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Help with calculating Hz in a siren curcuit

Discussion in 'Electronics Homework Help' started by SaintAralor, Oct 9, 2016.

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  1. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Hi, this isn't really homework, but this seemed to be the best place to post it :)

    Could anyone help with how to calculate Hz in a siren system? Here is a picture of the system: http://imgur.com/a/N9E6Q

    The Hz of the first astable 555 circuit is supposed to be 1 Hz and the other system is supposed to be 1kHz.

    I used this formula: http://imgur.com/a/SZipc (the one on the top called "to calculate frequency")

    I tried to put 3.3u as the c value and 0.01u (converted to Farad of course). I also tried to put both in series and in parallel, but i couldnt get 1Hz!

    What am i doing wrong here?

    All help much appreciated, and thanks for your time!
     
  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Putting those caps in series or in parallel is wrong. Check the schematic.
    Welcome to EP!

    Edit: Replacing the 1meg resistor with 270k will get you closer to 1Hz.
     
    Last edited: Oct 9, 2016
  3. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Yea figured as much, but i just tried it anyway because i couldnt get it to work ;)

    The schematic as is should get those reults of 1hz and 1khz, and i just don't see how they got that
     
    Last edited: Oct 9, 2016
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    The frequencies are 0.36Hz and 327Hz to 1kHz according to LTspice.
     
  5. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Hmm, do you know what formula you would use to calculate that?
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    Not off-hand. Have you tried this calculator? It agrees with the LTspice simulation.

    Googling '555 timer formula' will answer your question.
     
    Last edited: Oct 9, 2016
  7. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Yea thats what i get on my own as well.. Does this mean that my book is wrong? seems weird
     
  8. SaintAralor

    SaintAralor

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    0
    Oct 9, 2016
    Ive asked on a few other forums as well, and it seems that most agree that it should be 2 and 0.5k, so i guess im rolling with that, and just assuming the book is wrong. Thanks a lot for your help, ive really been pulling my hair out over this ;)
     
  9. Alec_t

    Alec_t

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    Jul 7, 2015
    Looks like it. Can you post the formulae that the book uses?
     
  10. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Thats the problem, the book doesnt show what formulae it uses.
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    Not a good book then :(.
     
    Last edited: Oct 9, 2016
  12. SaintAralor

    SaintAralor

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    Oct 9, 2016
    Agreed haha :)
     
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