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Long stretches of steel fence can be monitored using the simple circuit below. An
insulated wire is woven through the links in the fence. Where the fence begins,
the circuit is connected to an earth ground and the security wire. At the end of
the fence, the wire is connected to one of the steel posts, held in the ground.
The typical earth ground resistance will be in the 1000s of ohms range, which is
low enough to keep the n-channel FET turned off in the circuit below.
If the wire is cut or broken, the earth ground
circuit path is interrupted, allowing the transistor to turn on. This will
activate a very loud piezoelectric type siren. I show the use of a cheap 9v
battery but a long lasting 12v battery can also be used. The circuit’s
current drain is a very low 2 microamps during normal operation, so any battery
will last many years. The popular 2N7000 MOSFET transistor is good for about
200ma. I would suggest an n-channel device with a higher current rating, if
you plan on using a louder, more powerful siren alarm. As an example, the
IRFZ34 device could drive a 10 amp siren. All of these parts are available
from www.mouser.com.
Click on Drawing Below to view PDF version of Schematic
