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Chain Link Fence Alarm
February 22, 2009 |
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. |
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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. |
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Click on Drawing Below to view PDF version of Schematic |
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eMail David A.
Johnson, P.E. about this circuit |
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