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    Wily Widget [Issue 2, October 2009]                                                                                            Previous Issues     
  

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.

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.

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. 
       

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.
 

Wily Widget's Wire Fence Alarm Schematic

Wily Widget the Lone Inventor     Gadget & Gizmo


October 2009      Issue 2

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Circuit
Diagnosis
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Good Idea
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New Products Rants &
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What the World
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Wily Widget


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