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Wily Widget [Issue
2, October 2009]
Previous Issues
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Fence Alarm
It seemed to
Wily that frantic phone calls from clients always tended to come in the morning. He
had just taken his first sip of morning coffee when the phone rang. It was from a
cattle rancher in Montana. It was deer hunting season in Montana and this rancher
was fed up with hunters cutting his barbed wire fence, to gain entrance onto his properly.
He called and wanted to know if there was any way he could receive an alarm signal
whenever the fence was cut. The rancher had looked on the Internet and couldn’t find
anything that seemed appropriate for him. After talking to several people, Wily’s
name was suggested.
Wily said he might take on the project, but needed some
more information. The fence was a classic cedar post with 4 barbed wires. The
fence posts were spread out 12 to 16 feet. Most often, the hunters would shoot a
deer on the rancher’s property from the road and then cut the fence, so they could drive
up to the carcass to haul it away. They often did this opportunity hunting early in
the morning and were in and out before the rancher even got up. Shooting from the
road toward his ranch was against the law and so was cutting the fence. But that did
not stop the hunters from doing it anyway. The rancher wanted to know the second the
fence was cut, so he could confront the poachers. |
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The fence section most
often cut bordered a gravel county road, which ran along one section of the ranch’s
land. The fence line ran about 2.5 miles along the road. There was another
section 0.25 miles from the road to the ranch house. The other fence sections
bordered a neighbor’s land with no roads, so it didn’t need an alarm. |
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Wily gave the rancher
a rough estimate for a working unit. It was more than the rancher wanted to
spend but in the end he decided that it would be worth it. |
Wily told the rancher that a
single 22ga insulated wire, with outdoor UV rated insulation, would need to be
installed along the fence to be monitored. Wily figured that the wire should be
placed along the second wire from the top. This position should keep small
animals from chewing on the wire and would be less likely to be broken from deer
jumping over the fence. |
Wily had the rancher pound into
the ground two 8 foot long copper rods. These were the type usually
installed next to a home’s circuit breaker box and were available at any home
supply center like Lowes or Home Depot. He had him install one ground rod at
the end of the 2.5 mile long fence and a second near his ranch house. The
insulated wire was wired to the copper rod at the end of the fence. |
Once the wire had been laid and the
copper rods placed in the ground, Wily had the rancher measure the resistance
between the wire and the ground. It measured about 10K ohms but Wily
figured that in dry seasons, it could be much higher. Wily designed a circuit
that would monitor the resistance and when it exceeded 100K ohms, it would
activate a very loud siren alarm. |
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The rancher decided he
wanted the unit to be powered from a wall adapter so he would not have to deal
with batteries. Wily provided him with a source for such an adapter and for
a very loud siren alarm, which at 12v would wake the dead. |
Wily built up the unit in a small
plastic box, so it could be placed inside the ranch house. He installed a
power jack for the +12v input and a terminal block for the earth ground and sensor
wire connections. Two more terminals were connected to the siren.
Wily also installed a small normally closed pushbutton switch. When
pressed, the button would test the circuit by simulating a broken fence wire.
Pressing the switch should sound the alarm. |
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Wily Widget's Wire Fence Alarm Schematic |
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Wily
Widget the Lone Inventor
Gadget & Gizmo |
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