Alarm. - Here is Circuit Schematic Chain Link Fence Alarm using 2N7000 MOSFET. 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.