. I've tried a
"short" coil that I wound myself and an op amp
(my first experience with them). But I can't
seem to receive and amplify the signal.

The goal is to amplify it and then send it to a 567 tone decoder to
detect the perimeter.

Commercial Fence
I considered purchasing and modifying a commercial "radio fence" used
to keep dogs in the yard- but decided I'd learn more about RF by doing
it myself. See http://www.radiofence.com - they start at about $100.
Schematics for them are available from their patent - search for radio
fence at http://www.patents.ibm.com/ibm.html

Long Wave Radio
I have recently done a little research about Long Wave and found:
It looks like longwave radio propagation is great for my application.
Folks use it to penetrate the ground. They use it in caving
expeditions for rock penetration. The military uses it for
underwater penetration and utility companies use if for buried
cable location (BINGO).

I looked at FCC regulations part 15 (the U.S. organization the
governs the air waves)
the 160-190 kilohertz band is availble, no licence for less than
one watt (but a less than 15 meter antenna) :(

the 9-490 khz is available for cable location, peak 10 watts
but only temporary use.

I think I'll keep building in the 20 khz range for now to allow
the use of "audio" parts. In a pinch I think I could send the
output through my 100 watt stereo amplifier and control the
power with the volume control. I could probably change
to the 160khz band if necessary.

I think I know one reason why I'm having so much trouble
building a good reciever antenna- a quater wave length is
a little over 2 miles!

Wasn't someone on this list experimenting with a radio fence?

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

-Monta "still mowing the hardway" Elkins

"And the world will be better for this: That one man scorned and
covered with scars, still strove with his last ounce of courage;
to reach the unreachable stars"
System Administrator - Blacksburg Electronic Village - www.bev.net
monta nospam at bev.net - http://www.bev.net/project/people/monta