How Can You Build a Locker Alarm That Activates When Wires Separate?

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EllBe
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Please help me with my school project!

I'm in AP Physics in High School right now, and the teacher just assigned our class to design a locker alarm. It must turn on when the locker is opened and turn off when the locker is closed. However, there's a restriction: the alarm must turn on when 2 wires separate, not when they come together. I'm pretty stuck with this project because of the restriction...anyone has any idea on how to build the alarm in a way that would satisfy the restriction?? Thank you very much!
 
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EllBe said:
I'm in AP Physics in High School right now, and the teacher just assigned our class to design a locker alarm. It must turn on when the locker is opened and turn off when the locker is closed. However, there's a restriction: the alarm must turn on when 2 wires separate, not when they come together. I'm pretty stuck with this project because of the restriction...anyone has any idea on how to build the alarm in a way that would satisfy the restriction?? Thank you very much!

Welcome to the PF.

It's your school project, so we can only offer small hints.

Are you familiar with transistor circuits yet? You could have a transistor "sense" when the input "2-wire" circuit is opened...
 


You could use a dp relay and wire it up so the relay breaks the alarm circuit when the wires are closed and completes the alarm circuit when the wires break.
 


rollcast said:
You could use a dp relay and wire it up so the relay breaks the alarm circuit when the wires are closed and completes the alarm circuit when the wires break.

Good hint. That's probably simpler for him if he's not familiar with transistors.

Let's all wait for the OP to come back with a reply. They should be able to get quite far with the relay hint.
 


Thank you for the hints! :)

The teacher didn't tell us much about the transistors though we have to use one for this project. She provided us with resistors, a NPN transistor, LED, buzzer, wire, battery and the battery connector; and we have to use all of them.

Can you explain for me what's the dp relay?? xD is it the differential protective relay?? ty
and I'm a girl btw :P
 


EllBe said:
Thank you for the hints! :)

The teacher didn't tell us much about the transistors though we have to use one for this project. She provided us with resistors, a NPN transistor, LED, buzzer, wire, battery and the battery connector; and we have to use all of them.

Can you explain for me what's the dp relay?? xD is it the differential protective relay?? ty
and I'm a girl btw :P

A DP relay is a double-pole relay:

http://en.wikipedia.org/wiki/Electrical_relay

It's good that you can use a transistor -- the circuit will be lower power that way (the standby/off power consumed).

Do you have an example circuit of an NPN transistor being used to drive your buzzer? That should be pretty easy to find in the buzzer datasheet or by doing a Google Images search...