Help with Secured Wire Loop Circuit - 2N7000/2N3906 Transistor Questions

[Inspiron1]

Hey guys. Can someone help me with this circuit?
http://www.discovercircuits.com/H-Corner/secwireloop.htm

Where it says "beeper" do they mean a speaker or do they mean a buzzer?

And can I replace the 2N7000 transistor with a 2N3906?

 

[Inspiron1]

Simpler question: What does the D G S mean around the transistor?

Shouldn't it say E B C?

 

[Bob S]

D G S means drain gate source. The 2N7000 is an N channel FET (field effect transistor). Type in "2N7000 datasheet" in Google search to find one. The beeper is one of those units that beep when you apply a dc voltage. You could use a transistor instead of a FET, but the current to power the base will be more, so the drain current from the battery will be more.. The circuit is laid out for a NPN; the 2N3906 is a PNP.

 

[Inspiron1]

Will someone help explain how this circuit works? I don't understand what the transistor does.

 

[dlgoff]

Hey Inspiron1,

The security loop wire grounds (neg. side of battery) the gate keeping the transistor from conducting. When it is opened (breach of security), the gate voltage goes to 9 volts which causes a current to flow through the buzzer via the transistors drain to source conduction.

 

[bipolar]

It can be a speaker or a naked piezo element. When the loop is broken, the circuit will oscillate with the capacitor on the gate.