Here is the circuit diagram of a very simple wire loop alarm based on a N-channel enhancement FET 2N700, a buzzer and few passive components.In normal condition the gate of 2N700 (Q1) is connected to the ground through a 100K resistor (R2) and the wire loop. When the wire loop is broken , Q1 will be biased to ON state and the buzzer is activated.The circuit consumes very less standyby current and so the battery will last for a very long period of time.
- Assemble the circuit on a good quality PCB.
- The circuit can be a powered from a 9V PP3 battery.
- K1 can be a miniature 9V buzzer.