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Homework Help: AND and OR circuits

  1. Oct 31, 2013 #1
    I can't answer Q3 of:

    A spin-drier has a start-stop switch, but the motor will only spin the drum if the lid is closed. Closing the lid closes an SPST switch inside the machine, and the motor can then be started with the start-stop switch.

    2) Draw a diagram of a simple circuit for this (done - with battery, SPST, push switch and motor in series)

    3) Add a buzzer to your circuit diagram so that the buzzer will sound if the lid is not closed and the start-stop switch is operated (remember that a buzzer requires much less current than a motor does).

    I cannot work out Q3. If I add a cell and buzzer in parallel with the push switch it would sound when the lid is open and when the lid is closed so this wouldn't work.

    Any help would be much appreciated!
  2. jcsd
  3. Oct 31, 2013 #2


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    Hi there
    welcome to PF :smile:

    Is this homework ??
    looks much like it

    How about starting with telling us what you can figure out
    have you attempted to draw the circuit ? .... show us so we can then guide you

  4. Oct 31, 2013 #3
    I can't draw it on these forums can I? I am a first year teacher who hasn't done physics for 16 years. I want to go through this with my class tomorrow. If you could guide me then I will be able to understand it. Thanks.
  5. Oct 31, 2013 #4


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    Hint: What is the voltage across the lid push button when the lid push button is open/closed.
  6. Oct 31, 2013 #5


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    No, but you can draw it elsewhere and either attach the image to your post, or host it on the web and just link to the image (preferably using [noparse][/noparse] tags, so that the image is displayed as part of the post).
  7. Oct 31, 2013 #6
    Ok, I'm afraid I don't have time to draw and attach/scan - it's already evening here (Kenya), my work day is over and my elementary scholarship class starts at 7am tomorrow. I'd really like to do it with them if I can get an answer to this question tonight.

    Watters - the voltage will be nil when the switch is open and something when the switch is closed. How does this relate? I assume that if I add the buzzer with a separate battery in parallel with the push switch, some current will still flow around the parallel circuit when the large switch is on, so this wouldn't work.
  8. Oct 31, 2013 #7


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    That's not correct.

    With the main ON/OFF switch ON and the lid switch is OPEN (=off) the voltage across the lid switch will be the battery voltage. When the switch is closed the voltage will be zero because the switch is a short circuit.

    There is another hint in the problem..

    You have the right idea but won't need another battery for the buzzer.
  9. Oct 31, 2013 #8
    Ok, thanks CWatters, so what's the solution to the problem? Obviously the buzzer won't sound with the lid switch open as it is a series circuit, if I am right.

  10. Oct 31, 2013 #9


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    Just put the buzzer in parallel with the lid switch...

    With the main switch ON and the lid open current flows from the power supply, through the ON/Off switch, through the buzzer and then through the motor back to the battery. Not enough current will flow to turn the motor because the buzzer draws little power (meaning it has quite a high resistance. Most of the supply voltage appears across the buzzer and very little across the motor. For more detail google "Potential Divider circuit"). The buzzer sounds.

    With the main switch ON and the lid shut current flows from the power supply, through the ON/OFF switch, through the lid switch, through the motor (causing the motor to run) and back to the battery. The buzzer does not sound because the voltage across it is zero (It is shorted out by the lid switch).

    Attached Files:

    Last edited: Oct 31, 2013
  11. Oct 31, 2013 #10
    Ah ok makes sense. Thanks a lot for your reply.
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