Wire loop alarm

Description.

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.

Circuit diagram.

wire loop alarm circuit

Notes.

  • 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.
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