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Circuits homework problem

  1. Oct 22, 2012 #1
    1. The problem statement, all variables and given/known data

    I actually have posted this question in another part of this forum, but due to lack of responses, I felt maybe I posted it in the wrong section.

    Can you all take a looksie please?

    https://www.physicsforums.com/showthread.php?t=645676
     
  2. jcsd
  3. Oct 22, 2012 #2

    Simon Bridge

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    Re: Circuits!

    You seem to have all the information you need to design the circuit in that thread.
    What's the problem?
     
  4. Oct 22, 2012 #3
    Re: Circuits!

    I'm somewhat lost as to HOW to make it. My thinking was to use the 100 ohm wire to surround the window. The buzzer would be parallel to one side of the window and the voltage source will be parallel to another side of the window. There will be resistors in series in between the wire connecting the buzzer and the voltage source.

    When someone breaches the system, shouldn't the voltage drop through the resistors?
     
  5. Oct 22, 2012 #4

    Simon Bridge

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    Re: Circuits!

    You're thinking of putting a loop of wire around the window?
    How is that supposed to work?
     
  6. Oct 22, 2012 #5
    Re: Circuits!

    I was thinking it would act as a small circuit of a sorts. But I've been questioning it as it seems off.
     
  7. Oct 22, 2012 #6
    Re: Circuits!

    Perhaps the loop around the window can be connected Tina smaller loop that us attached to the window itself?
     
  8. Oct 22, 2012 #7
    Re: Circuits!

    Actually come to think of it. Putting a loop AROUND the widow won't do anything. If anything the loop should be put around the glass so that when the window is closed it's kind of shorting the circuit. The moment I lift the widow if should allow voltage to flow.
     
  9. Oct 22, 2012 #8

    Simon Bridge

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    Re: Circuits!

    OK - I think I see where you are struggling.
    Try turning the window into a switch.

    Your circuit diagram should include the switch in such a way that, when it opens, the alarm sounds. That help?
     
  10. Oct 22, 2012 #9
    Re: Circuits!

    I would use a switch but that's something we're not allowed to use. No switches or anything of that sort....
     
  11. Oct 22, 2012 #10

    Simon Bridge

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    Re: Circuits!

    The wire is a switch.
    You have to think outside the box - just because it dosn't have a label on it that says "switch" does not mean it can't be used as one.
    Anything that can be used to conditionally break the circuit is a switch. I mean - you start out with the circuit unassembled - so no current flows right? When you fit in the last component - you turn the power on: that was a switch.

    What are you supposed to use to fix the alarm system in place on the window with?
    Are you allowed to cut the wire?
     
  12. Oct 22, 2012 #11
    Re: Circuits!

    I see what you're saying! Being that the window is a switch itself, when said switch is closed electricity will flow without Interruption. However a disruption will cause an alarm to sound.

    I'm not sure how to put it in electrical components is what in stuck on I suppose
     
  13. Oct 22, 2012 #12
    Re: Circuits!

    You haven't let us know all the details. There has to be a reason why 12 AAA batteries are specified. Do you have the characteristics of the buzzer?

    If the burglar cuts the wire along the top frame (say through the top resistor), how does the peizo get the power to sound off?

    I can see two ways to secure a window with just wire: one is to detect broken glass by running wire around the glass at a smallish outline and the other is to detect an open window by running wire from the frame to the window edge, maybe in more than one place. Both depend on breaking the wire to detect a problem (as you already mentioned), which means your circuit needs to detect an open circuit from a safe short circuit. It also means you can use the same length of wire to circle the glass and frame, and perhaps it can be run past several windows and doors, since any break anywhere should set off the alarm. If you need to have an idea of where the break occurred, then you need separate wires to detect problems in separate zones.

    The fellow in the other thread already mentioned this, but to spell it out, you want to short out the buzzer with the wire so that 0 volts appears across the buzzer under normal operation. When the shorting wire is broken, the buzzer sees some voltage across it and sounds off.

    Next you might want to look at how to make things tamper proof. Can the burglar do anything to foil the alarm if he has access to the wires running along the window? (I don't see how to detect this without using an amplifier using a wheatstone bridge to compare the resistance of the wire network to a reference). The thing is powered using AAA batteries... can you do something to extend battery life or detect a low power condition to alert the customer to replace batteries (again, normally I would say make the buzzer chirp but you can't do that with just resistors and sounding off the buzzer constantly may be a way to kill the battery while the customer is away from home).

    Then see if there are any gotchas... you mentioned adding resistors in the loop to prevent false alarms. I think the place you read this from was concerned about flux through loops of wire or maybe changing capacitance between objects moving with respect to the wire. This is not going to be enough energy to sound off a buzzer and would only matter to sensitive pickups. For example, if the amplifier and wheatstone bridge I mentioned was in play, it may detect induced voltages along the wire and falsely set off an alarm.
     
  14. Oct 22, 2012 #13

    Simon Bridge

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    Re: Circuits!

    That's the idea - draw the circuit diagram that does this first, then worry about implementation.

    One example would be to put some wire across one corner of the window - when someone opens the window, the wire breaks ... you'll need to do better than this though.

    Presumably you have to fix the device to a window with something??? I mean - the wire wouldn't have stayed on the window-frame by itself.

    You do need to think about a threat model too - and maybe ways to get around your initial design (so you can improve it). But you should get the basic one working first.
     
  15. Oct 23, 2012 #14
    Re: Circuits!

    I'm sorry if this isn't glaringly obvious to me...circuits isn't my strongest suit. I'm doing good in all courses (As and Bs) except this. I'd like to develop more on the idea of having a circuit around the widow connected to a small safe circuit.
     
  16. Oct 23, 2012 #15
    Re: Circuits!

    I drew an outline as suggested above:
     

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  17. Oct 23, 2012 #16
    Re: Circuits!

    Your circuit sounds the buzzer constantly :)

    A 'safe circuit' connected at one point cannot alter the operation of your window circuit. The current flowing through the buzzer is controlled by the voltage across the buzzer, which is set up by the battery connected across it (and the resistor). A handle on one point that can raise or drop the absolute voltage of one side of the buzzer cannot change the voltage across the buzzer set up by the battery. The battery has one terminal connected to that point and then adds a voltage to that at the top terminal, to set up the voltage difference seen by the buzzer.

    To interrupt the buzzer, a 'safe circuit' would need to either alter the voltage across the buzzer (which requires a handle on two points) or physically detach the buzzer to break the circuit. You do have a means to physically break the circuit -- the broken wire -- but if you placed this window wire in series with the buzzer, your alarm system will not work as intended. The circuit under normal operation would constantly sound the alarm and only stop when the wire was broken.

    So you should look at how to alter the voltage across the buzzer. There are two ways to do this: one is by connecting something in parallel (for example, what happens if you attach a piece of wire across the buzzer) and the second is by inserting something in series (for example another battery) that reduces the voltage across the buzzer. One of these ways is going to be more power efficient and may not be doable given the materials you have to work with but you should look at both just to gain a better understanding of circuits.
     
    Last edited: Oct 23, 2012
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