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More  Alarms, Security

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.  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

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